LIBRARY Want FUE : , St =i tom iF i ~~ * Digitized by the Internet Archive in 2010 with funding from University of Toronto http://www.archive.org/details/distributionofveOOlivi THE DISTRIBUTION OF VEGETATION IN THE UNITED STATES, AS RELATED TO CLIMATIC CONDITIONS BURTON E. LIVINGSTON AND FORREST SHREVE Su, tee lbS8Uo a PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON 1921 '? i CV Orde enya i” ' aA he : Pv 17 a Ole” a! eee rab } er , : e » ae ; ; bd iy oa ‘ os sea Pht AAT D Ty, CARNEGIE INSTITUTION OF WASHINGTON ia PUBLICATION No. 284 . if ; = bf s mA Lad . es 7 fees ih i q t : re »* 4 iy rtf HED Alig st = en ' Pir Ste ar ke ok! x. : , i J ) Fi i’ j cA. 44 4 5 > } rth wi pe aired 333 ~ PRESS OF GIBSON BROTHERS, INC. atte asth> Gey F : WASHINGTON, D. C. . ie a ‘ CONTENTS. LE FBTR SS SAY eel aaa Aah Bice oO SU nee Co) Pee SR a RL Part I, Tue VEGETATION OF THE UNITED STATES. INTRODUCTION: : I. The distribution of vegetation in general, as related to climatic conditions. . Il. Study of the distribution of individual species....................0000- Ill. Manifold operation of environmental conditions......................-- Merete Me -Mee MI OF PMAMLEA oye Cicioc fercts cldcsue. cl nga. ins aero kD vie ara We Mee OE bises TaBLE 1.—Analysis of Drude’s criteria for distinguishing PLOW MH UOREASE & nti Taire Shs, o marche a Gat ae Ce s)/el\s)\ 8) ,sy (8) 6) @ law aa eile iene) (a(\ a ver le:s),s aie re: \@ (6 Le: (6, 18! ©) a. Ferie'(6 (a0 \e)\0 @ (@\e ea oleae) e DISTRIBUTION OF VEGETATION IN THE UNITED STATES: I. Methods used in securing and presenting the distributional data......... II. Leading vegetation types of the United States and their geographical areas III. Distributional areas of conformic groups of plants..................05- IV. Distributional areas of selected individual species.................0205- Part IJ. ENVIRONMENTAL CONDITIONS. OMETEL NTS CCRT ONG Hy at hae has she LE aI A ID es eee ee re a i es ee GENERAL INFLUENCE OF THE ENVIRONMENT ON Puant LIFE: I. External and internal conditions and plant activity...................0. Mecameory OW piysiolomicnl Timnitey.. 6. josie cis eet Fae d Ske die ss a oo 2 « Couecs gone leons III. Relation of plant distribution to the physiological limits of the various EVID INGREAL DRBSOM See cic tanis 2. tices, SMa sR Poem is uate vee IV. Genetic continuity of protoplasm and its cyclic activities, in connection Wn PFODIEMIs Of CIStTIDUGION... os vio, t's 2 oe tao be ale Re oem Reo. oe eens Cuter ENVIRONMENTAL CONDITIONS AND THE GENERAL NATURE OF THEIR Errects Upon PLants: 1. General classification of environmental factors...............0000eeeee II. Moisture: 1, Water requirement within the plantic.<:. dc dacisd se. ac cece ad cews 2. Supply Obwater To thes plata sed sa fei ica ast ee mios bee alse aesib oe 3. Relations between water-requirement and water-supply........... III. Temperature: 1. Temperature requirement within the plant...................00. 2. The relation of temperature within the plant to conditions of PTY ATOMUAANU, 21 eerie aactiee oie Deel. SiS .< pial! thea e's ¢ Fie aes 3. The duration aspect of the temperature relation.................. IV. Light: ie Ganeral nate of Weng ays saute ok Fis cyiereiela cna sees b hives oo oma oe PeMEectiGtelie hia pOn, plats itis veeevee a ceret seh sents a cisielerers icke a mires 3. Duration aspect of light relation of ordinary plants............... V. Chemical conditions: 1. Requirement of material within the plant....................05. 2. Material exchanges between the plant and its surroundings........ ay Chemienl environment im nap ite t28 bcs bd gle san esc dae ene 4, Duration aspect of chemical conditions. ................s.sesesces VI. Mechanical conditions: Bp CHC EAL CORBI CERIO hath Sythe Sven wigiin ares «a sid aie da) Gis ee eae foe 2. Destructive influences of mechanical conditions................+- 3. Favorable influences of mechanical conditions................000% VII. Interrelations of the environmental conditions................00+0008 VIII. Experimental determination of relations between plant activity and ENvArON MeN tal CONGIGIONS we. cyoppaptate siatapsbaae = sie Jalal 498 2... Discussion of the; ODSERVALIONS: emheisrcndatitel > adeiac/clctdcimiets arate 515 1V. Conditions that probably determine the life-zones of Merriam. 1. ‘Observations troni the Charts. acces cine erase ieee ores teeta eee 519 2. Discussion/of the: observations «seas veel ae a bast tet. olde 526 V. Conditions that probably determine the distribution of growth-forms and the ecological distribution of individual species. 1 GG Eb=lor mas: SoA kate Sek oi ek eres fone a ere eee et ora es ee 529 9; Species... ikiseds Ab eee. belo cae deca SU ae 537 VI. Correlation of vegetational areas with generalized climatic provinces. 1. IntrogUctery ojos crane «ata ambient Asie tials, a0 1gnl a 2 oe 570 2. "Teniperanture! provinieGs: 500.0 hen dn Gene laa os soe cs Sa ee ee 571 8. Moisture provitioagiints Aico MAA td) HARE 572 4, Temperature-moisture provinces based on product index.......... 576 5. Two-dimensional climatic provinces..................-..eeeeeee 578 OMCLUBION: (h..3 0 isola ecu le Bae ae remo wi Ae Tlie tow Pain a Seid oye als re 581 DImRATURE: REFDRENCHS2hl) 3 fad (dons soda he ee aida s alles aehd vis & tM ieee 587 PREFACE. The differences in plant life which exist between distant or even between nearby localities must have come under the notice of man in the earliest semicivilized stages of his existence. Human depend- ence upon the products of the vegetable kingdom has served to maintain throughout all historic time a vivid realization of the vegetational differences encountered with changes of latitude, alti- tude, and proximity to the sea. The later stages of modern civiliza- tion have done extremely little to liberate man from his dependence upon plants, although the development of methods for the preserva- tion and transportation of food has given him greater freedom of movement into the jungle, the polar regions, the desert, and the modern city. Many of the activities of the last 150 years have been such as to increase our interest in the distribution of plants and in the nature of the plant populations which characterize different regions, different soils, or different topographic situations. Innumerable bands of ex- plorers and collectors have penetrated all parts of the world, bringing back materials upon which we have been able to base a knowledge of the flora and the larger aspects of the vegetation of all but the most inaccessible portions of the globe. The extensive introduction of economic and ornamental plants into new regions and even into new continents has awakened an interest in the possibility of still further introductions and in a study of the causes of the success or failure of such as have been made. The increasing value of all the products of the forest has led to the planting of trees on a large scale within their native regions and to the experimental introduction of trees from dis- tant countries, as well as to attempts to improve natural forest stands. The increasing population of the world has augmented the value of its agricultural lands and has given importance to the study of the proper- ties of the soil in their relation to plants. The search for new agri- cultural regions and for crop plants adapted to the conditions in newly settled areas has led to an interest in natural plant growth as an index of the most promising soils or of the most suitable crops to be cultivated. Hand in hand with this widening utilization of the plant products of the world has gone a rapid development of the scientific study of plants in relation to their natural environment. During the first half of the nineteenth century there was a rapid accumulation of facts regarding the composition of the floras of the outlying portions of the earth, and these facts were almost as rapidly marshaled into an ordered knowledge of the great floristic regions. In this immense task the names of Humboldt, Schouw, Grisebach, de Candolle, Hooker, and Engler are intimately associated with the greatest accomplishments. The interests of plant geography in this stage of its development were Ix x PREFACE. largely confined to the distribution of species. Each species was of importance, because its distribution threw light upon the floristic affinities of the region in which it grew. Only a historical interest now attaches to the discussions of ‘‘centers of creation”? which occupied de Candolle and his contemporaries. The inevitable questions as to the origin and significance of the floral regions of the world were immediately given a new trend by the publication of The Origin of Species. Two features of the Darwinian conceptions and method were destined to be of the most fundamental influence upon the further development of plant geography and its later outgrowths. The first was the demonstration—which it is now difficult for us to realize as so recent—that the present features of plant distribution have grown out of the distributional features of the past, and the second was the emphasis which it laid upon the importance to the plant of the entire complex or constellation of its environmental conditions. In the hands of Darwin the great store of distributional facts served as a source of material for aiding in the demonstration of his new principle. .The multifarious structures of plants, as yet incompletely investigated by physiologists and anatomists, assumed a new signifi- cance as having an important réle in the existence of the individual and the race. For 20 years after the appearance of The Origin of Species there was keen activity in the reinterpretation of distributional facts and in the fresh interpretation of plant structures as related to environ- mental conditions. The study of plant distribution and of the differ- ences between the great floristic areas was now carried on as a dynamic subject, correlated with our knowledge of the geological past, and interpreted in terms of the importance of areas and periods of evolu- tionary activity, of paths of migration, of barriers, of the importance of isolation, and of relict forms. This path of investigation was tra- versed up to the point at which it became obscure and difficult. Its culminating achievements are recorded in Engler’s Entwickelungs- geschichte der Pflanzenwelt, a work which would even now admit of only minor revisions, 39 years after its first appearance. The fresh interpretation of plant structures, to which stimulus was given by the work of Darwin, was due to a new appreciation of the importance of these structures in relation to environmental conditions; but it was unfortunately the course of events for many years that the structures themselves received attention, to the great neglect of the environment. The principal attempt of the outdoor workers who had become imbued with the Darwinian conceptions was to attach a significance to every structure and habit in plants, no matter whether such significance had been experimentally demonstrated or merely seemed to be highly plausible. One of the most energetic and ingenious of these workers was Kerner von Marilaun, to whom we owe many acute observations, PREFACE. Xi interpreted by an imagination of unrivaled vigor. It was charac- teristic of this epoch that chief stress was laid upon the living environ- ment, or ‘‘biological factors,’ while little or no attention was given to the fundamental physical factors. Much careful work was done relative to the importance of insects for pollination—the structures in plants which serve to attract insects of a beneficial character or to repel harmful insects, mammals, snails, or toads. It is impossible, however, to overestimate the value of this period, in which travel and outdoor observation received such a great stimulus. Many facts were assembled, and the value of these was by no means vitiated through the frequently wrong interpretations that were placed upon them. The rapidly approaching completion of our knowledge of the floras of the world, and the inevitable slowness of all further investigations as to their origin and geologic history, have led to a great growth of interest in the natural assemblages of plants—in those plant communi- ties, large and small, which we designate as vegetation. For 25 years there has been an increasing interest in the study of vegetation. This has been partly an outgrowth of the relatively finished condition of the science of floristics and partly a result of the readjustment of the principles of the interrelation of the plant and its environment. The study of vegetation has already passed through the descriptive phase which ushers in every new branch of science, into its period of greatest fruitfulness, and has brought its leading problems to the point at which they demand for their solution a precise knowledge of the functional activities of the plant and an equally precise knowledge of the environ- ment. The subject of plant distribution and that of the relation of the plant to its environment are inseparable, and the study of vegetation during the past 20 years has been marked by a rapid coalescence of these two fields. In short, the old field of plant geography and the post-Darwinian field of environmental study have been brought together, and have pushed their problems to a point at which physio- logical facts and methods are of first importance for the next steps in their solution. The modern study of plant ecology may be looked upon as plant geography which has drawn its major outlines and has begun to give attention to details, or it may be regarded as a study of the relation of plant and environment in which the plant is viewed as a functioning organism and the environment as a physical complex. The study of the environmental control of the activities of a single species of plant has many differences from the study of the control of a plant population, in so far as concerns the more general features of the controls in each case, but in final analysis the two problems merge into each other. It is precisely this fact that has come to be generally recognized and has resulted in the coalescence that has been alluded to as forming the subject-matter of ecology. XII PREFACE. Both communities and species may be studied with respect to the phylogenetic relationship of the species concerned and their places in the natural system of classification. The phylogenetic study of individual species has formed the sharply defined field of taxonomy, but the study of communities from the standpoint of the phylogenetic relationship of their component individuals has long been such a prominent part of plant geography that it has remained as a large element in ecological activities. The investigation of the causes which determine the distribution of plants and plant communities is essentially a physiological task, in which it is necessary for us to regard the plant as a functioning organism and to give little attention, for the time being, to the fact that it has a descent-kinship with other plants. We must keep the plant in mind as an aggregation of coordi- nated physiological processes, continually. controlled by a complex of environmental conditions. It is only by a sharp separation of the phylogenetic and the physiological considerations of the plant that we can hope to investigate with success the relation of plants to their environmental controls. The physiologist has thus far been mainly interested in the individual processes of the plant as affected by the environmental conditions acting singly. The ecologist is interested in the collective activities of the plant, as controlled by the entire set of environmental conditions and as measured by the dispersal, estab- lishment, growth, reproduction, and survival of the plant in a state of nature. He is further interested in the assemblages of plants which occupy the same natural situations or habitats, which appear to be subjected to closely similar sets of environmental conditions and appear to meet these environmental complexes by closely similar or dissimilar types of physiological behavior. In short, the physiologist has mainly investigated the absolute value of conditions by the pre- arranged and controlled methods of experiment, while the ecologist investigates the relative value of these conditions as they cooperate to influence the plant, endeavoring to determine which of them are effective in determining habitat and distribution, and what intensities are of critical importance in this connection. He is also especially interested in the combination of the many different kinds of environ- mental conditions, as these form the infinite variety of environmental complexes furnished by nature. While plant geography is an old science, with a large literature, and while the newer science of ecological plant distribution already pos- Sesses numerous monographs that present types of vegetation, plant associations, etc., as related, in a general way, to environmental con- ditions, yet these studies have usually been primarily descriptive of the vegetation itself, and but little has yet been accomplished in the way of corresponding descriptions of the environmental conditions that are observed to be concomitant with the various forms of vegeta- PREFACE. XIII tion. Much less has it been possible to discover quantitative relations between vegetation characters on the one hand and environmental conditions on the other. Before such relations can be looked for it is obvious that environmental conditions must be described in more or less quantitative terms, and similarly quantitative descriptions of the corresponding vegetational forms must also be available. The present publication is a first attempt to bring these two kinds of descriptive knowledge together for the geographic area of the United States. Be- cause of the newness of the point of view, if not of the subject, but little detailed discussion of the reasons for the actual quantitative relations that exist between plants and their surroundings is here attempted. We have generally been content to point out the kinds of observations that appear to be needed and to bring together such observations and descriptive deductions as we have been able to obtain, both with reference to the vegetation and with reference to those of the environ- mental conditions that are measurable, and for which measurements are at hand. It is obvious at once that the subterranean conditions of plant habitats have not yet received enough attention from the present point of view to make even a tentative description of these conditions possible; the soil studies that have been made are either not sufficiently quantitative or else they deal with features that are not directly related to plants, or the relation of which to plants is not yet clear. This being the case, the main environmental conditions that thus far lend themselves to quantitative study, albeit in a very superficial way, are those that are effective above the soil surface. These features com- prise those conditions that are generally termed climatic. Therefore our study has dealt almost wholly with climatic features, and the rela- tionships between vegetation and climate are the main relationships with which we have been constrained to deal. It is almost certain that the causal relationships between plants and their environments can not be satisfactorily discussed in the majority of cases until sub- terranean conditions are given at least as thorough treatment as we have been able to give to the aerial conditions, so that any apparently definite conclusions that seem to emerge from our comparisons must be held tentatively until suitable methods for the quantitative study of soil conditions have been devised and generally applied. Another aspect of the causal relations that obtain, or have obtained, between plants and the environmental complexes of their habitats brings what has been termed the historic factor into prominence, and this factor involves conditions of the remote past, both aerial and sub- terranean. With this aspect we do not deal seriously in the present publication. On the whole, then, our aim has not been to discover true causal relationships between the two categories of observations here con- XIV PREFACE. sidered, but, rather, simply to describe some of the vegetational and climatic features of the country, in such a way as to emphasize the desirability of pushing this sort of study forward, and to make clear what sort of observations and what sort of deductions therefrom seem to give promise in this direction. Our work is primarily descriptive, as most ecological work must be for a long time to come, and the dis- covery of simple concomitancy is our nearest approach toward the establishment of causal relations. We have been led to the view that ecological science can be most rapidly advanced through this general method of quantitative comparison and by the placing upon record of such cases of concomitancy (between plants and their surroundings) as this method is able to bring forth. Our attitude toward plants has been that of the physiologist, and we have tried to bear constantly in mind the conception that vegetational characters are simply expressions of the activities of individual plants. We maintain that all discovery of true causal relations in ecology must depend finally upon this point of view. Our attitude toward climatic conditions has been somewhat, though not wholly, like that of the climatologist; with meteorology and the causes of climatic features we have had nothing to do. Attention has been centered, as far as possible, upon those particular climatic features that directly affect plant activity. Thus most of our climatic discussions bear either upon temperature or moisture conditions. We have tried to consider climate in relation to plant growth in much the same way as the experimental physiologist considers the relations between his cultures and their surrounding conditions. Many features of the vegetation and many climatic conditions have been omitted or have received scant consideration in the present publication. Those have been more seriously considered which seemed to give the greatest promise and for which the needed data were most readily available. The work has grown from very unpretentious beginnings made over a decade ago, and its ramifications into aspects not at first thought of have been controlled partly by a priori judgment as to what appeared more or less promising, partly by availability of the requisite observations and partly by our own limitations as to time and energy as well as ability. Many other features or dimensions of climate and of vegetation might have been dealt with, and the reader will find here many suggestions for investigations, the carrying out of which would require from a few hours to many years. In short, em- phasis should be laid on the fact that the present study is to be regarded only as a beginning along a line that holds forth very great promise. The real conclusions from our work are to be drawn by others as this kind of study is pushed forward. It may be in place here to give a little space to our reasons for attributing to quantitative physiological plant geography such great PREFACE. XV importance and promise for the future as we frankly do. Ecological science has won its way in a comparatively short time, and now finds itself in the front rank of those lines of intellectual effort that con- stitute biology in the broad sense. Following Warming and Schimper, the biological world has rapidly become very thoroughly interested in the occurrence and behavior of organisms under natural conditions and in the reasons for this occurrence and behavior. This widespread interest may be taken as evidence that ecological study isnow generally regarded as fully as worth while as are taxonomy and phylogeny. Since ecological problems are dynamic ones by their very nature, the quantitative aspect of ecological description and the dynamic relation of different sets of conditions within and without the plant must receive the main attention as soon as a superficial acquaintance with the field has been attained. Plant geography can progress but little farther by qualitative observational methods, and the physio- logical and quantitative point of view must, of necessity, finally pre- vail. Our aim has been largely to make some planned preparation for this newer development, which has already gained considerable head- way. Another and more obviously practical reason for regarding the physiological ecology of animals and plants as of very great promise lies in the fact that the art of animal and plant production (agriculture) rests almost wholly upon this branch of biological science. The problems with which the physiological ecologist deals are the same problems as have to be solved by the agriculturist. One may study natural vegetation or the distribution and environmental relations of wild animals and the other may give his attention wholly to agri- cultural crops and the rearing of domestic animals, but the problems and the general methods by which solution may be obtained are the same in both cases. The interpretation of crop production in terms of climatological conditions has already attained to great importance. The government of Russia long maintained an organization for the study of agricultural climatology and the results warranted great expectation. During the years in which we have been engaged upon the present investigation, the Canadian government has copied many features of the Russian organization, and this branch of the Dominion Meteorological Service is now well established. Finally, the obvious importance of climatology in agriculture has been emphasized in the United States through the establishment of a special division of agri- cultural meteorology in the United States Weather Bureau. Also, it should be remarked that much of the art of forestry rests upon the science of physiological plant geography; so much so that students of forestry already clearly realize the need for studies of the kind suggested by this publication. XVI PREFACE. The studies here reported were carried out under the auspices of the Department of Botanical Research of the Carnegie Institution of Washington. They were begun when both the authors were located at the Desert Laboratory. We are greatly indebted to many persons who collaborated and assisted in various ways. Especially should be mentioned Grace J. Flanders, who carried out most of the climato- logical computations upon which the work rests and who assisted very much in the preparation of the climatic charts. Others who helped with computations are J. W. Shive and H. E. Pulling. . The cooperators in our series of evaporation observations, whose names are given in table 13, should also be mentioned here. It is interesting and significant to remark that this series of studies, in which these individuals so kindly assisted, formed the point of departure from which the whole study, as here presented, has been developed. ol eee een ne ay? BASE FROM U.S. GEOLOGICAL SURVEY PLATE 1 EXPLANATION California Microphyll Desert. LI Great Basin Microphyll Desert. ¥, 4 2 3 a a 2 all ol & 2 Arizona Succulent Desert. J Texas Succulent Desert. J Pacific Semi-Desert. Desert-Grassland Transition. Graasland. Grassland—Deciduous-’orest. Transition. ted aay RI x rls M a ea Southeastern Evergreen- Deciduous Transition Forest. Southeastern Mesophytic Evergreen Forest. Northeastenn Evergretn Deciduous Transition Forest. Northern Mesophytic Evergreen Forest. Western Xerophytic Evergreen Forest. Northwestern Hygrophytic Evergreen Forest. | | 8 as” | —----——- -— ———— — —_—~-— - = = = = Alpine Summits. VEGETATION AREAS OF THE UNITED STATES oi . Compiled by Forrest SHREVE. Scale 1: 9,600,000. Swamps and Marshes. : —— af Sa a gene ee ee : PART I. THE VEGETATION OF THE UNITED STATES, INTRODUCTION. I. THE DISTRIBUTION OF VEGETATION IN GENERAL, AS RELATED TO CLIMATIC CONDITIONS. This publication constitutes an attempt to correlate the distribution of the vegetation of the United States with the distribution of some of the climatic conditions that appear to be most important to plants. It has long been a matter of common information that such a correla- tion exists, and some of its most obvious features have commanded popular attention from the earliest settlement of the country. The influence of a low and uncertain rainfall in inhibiting the occurrence of trees in certain portions of Kansas and Nebraska, for example, and the influence of the high winter precipitation of the Pacific Northwest in permitting the occurrence of a heavy forest in that region, are mat- ters that have come to the attention of every one familiar with those regions. It has been the aim of our work to make a somewhat thor- ough investigation of such correlations as these by bringing together a carefully elaborated set of climatological data and a representative set of data with respect to the occurrence of certain characteristic species of plants, in addition to the facts of the distribution of typical vegetations. We have sought, by appropriate means, to ascertain the extremes of each climatic feature for each of the vegetational or dis- tributional areas. In short, we have determined the maximum and minimum values of each climatic feature for such well-known regions as the Great Plains, the Gulf pine-belt, or for such well-known species as the Sitka spruce (Picea sitchensis), the sage-brush (Artemisia tri- dentata), and the small cane (Arundinaria tecta). Our desire is not only to set forth the basal facts upon which we have worked and such features of correlation as we have been able to dis- cover, but also to clarify some of the conceptions fundamental to such work and to stimulate a greater interest in it. It is particularly desir- able that our work should be regarded as a preliminary and extremely general investigation of this subject for the United States, and that more exact studies of smaller areas should be carried out in order to study more thoroughly the relations with which we have dealt. It will of course be possible to use the climatological data which we have gathered for the study of other subdivisions of the vegetation than those that we have used and for the determination of the climatic con- trols for other species than those we have selected. It is also to be hoped that the United States Weather Bureau and other agencies will make it possible, at no distant date, to draw other climatic maps than those we have been able to construct from the data now available. 3 4 _THE VEGETATION OF THE UNITED STATES. While the determination and delineation of climatic conditions is capable of being given mathematical precision in a number of different ways (as will be shown in Part II), the classification and geographical delimitation of vegetation requires a preliminary discussion of the point of view from which such work may be done and of the concep- tions on which it may be based. The term “vegetation,” meaning the total plant population of an area viewed from the anatomical and physiological rather than the taxonomic and floristic standpoint, is easy of definition and clear in its meaning. So complex, however, are the natural plant assemblages of almost every locality that an attempt to classify them, even in the most preliminary manner, requires at once the use of personal judgment as to criteria of classification and as to the relative weight to be accorded to characteristics that are totally distinct in kind. Several workers have devised methods for giving mathematical definiteness to the description of plant assemblages in terms of floristies, by count or estimation of the number of individuals of each species involved. To give this same assemblage, however, a definition which might serve to compare adequately the physiological characteristics of its component individuals with those of the individuals in another near or distant assemblage, is at present very far from possible. We are merely able to define such an assemblage as being “salt marsh,” “arctic tundra,’ “coniferous forest,”’ or the like. These categories are sufficiently definite in their meaning to give us a mental picture of the size, gross anatomy, density of stand, seasonal activities, and other features of the plants concerned; they are not adequate, however, for a strict comparison of the salt marsh and the arctic tundra in physio- logical terms, nor for the comparison of two salt marshes that are widely distant. The study of vegetation as such has been, on the whole, greatly obscured by the fact that it has never been completely divorced from the study of the flora. Too much emphasis can not be laid, at the present time, on the radical distinctness of the work of physiological plant geography, on the one hand, which attempts to relate the oceur- rence and distribution of species as physiological entities, to the factors of environment, and the work of floristic plant geography, or phytogeo- graphy, on the other hand—which attempts to reveal the geological history, the movements, and vicissitudes of species as phylogenetic entities. The floristic flavor which plant geography and ecology have always possessed may be largely accounted for by the fact that all plant-geographical interest has sprung historically out of floristics, and by the fact that we are in the position of not being able to men- tion a plant of particular identity without using its technical Latin name, which is solely an abbreviated expression for denoting the place we believe it to occupy in the phylogenetic scheme. No one will deny INTRODUCTION. 5 that the Latin name is a convenient means of expressing genetic rela- tionship, that it is a convenient designation for speaking about species in any connection whatever, and that it will continue to have these uses even after the student of genetics has been driven to use some numeri- cal scheme of designation for the forms in which he is particularly interested. Nevertheless, in order to come squarely to face with the problems of physiological plant geography, we shall have to lay aside much that floristics has taught us, and shall have to ignore phylogeny, except in so far as it shows us that plants of close kinship often have the same or similar anatomical and physiological characteristics. The first writer to insist upon the physiological point of view in plant geography was A. F. W. Schimper, whose monumental Plant Geog- raphy, which appeared 22 years ago, has done much to stimulate interest and activity in what we may designate as causational or etiological plant geography. We have approached our problems in plant geography with the men- tal conception that they are merely problems in physiology, with all of the environmental conditions fluctuating and uncontrolled, but nevertheless measurable, and with all the activities of the plant in normal performance and also measurable, not by auxograph and bal- ance, but by such features as distributional extent, habitat occurrence, communal behavior, relative abundance, size, seasonal behavior, ete. The observation, description, and classification of the innumerable types of vegetation which clothe the earth have been carried on in great detail for some of the areas of Europe, Africa, and North America, and have been outlined for the whole globe. These observations and descriptions range from the hasty and incomplete work of pioneer explorers, who were perhaps making many kinds of observations at the same time, to the most painstaking charting of the location of individual plants over larger or smaller areas. The classification of plant communities from non-floristic standpoints has been made, in connection with their description, by numerous workers in many countries, and almost innumerable schemes of vegetational classifica- tion and nomenclature have been proposed for general application. The fundamental defect of these attempts at the classification of such a complex body of material is that they are all largely subjective in character. As a result of this, the total amount of disagreement among students of vegetation is at least as great as the amount of agreement. Perhaps no ten workers could be found all of whom would place the same plant community in the same general vegetational category or would propose less than 6 or 8 different technical designa- tions for it. There is a strong desire among all classes of plant geog- raphers to come into closer agreement in these matters, but it may well be asked whether, in default of fundamental and universal criteria of classification, such agreement is possible among men of different 6 THE VEGETATION OF THE UNITED STATES. observational experiences, and, indeed, whether such agreement would actually yield substantial advantages. The classification of vegetations is thus seen to be by no means an easy task, and it is no marvel that there is lack of agreement among those interested in it. Plant communities are usually made up of many species, and these species are usually of distinct floristic relationship, or dissimilar geographic range, and of varied physiological require- ments and behavior. Furthermore, a particular community does not range far without the acquisition of new members and the loss of old ones. The classification of such a complex material requires the adop- tion of arbitrary standards, and consequently leads to an unnatural system or else to one which is only of local applicability. Physiographic ecology affords a genetic basis for the classification of vegetation, and the logic of such a system has much to recommend it. Physiographiec criteria are not, however, of universal applicability for the classing of vegetation, and lead to unnatural interpretations of vegetational phenomena in many regions, particularly in the tropics, in deserts, and in regions with diversified geological and soil conditions. Furthermore, physiographic ecology is inclined to lead to a false sense of satisfaction with the assembled results of the study of a series of successions, and has often failed to stimulate a study of the physical causes which underlie the observed successional phenomena. Several Scandinavian and German botanists have rightly urged that the classification of plant communities should be based upon the recognition of certain distinctive types or forms of plants. These “biological types,” “life-forms,” ‘‘vegetation forms’ or ‘growth- forms’ are recognized without regard to phylogenetic relationships, and serve to distinguish such groups as deciduous broad-leaved trees, evergreen coniferous trees, perennial grasses, sclerophyllous shrubs, etc. Several systems of growth-forms have been proposed, each more elaborate and complete than the next preceding. In so far as these systems represent an attempt at a physiological classification of plants they are highly commendable, and must lie at the foundation of physi- ological plant geography. Unfortunately, however, our knowledge of the physiology of plants is chiefly based on the behavior of a small number of kinds of plants, mostly cultivated forms growing under sets of conditions at best only partially controlled or measured, and the existing classifications of growth-forms have been based on the inference that the gross anatomi- cal features of plants are an index of their major physiological char- acteristics. We have not yet secured enough evidence to test this inference in more than a general way. We know that there are marked differences between the annual march of transpiration and photo- synthesis in evergreen conifers and in deciduous broad-leaved trees, and that there is no contradictory significance in the fact that such INTRODUCTION. 7 dissimilar trees often grow side by side. We know that arborescent cacti and microphyllous trees differ markedly in the character and activity of their root-systems, and that the annual course of absorption and transpiration in the two types is very unlike, in spite of the fact that the two are constant associates. In view of the incompleteness of the knowledge which might afford a basis for the classification of life forms, it behooves us to recognize a limited number of such forms and to extend the list only on securing evidence of the divergent behavior of groups of plants. The difficulties inherent in the classification of vegetation have led some of the German and British botanists to use the physical charac- teristics of the habitat either as the sole criterion for classification or else as a secondary criterion, employed together with the charac- teristics of the plants themselves. Such a procedure may be looked upon as an indirect method of securing evidence of the physiological character of the plants involved and is logically allowable only on such ground. In any case in which plants of identical growth-form occur in two situations of very unlike physical conditions, an excellent opportunity is afforded to investigate the comparative physiology of the plants. If, as is very unlikely, the two groups should be found to be physiologically alike, there would then be no ground whatever for the separation of the two vegetations in classification. If physiological distinctions should be discovered, these, rather than the unlikeness of the habitats, should then form the basis for separating the two vege- tations. Our deepest concern for the development of plant geography is that its activities may be diverted from the description and classifica- tion of vegetation on subjective grounds and that they may be directed toward experimental work so planned as to yield an actual physio- logical basis for the classification of vegetation. Starting with the small body of experimental data which now makes it possible to recog- nize certain groups of plants with a general coherence of physiological behavior, it will become possible in the course of time, and of much hard work, to extend our knowledge far enough to actually under- stand the different types of vegetation which we now photograph, map, list, and name. II. STUDY OF THE DISTRIBUTION OF INDIVIDUAL SPECIES. The investigation of correlations between climatic conditions and vegetation has been extended, in our work, to the relations between the climate and the distribution of certain individual species of plants. We are here treading upon fresher ground, on which extremely little work has been done. To attempt to define the factors which are responsible for the geographical distribution of any one plant species 8 THE VEGETATION OF THE UNITED STATES. is, indeed, to face a problem of considerable complexity. Even in the presence of the results of our own work we are not prepared to main- tain that the distribution of all species is at the present time strictly controlled by the complex of physical conditions for which we have tried to derive numerical values. We are in the position, however, of being convinced that certain species are thus controlled, the evidence in this direction being partly our own and partly due to an analysis of the work of others. It is very generally maintained that the distribution of many plants is due to certain “‘historical factors,” which is merely to say certain physical conditions which have operated in the past, or the conditions which determined the distribution of the ancestral stock of the plant in question. When we speak of the historical factors and the present factors operative in determining plant distribution, we must bear in mind that we are embracing in the former term two very dissimilar things which have registered a combined effect. We must look to evolutionary history and to paleobotany to tell us where a particular genus originated, at what epoch, and from what stock. We must look to paleobotany and paleoclimatology to tell us what have been the movements, the extensions, and the retreats of this genus. The initiation of a species is, from our standpoint, purely an evolutionary event; the history and fate of the species after its initiation are con- sidered as dependent upon the changes in orography or climate which it may encounter, always with the possible cooperation of physiological changes in the stock which are unaccompanied by morphological modifications of diagnostic value. It is conducive to clearer thinking, therefore, to distinguish between the evolutionary factors and the paleoclimatic factors which compose the historical factor. This dis- tinction has its principal value in compelling us to regard the present distributional phenomena of the earth as merely a momentary stage in the prolonged and incessantly active procession of change due to secular or sudden changes of climate, or to destructive or constructive events in surface geology, and discontinuously marked by evolutionary activity. We are made to realize that there is no gulf between the climate of the past and that of the present, and that there has been no sudden, extensive, or unaccountable readjustment of distributions. The role of the evolutionary factor can not be escaped in any con- sideration of distribution. For example, we owe it to facts in the history of the great plant stocks that Yucca arborescens occurs in the Mohave Desert of Southern California and that Aloe dichotoma, somewhat similar to it in form, grows in the deserts of Southwest Africa. It is likewise a part of phylogenetic history that very many plants were formerly confined to particular regions, whereas they now have been introduced overseas, into climates which prove to be wholly congenial to them. However, the reasons for the distributional ranges INTRODUCTION. 9 of Yucca and Aloe in their respective continents are to be sought in the operation of the environment, and the spread of an introduced plant in a new continent must be controlled by environmental con- ditions just as it was controlled in its native continent. Evolutionary activities may be thought of as supplying the raw materials out of which the physical environment has made the present distributional complex of the earth’s surface. We can not hope at present to understand why a strong development of the genus Yucca has occurred in the southwestern United States and a rich develop- ment of Aloe in South Africa, and the answer to such a question, if forthcoming, would have only a remote relation to the study of the present influences which are limiting the distribution of the members of these two stocks in their respective continents. The réle of paleoclimatic factors is also an immanent one in the determination of plant distribution. Many species have a known ancestry and a known ancestral range, at the same time that they now inhabit areas of such small size that it is difficult to believe that any present physical conditions are restricting them. The history of these plants has been one of extinction and retraction, due to changes in their old extended environment. They are now unable to regain their old ranges, or even to spread over relatively short distances. These species may well be looked upon as physiologically and genetically decadent, and either decadent because they are geologically old or else old because they have failed to change their requirements to that slight extent which might permit a greater extension, or to that greater extent that would have reacted upon form in such a manner as to make us regard the resulting plants as distinct species from the old ones. In addition to the old and restricted, or relict, species we have another class of plants of limited range, those which are apparently just appearing on the scene and have not yet had time to occupy the entire area in which they might be expected to find congenial condi- tions. These plants are generally members of large genera, such as Opuntia, Antennaria, and Crategus, or else of genera which have been recently subdivided, whereas the relict species are usually members of small genera. It is not always easy to decide whether a given restricted distribution is of the relict type or of the ‘‘novitiate” type, as we may call the emerging species. Our decision, in the lack of other evidence, is apt to be based upon the existence or absence of fossil records of the species or its allies, or else upon the size and phylogenetic position of the genus or family. Such a restricted conifer as Pinus mayriana may well belong in either class, and the members of many small or monotypic genera of Mexican Compositz may also stand in a doubtful position. A very considerable number of our plants belong neither to the relict nor to the novitiate class. They are of such age that they have had 10 THE VEGETATION OF THE UNITED STATES. time to extend themselves as far as the subtle factors of their environ- ment, the crude power of great barriers, or the operation of suitable agents of dispersal have permitted, and their course has not run so far that climatic and orographic calamities have overtaken and restricted them. This great class of physically controlled plants can not be specifi- cally enumerated at the present time. There are many good reasons for believing it to contain the bulk of the species which form the dominant element in all vegetation, whether these are plants of extended range and frequent occurrence, plants of more restricted range, or plants whose occurrence is determined by complexes of con- ditions which are themselves rare. We may find areas of vegetation in which a relict plant is dominant, as is true of the groves of Cupressus macrocarpa near Monterey, California, or others in which a novitiate plant is very abundant, as is true of the Cababi Hills in southern Arizona, where an undescribed Opuntia, known nowhere else, is extremely abundant. Although cases of this sort are fairly common, they are usually readily detected by a more thorough study of the adjacent regions. Such trees as Liriodendron, Taxodium, and Liquid- ambar are known to have undergone distributional recessions, but they occupy such large areas at present that they can scarcely be classed as relict species, and can surely be placed among the plants whose distributional controls are worth looking for among factors at present operative. Whether the physically controlled plants form a large or a small percentage of our flora, we can at least state with some assurance, based upon the correlation of distribution and climatic conditions, that they form the predominant part of our vegetation. The great bulk of the trees, shrubs, grasses, root-perennials, and other plants which make up the dominant natural vegetation of the world may safely be held to have had their present distributional limits imposed by physical factors which are either now operative or were operative in very recent time. Such factors may be acting directly through the conditions of climate or may be acting indirectly through soil conditions, through geographic or physiographic changes, through the influence of associated plants, through animal, fungal, or bac- terial enemies, through fire or mechanical agencies, or through the means of animate or inanimate agents of dispersal. Since these indirect factors of environment can affect plants only through the same kinds of physiological influences as are exerted by the direct factors of the climate, they are at bottom of the same nature, whether we allude to them as ‘‘biotic’’ factors, “mechanical factors,’ or what not. In the isolated desert mountains of southern Arizona the great bulk of the species are physically controlled in their distribution, as is abundantly shown by the universality of each of these species through- out a given altitudinal range or a given set of habitats, and by the definiteness with which it is limited by rather sharply drawn lines. INTRODUCTION. 11 When one of these mountains is compared with another the floras are found to be closely similar but not identical. Certain species range over the western cordillera of Mexico or over the central Rocky Mountains and have reached certain of the isolated desert mountains without having reached all of them. At least a few cases are known in which a given species is common throughout several mountains and is known from only one or two restricted localities in another moun- tain, although it is there surrounded by an area possessing a favorable environment for it. In view of the universality of the distribution of most of the mountain species, these cases appear to be the opening invasions by which new components are being added to the flora. They are in a way analogous to the novitiate species of larger areas, and their presence in no way vitiates the evidence for the physical control of the distribution of the major portion of the mountain flora. In determining the limits of the types of vegetation which are shown in plate 1, no consideration whatever has been given to the ranges of individual species of plants. It is true, nevertheless, that each of these great vegetations possesses many species, particularly among its dominant forms, which are roughly confined to the area occupied by the vegetation itself. Picea sitchensis, for example, isnearly coincident in distribution with the Northwestern Hygrophytic Evergreen Forest; Pinus teda extends very little beyond the range of the Southeastern Mesophytic Evergreen Forest, and Bulbilis dactyloides extends over nearly the same area as the Grassland and the Grassland-Deciduous Forest Transition. Other forms are nearly coincident with groups of areas, as is the case with Covillea tridentata, which is found in the California Microphyll Desert, the Arizona Succulent Desert, and the Texas Succulent Desert. Since the limits of groups of dominant plants have been unconsciously and necessarily taken into account in delimiting the vegetational areas, it is to be expected that there should be many cases in which the physical conditions concomitant with the distribution of a given type of vegetation and those concomi- tant with the distribution of some of its dominant species should prove to be nearly identical. We regard the view that vegetation is determined in its distribution by complexes of physical conditions, as established beyond all cavil by the work of a large number of men, if indeed it is not almost axio- matic. That the distribution of individual species is also controlled by physical conditions is equally well demonstrated, so long as we con- fine our attention to the common forms which are important elements of the vegetation. There are doubtless many of the novitiate and relict species of plants which find the physical conditions of the present time a barrier to their spread, but such cases have not yet been demon- strated. We have, therefore, confined our consideration of individual species to forms which are extremely abundant, except in a few cases which will be noted. 12 THE VEGETATION OF THE UNITED STATES. III. MANIFOLD OPERATION OF ENVIRONMENTAL CONDITIONS. Our knowledge of the modes in which the nature of the environment may affect the activities of plants, and thereby affect or determine their distribution, is sufficiently great to give a profound impression of the complexity of the problem of the physical control of distribution. The experiences of agriculturists and horticulturists, the work of plant physiologists, and the observations and deductions of ecologists, have all combined to give us a very large body of facts relative to the mani- fold. features of the physical environment which may be of critical importance for the spread or survival of particular plants under par- ticular conditions. We are very well informed, both empirically and experimentally, with regard to many of the conditions which are harmful to cultivated crops and the climatic conditions which make it impossible to cultivate them beyond certain well-defined boundaries. A large part of this information relates to the ability or inability of plants to withstand frost or freezing temperatures, and to their ability to survive and grow under given limitations of water-supply. Enough has been done in the selection and breeding of economic plants to show that closely related forms may often exhibit great differences in ability to withstand low temperatures, low soil-moisture content, great ranges of soil texture, and the like. Enough is known of the distributional limits of ‘plants which are associated in given regions to indicate that these are not limited by the same sets of condi- tions; they range to different distances and into regions of diverse character in such manner as to indicate that they have very dis- similar distributional controls. In spite of the manifold nature of the environmental controls and the well-known diversity among plants with respect to the nature and intensities of the conditions which control them, it is possible, never- theless, to distinguish certain classes of controlling conditions. The broadest line that may be drawn is the one separating the simple or direct factors of a climatic character and those which are not directly attributable to the climate. Later pages will be devoted to an elabora- tion of some of the major types of direct climatic conditions, but little will be said hereafter regarding the non-climatic conditions, such as the nature of the soil, such ‘“‘biotie factors”? as competing plants and preying animals, and such mechanical factors as the influence of wind (in causing mechanical injury), lightning, fire, landslips, inundations, active erosion, and other agencies of very real importance but usually of local or comparatively infrequent occurrence. So much is known regarding the importance of the character of the soil that it is custom- ary to speak of ‘‘climatie and soil conditions” as if the two were of co- ordinate importance. The rdle of the soil in maintaining a water- supply for plants is of vastly greater importance to them than any of INTRODUCTION. 13 the other réles which it plays. Although the texture of the soil is of prime importance with relation to the penetration, movement, and con- servation of a water-supply for plants, it is fundamentally the climatic elements of rainfall and evaporation that determine what the soils of a given texture are able to do in presenting a moisture-supply of a given amount. We are compelled, therefore, to regard the soil as a medium through which the climate acts upon plants. The supply of moisture to the plant, due primarily to climatic conditions, is secondarily determined by the soil, just as the loss of moisture from the plant is determined through the medium of the atmosphere. The soil is a medium which differs from place to place independently of the climate, while the atmosphere is alike in all places, except in so far as it is directly affected in its movements, temperature, and moisture- content by the primary conditions of. climate. Such a view of the réle of the soil in forming a portion of the environ- ment of plants takes no account of the cases in which the chemical nature of the soil and the amount and character of the salts and other solutes in the soil-water become factors of great moment. In the con- sideration of saline and alkaline areas, and certain limestone and ser- pentine regions, it is necessary to do more than investigate the texture of the soil in its réle as a stabilizer of the climatic moisture conditions. It has been customary to speak of competition as if it were a distinct condition of elemental character, capable of admitting or excluding a given plant to a given area in much the same manner as that in which a purely climatic condition would operate. The results of competition are registered upon a plant, however, in exactly the same manner as the results of a given climatic condition or set of conditions. Com- petition may exclude light, may restrict water-supply, or may operate in any one of a number of ways. The end-effects upon the processes of the plant are exactly such as might be exerted through climatic agencies, except it be in those cases in which there is an addition of toxic root-excretions to the soil. Even in such cases, the toxic substances act as chemicals and the plants producing them are not directly effec- tive. Competition may be of importance in determining the com- position of small areas of vegetation, but even then the competing plants must be regarded as struggling not with each other, but with physical conditions which are of precisely the same general nature as the conditions due in other places solely to climatic causes. The cases in which plants grow so closely as to exert an effect on the environ- mental conditions are similar to the cases in which the major plants modify the climate for the minor plants. Both of these cases must be left out of consideration in an attempt to determine the larger features of the réle of climate in relation to vegetation. It is possible to lay down a program for the study of distribution and its controlling conditions, applicable almost. equally well to a given 14 THE VEGETATION OF THE UNITED STATES. species or to a given type of vegetation, and idealistic only in that it would require extension or modification to fit the necessities of any given case. Such a program, briefly outlined, is as follows: A. The securing of the distributional facts. 1. Securing an exact knowledge of the geographical range of the given plant. 2. Determining the ecological distribution of the plant. a. Its region of greatest abundance. b. Its region of greatest size. c. Its region of most rapid growth. d. Its region of greatest productivity. e. Its region of greatest catholicity of habitat. 3. Determining the behavior of the plant at the limits of its range. a. The character of its limital habitats. b. The evidences for its limitation. B. Ascertaining the apparent climatic controls on a correlational basis. 1. Determining the isoclimatic lines which follow nearest to the geographical limits of the plant form considered. 2. Determining its habitat behavior with respect to climatic elements discovered in 1. 3. Determining its comparative behavior at different portions of the periphery of its geographical range. C. Ascertaining the actual climatic controls by experimentation. IV. GROWTH-FORMS OF PLANTS. When we undertake to regard the vegetable kingdom from the ecological standpoint, and to investigate the importance of the physio- logical characteristics of plants as related to their distributional fea- tures, it is clear that considerations of phylogenetic relationship become of little importance. If we attempt to arrange the multifarious plant-forms of the earth in a series of groups according to their physio- logical affinities, so as to bring together the plants which have solved the same problems of environmental adjustment in the same manner, we shall have to depart very far from the families and genera of the natural system of classification. This is very obvious from a consideration of the diversity that exists in some of the large plant families. In the Composit, for example, we have trees, shrubs, and herbs, terrestrial plants and aquatics, large-leaved, small-leaved, and leafless perennials, mat- forming or cushion plants, slender climbers, etc. In the southwestern United States we have, conversely, a group of small-leaved or leafless woody perennials which are green-stemmed and richly branched and bear a close resemblance to each other, in so far as vegetative structures are concerned, in spite of the fact that they may belong to entirely different families. Among these plants are Thamnosma montanum (Rutacee), Keberlinia spinosa (Keeberliniacese), Holacantha emoryt (Simarubacex), Canotia holacantha (Celastracere), and Parkinsonia microphylla (Leguminose). Among the criteria used in the phylogenetic classification of plants are: the structure of the flower, the developmental history of the floral INTRODUCTION. 15 organs, the systems of arrangement of leaves (phyllotaxy), the stelar anatomy of the stem, the existence and character of vestigial structures, and the recapitulation of ancestral features in the early life history of the plant. The facts which fall under these categories have been of long-standing use in phylogenetic classification, but have no direct bearing upon the present relation of the plants to their environmental conditions. Therefore these are not the criteria that would be useful in classifying plants for the purposes of ecology and plant geography. For these purposes we require a classification which shall give first attention to the vegetative rather than the reproductive organs of the plants, and to those features and structures which have to do most obviously with their relation to the conditions of the soil and atmos- phere. When plant geographers first began to break away from floristic considerations and commenced to consider plants collectively, as vegetation, they felt the need of a means by which it would be possible to express physiological relationships. It was a very difficult thing to depart from the point of view by which plants could be placed in such definite categories as the Saxifragacez or the Liliacez, or in such groups as “arctic circumpolar” or “littoral pantropist.”’ It was still more difficult to attempt, for example, to arrange the plants of heath, moor, tundra, and alpine meadow, in a series of groups that would bring out their physiological affinities. The very instant that we distinguish between the vegetation of any two areas we have taken into account, consciously or unconsciously, certain features of differ- ence between the plants of these areas. We notice the difference between the soft carpet of short grass which lies just above mean high tide in a brackish marsh and the tall, coarse grass which inhabits the quiet shallows below the high-tide line. We notice the difference between the forests of the southern Alleghenies and those of the Gulf Coast. In each case we have had our attention called to certain differences in the gross anatomy of the plants involved. In spite of the fact that the plants which characterize the two areas of marsh are both grasses, we recognize in them plants of different type, just as we distinguish the pines of the Gulf Coast and the oaks and chestnuts of the Alleghen- lanregion. It is these obvious differences between plants, conspicuous even to the man who knows no Latin names for them, that form the basis for all the distinctions between vegetational areas. Considerable attention has been given, from time to time, to the definition of these anatomically and physiologically distinctive types of plants which are best designated as growth-forms. These attempts have a very fundamental importance to plant geography, for, although many of them have been extremely crude, they represent an attempt to express an ecological similarity that exists between many plants of distant phyletic relationships. They represent an effort to establish 16 THE VEGETATION OF THE UNITED STATES. categories in which we can place the rich variety of types that the plant organism has assumed. They constitute the beginnings of an ecological classification of plants from a physiological standpoint. Everyone must be aware that such a classification should have its beginnings in physiological work and not in the descriptive work of plant geography. It is not strange, however, that need for it should arise in geographical work and should be felt more by plant geographers and ecologists than by most physiologists. In a brief review of the attempts that have thus far been made to establish systems of growth-forms that will be of service in plant geography, it will suffice to mention only a few. The first was proposed by Humboldt! in 1805, in connection with his effort to determine the features that give distinctive character to the vegetation of different altitudes in tropical America. Humboldt saw, in the types which he recognized, the distinctive vegetational units that serve to bring about the physiognomic diversity of the different regions of the earth, rather than groups of possible physiological affinity. His list of 19 types included the coniferous tree, the palm, the cactus, the tamarind- like tree, grasses, aroids, and the like. Grisebach’ described 60 vege- tative forms, and his classification, like that of Humboldt, had to do largely with the conspicuous types of plants which determine the physiognomy of vegetation and aid in differentiating the great floral regions of the earth. Following upon these early classifications have come the systems of Drude,* Krause,* Pound and Clements,’ Raun- kiir,® and Warming.’ The systems proposed by these men are far more elaborate than the earlier ones; they embrace all cryptogamic as well as phanerogamic plants; they include aquatics as well as land plants; they take into account seasonal behavior as well as form and differentiation, and, what is best of all in an attempt to devise a natural system, they introduce subordinate categories. The system of growth-forms most widely used at the present time, and the one that seems to have attracted the most attention to this subject, is that proposed by Raunkiir. His system is based entirely on the character of the perennating organs of plants and their position with respect to the substratum. His five groups are as follows: 1Humboldt, Alexander von., Essai sur la Géographie des Plantes, Paris, 1805. *Grisebach, A. R. H., Die Vegetation der Erde, Leipzig, 1872. ®Drude, O., Deutschlands Pflanzengeographie, 1896, and earlier papers. ‘Krause, E. H. L., Die Eintheilung der Pflanzen nach ihrer Dauer., Ber. d. deut. Bot. Ges. 9: 233-237, 1891. 5Pound, R., and F. E. Clements, The phytogeography of Nebraska, Lincoln, 1898. ®Raunkiir, C. Types biologiques pour la géographie botanique, Bull. Acad. Roy. Se. Dane- mark, Copenhague, 1905.—Livsformernes Statistik som Grundlag for biologisk Plantegeografi. Botan. Tidssk., 29, Kjobenhavn, 1908 (translation in Beih. Bot. Centralbl. 87, 1910).—Formations- underségelse og Formationsstatistik. Botan. Tidssk., 30, Kjobenhavn, 1909 (English abstract in Bot. Centralbl. 113:662, 1910). 7Warming, E., Om Planterigets Livsformer, Festekr. ugd. af Universitet, Kjobenhavn, 1908. INTRODUCTION. 17 Phanerophytes: Trees and shrubs with buds exposed on branches. Chamephytes: Plants with their dormant buds on the surface of the soil or just above it (30 cm.). Hemicryptophytes: Plants with buds in the surface layer of the soil. Cryptophytes: With,subterranean dormant buds. Therophytes: Perennating as seeds; annuals. This classification expresses the physiological diversities of the vege- table kingdom in a very inadequate manner. It lays stress upon the resting organs, with total disregard of what we may term the “work- ing organs.” Its author has more recently’ proposed a subdivision of his group ‘“Dhanerophytes,” based on the size of the leaves. These six “size- classes’ make it possible to use somewhat more definite terms in descriptive plant geography, but they do not satisfy the requirement for a more precise knowledge of the physiological significance of leaf- size. The fact that the transpiring power of leaves is not definitely related to their size is one of the considerations which makes this criterion of doubtful value even for a preliminary classification of growth-forms. Raunkiar, Paulsen, and other workers have used the above system of growth-forms to derive what they have designated as ‘“‘biological spectra.’’ By this method the entire flora of a given region is appor- tioned among the five classes of the system, and the values are thus secured for the percentage of the total flora which is formed by each class. These spectra possess little value to the student of vegetation, inasmuch as they are based upon a consideration of the flora rather than the vegetation. The biological spectrum of a pine forest with 175 species of root perennials growing in its shade would be very slightly changed by the removal of the pines, although this would effect a very profound change in the character of the vegetation. The securing of the biological spectrum for a given number of the com- monest plants of an area, as has been done by Taylor? for Long Island, gives results of some value, but their ecological importance is still limited by the inadequacy of the classification. The most carefully elaborated system of growth-forms is that of Drude, proposed in his Oekologie der Pflanzen.? This system is thoroughgoing and complete at the same time that it is eminently natural, in the sense that it comprises almost no subjective or phylo- genetic distinctions. The principal subdivision of the vegetable king- dom is into terrestrial, aquatic, and non-vascular plants, and the total 4Raunkiir, C., Om Bladstérrelsens Anvendelse i den biologiske Plantegeografi, Bot. Tidssk 33: 225-240, 1916.—Translation by G. D. Fuller and A. L. Bakke in The Plant World 21: 25-37, » Taylor, Norman, Flora of the vicinity of New York, a contribution to plant geography, Mem. New York Bot. Gard., v. vi +683 p. New York, 1915. ®Drude, Oscar, Die Okologie der Pflanzen, 308 p., 80 figs., Braunschweig, 1913. 18 THE VEGETATION OF THE UNITED STATES. number of growth-forms recognized is 55. The number of growth- forms apportioned to each of the various classes of plants is shown by the following table: I. Terrestrial plants (38): II. Aquatic plants (6): PEER Str tee. OTe Sta ES Bhar are if Amp BiDIOUB! 20.0205 25 es oe 3 PUTTS Ares cacee ae kette ce, aent rae % 9 Submerged... ik eee 2 Climbers! aves acbe dt de peek 4 FlOsting.:2.:.:2)2 57-45. pe ce L Parasites and saprophytes...... 2 | II]. Non-vascular plants.............. 11 ARAROS. hatha. xe ee cts lve 3 UIC CIGMLSin a, cha peek t oy eydlah tiers 3 Small perennials............... 4 PATA Ne road oak Pee © lew siete 3 R The importance for us of a carefully elaborated and natural system of growth-forms such as that of Drude lies not so much in its details as in the criteria on which it is based. Some of the gross anatomical or physiognomic criteria are of profound and obvious physiological importance, such as the major distinction between terrestrial and aquatic plants, the distinction between perennials and annuals, and that between succulent and non-succulent forms. Other criteria are of known physiological importance, such as the distinction between saprophytic, parasitic, and autonomous plants, or between the decidu- ous and perennial habits of leaves. When, however, we approach such distinctions as those between broad and narrow leaves, between pov- erty and richness of branching, and between the possession of rhi- zomes and that of bulbs, we are on extremely controversial ground. There is much evidence to indicate that the form and size of leaves has been overestimated as a criterion of importance in the ecological classification of plants. Paleobotanical evidence shows that many unusual forms of leaf, such as those of Liguidambar, Platanus, and Artocarpus, have persisted through long periods of time. The fact that these trees have undergone extensive migrations and recessions, undoubtedly encountering substantial changes of environment, affords some basis for a belief that leaf-form is often as conservative as the structure of the floral organs’. The importance of mere leaf-size in relation to water-loss has also been overestimated, as it has been shown that the transpiring power of a leaf bears no invariable relation to its size. This explains the existence, side by side in the deserts of southern Arizona, of such plants as Franseria ambrosioides, with leaves from 25 to 40 sq. em. in area, and such plants as Hymenoclea monogyra and Baccharis emoryi, with leaves from 1 to 2 sq. em. in area; or the con- comitant occurrence, in relatively dry habitats in the mountains of Jamaica, of Bocconia frutescens, with leaves often 200 sq. cm. in area, and Micromeria obovata, with leaves less than 0.25 em. in area. 1For a discussion of this topic from the paleobotanical standpoint, see: Berry, Edward W., The Lower Eocene Floras of Southeastern North America, U. 8. Geol. Surv., Professional Paper 91, 351 p., 1916 (p. 73). INTRODUCTION. 19 The nature of the growth-forms recognized by Drude has been examined with a view to determining what features of plant structure have been used by him as criteria for his subdivisions. The chief of these criteria are listed in table 1, together with the divisions based upon these criteria, and the environmental conditions to which these features seem, in the present state of our knowledge, to be most closely related. An examination of this table will show that Drude has used, in the main, criteria to which a definite physiological importance can be attached, or to which, in some cases, several lines of importance can be ascribed. The definitions which Drude has given some of his growth-forms employ the words “dicotyledonous” and ‘‘monocotyl- edonous.”’ It is difficult to decide whether these words indicate a recognition of phylogenetic divisions or whether they are used as a brief and convenient means of distinguishing types of stem, of leaf, and of branching, which may have a physiological as well as a phylogenetic significance. TaBLe 1.—Analysis of the criteria used by Drude in distinguishing growth-forms. Environmental conditions to which it is Criterion and subdivisions based upon it. MEET CH BUTUDS!): fs -ciie\. sels a Wek e'raaid o e Length of life: Perennial (or biennial); an- nual. Status: Autonomous; climbing; epiphytic; parasitic, saprophytic (?). Stem: Caulescent; acaulescent.............. related. General favorableness of all conditions. General favorableness of all conditions. Source of food materials. Ratio of material expended in mechanical tissues to extent of leaf surface. Ratio of material expended in mechanical tissues to extent of leaf surface. Exposure to atmospheric factors. Ratio of material expended in mechanical tissues to extent of leaf surface. Exposure to atmospheric factors. General favorableness of all conditions. sonal incidence of water-supply. General water-relations. General water-relations (phylogeny). Seasonal distribution of rainfall. General water and temperature conditions (phylogeny). General water and light conditions. Habit of stem: Erect; procumbent.......... Type of stem: Woody; succulent; herbaceous Sea- Memtenty- leafless... -......0.0....620 ees Shape of leaf: Broad; needle-like............ Type of leaf: Deciduous; perennial.......... Branching: Absent (palms); poor (screw- pines); rich (polster plants). Arrangement of foliage: Generalized; uni- centric. Type of subterranean organs: Rhizome; woody root; bulb. Incidence and duration of cold or dry seasons. We are here brought to face the difficult question as to whether the distinction between the dicotyledonous and monocotyledonous types of stem should be maintained in a classification of this kind. Is the distinction to be regarded as a purely phylogenetic one, or is there sufficient difference between the physiological efficiency of these very dissimilar organs of conduction and leaf display to warrant separating them? A similar question is raised as to the physiological importance of the parallel-veined and net-veined condition of leaves. Again, 20 THE VEGETATION OF THE UNITED STATES. should the unicentric foliage of a Yucca be distinguished from the similar leaf arrangement of Echeveria, in which the leaves are separated by internodes? Should the sessile foliage of Agave be regarded as per- forming its functions in precisely the same manner as the similar leaf- rosette of Aloe, which is raised well above the ground on a stout stem? It is only to future investigations that we can look for knowledge that will enable us to draw a line between the structural features that are of physiological or ecological importance and those that are due to what we might designate as evolutionary inertia. It is still impossible for us to distinguish between structures that are vestigial, in the sense that they. no longer perform an office that they were able to perform in the early history of their race, and structures or structural features that arose fortuitously and never served a vital function, at the same time that they were not of such a.nature as to be eliminated by selection. It does not require an examination of the physiological significance of the criteria used in any of the classifications of growth-forms to discover the fact that the water-relations of plants have done far more to influence their external form than have any other set of relations to environmental conditions. Anyone familiar with the cultivation of plants could predict with great certainty the relative water require- ments of Parosela spinosa, a hoary, small-leaved tree of the Colorado Desert, and such a tree as the red maple. It is not an invariable rule, however, that the water requirements are obvious, as witness the close similarity of Baccharis scoparia of the Jamaican mountains and Bac- charis emoryi of the Colorado Desert, or the general similarity of the grasses of dunesand swamps. The water requirements of a plant may, however, be much more commonly read from its outward form than may its temperature requirements. There is nothing, for example, in the appearance of Pinus divaricata and Pinus caribea to indicate that the former grows in the cold taiga of Canada and the latter in the West Indian Islands and Florida. Schimper recognized the importance of giving equal weight to the water and temperature requirements of plants in grouping them for ecological purposes. Heaccordingly divided each of the general classes of plants which are recognized on a basis of their water-relations— xerophytes, mesophytes, and hydrophytes—into three classes based on temperature requirements—microtherms, mesotherms, and mega- therms. In this manner nine categories were secured, in which it was possible to place plants only after securing some knowledge of their habitat requirements. The only logical basis on which we can proceed to a classification of the vegetation of the world is one in which we take account of the nature of the vegetation itself, and give no weight whatever to any of the natural conditions or circumstances by which vegetation is affected. It is for this reason that importance attaches to the study of growth- INTRODUCTION. 21 forms. If we wish to understand vegetation we must understand the individual species of which it is composed. If we wish to understand the relation of each plant species to its environment we must under- stand the nature of its functions and the character and role of each of the organs through which they are carried on. Whatever features of the gross anatomy of plants may be discovered to have no apparent importance in any aspect of their adjustment to environment will have no place in shaping our ultimate system of growth-forms. Progress toward such an ultimate system is beset by two dangers: that which would lead us to be satisfied with a system which is too simple, and that which would lead us to adopt a system in which anatomical features of questionable importance would be recognized along with those of demonstrated importance. Vegetational units have been grouped or classified by various workers according to the nature of the habitats in which they are found, according to their floristic make-up, and according to their successional relation to one another. A large body of work has been done by these methods, giving us a substantial part of our knowledge of the vegeta- tion of the globe. The only one of these methods which is purely vegetational is the last. If it were possible to demonstrate changes of vegetation in a state of nature which were not accompanied by changes of environmental conditions, it would indeed be necessary to give strict attention to the stages of succession in making any attempt to correlate vegetation and conditions. If it were true that identical conditions might sometimes present different vegetations, our problem of correlation would be made still more complicated than it already is. It has been amply shown, however, that successional changes of vege- tation are both preceded by and accompanied by changes of environ- ment. The well-known work of Cowles has shown the importance of the changes of soil-moisture which accompany physiographic develop- ment, and the work of Fuller, of Gates, of Weaver, and of Cooper has shown the importance of other conditions of both soil and atmos- phere. In addition to the physiographically initiated changes in the environment are those initiated by the vegetation itself, supplying conditions favorable for invasion by a new group of species. Although a large amount of work has been done in describing successions and in relating successional stages to each other, it is only recently that the workers just cited have made a beginning in the investigation of the physical conditions which underlie the separate stages. As soon as we begin to study the relation of physical conditions to successional stages, the relation of these stages to each other sinks to a position of minor importance, and our work emerges upon the broad field of causational plant geography. The imperfections of our present knowledge of the physiology of plants and the consequent imperfections of our system of growth- 22 THE VEGETATION OF THE UNITED STATES. forms are carried on into our classification of vegetation. The fact that the water-relations of plants are more easily known from external criteria, and the fact that they have been more thoroughly investigated, have not only influenced our prevailing system of growth-forms, but have determined the nature of our vegetational units. The classification of growth-forms and the classification of vege- tation are like all other scientific efforts to reduce natural phenomena to a logical system, in that the classification possesses its chief value as a@ concise expression of the results of research. A classification of growth-forms which had been highly perfected by our present methods and knowledge would still be roughly made from the point of view of the ecologist and physiologist of tomorrow. It is perhaps idealistic, and is surely premature, to hope that we may one day have an eco- logical classification of the vegetable kingdom on a physiological basis. Such a classification will merely be the perfecting of the begin- ning which has been made by Drude and his predecessors. It will not be possible without a great deal of physiological work that is not yet so much as planned, and it will not be of more than academic interest unless it is constructed from a broad ecological point of view. V. PLANT COMMUNITIES. The study of vegetation is essentially a study of plants which are growing together in a state of nature, it is an investigation of all the phenomena which these plants exhibit as an aggregation, as dis- tinguished from the behavior of any one of them when considered. alone. The natural assemblages of plants which characterize given areas have been assiduously studied by a very large number of workers in all portions of the world. The contrast between the small aggre- gations of local character, the larger ones of more general occurrence, and the still larger ones of very wide distribution has given rise to the recognition of various ranks of aggregation or association and to the study of the relationship existing between aggregations of different rank. By common consent among plant geographers and ecologists, the term ‘‘community” has been adopted as a general designation for any assemblage of plants regardless of its rank in the formal schemes of classification. In our work with the vegetation of the United States we have had to do with the climatic conditions influencing certain of the plant communities, and we have been under the necessity of deciding upon the criteria to be used in differentiating the communities, as well as under the need of disregarding, for our immediate purpose, certain other communities which stand in definite relation to climatic con- ditions, as well as the communities which are affected by climate chiefly through the medium of the soil. We have, perhaps, been some- INTRODUCTION. 23 what informal in our handling of this eminently formal subject, upon which so much has been written and so much has been enacted by botanical congresses. As already intimated, the study of vegetation has resulted in the recognition of different degrees of communal existence among plants. These degrees have been designated by names, among which may be found the words formation, region, zone, society, association, district, consocies, group, belt, strip, and a score of others. Scarcely any two workers have used the same term in precisely the same sense, and few of the workers have defined their terms in such a manner as to enable a botanist to recognize one of the communities in case he should find himself in its midst. There has been an organized effort in recent years to secure a general and international agreement regarding the classification and nomenclature of plant communities. The extensive areas such as the sagebrush plains of the Great Basin, the grasslands of Nebraska and Kansas, or the pine forests of the Atlantic Coastal Plain are designated as formations. The smaller and less markedly differentiated areas within a formation are designated as associations, as, for example, the forests of shortleaf pine in New Jersey, those of loblolly pine in Maryland and Virginia, and those of longleaf pine in the Gulf States, all lying within the Coastal Plain formation. The smallest units of vegetation are termed societies, and these are of small area and represent portions of the association in which a definite aggregation of species is to be found. This classification of communities is simple and natural and has much to commend it for general use in describing vegetation. It is to be noted that, just as the formation is defined in terms of growth- forms which are found to be most common and characteristic in that community, so the association is defined partly in terms of the growth- forms present and partly in terms of species, while the society is defined chiefly by the species which it contains. The most important criterion to be employed in the distinguishing of communities is always the kind or kinds of growth-forms which are present, and this is a criterion which can be used for societies as well as for formations. A community may present a single growth-form, represented by a single species or by a group of species, as is true of many saline marshes and of very many forest areas. It may present an intermingling of two or three growth-forms, as is true of those saline marshes that contain grasses, the succulent Salicornia and the large- leaved Statice. In certain localities there occur communities which are made up of a very large number of growth-forms, as, for example, in the Karroo Desert of South Africa and in the deserts of Tehuacan, Mexico. Communities may be of such a character as to contain only plants of a single order of size, as the short-grass prairies of Nebraska, or they 24 THE VEGETATION OF THE UNITED STATES. may contain individuals of a particular height, together with other smaller individuals, as is true of the fresh-water marshes of the tribu- taries of Chesapeake Bay. By far the most common condition is that in which several successive stages of height are found together, so that the vegetation is said to possess a stratification with respect to the foliage of the plants concerned. This is found in every forest and reaches a splendid climax in the tropical rain-forests. In the com- munities which possess several strata of plants, the highest one has been designated the “facies,” but its members will here be alluded to as the “major” plants of the community and the several lower strata as the “minor” plants. A further salient feature of communities depends upon whether the plants cover the ground closely or stand in an open formation. We may discover open or closed communities among plants of every stature, from the smallest grasses to trees of a height of 50 or 60 feet. The greatest density of stand is reached by trees only under conditions which favor the attainment of a greater size than this. In open com- munities there may be plants of different heights, but in such cases the low plants are not dependent on the large plants for the conditions which render their existence possible. Even in open communities there may be a certain degree of stratification under the largest in- dividuals as is observable in southern Arizona, where many small annuals occur abundantly beneath the trees as well as away from them. Last among the criteria of the community is the number of species comprised in it. Even if there is a uniformity of growth-form through- out the community, there is importance, from our standpoint, in know- ing whether this uniformity is caused by the existence of a single species or of many. It is customary to designate the species which is most common in a community as the “dominant” one and the other species as “subordinate.” The salient features by which we distinguish types of vegetation are, then, to sum up: the growth-forms involved, the presence of one or many strata of plants, the open or closed condition, and the degree of simplicity or complexity of the specific content. These are all features which must be looked upon as products of the environmental conditions just as truly as are any of the structures or physiological reactions of the individual plants themselves. The presence of a single growth-form or of many, the existence of a low carpet of plants or of a lofty forest, the openness or density, and the dominance of a single species or the successful association of many, are all features which are determned by the environmental complex just as truly as is the rate of growth or that of photosynthesis for an individual plant or species. The only other criteria that have been used in defining communities are the physical nature of the habitat and the specific identity of the INTRODUCTION. 25 plants concerned, each of which has been sufficiently discussed to indicate that they are counter to our purpose. In making a general study of the relation of climate to vegetation for as large an area as the United States, it has been necessary for us to disregard the small communities which are unlike the general vege- tation of the surrounding region, such as the local prairies of Arkansas and Mississippi and the bands of trees that border the rivers of the western plains. These require special treatment, in which soil condi- tions can be given a thorough investigation. It has also been neces- sary for us to leave out ‘of reckoning all of the minor plants in the stratified communities, inasmuch as the conditions under which they live are unlike those for the major plants. The climatic conditions under which the major plants exist are modified by them in such a manner as frequently to give the minor plants a very different environ- mental complex. By means of these omissions we have done a great deal to simplify our problem from the standpoint of the demarcation of vegetational areas. It has been desirable, furthermore, to consider only the most general features of the vegetation, because we have only very general data as to the distribution of the climatic factors. The subsidiary communities of every region are so largely controlled through the soil conditions that it would have carried us beyond our investigation of climatic controls to have entered upon a consideration of them. The statement that the occurrence and geographical range of all plant communities is controlled by the physical, and rarely the chemi- eal, characteristics of the environment is almost axiomatic. The operation of these controls has been observed from time immemorial by men of no technical training but of keen powers of observation, and the knowledge of them has become more exact among the practical pioneers in the agriculture and forestry of many lands where the natural vegetation has been used as an index of the cultural capabili- ties of given situations. Any skepticism regarding the physical con- trol of communities would be dissipated by an extended course of travel in diversified regions, or equally well by a careful reading of Schimper’s Plant Geography, to say nothing of an examination of the many scattered papers which give proofs in regard to particular instances of such control. We can, in brief, put it down as a law of plant geography that the existence, limits, and movements of plant communities are controlled by physical conditions. The conditions that control the movements of the community are those of the soil; the conditions that control the broader geographical limits are almost solely those of the climate. The existence of the community and the extent of the area occupied are, of course, controlled by conditions of both soil and climate. 26 THE VEGETATION OF THE UNITED STATES. VI. DELIMITATION OF VEGETATIONAL AREAS. The botanical areas that have formed a basis for the correlations discussed in the following pages have been outlined in such manner as to show either the distribution of particular types of vegetation or else the ranges of individual species or groups of species. The deter- mining of the distributional area of a given species is a relatively simple matter, depending for its accuracy only on the exploration that has been carried out and the records of occurrence that are available. The delimitation of vegetational areas, however, demands a careful scrutiny, of criteria and methods such as we have attempted to give above. "We have endeavored primarily to classify and map the vege- tation of the United States upon a basis which is purely vegetational, without regard to floristics, climate, topography, or other features, however closely these may seem to be associated with the vegetation. The effort to observe this requirement, for the sake of the logical soundness of our work, is nevertheless far from removing all of the difficulties which beset an attempt to classify and map the vegetation of a large area. It is difficult for the worker to avoid a subjective treatment of his material and to escape the bias which his own particu- lar experiences or field of observation may have given him. A more tangible set of difficulties arises in deciding where to draw the lines of demarcation in subdividing a set of intergrading vegetations, and where, on the map, to place the lines of separation between vegeta- tions that merge into each other over areas of great extent. This difficulty has been met by drawing lines on the chart through all transitional regions, these lines being so drawn as to be regarded as connecting the points that exhibit the same stage in transition, after the manner of isotherms and other isoclimatic lines. After a series of vegetational areas has been distinguished and delimited, and each has possibly been subdivided, we must refrain from regarding these divisions or subdivisions as of coordinate value, for there is no means of putting the degree of their relationship to a test. The subdivisions of the forest areas and those of the desert areas appear, on the printed page, to be of the same rank in classification, but we have no actual knowledge upon which we can base such a supposition. We have already seen that the features of outward configuration which are considered in distinguishing growth-forms have to do, to a predominant extent, with the water-relations of plants. When we examine into the other features which we use in distinguishing vegeta- tions, such as the height of the dominant plants, the density of stand, and the simplicity or complexity of the stand, we are impressed with the fact that these features stand also in dependence upon water- relations. INTRODUCTION. Hd These considerations force us to realize that the most commonly used and most natural subdivisions of vegetation are based upon cri- teria which have to do with the relations of the communities to water- supply and water-loss. /It is quite true that the water-relations of plants have more to do with the control of the local and general dis- tribution of vegetation than have any other conditions. This is not true of the local and general distribution of the species themselves, for we here find temperature relations playing a strong role. ‘For the pur- poses of our investigation into the correlations existing between vege- tation and climate it is therefore significant that we are under the necessity of using a classification of vegetation which rests so largely upon a basis of the water-relations of plants. We might foresee from this fact that strong correlations would be discovered between vegeta- tion and water conditions and weak correlations between vegetation and temperature conditions. In spite of the efforts of Schimper and of others to give the tempera- ture-relations of plants a place in vegetational distinctions, by the recognition of microtherms, mesotherms, and megatherms, we are as yet unable to place a given species in its proper thermal category with- out possessing facts which are still lacking for all but a very few plants. We can not tell a megathermous plant from a microthermous plant when we see them growing side by side, and it follows that we can not go into the field in Georgia or Texas and pick out the plants in each habitat which have great or small temperature requirements, as we ean rather satisfactorily distinguish those of great or small water requirement. These categories are consequently of no present use in delimiting vegetational areas. The system of life-zones which was worked out by Merriam! for the United States, and has been elaborated by members of the United States Biological Survey, constitutes an effort to delimit biological areas primarily with respect to the influence of temperature. With such a classification of biological areas in hand it is not possible, how- ever, to make an impartial effort to determine which of several climatic factors is primarily concerned in conditioning the existence and limits of the areas. This attempt would indicate a very close correlation of biological areas and certain temperature conditions, because the climatic maps showing these temperature conditions were used as a basis in the original form of the life-zone map. If we were to make a map of the mean rainfall of the growing-season in the United States it would be found to possess certain isohyetal lines which corresponded closely with the distribution of certain plants or vegetations. If we were, then, to modify the rainfall map in slight 1Merriam, C. Hart. , Laws of temperature control of the geographic distribution of terrestrial animals and plants, Nat. Geog. Mag., 6:229-238, 3 maps, 1894.—Life zones and crop zones of the United States. U.S. Dept. Agric., Biol. Surv. Bull. 10, 1898. 28 THE VEGETATION OF THE UNITED STATES. particulars so as to make it conform more closely to the distribution of vegetation, using vegetation rather than additional rainfall data as a basis, we would secure a map of great interest and value as a delinea- tion of the vegetation of the United States. This map would be of no value for a correlational study, however, since it would be in inherent and foreseeable agreement with the map of mean rainfall of the growing season, which is very similar to all other maps of moisture conditions. The drawing of this map would resemble in all particulars the con- struction of the life-zone map of the United States, both with respect to the manner in which it was made and with regard to its unsuita- bility for our purposes. It may also be emphasized in this connection that, although the temperature conditions and the moisture conditions of climate are con- sidered as distinct in analytical studies, yet they are not truly inde- pendent of each other. Since evaporation and precipitation are so largely influenced by temperature, neither the chart of rainfall nor that of temperature is wholly without indications of the influence of one condition upon the other. It frequently happens, for example, that a region of low temperature is one of high soil-moisture content, low evaporation, etc. These considerations will receive more attention in Part IT. DISTRIBUTION OF VEGETATION IN THE UNITED STATES. I. METHODS USED IN SECURING AND PRESENTING THE DISTRIBUTIONAL DATA. The botanical data on which we have based the correlations that are to be discussed in the succeeding pages are presented in carto- graphic form in plates 1 to 33. A detailed map of the vegetation of the United States (plate 1) has been executed as a basis for our cor- relations of climate with the vegetational areas of the country as a whole.! The features of the map will be discussed in a succeeding sec- tion, together with a general account of the vegetation of the 18 sub- divisions which it recognizes. This map is of very uneven merit for the different parts of the United States, owing to the fact that there is an abundance of literature for certain portions of the country, while there are very few descriptive treatments or maps of the vegetation for other portions. As it stands, however, this map is somewhat too detailed for use in correlation with the climatological data that we have been able to secure. For this reason we have deemed it desirable to make a generalized map based upon the detailed one (plate 2). The latter contains 18 vegetational areas, whereas in the former the number has been reduced to 9 by a combination of the areas which are most similar in character. Even after this is done there are some of the vegetational areas of the United States for which we have only a very small number of climatological stations. In order to investigate the correlations between climatic conditions and the distribution of certain common growth-forms, a series of 7 maps has been drawn, showing the cumulative occurrence of these forms (plates 3 to 9). The maps have been prepared by the method used by Transeau in his investigation of the forest-centers of the eastern United States.? It consists merely in indicating on a single map the distributional limits of all of the plants involved. The area in which all of them are found together represents the region of maximum development of the particular group which has been selected. In some cases these maps have been drawn for all of the species of a particular growth-form, whereas in other cases they have been drawn for a repre- sentative group of the most common species of a particular growth- form. These maps are of value, not only in showing the center of development of a particular form or group of species and in showing the extreme limits of the form, but also in showing the manner in which the abundance of the given form shades off in different direc- tions from the center. 1Shreve, Forrest, A map of the vegetation of the United States, Geographical Rey. 3: 119- 125. With map. 1917. This map is reproduced here as our plate 1. *Transeau, E. N., Forest Centers of Eastern America, Am. Nat., 39: 875-889. 1905. 29 30 YHE VEGETATION OF THE UNITED STATES. Three maps have also been prepared showing the ecological dis- tribution of individual species of plants (plates 8, 9, 10). On these maps an effort is made to show the features of distribution in such a way as to indicate the regions of greatest abundance and greatest catholicity of habitat, the regions of frequent occurrence, the regions of rare occurrence, and the extreme geographical limits of the species. Maps of this character afford a picture of the distribution of a single species which is similar to the picture afforded by the maps of cumu- lative distribution of groups of related growth-forms. As a basis for the correlation of climatic conditions with the ranges of individual species, 70 plants were selected, the distributional areas of which are shown by groups on plates 13 to 33. Some of the species selected for this purpose were chosen because they are common and dominant elements in important vegetations of wide extent. Others were selected because their geographical ranges seem to be typical of those exhibited by a large number of species. Certain other species were selected because of the interest which attaches, from our point of view, to the character and particular direction of their distribution. Several of these plants extend across the continent from the Atlantic to the Pacific, either in the northern or in the southern part of the United States, in such a manner that they cross the principal boun- daries between vegetational areas. Distributions of this character seem to indicate that the plants in question are probably controlled by temperature conditions rather than moisture conditions. Several plants of this character and of more limited range were selected, be- cause they are commonly found in swamps or marshes and may there- fore be thought of as growing through a wide range of atmospheric conditions, at the same time that they are subjected to a relatively uniform set of soil-moisture conditions. It is to be anticipated that plants of this character differ markedly in their distribution from those whose range is greatly influenced by soil-water conditions. The construction of the vegetational maps has involved the examina- tion of a large body of ecological, floristic, and geographical literature. It was originally our plan to publish a complete list of the sources that were used in the compilation of these maps, but the appearance of Harshberger’s Phytogeographic Survey of North America has since made it superfluous to do so, inasmuch as this author has given a very thorough bibliography of the literature of American vegetation, in- cluding nearly all of the publications that we have used.’ In the construction of the maps of vegetation we have been heavily indebted to the maps published by the United States Forest Service, to the maps of grazing-lands published by the United States Bureau of Plant Industry, and to the detailed maps which have been published 'Harshberger, J. W., Phytogeographic survey of North America, Die Vegetation der Erde, 13: 863 p., 32 figs., 18 pls., 1 map, Leipzig, 1911. DISTRIBUTION OF VEGETATION IN UNITED STATES. 31 for several States. We have not only drawn upon ecological literature and the publications of the United States Geological Survey, but have been greatly aided by consultation of the photographic illustrations in numerous works of a non-botanical character, and we are particularly indebted to many of our colleagues, who have generously given us the information at their disposal. In the preparation of the maps showing the cumulative distribution of growth-forms and the distribution of individual species we have used all of the manuals, floras, and local lists that it was possible to secure. We have not consulted the specimens that are to be found in any of the large herbaria, but have depended solely upon published statements of occurrence. We have done this because it is so frequently possible to secure, in ecological literature or in the publications of the United States Biological Survey, statements regarding the ranges of plants which would in all probability be represented in the herbaria by only a few collections, on the labels of which the information regarding the ecological occurrence would be extremely meager. Some of the ranges of individual species are naturally outlined much more accurately than others. The distribution of trees is in general much better known that that of herbaceous plants, and the distribu- tion of grasses is better known than that of plants whose economic importance is not so great. Some of the ranges exhibited in our maps have been based on extremely few stations, as is true particularly of such plants as Flerkea occidentalis and Trautvetteria grandis. It may be taken for granted that all of our distributional areas which are represented by smooth and wide-sweeping lines are in general based upon less precise information than are the areas limited by very sinu- ous lines. The limitations of our method of mapping have required that the range of numerous mountain plants occurring in the Western States be exhibited by passing a bounding-line around the entire region in which they are found locally at their appropriate elevations. In similar manner several of the aquatic and palustrine plants have been plotted as if growing continuously throughout extensive stretches of country in which they are really of very local occurrence. This is notably true for Sium cicutefolium and for Cephalanthus occidentalis, which is not actually known in New Mexico and is very uncommon in Arizona, although it reappears in great abundance in interior Cali- fornia. The scale of our maps has made it necessary to include in the ranges of very many of our plants certain extreme coastal locations in which they are actually absent. This matter is mentioned only because several important climatological stations are located on the coast in areas which differ very markedly from the adjacent mainland in the character of their vegetation. This is true of Key West, Cape Hatteras, and Point Flattery. Further details regarding the vege- tational maps will be given in the succeeding pages. 32 THE VEGETATION OF THE UNITED STATES. II. LEADING VEGETATION TYPES OF THE UNITED STATES AND THEIR GEOGRAPHICAL AREAS. In subdividing the vegetation of the United States we have used primarily the old distinction of desert, grassland, and forest. The distinguishing of these formations involves practically all of the criteria that have been discussed in previous pages. Our ultimate units have resulted from a subdivision of the desert and the forest, but we have not attempted to subdivide the grassland, partly because of lack of descriptive literature for all parts of the grassland region, and partly because of the extreme complexity exhibited by this region, particu- larly in its central portion. The desert areas have fallen into two groups which may be designated as Continental Desert and Coastal Desert. The latter include the semidesert regions of coastal California and of extreme southern Texas, each of them regions in which truly desert areas are found together with areas which scarcely merit this designation. The Continental Desert falls naturally into two regions, one of which is dominated by sclerophyllous shrubs and the other by a mixture of such shrubs and succulent or semisucculent plants. The sclerophyllous desert has been subdivided into the Great Basin Desert and the California Desert, while the succulent desert has been subdivided into the Arizona and Texas areas. The two bodies of sclerophyllous desert are adjacent and merge into each other, whereas the succulent deserts are separated by regions of dissimilar character. The Grassland area is bordered on the southwest by the Desert- Grassland Transition and on the east by the Grassland-Deciduous Forest Transition. The forested portion of the United States has been subdivided into the Deciduous Forest and 4 areas of Evergreen Needle-Leaved Forest. Two of these evergreen-forest areas are mesophytic in character, one of them xerophytic, and one of them hygrophytic. We have included in the Northern Mesophytic Evergreen Forest all of the needle-leaved forests of the northeastern States as well as of the Rocky Mountain and Pacific regions, with the exception of the extreme Northwest. Although this large forest region exhibits marked differences in its floristic make-up and in minor features of its physiognomy and ecological characteristics, there are nevertheless more reasons for maintaining it as a single area than for separating it into minor sub- divisions, so far as the purposes of our work are concerned. The Southeastern Mesophytic Evergreen Forest, the Western Xerophytie Forest, and the Northwestern Hygrophytic Evergreen Forest are all regions of distinctive character which are neither similar to the Northern Mesophytic Evergreen Forest nor to each other. Two transitional areas have also been outlined, connecting the Deciduous Forest with the Evergreen Forests to the north and to the south of it. DISTRIBUTION OF VEGETATION IN UNITED STATES. 30 We have also recognized as alpine summits all of the areas lying above timber-line, and as Swamps and Marshes the areas with saturated soil in the Atlantic Coastal Plain. In selecting names for the vegetational areas thus distinguished we have felt it desirable to use designations which have to do with the ecological character of the plants found in these areas. We have avoided the use of such words as coniferous, which alludes to a mor- phological feature, and have also avoided using any designations which would imply that the vegetation of a particular area is controlled by a particular physical condition, as, for example, the use of such terms as ‘‘monsoon forest”’ or ‘‘pinelands of the odlitic limestone.’”’ We are well aware that the names that we have used are neither brief nor convenient, but they have been selected with the cautions that have been mentioned, for purposes of ecological consistency. An effort has been made to draw all of the vegetational boundaries on the map shown as plate 1 with relation to the original plant covering. This primeval condition has been so greatly disturbed over large portions of the country, particularly in the Northeast, that it is now very difficult to be certain as to the limits between the virgin forest formations. Outr map is therefore undoubtedly much less accurate for the northeastern portion of the United States than it is for the Western States, in which the vegetational differences are more marked, the country is less disturbed by human activities, and the published ecological descriptions of vegetation are much fuller and more accurate. The following paragraphs serve to give a brief characterization of each of the vegetational areas found in our detailed map, plate 1: California Microphyll Desert—The southernmost part of Nevada and the interior portion of California form an area in which the vege- tation is closely related to that of the Great Basin by reason of the fact that the dominant plants in each are microphyllous shrubs. In the California Desert the creosote-bush (Covillea tridentata) and the sand- bur (Franseria dumosa) are the most common plants and the dominant ones over extensive areas. A large number of deciduous shrubs are found, large semisucculents occur throughout the more elevated por- tions of the desert (Yucca brevifolia, Y. arborescens), and stem-suc- culents (Opuntia spp.) are found in small numbers. In both the Great Basin and the California deserts the plants which perennate by under- ground parts are very rare and grasses are uncommon, while short- lived annuals are abundant in the spring months. Great Basin Microphyll Desert—This desert occupies the floor and low mountains of the Great Basin from southern Washington to southern Nevada and eastward to Colorado, lying at elevations of 1,000 to 5,000 feet. Throughout practically the whole of this area the vegetation is dominated by a single species, the sagebrush (Arte- ¢ misia tridentata), a small-leaved evergreen shrub which sometimes 34 THE VEGETATION OF THE UNITED STATES. occurs in very open stand with a height of a few inches or at other times grows so thickly as nearly to cover the ground, reaching a height of 4 feet or more. Together with the sagebrush are to be found several other microphyllous shrubs of similar size and distribution (Sarco- batus vermiculatus, Grayia spinosa, Coleogyne ramosissima, Kunzia tridentata, Tetradymia glabrata, etc.). The Great Basin desert is dis- tinctively a region of microphyllous shrubs, in which succulent plants are rare and confined to the highest mountain-slopes. The simplicity of the vegetation and the uniformity in the character of the several shrubs which play a secondary réle in it are features very unlike the other desert areas about to be described. Texas Succulent Desert.—The central valley of the Rio Grande and the valley of the Pecos River form a desert area in which extensive tracts are dominated by evergreen shrubs in open stand, while other areas are chiefly occupied by deciduous shrubbery (Acacia, Flourensia, Brayodendron). Large areas are covered by the low leaf-succulent lechuguilla (Agave lechuguilla), but not to the exclusion of the shrub- bery. Other areas are dominated more conspicuously by the sotol (Dasylirion texanum), a plant with perennial foliage and with a store of water and reserve materials in its stout stem. Succulents are abundantly represented in this desert, but chiefly by small species which do not play an important part in the physiognomy of the vege- tation. Perennial bunch-grasses are common in certain portions of the Texas Desert, and the number of plants perennial by underground succulent or semisucculent parts is larger than in the Arizona Desert. This is distinctively a region of leaf-succulents and semisucculents as contrasted with the Arizona region of stem-succulents. Arizona Succulent Desert—This area comprises the southern por- tion of Arizona drained by the Bill Williams and Gila Rivers and lying below 4,000 feet. In this desert the vegetation is largely made up of microphyllous shrubs, but there is everywhere a rich commingling of other types of non-succulent plants and of several types of succu- lents. The vegetation is open and low, but of irregular height. The sclerophyllous shrubs comprise the evergreen creosote-bush (Covillea tridentata), deciduous acacias (Acacia paucispina, A. constricta), and bitter bush (Flowrensia cernua), the drought-deciduous ocotillo (Fou- quieria splendens), several small-leaved or leafless trees and shrubs with green bark and stems (Parkinsonia, Canotia, Holacantha, Keberlinia, Ephedra), as well as the columnar giant cactus (Carnegiea gigantea) and numerous species of flat-jointed and round-jointed cacti (Opuntia). This desert is by no means poor in perennial grasses, and the seasonal rains are followed by the appearance of carpets of annual grasses and other herbaceous plants. The leafy succulents are rare in this desert, except at its upper edges around the higher mountains, and the plants with subterranean water-storing organs are very infrequent. DISTRIBUTION OF VEGETATION IN UNITED STATES. 35 Texas Semidesert (Mesquital-Grassland Complex).—The valley of the Rio Grande below the Balcones Escarpment and the River Neuces presents a region in which the deciduous compound-leaved leguminous shrubs and trees form the dominant vegetation, interspersed with areas of more or less open grassland. The shrubs and trees form a more or less closed scrub from 3 to 6 feet high where the shrubs pre- dominate and a more open one from 15 to 25 feet high where the trees are most abundant. The commonest of the trees is the mesquite (Prosopis glandulosa), which has spread somewhat over adjacent areas since the advent of the white man. With it the evergreen broad- leaved live-oak (Quercus virginiana) is a minor component of the vege- tation. In the scrub the dominant species are huisache (Acacia farnesiana), guajillo (Acacia berlandierei), and other microphyllous forms (Momesia, Parkinsonia, Condalia, Sesban, etc.), while the succu- lents are confined to the frequent occurrence of a prickly-pear cactus (Opuntia lindheimert) and a yucca (Yucca treculeana). Ephemeral herbaceous plants are also present here. Pacific Semidesert (Chaparral-Encinal-Desert Complex).—The Pacific. semidesert region is best designated thus in spite of the fact that it comprises many small areas which are far from being desert. There is no other portion of the United States in which such profound differ- ences of vegetation exist within such small areas, and in which it would be more difficult to map on a small scale the complex allocation of these areas. Over the low hills and around the bases of the Coast Ranges is to be found chaparral, varying from place to place in height and density; in the valleys and on the north faces of some of the hills are to be found groves of evergreen or deciduous oaks; while in other valleys, particularly the broad valleys of the Sacramento and San Joaquin Rivers, and on the interior hills of the Coast Ranges, are to be found some of the most truly desert areas in the United States. The chaparral is sometimes a very low aggregation of shrubs, or some- times reaches a height of 6 or 7 feet. It is also variable in its density, but is commonly so close-set that it can be traversed only very slowly. It is made up predominantly of evergreen shrubs (Ademostoma, Arctostaphylos, Heteromeles), but partly of deciduous shrubs (Ceano- thus spp.). The encinal is made up of evergreen oaks (Quercus agrt- folia) and deciduous oaks (Q. lobata, Q. wislizeni), with the digger pine (Pinus sabiniana) an occasional component of it. In the desert of the interior hills and the great valley is to be witnessed a region almost totally devoid of perennial plants, in which the only cover of vegetation is due to the herbaceous annuals that appear in the spring. Desert-Grassland Transition.—This transition region comprises the Llano Estacado of Texas and certain plateau lands in Arizona and New Mexico above 5,000 feet in elevation. It is a region that is inter- mediate in all important respects between the Grasslands to the east 36 THE VEGETATION OF THE UNITED STATES. and the Desert regions to the west. It is essentially a very open stand of perennial grasses, together with the herbaceous annuals or peren- nials of the Desert and an extremely scattered stand of succulent or semisucculent plants. In the Llano Estacado and in southern New Mexico and Arizona the latter group comprises sotol (Dasylirion) and bear-grass (Nolina), while farther north in Texas and New Mexico the commonest succulent or semisucculent forms are a yucca (Yucca glauca) and a round-jointed cactus (Opuntia arborescens). In northern Arizona and northwestern New Mexico there are low, shrubby sages (Artemisia), Mormon tea (Ephedra), and other scattered bushes, and small cacti (Opuntia hystricina, O. whipplei). Throughout the Transi- tion region the grasses are omnipresent, sometimes forming nearly as dense a carpet as they do in the Grassland itself. There is a con- siderable variety in the grass flora, but the commonest forms are species of Bouteloua, Hilaria, Bulbilis, and Aristida. Grassland.—The Grassland region extends from central Texas to the Canadian boundary, merging on the east into the transition region which separates it from the Deciduous Forest, and on the west either merging into the Desert-Grassland Transition or else terminating at the eastern base of the Rocky Mountains. Smaller detached areas of Grassland also surround the northernmost salients of the desert. Throughout the Grassland region the vegetation is dominated by a more or less continuous cover of perennial grasses—in some localities by a dense sod, in others by an open sod, and in still others by an open stand of bunch-grasses. The types of grasses which form the grassland are varied, both in the region as a whole and in any small portion of it. A score of grass species form the great bulk of the vegetation, several of them being of very widespread occurrence throughout the region, as Bouteloua oligostachya, Bulbilis dactyloides, Keleria cristata, and “species of Andropogon, while others are confined to different portions of the area or to particular soils, as the species of Hilaria in Texas, the species of Sporobolus and Stipa in Kansas and Nebraska, and the species of Agropyron in the northwestern part of the area. In addition to the score of commonest species, there are a hundred or more that are either frequent over large areas or common over smaller portions of the area. From a floristic standpoint the Grassland presents two grada- tions, one encountered in going from the eastern edge toward the Rocky Mountains, the other encountered in going from south to north through its entire length of over 1,200 miles. From a vegetational standpoint, however, this is allaregion of great uniformity. Its prin- cipal variations are in the relative density or openness of the grassy cover, in the character of the areas in which grasses are sparse or absent, and in the frequence of plants other than grasses. It may be said, in general, that the carpet of grasses is most evenly closed along the eastern edge of the area and in the central portion. In central Texas DISTRIBUTION OF VEGETATION IN UNITED STATES. ay and in the extreme north much of the Grassland is relatively open, particularly in the sandhills of Nebraska, where Andropogon scoparius is predominant, and in the portions of the region lying in Washington and Oregon, where Agropyron spicatum and Poa sandbergii are pre- dominant. In the highly eroded ‘‘bad lands,” such as occur in southern South Dakota, low, shrubby perennials are predominant, as the rabbit- brush (Chrysothamnus graveolens), the white “sage” (Eurotia lanata), and greasewood (Sarcobatus vermiculatus). Throughout the portion of the Grassland which lies nearest the Rocky Mountains there is a complicated patchwork of vegetation, in which closed grassland, open grassland, and open stands of low bushes, with or without grasses, are found to alternate in habitats of different character. The shrubby perennials found in such areas are chiefly those which have just been mentioned, together with species of true sage (Artemisia tridentata, A. frigida). The Grassland is locally invaded by plants of the types which are dominant in all of the surrounding regions. In the south- western portion of the area arborescent round-jointed cacti (Opuntia arborescens) are sporadic, and also yucca (Yucca glauca), which is likewise common in the sandhills of Nebraska. Throughout the area a low prickly pear (Opuntia missouriensis) is abundant on coarse soil, particularly in the Bad Lands. In the vicinity of the mountains the Grassland is invaded by shrubs (Cercocarpus, Quercus, Symphoricarpus), and in some localities even by coniferous trees (Pinus ponderosa), while the bottomlands of the rivers are the westernmost localities for many eastern broad-leaved deciduous trees. In every portion of the Grassland there are to be found very many types of low annual or root-perennial plants other than grasses. Among these certain composites are perhaps the most abundant, as Grindelia squarrosa and Chrysopsis villosa, although there are many plants of other types. The seasonal habits of the grasses and of these associated non-gramineous plants are such as to give rise to one of the most striking characteristics of the Grassland, namely, its different aspect in different portions of the growing-season, and the difference in the conspicuously predominant plants in different months. Grassland-Deciduous Forest Transition This is the broadest and most extensive of the transition areas, but is so purely transitional in its character that it does not merit recognition on any other basis. Its eastern limit has been fixed along the line at which the Deciduous Forest ceases to be an unbroken formation and begins to exhibit the islands of grassland locally known as “oak openings.”’ The western limit has been placed where timber ceases to occur on the upland. The transition area is seen, therefore, to increase in the amount of timber found on going east and to increase in the extent of grassland exhibited on going west. 38 THE VEGETATION OF THE UNITED STATES. In the northern part of the Transition belt the commonest trees are black oak (Quercus velutina) and bur oak (Q. macrocarpa); in the southern part the commonest are post oak (Quercus minor) and blackjack (Q. marilandica). The grasses originally most abundant in the central part of the belt were Andropogon furcatus, Sorghastrum nutans, Andropogon virginicus, and Sporobolus cryptandrus. Deciduous Forest—The Deciduous Forest formerly occupied the lower elevations of the Northeastern States, the summits and slopes of the southern Allegheny Mountains, the Piedmont region, and the valleys of the Ohio, Cumberland, and Tennessee Rivers, with exten- sions into southern Texas and into northern Michigan and the Dakotas, and an attenuated edge that merges into the transition area toward the west. There is no one of the vegetational areas of the United States that has been more completely and profoundly altered by man than has this one. In fact, it is difficult at the present time to secure reliable information as to the exact original extent of this type of forest in the Northeastern States. The virgin stands of deciduous forest were made up solely of deciduous broad-leaved trees over extensive areas, and these forests were both dense and of a stature as great as 100 feet, or even more. At the western edge of the Deciduous Forest its con- tinuity becomes interrupted by open areas, while in the Texan exten- sion of it the stand of trees is even, but very open. In all of the moun- tainous or hilly portions of the area the needle-leaved evergreens frequently become components of the Deciduous Forest, and on steep bluffs, rocky slopes, and limestone ledges the needle-leaved trees are sometimes predominant. The floor of the Deciduous Forest is some- times thickly covered with shrubbery and young trees, or is often open and more conspicuously occupied by herbaceous perennials, among which the chief vegetative activity takes place in the spring, before the complete unfolding of the foliage of the trees. The number of tree species participating prominently in the make-up of the Deciduous Forest is large, and very many of the commonest ones are found almost throughout the area, as the white oak (Quercus alba), black oak (Quercus velutina), pignut hickory (Hicoria glabra), beech (Fagus ferruginea), and tulip-tree (Liriodendron tulipifera). The Appalachian region is the place in which the Deciduous Forest reaches its finest development, both in respect of density and stature of the stand and with regard to the number of tree species participat- ing in its composition. The most common trees of that region are the widespread ones which have just been mentioned, and also chestnut (Castanea dentata), chestnut oak (Quercus prinus), scarlet oak (Quercus coccinea), shagbark hickory (Hicoria ovata), Spanish oak (Quercus digitata), sugar maple (Acer saccharum), and red maple (Acer rubrum). On passing northward from the center of the Deciduous Forest, the number of tree species becomes smaller, as many are left behind and sll eee ae eee 2 oe DISTRIBUTION OF VEGETATION IN UNITED STATES. 39 few new ones are met, as birch (Betula lutea) and popular (Populus balsamifera). On passing southward, the commonest forms of the Alleghenian region are found to be confined to the mountainous dis tricts, while their place is taken on the Piedmont and in the Mississippi Valley by a still larger group of species, many of which are found not only on the upland, but in the half-swampy areas of the level regions. Prominent among these trees are Spanish oak (Quercus digitata), water oak (Quercus nigra), willow oak (Quercus phellos), black gum (Nyssa sylvatica), red gum (Liquidambar styraciflua), and blue-jack oak (Quercus brevifolia). The most southwesterly portion of the Deciduous Forest in central Texas is made up almost solely of post oak (Quercus minor) and black- jack oak (Quercus marilandica), while the most northwesterly islands of deciduous forest in the Dakotas are made up chiefly of bur oak (Quercus macrocarpa). Southeastern Evergreen-Deciduous Transition Forest—This transi- tion lies between the coastal Evergreen Forest and the interior Deciduous Forest, occupying hilly and broken land, except at the extreme western end. Throughout this area there are small bodies of pure evergreen needle-leaved forest and other bodies of pure deciduous forest, but the vegetation consists in the main of an admixture of the two types of trees in such percentages that neither dominates strongly over the other. As in all other transition regions, the local conditions of soil and topography often determine the precise composition of the forest. East of the Mississippi the Evergreen-Deciduous Transition is formed chiefly of the loblolly pine and the species of deciduous oaks that will be mentioned in connection with the inner portion of the Southeastern Evergreen Forest. West of the Mississippi the loblolly pine occurs near the Gulf coast and is chiefly replaced by the shortleaf pine (Pinus echinata) above the Neches River, while the same species of deciduous oaks accompany each of the pines. Southeastern Mesophytic Evergreen Forest—This region stretches from Long Island to Louisiana along the Coastal Plain, with an exten- sion into peninsular Florida, and with outlying areas in central Ala- bama and in Arkansas, Louisiana, and Texas. This area is dominated by evergreens, and a secondary réle in the vegetation is played by deciduous broad-leaved trees and by evergreen broad-leaved trees. The forest stands of this region are nowhere dense in the same sense as are some of the evergreen stands of Montana or Maine; indeed, many of the pine stands in all parts of the area, and particularly in Florida, are rather open. Some of the heaviest stands are found in Louisiana and Texas and the lightest are those of Florida and New Jersey. As a rule the pinelands which lie nearest the coast, particu- larly in the Gulf States, are the purest, while those of Maryland and the Carolinas, as well as the interior areas of Alabama and Arkansas, 40 THE VEGETATION OF THE UNITED STATES. are much richer in deciduous broad-leaved associates. In the pine forests of New Jersey the floor is extremely shrubby, and this is often the case as far south as South Carolina. In southern Georgia and the Gulf region the floor is much more open. In both of these cases it is difficult to decide how much the normal conditions may have been disturbed by fire and by clearing. Under present conditions, at least, the Gulf pinelands present a very clean floor, closely carpeted by grasses, palmetto, pitcher-plants, and a multitude of other herbaceous species. Along the branches and other depressions there is a dense stand of shrubbery and a slightly different type of forest. The half dozen species of pine which dominate the different sections of the Southeastern Mesophytic Evergreen Forest are very similar in their appearance, and there is consequently a general resemblance between the pinelands of the entire Coastal Plain. In Long Island and New Jersey the scrub pine (Pinus rigida) is the dominant species, and is scarcely found elsewhere in this forest. In Maryland, Virginia, and North Carolina the loblolly pine (Pinus teda) is the leading species, while in peninsular Florida the Cuban pine (Pinus caribea) is the chief form. Throughout the remaining major portion of the forest the long- leaf pine (Pinus palustris) is always the dominant tree, or at least abundantly represented in company with the loblolly pine. Through- out the Gulf region the slash pine (Pinus heterophylla) is found in moist soils and the pond pine (Pinus elliottii) in wet soils. Throughout the interior portions of this forest the shortleaf pine (Pinus echinata) is a characteristic tree, seldom found in company with the longleaf pine. In the northern and interior portions of the Southeastern Forest, the broad-leaved deciduous oaks are frequent associates of the pines. The post oak (Quercus minor) and the Spanish oak (Quercus digitata) are common from Maryland to Texas, and the blue-jack oak (Quercus brevifolia) and turkey oak (Quercus catesbei) from Georgia to Missis- sippi. The red gum (Liquidambar styraciflua), the black gum (Nyssa biflora), and the red maple (Acer rubrum) are also common broad- leaved deciduous elements of this forest, although usually most com- mon in poorly drained soil. From South Carolina to Texas the branches and moist depressions of the forest are characterized by many ever- green broad-leaved trees. Some of these are always evergreens, as the magnolia (Magnolia grandiflora) and the live oak (Quercus virgin- tana), while others are evergreen in the Gulf region and deciduous further north, as the water-oak (Quercus nigra) and the laurel oak (Quercus laurifolia). Throughout the portions of the area which are poorest in associated deciduous trees there are a number of evergreen broad-leaved shrubs which form a conspicuous element of the vegetation. The most notable of these are the gallberry (Jlex glabra), the red bay (Persea carolina), the waxberry (Myrica caroliniana), the ti-ti (Cliflonia monophylla), . —— oe a ele ee eae SE ee eee DISTRIBUTION OF VEGETATION IN UNITED STATES. Al and the sweet illicium (Illiciwm floridanum). In the extreme coastal region and in peninsular Florida the saw palmetto (Serenoa serrulata) is more conspicuous than the shrubs, although there is an increasing number of species of the latter, including two dwarf oaks (Quercus pumila and Quercus minima). Northeastern Evergreen-Deciduous Transition Forest—This type. of forest fringes the Northern Mesophytic Evergreen Forest from Minne- sota to Maine and southward along the Alleghenies. It is sometimes well marked as a nearly equal admixture of deciduous and evergreen needle-leaved trees, but on its southern and northern edges it merges into the larger types of forest, except where sudden changes of soil break the influence of climate. The most important deciduous con- stituents are sugar-maple (Acer saccharum), beech (Fagus atropunicea), birch (Betula spp.), and basswood (Tilia americana). The commonest evergreen trees are hemlock (T'suga canadensis), white pine (Pinus strobus), balsam fir (Abies balsamea), and jack pine (Pinus divaricata). Northern Mesophytic Evergreen Forest.—This forest occupies portions of the northern Pacific coast, all but the alpine portions of the Rocky Mountains above an elevation of 6,000 to 7,000 feet, and the higher summits of all the coastal and inland mountain ranges of the Western States. It also occurs in northern Minnesota, Wisconsin, and Michi- gan and extends from Maine over the higher elevations of the Appa- lachian region through New York, Pennsylvania, and West Virginia south to North Carolina. This widespread forest is essentially similar in its physiognomy throughout. It is dominated in all portions by the needle-leaved ever- green tree, although it is by no means free of an admixture of broad- leaved deciduous trees, and the latter are particularly common along the streams or as small trees beneath the canopy of evergreens. This forest is usually between 50 and 100 feet in stature, and it is commonly so dense that the entire ground is in shade, although this is notably untrue of the forests at elevations approaching timber-line on high mountains, and of those which approach the lower limit of timber on the desert mountain ranges of the interior. The heaviest stands of this forest are almost devoid of either shrubby or herbaceous under- growth, but are carpeted by beds of moss. In the more open stands there is usually considerable shrubbery, and when this exists it is made up of deciduous plants. In spite of the essential similarity of the Northern Mesophytic Evergreen Forest, throughout its extensive range of occurrence, it is made up of a large number of tree species, and its composition varies greatly from State to State, and especially when the eastern and western portions of the forest are compared. Owing to differences in habit of growth that exist between different coniferous trees there are some very striking differences in the physiognomy of the forest that 42 THE VEGETATION OF THE UNITED STATES. are to be thus explained, as, for example, the contrast of the open yellow-pine forest with the very dense lodgepole-pine forest. In the mountains of the Western States there are also considerable differ- ences in the coniferous forests due to altitude, and these differences are visible in the floristic composition as well as in the physiognomy. The leading tree in the composition of all the western forests of this character is the western yellow pine (Pinus ponderosa), which often forms extensive pure stands at middle altitudes, is replaced by other species at the highest elevations, and is sometimes the tree of lowest range, as in the northern Rocky Mountains, or is mingled with the oaks, junipers, and nut pines of the Western Xerophytic Evergreen Forest at its lower range. The tree which most frequently replaces the yellow pine in the domination of this forest is the lodgepole pine (Pinus murrayana), which is extremely abundant in pure stands in Idaho and Montana, occupies a belt of dominance from 3,000 to 5,000 feet in portions of eastern Washington, is almost the sole forest tree in the Big Horn Mountains of Wyoming, grows with the Douglas fir (Pseudotsuga mucronata) on the western side of the Coast Range, and is locally dominant in the Rocky Mountains of Colorado. The most hygrophilous portions of the Mesophytic Evergreen Forest are the coastward slopes of the mountains of Oregon and the western slopes of the Rocky Mountains in Montana and Idaho. In the former of these regions the lowest belt of forest is formed by yellow pine, Douglas fir, and lodgepole pine; above 5,000 feet the Douglas fir is the dominant tree, growing with yellow pine, sugar pine (Pinus lambertiana), white fir (Abies concolor), lodgepole pine, and noble fir (Abies nobilis); while the higher forested elevations are dominated by the alpine hemlock (TJ'suga pattonii), together with lodgepole pine, noble fir, white-bark pine (Pinus albicaulis), western white pine (Pinus monticola), and alpine fir (Abies lasiocarpa). The moister forests of the northern Rocky Mountains lie between a pure stand of yellow pine at lower elevations and an open stand of alpine fir and white-bark pine above. The commonest trees of this forest are western white pine and western larch (Larix occidentalis). The Douglas fir is frequent and the alpine fir (Abies lasiocarpa), the lodgepole pine, and giant cedar (Thuja plicata) form a small percentage of the arborescent vegetation. In the Sierra Nevada the lowest coastward fringe of forest is formed by an open stand of the digger pine (Pinus sabiniana), and the main body of the forest is formed of yellow pine, incense cedar (Libocedrus decurrens), sugar pine (Pinus lambertiana), Jeffrey pine (Pinus jeffreyt) and white fir (Abies concolor). At the highest timbered elevations lodgepole pine, Jeffrey pine, and red fir (Abies magnifica) are the char- acteristic trees. In the San Bernardino and San Jacinto Mountains the forest below 6,000 feet is composed almost solely of yellow pine, —— ——_— a DISTRIBUTION OF VEGETATION IN UNITED STATES. 43 and above that elevation of an admixture of yellow pine, white fir, and sugar pine. In the Rocky Mountains of Colorado the principal areas of forest are dominated either by yellow pine or lodgepole pine, or more rarely by an admixture of them. The higher elevations are characterized by Douglas fir (Pseudotsuga mucronata), Engelmann spruce (Picea engelmanni), and by the fox-tail and limber pines (Pinus aristata and P. flexilis) and the Parry fir (Picea parryana) and alpine fir (Abies lasiocarpa). The forest in the Black Hills of South Dakota and in the desert mountains of Arizona and New Mexico is chiefly formed by yellow pine. The Mesophytic Evergreen Forest of northern Minnesota is a com- posite of white pine (Pinus strobus) on the deeper soils; of Norway pine (Pinus resinosa) and jack pine (Pinus divaricata) on the lighter soils; with tamarack (Larix laricina), black spruce (Picea mariana), and white spruce (Picea canadensis) in wet soils; and arborvite (Thuja occidentalis) in the bogs. The deciduous broad-leaved trees are more conspicuous here than in any portion of the western half of this forest, bur oak (Quercus macrocarpa), basswood (Tilia americana), and sugar maple (Acer saccharum) being the commonest species. In Maine the principal trees in the evergreen forest are spruce (Picea nigra), balsam fir (Abies balsamea), white pine (Pinus strobus), and hemlock (7'suga canadensis). This group of trees is also charac- teristic of the coniferous areas of the other New England States, of the Catskill and Adirondack Mountains in New York, and of the moun- tains of Pennsylvania. The small coniferous areas on the mountains of North Carolina are chiefly composed of black spruce (Picea mariana) and the Fraser fir (Abies fraseri). Western Xerophytic Evergreen Forest—The Xerophytic Evergreen Forest is a dwarf and open form of ‘‘woodland”’ or ‘‘semi-forest”’ that characterizes the edges of the Mesophytic Evergreen Forest through- out the southern half of the western portion of that forest. The Xerophytic Forest seldom covers extensive areas, except in northern Arizona, and in all localities it becomes more open at the lower edges, where it meets the Desert or the Desert-Grassland Transition, and more closed at the upper edge, where it merges with the Mesophytic Ever- green Forest. The Xerophytic Forest is similar to the desert in that its dominant plants are widely spaced, leaving much unoccupied ground. It is, again, similar to the desert and unlike the other forest areas in the small stature of its trees, which never exceed 50 feet and frequently attain less than 25 feet in height. The two types of tree which dominate the Xerophytic Forest are the nut pine (Pinus edulis, P. cembroides, P. parryana) and the juniper (Juniperus utahensis, J. californica, J. 44 THE VEGETATION OF THE UNITED STATES. occidentalis, J. pachyplea). Sometimes these two types are equally mingled, or more frequently one of the two is predominant. The species mentioned do not greatly overlap, but occupy different areas within the Xerophytic Forest. With these coniferous trees grow also certain evergreen broad-leaved oaks. In the Great Basin and in Colo- rado the réle played by the oaks is a minor one, but in New Mexico it is more important, and in southern Arizona several arborescent species of evergreen oaks are frequently as common as the conifers, or more so. The Xerophytic Forest also contains numerous conspicuous shrubs of different types (Cercocarpus, Cowania, Artemisia, Ephedra, ete.), as well as such succulent and semisucculent plants as the yuccas and agaves, and conspicuous bunch-grasses and other perennials of inter- mittent or seasonal activity. Northwestern Hygrophytic Evergreen Forest.—This forest occupies the coastal region of Washington, Oregon, and extreme northern California, and an isolated portion of it lies on the western slopes of the Cascade Range in Oregon. This area exceeds any portions of the Mesophytic Evergreen Forest in density of stand and in the stature of the trees, which very frequently exceed 100 feet in height. The heavily shaded floor of the forest is covered with fallen trunks and limbs, overgrown with mosses and hepatics, and underlaid by a deep bed of humus. The deciduous trees are few and small, but a number of evergreen ericaceous shrubs are common on the floor of the forest, as are also ferns and large-leaved herbaceous plants. The density, tall stature, and vigorous activity of the Hygrophytic Forest give it a very distinctive physiognomy and betoken a set of environmental conditions unlike those of the Mesophytic Forest, in accordance with which it possesses a number of distinctive tree species. The tree which is of most general occurrence throughout the area is the Douglas fir (Pseudotsuga mucronata), which is also found far beyond the limits of this forest. It is accompanied in nearly equal admixture in many localities by the black hemlock (7'suga mertensiana). Other species common in this forest are the Sitka spruce (Picea sitchensis), white fir (Abies grandis), giant cedar (Thuja plicata), amabilis fir (Abies amabilis), noble fir (Abies nobilis), redwood (Sequoia semper- virens), and western white pine (Pinus monticola). The highest ocean- ward elevations of the Coast Range are similar to other subalpine areas in the coniferous forests, and are characterized by an open stand of symmetrical conifers, branching to the ground. Alpine Summits.—The principal alpine summits are those of the Caseade, Sierra Nevada, and Rocky Mountains, although small areas occur elsewhere. Their vegetation is composed of such dwarfed or prostrate trees as may be able to exist above timber-line, together with low, matted, or polsterform perennial plants with large roots. Meadows or the margins of lakes above timber-line are the habitats of DISTRIBUTION OF VEGETATION IN UNITED STATES. 45 numerous herbaceous species, usually less highly specialized than those growing in rocky soil or crevices. On the highest mountains of the United States the vegetation is often limited to mosses and lichens or is locally absent. Swamps and Marshes.—These terms comprise an extremely varied series of communities, partially dominated by trees and partially by coarse grasses, sedges, or other palustrine plants. The words “swamp” and ‘‘marsh”’ are both somewhat objectionable for use in the present connection because of their intrinsic reference to the nature of the habitat. The distinction between swamp and marsh has so long been drawn in popular speech and scientific writing, however, that the words are used here as terms descriptive of the vegetation rather than designations that imply the feature of the environment which deter- mines the vegetation. The areas of swamp and marsh are so intri- cately interwoven that no effort has been made to separate them. The greatest development of swamps and marshes is to be found along the shores of the Atlantic Coastal Plain, although there are smaller areas of marsh on the Pacific Coast and scattered areas of swamp throughout the glaciated region. The saline marshes are dominated by nearly pure stands of halo- phytic grasses, while the fresh or brackish marshes are inhabited by very diverse populations of herbaceous perennials and annuals. The swamps of the Southeastern States are composed of a particularly rich assemblage of deciduous broad-leaved trees (Nyssa, Acer, Mag- nolia, and Quercus) or of nearly pure stands of the deciduous needle- leaved bald cypress (T'axodiwm distichum). Although the map of the vegetation of the United States which has just been described (plate 1) is not so detailed as might be possible or desirable, it was found, nevertheless, as has been noted, that many of its smallest areas and the sinuosities of many of its major boundaries would be meaningless when brought into comparison with the rela- tively small number of stations from which we were able to secure climatic data. We have therefore prepared a generalized map of the vegetation of the United States, shown as plate 2, which was executed with special reference to the number of stations represented in our accumulations of climatological data. The detailed map has been published and described for the sake of giving the basis upon which this more generalized map has been drawn. The number of areas has thus been reduced from 18 to 9 by a reduction of the four desert areas to one, by a consideration of the two semidesert areas as one, by the elimination of the transition areas from desert to grassland and from deciduous forest to the forest areas to the north and south of it, and by disregarding the alpine summits and swamps and marshes. We have chosen to separate our study of climatic correlations for the eastern and western portions of the Northern Mesophytic Evergreen Forest. Although these two areas are ecologically alike, it seemed 46 THE VEGETATION OF THE UNITED STATES. desirable to give them separate study, in view of the fact that they are so widely separated, at least within the geographical limits of the United States. The generalized map (plate 2)' is, therefore, a simplification of the detailed vegetation map, in addition to being a generalization from it in the sense that the lines between the plant formations have been smoothed, although their location has in no case been changed in such manner as to throw any of our leading climatological stations into vegetations other than those in which they actually belong. The areas represented on the generalized map of the vegetation have been designated as follows: . Desert. . Semidesert. . Grassland. . Grassland-Deciduous Forest Transition. . Deciduous Forest. . Northwestern Hygrophytic Evergreen Forest. . Western Section of the Northern Mesophytic Evergreen Forest. . Eastern Section of the Northern Mesophytic Evergreen Forest. . Southeastern Mesophytic Evergreen Forest. OONIMSPWNH Ee III. DISTRIBUTIONAL AREAS OF CONFORMIC GROUPS OF PLANTS. Under this heading we desire to discuss briefly the groups of con- formic plants (plants of the same growth-form) which we have used in the correlations dealt with on the following pages. Four groups of such plants have been charted and are exhibited in plates 3, 4, and 5. It seems desirable to give here some of the detailed data upon which these maps have been based. CUMULATIVE DISTRIBUTION OF EVERGREEN BROAD-LEAVED TREES. The evergreen habit in broad-leaved trees is commonly regarded as one that has developed in moist, warm climates, and this view is confirmed by the predominance of trees of this type in the tropical rain-forests of both hemispheres. We have endeavored to define both the term “‘tree” and the term “‘broad-leaf” as definitely as possible for securing the list that we have used. We have regarded as trees only those woody plants which have a well-defined trunk and a height of 20 feet or more, and have regarded as evergreen all of those trees which retain some of their leaves throughout the year, at least holding the old ones until the time of appearance of the new leaves. The needle- leaved evergreen trees have not been included in this class. The trees of this group merge into shrubs and in such a manner that it is extremely difficult to draw a hard-and-fast line between them, and indeed some of the species which are arborescent in one portion of their = 1On Plate 2, and also on plates 6, 7, 11, 34-37, 39, 42-72, the description “Southeastern meso- phytic forest’ should read ‘‘Southeastern mesophytic evergreen forest."’ "UO1YBIOTOA OY} JO SUOISTAIPAnS 10 sod4y [eLoUDs our SULMOYS ‘soywIg pojUA oY} Jo duu uoryeyoSoA pozlpvsouer \ g 38010} uveI1FI9A9 uonrsues — oH Aydoss Ay 78910} snonpisep Wia}somyyION =puryssei5 : WY Ld yse10} I Aydosour us9]SeaTINES 1 puryssery AS WK PY YY N 7 WS Z : \Y XS x QQ \ van SAS ~ \ \ a : : > ul x —— = ay x > a bk b = AY a 4 44h fy “YA Z 4 4 (aaa ‘ 2 * a Ib ae se ee | = — ee TT eh see aes Nae OT kO .GOT SL oth 6L JAS 88 $8 48 68 Ab CO i DISTRIBUTION OF VEGETATION IN UNITED STATES. 47 range are shrubs in another. It is also well known that a number of trees which are evergreen in southern latitudes are somewhat decid- uous near the northern limits of their ranges. Evergreen broad-leaved trees are found in the United States only on the Pacific coast, in the mountains of the extreme Southwest, and in the southeastern portion of the United States, reaching their greatest abundance in peninsular Florida. Those of the Pacific region are either confined to the Pacific coast of the United States or are found only in extreme northwestern Mexico. The evergreens of the south- western mountains are largely trees which have the major portion of their range in the Sierra Madre region of Mexico. Those of the south- eastern United States are partly peculiar to that region and partly trees of wide distribution in the West Indies, this being notably true of those found in extreme southern Florida. In the western states the greatest extension given the area of this growth-form, at least to the north, is due to the extended range of Arbutus menziesii. In the east the maximum extension of this type is due to the ranges of [lex opaca and Magnolia glauca, which extend north in the Coastal Plain as far as Massachusetts. The member of the southeastern group which extends farthest west is Quercus virginiana, which is found in western Texas. The inclusion of the evergreen shrubs Rhododendron maximum and Kalmia latifolia would have extended the region of occurrence of this growth-form into the southern Alleghenies and farther into the Northeastern States. With the exception of these shrubs, the ever- green habit is rather poorly represented among the shrubs in the deciduous-forest region, although the evergreen broad-leaved habit again appears in the north as characteristic of numerous bog shrubs. In the construction of the map of cumulative occurrence of broad- leaved evergreens 129 species have been used. Of these species, 25 occur in California and the Southwestern States, 25 in the Eastern States exclusive of peninsular Florida, and 79 in the last-named region. The western and eastern groups do not overlap, except in so far as Quercus virginiana is sometimes found in western Texas in the same region with evergreen oaks characteristic of the Mexican Cordil- lera. Our eastern group of evergreens merges into the group for peninsular Florida in a manner which it is impossible to describe accu- rately on the basis of existing literature. Several of the evergreens of the Southeastern States do not range to the extreme southern end of Florida. Twelve of our 25 eastern species have been eliminated with certainty from the number credited to southern Florida. Out of the 79 species which we are listing for peninsular Florida, only 65 are found in the Everglade region exclusive of the keys, while 26 are confined to the keys and their adjacent shores. The complicated distribution of many of these trees in peninsular Florida has made it necessary for us to map that region in a somewhat conventional manner. 48 THE VEGETATION OF THE UNITED STATES. Following is given a list of the species of evergreen broad-leaved trees which have been used in the compilation shown on plate 3. In order to identify these with greater certainty, author names are given: EVERGREEN Broapv-LEAVED TREES OF THE UNITED STATES. Western group: Southeastern group—continued: Arbutus arizonica (Gray) Sarg. Arbutus menziesii Pursh. Arbutus texana Buck. Castanopsis chrysophylla (Hook) A. DC. Ceanothus spinosus Nutt. Ceanothus thyrsiflorus Esch. Ehretia elliptica DC. Fremontodendron californicum (Torr.) Cov. Garrya elliptica Dougl. Myrica californica Cham. Prunus ilicifolia (Nutt.) Walp. Quercus agrifolia Nee. Quercus arizonica Sarg. Quercus chrysolepis Liebm. Quercus densiflora Hook. and Arn. Quercus emoryi Torr. Quercus engelmanni Greene. Quercus hypoleuca Engelm. Quercus oblongifolia Torr. Quercus reticulata Humb. and Bonpl. Quercus wislizeni A. DC. Rhus integrifolia (Nutt.) Benth. and Hook. Sophora secundiflora (Cav.) DC. Umbellularia californica (Hook. and Arn.) Nutt. Vauquelinia californica (Torr.) Sarg. Southeastern group: Bumelia angustifolia Nutt. Bumelia cassinifolia Small. Bumelia lanuginosa (Michx.) Pers. Bumelia lucida Small. Bumelia tenax (L.) Willd. Bumelia texana Buckl. Cliftonia monophylla (Lam.) Sarg. Cyrilla racemiflora L. Gordonia lasianthus (L.) Ellis. Ilex cassine Walt. Ilex myrtifolia Walt. Ilex opaca Ait. Magnolia glauca L. Magnolia grandiflora L. Osmanthus americanus (L.) B. and H. Persea borbonia (L.) Spreng. Persea pubescens (Pursh.) Sarg. Prunus caroliniana (Mill.) Ait. Quercus laurifolia Michx. Quercus nigra L. Quercus virginianas Ell. Sabal palmetto (Walt.) R. and S. Symplocos tinctoria (L.) L’Her. Vaccinium arboreum Marsh. Xanthoxylum fagara (L.) Sarg. Peninsular Florida group: Alvaradoa amorphoides Liebm. Amyris balsamifera L. Amyris maritima Jacq. Anamomis dicrana (Berg.) Britton. Anona glabra L. Avicennia nitida Jacq. Bourreria havanensis (R.and§.) Miers. Bourreria virgata (Sw.) D. Don. Bucida buceras L. Bumelia angustifolia Nutt. Calyptranthes pallens (Poiret) Griseb. Canella winteriana (L.) Gaert. Capparis jamaicensis Jacq. Chrysobalanus icaco L. Chrysophyllum oliviforme L. Citharexylon cinereum L. Coccolobis laurifolia (Jaeq.) Sarg. Coccolobis uvifera (L.) Sarg. Colubrina reclinata (L’Her.) Brongn. Conocarpus erecta L, Cordia sebestina L. Crescentia cujete L. Crescentia cucurbitina L. Cupania glabra Sw. Dipholis salicifolia (L.) A. DC. Drypetes diversifolia Urb. Drypetes lateriflora (Sw.) Urb. Eugenia axillaris (Sw.) Willd. Eugenia buxifolia (Sw.) Willd. Eugenia confusa DC. Eugenia longipes Berg. Eugenia rhombea (Berg.) Krug and Urban. Exostema caribeum (Jaeq.) Griseb. Exotheca paniculata (Juss.) Radlk. Ficus aurea Nutt. Ficus brevifolia Nutt. Genipa clusiifolia (Jaeq.) Griseb. Guaiacum sanctum L. yuettarda elliptica Sw. Guettarda scabra Vent. Gyminda latifolia (Sw.) Urban. Gymnanthes lucida Sw. Hippomane mancinella L. Hypelate trifoliata Sw. Icacorea paniculata (Nutt.) Sudw. Ichthyomethia piscipula (L.) A. § Hitch. DISTRIBUTION OF VEGETATION IN UNITED STATES. 4g EVERGREEN BrROAD-LEAVED TREES OF THE UNITED STATES—continued. Peninsular Florida group—continued: Peninsular Florida group—continued: Tlex krugiana Loesn. Sapindus saponaria L. Jacquinia keyensis Mez. Schaefferia frutescens Jacq. Krugiodendron ferreum (Vahl.) Urb. Schoepfia chrysophylloides (A. Rich.) Laguncularia racemosa (L.) Gaertn. f. Planch. Lysiloma bahamensis Benth. Sideroxylon fcetidissimum Jacq. Mimusops parvifolia (Nutt.) Radlk. Simaruba medicinalis Endl. Ocotea catesbyana (Michx.) Sarg. Swietenia mahogoni Jacq. Oreodoxa regia H. B. K. Terebinthus simaruba (L.) W. F.-. Picramnia pentandra Sw. Wight. Pithecolobium guadalupense Chapm. Thrinax microcarpa Sarg. Prunus spherocarpa Sw. Thrinax parviflora Sw. Pseudopheenix sargentii Wend. Torrubia longifolia (Heimerl.) Britton. Psychotria undata Jacq. Trema floridana Britton. Rapanea guianensis Aubl. Xanthoxylum coriaceum A. Rich. Reynosia septentrionalis Urb. . Xanthoxylum flavum Vahl. Rhacoma crossopetalum L. Ximenia americana 1. Rhizophora mangle L. Zygia unguis-cacti (L.) Sudw. Rhus metopium L. CUMULATIVE DISTRIBUTION OF MICROPHYLLOUS TREES. (PLATE 3.) - This group comprises plants which are trees in form and reach a height of 15 feet or more, being characterized by leaves which are either simple and very small or have pinnate or bipinnate leaves with small leaflets. Several species have been comprised which have green stems and leaves which are of very short duration or wholly absent. The members of this group merge into the much larger class of shrubs in the southwestern United States which possess a similar character. Eight of the species which have been used are extremely common as shrubs, but frequently become trees within the limits of our definition. Microphyllous trees are most strongly represented in the United States in southern Texas and southern Arizona. The maximum north- ward extension of individuals of this group reaches northern Texas and the southern portion of Nevada, due to the range of Prosopis glandulosa. The cumulative distribution of this group, as well as of the group just considered, is shown in plate 3. The twenty-three species used in constructing this map are as follows: MicrRoPHYLLOUS TREES OF THE UNITED STATES. Acacia farnesiana Willd. Leucena pulverulenta (Schl.) Benth. Acacia greggiiGray. Olneya tesota Gray. Acacia wrightii Benth. Parkinsonia aculeata L. Brayodendron texanum (Scheele) Small. Parkinsonia microphylla Torr. Canotia holacantha Torr. Parosela spinosa (Gray) Heller. Cercidium floridum Benth. Pithecolobium brevifolium Benth. Cercidium torreyanum (Wats.) Sarg. Pithecolobium flexicaule Coulter. Condalia obovata Hook. Porliera angustifolia (Engelm.) Gray. Holacantha emoryi Gray. Prosopis glandolosa Torr. Keeberlinia spinosa Zuce. Prosopis pubescens Benth. Leucena glauca (L.) Benth. Prosopis velutina Wooton. Leucena greggii Wats. Dy, ‘Saut} 804) aptsino punoy aq 07 saI90ds yo saquinu WWIXELY DJBIIPUI SAUl| J9YIO ay) UO SeANny ‘sy awWanxe g a I {\ f\ Aes Ft OES ia zd 5 l) eh \ N.-4 | (} : I ; is ‘ we oo s 1a > 7 i‘\ \ } { > + oy | raw | | Fists iy LP oq Q \ a el = 5 «6 Sé S686 bul ae A A NS a < 9 ~~ ,69 os i J . i Pee Ai earlS we ARPS eee ¢ 801 901 401 601 AIT EIT SIT GLI 611 ZI £E1 Sl ‘90091990 UINUIIXEUI JO UOIZaI puv souvpuNqY BUISVIIOU! JO SOUOZ MOYS SOU] 1040 +S} {UNT] OUIII}XS MOYS Q POYIVUI SOUT] *saa1} SNONPloop UsoYsvayINOs CT Jo WONAIYSIP oATYBNUIND ba ae i a ZI € Sh | [) ie i ZI Je Zale oe UE ron} No A eee yp BretOL chOKuig 52 THE VEGETATION OF THE UNITED STATES. DECIDUOUS TREES OF THE SOUTHEASTERN UNITED STATES. (PLATE 4.) On the basis of the literature descriptive of the vegetation of the southeastern United States, a group of fifteen deciduous broad-leaved trees has been selected as representative of this vegetation form in that section of the country. The cumulative distribution of the fifteen trees which have been selected is shown in plate 4. These trees are of interest because they are extremely common in the Atlantic Coastal Plain and are nearly all either infrequent in the Piedmont and Allegheny regions or are absent there. These trees are, in short, representatives of the deciduous habit which have their maximum cumulative occurrence as well as their maximum abundance outside the deciduous forest area and in the heart of the Southeastern Mesophytic Evergreen Forest. Five of the species used are palustrine and nearly all of them occupy other habitats than those in which the evergreen needle-leaved trees are dominant. The following is a list of the species which have been used in the construction of the map shown in plate 4: Acer drummondii Hook. and Arn. Quercus brevifolia (Lam.) Sarg. Fraxinus caroliniana Mill. Quercus catesbzei Michx. Hicoria aquatica (Michx. f.) Britton. Quercus digitata (Marsh.) Sudw. Liquidambar styraciflua L. Quercus michauxii Nutt. Nyssa aquatica L. Quercus phellos L. Nyssa ogeche Marsh. Quercus texana Buckl. Planera aquatica (Walt.) Gmel. Ulmus alata Michx. Populus heterophylla L. CUMULATIVE DISTRIBUTION OF THE COMMONEST EASTERN DECIDUOUS TREES, (PLATE 5.) A selection has been made of the thirteen deciduous trees which are commonest in the Deciduous Forest area and are most widely dis- tributed throughout it. These are all large forest trees which are wholly deciduous throughout their ranges and are commonly found in upland habitats. ‘The maximum occurrence of this group is in the region extending from central New York to northern Alabama, com- prising the entire extent of the Allegheny Mountains. From this region, in which 13 of the species are found, the abundance of this group shades off to the east, south, and west, so that the area in which from 12 to 8 species are found covers the Coastal Plain of Virginia and Caro- lina, extends south to western Florida, and west as far as the eastern boundaries of Texas, Kansas, and Minnesota. The following is a list of the thirteen species that have been used in the preparation of this map, shown in plate 5: Acer saccharum Marsh. Juglans nigra L. Carpinus caroliniana Walt. Liriodendron tulipifera L. Castanea dentata (Marsh.) Borkh. Quercus alba L. Fagus atropunicea Ehrh. Quercus prinus L. Fraxinus americana L. Quercus velutina Lam. Hicoria glabra (Mill.) Britton. Ulmus americana L, Hicoria minima (Marsh.) Britton. DISTRIBUTION OF VEGETATION IN UNITED STATES. 53 THE COMMONEST EVERGREEN NEEDLE-LEAVED TREES OF THE SOUTHEASTERN UNITED STATES. (PLATE 6.) The four evergreen needle-leaved trees which are most widespread and most dominant in the Southeastern Mesophytic Forest are Pinus echinata, P. teda, P. palustris, and P. caribea. The ranges of these 4 pines have been superposed on a single map (plate 6). Pinus echinata possesses the most northerly range of this group, and it and Pinus teda exceed the distribution of the southeastern evergreen formation itself. The three most widely distributed species of this group reach their western limit at about the ninety-sixth meridian. The distribu- tion of P. palustris is closely coincident with that of the southeastern evergreen formation, while that of P. caribea lies entirely within that formation. These four species are all found in southern Georgia and northern Florida and the extreme southern portions of Alabama and Mississippi. The region of maximum occurrence of this group lies, therefore, in the heart of the southeastern evergreen area. THE COMMONEST EVERGREEN NEEDLE-LEAVED TREES OF THE NORTHEASTERN UNITED STATES. (PLATE 7.) The ranges of the 4 evergreen needle-leaved trees which are most generally dominant in the eastern section of the Northern Mesophytic Evergreen Forest are plotted together and are shown in plate 7. These trees are Pinus strobus, Tsuga canadensis, Abies balsamea, and Pinus divaricata. The region of cumulative occurrence of these trees corresponds closely with the distribution of the evergreen forest formation. ‘The southernmost extension of this group is found in the case of Tsuga and the northernmost in the case of Pinus divaricata. All four of these trees are found together in northern New England, northern New York, and in Michigan, Wisconsin, and Minnesota. The range of climatic conditions has been determined separately for each of these trees, owing to the fact that their regions of cumulative occurrence correspond so closely with the eastern section of the Northern Mesophytic Evergreen Forest. The same has been done with respect to the dominant trees of the Southeastern Evergreen Forest. THE ECOLOGICAL DISTRIBUTION OF PINUS TADA. (PLATE 8.) It is rarely that data are available on the relative abundance of a plant within its area of geographical distribution. Owing to the excel- lent work of Mohr,! we are able to use both the geographical and ecological distribution of the loblolly pine (Pinus tada). The map prepared by Mohr has been reproduced in plate 8 and shows three areas of varying abundance in addition to the region of scattered 1Mohr, Charles, Timber Pines of the southern United States, U. S. Dept. of Agric., Bur. For. Bull. 3, 1896. 54 dis ie fh OML te : a we J ne a) f% } =a Js pen et Za KS h 7 (4 a ae Ly =i a yi gi a (i Wags! 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SNL NYS SSS WSS SESS oR SANSA YY N RSS 2 se f SANT, NY SS i, I HTD oe ; I 4 nc Se itt Mtl in ie So a ! 4 ill SP settee ates . eteaneare tat See i it A A sae uth ts ny Seidl b eA Ly NP ‘S Wan sat Sl 7 2 co SSN N SAS .s S SANS AY SSAN RES NAA LS NSN ANN SAN h NS 4 2] p\ re Bb na au | : woe a5 ae P| “IYOJA! 109JB ‘(OOWIIINIVO Jo SaTYISUAP OAIFBIOI YPM “Opan} sNUI_ JO WOIYNGLSIP [BOLSO]OO re re ( r vy, Sa wae aig NYS ING SY b Latss lA a, a i (are, eles gi Bes. —{D h\ = = { UY = , ) tA 7 R 5 i= oh \ i rege are ae \ \\ PWV (\ TS 4a A S | Y & eas ‘ —- ooh ( ys el , NX Fas 3 ; as ile ARR " za’ Na ee ea aan es ea bea SiPrry glenn GOUT L Abae Oh ehh OL» AG» BB. 8B, of8 M0 16. 80 I 801 Sey t AN \ A\ | ASS > na Ver | } ‘ by “ Poo - “+ { A / | A - SRN NE xe macening 4, ~ W Figo ) — ; ae a a ~ me rea fal * - i . =< ' ~. / ees J f £ 4 cis = ( p {>L? 1 lat Gar J ‘DAYJOADS DIFLIYNA) PUB ‘s1suarinoss1u obpYog ‘wnyDrULID] wnrydjrs JO TONGLASIC (59 ay Ud9/I9q) FEMI VOIwS BIZ9LI91IND (89 94) U9MING) SISUALINOSSTUL OBEPI|OS ——— eel (doo any uri) winery wnryds ia ee Ph si a IN z = em a vi =—— ges Sec Smee Ewes fo a Ee saa Bays eat ay Ly HK ihe é | Wa \ ~ E > Z Bone Z eo ce Ae ime Ze eee Lil b SN I( coe ING): | acki eS ae NM] [=F a: I a a & ee - ae) Al I] | iP YS i St SK aN VAY. aa te ES =a ree j hs LA) — am Ry ame BoSOVET Unless ate et aunt Ak io ae AG Mr2 baascehueerce 8 £8 68 aa I LOI 3 he f wh, DISTRIBUTION OF VEGETATION IN UNITED STATES. 79 The members of this group are all shrubs of swamp and palustrine habitat and have been selected for use in our work with a view to bringing out the features of climatic control for plants which exist under a practically uniform set of soil-moisture conditions. Adelia is confined to the central portion of the Atlantic Coastal Plain. Jtea occupies a similar area, but extends throughout the Coastal Plain as far north as New Jersey. Decodon is found in palustrine situations throughout the eastern United States, and Cephalanthus occupies a similar area with a remarkable westward prolongation of its range into southern Arizona and the central valley of California. 11. MicropHyLLous DrEsrerRT SHRUBS. (PLATE 21.) Artemisia tridentata Nutt. Covillea tridentata (DC.) Vail (including C. glutinosa (Engelm.) Rydb.). These shrubs have been used in view of the fact that they are the most abundant plants in two of the most important of the desert areas. Artemisia is found throughout the Great Basin Microphyll Desert and extends beyond its limits to the east and northeast into the higher portions of the Grassland area. Covillea is the leading plant throughout the California Microphyll Desert and the Arizona and Texas Succulent Deserts, extending eastward into the Texas Semi- desert. 12. Cactt. (PLATE 23.) Opuntia polyacantha Haw. (including closely related varieties). Carnegiea gigantea (Engelm.) Britton and Rose (=Cereus giganteus Engelm.) The species of cacti which have been used were taken as examples of this great group of stem-succulent plants, the former representing one of the most widely distributed of the species found in the United States, and the latter one of the most restricted of the species which is in any place an important element of the vegetation. Opuntia poly- acantha is found throughout the Grassland region, extending from southern New Mexico to the Canadian boundary and occurring in small areas in Washington. Carnegiea is limited to southwestern Arizona at elevations below 4,000 feet. 13. CHARACTERISTIC COMPOSITES OF THE GREAT PLAINS. (PLATE 24.) Silphium laciniatum L. Solidago missouriensis Nutt. Gutierrezia sarothre (Pursh) B. and R. These suffrutescent perennials are found throughout the Grassland and Grassland-Deciduous areas and, in their range and relations to climate, may be taken as typical of a large number of similar plants. Silphium occupies the most easterly range of this group of plants, 80 THE VEGETATION OF THE UNITED STATES. extending as far as Pennsylvania and Florida, with the suspicion of its having been introduced along its easternmost limit. Solidago is closely confined to the Grassland and Grassland-Deciduous Transition area, with a northwestward extension in regions of this type through southern Idaho and eastern Washington. Gutierrezia is found in the western portion of the Grassland and in the higher portions of all areas in the extreme southwestern portion of the country. In addition to its natural habitats, this plant is an extremely common one in all areas in which the original cover of grasses has been disturbed by grazing or other unnatural conditions. 14. CHARACTERISTIC GRASSES OF THE GREAT PLAINS AND THE PRAIRIES. (PLatTEs 11 AND 25.) Bouteloua oligostachya (Nutt.) Torr. Bulbilis dactyloides (Nutt.) Raf. (=Buchlée dactyloides Nutt.). Keeleria cristata (L.) Pers. Agropyron spicatum (Pursh) Rydb. (=A. divergens Nees,=Festuca spicata Pursh. Not A. spicatum Scribn. and Smith). Hilaria jamesii (Torr.) Benth. Andropogon virginicus L. Bouteloua hirsuta Lag. Four of the members of this group of grasses characteristic of the Grassland and Grassland-Deciduous Forest Transition regions have already been used in constructing the map of cumulative occurrence of grasses (plate 12). The ranges of three other species are given in plate 25. Agropyron and its allies have been selected because of their importance in the formation of the Grassland of the northwestern States, Hilaria because of its importance in the southernmost exten- sion of the Grassland and the Desert-Grassland Transition, and Andro- pogon because of its importance in the easternmost portion of the prairies of the Grassland-Deciduous Forest Transition. 15. PALUSTRINE HERBACEOUS PLANTS. (PLATES 26 AND 27.) Sparganium americanum Nutt. (including 8. americanum var. androcladum (Engelm.) Fern. and Eames). Dianthera americana L. Sium cicutzfolium Gmel. These palustrine herbaceous plants are of interest for the same reason that has been mentioned in connection with palustrine shrubs. Spar- ganium is extensively distributed in the Eastern States, but its pre- cise range is not well known. Dianthera is found in the eastern half of the United States south of Wisconsin and outside the eastern half of the Coastal Plain. Sium is found practically throughout the United States, with the exception of the continental desert areas, its oceur- rence throughout a large part of this region being extremely infrequent. “WnaODUIpUNLD WNIYOYNG pur ‘wnuv9LWaUD WNUDb.LDdg ‘D}99) DLLDUAPUNLY Jo MOYYNGIASICT —_—_—- (189% NAA iS LO O S Y My Rag \ ye = = Opa &F 6 ee | aaa 3 Ae : ae } ha ) LL Gal ae 8 8 68 een 3 Elba NY 6 Ol "aes iu etl I DISTRIBUTION OF VEGETATION IN UNITED STATES. 91 22. PatRED SPECIES oF EASTERN AND WESTERN RANGES, RESPECTIVELY. - (PLATE 33.) Fleerkea occidentalis Rydb. Fleerkea proserpinacoides Willd. Trautvetteria grandis Nutt. Trautvetteria carolinensis (Walt.) Vail (=T. palmata Fisch. and Mey.). Cebatha diversifolia (DC.) Kze. (=Cocculus diversifolius DC.). Cebatha carolina (L.) Britton (=Cocculus carolinus (L.) DC.). There are a few cases in the flora of the United States in which a genus has two species, one of which is eastern and the other western in range. We have selected the paired species of each of the three genera, Flerkea, Trautvetteria, and Cebatha, with a view to investigating the climatic conditions characteristic of the present separated ranges of these pairs of closely related plants. There is some doubt in the case of the two species of Flerkia as to their specific distinctness. The two species of Cebatha apparently overlap in central Texas, whereas the other two species are widely separated. PLATE 33 92 Duos DY}Dg2) PUB ‘DYOfssoAyp DYIDA, ‘sIsUaUN04I DL.aeaNND.,], ‘sypuDsO piuayoannsy, ‘saprovnurdsasosd vays9014y ‘81pD}Uap1090 194490] JO TOLNGUysKCT ee ee ee me (era PIEO[D — ee eee ee me (ie posopoua Ape rns (10) DU} LITT Babs RS 2 \; al pes Ind VX as Lae a ae A YY eo = 4 Af Ae ae | suis | Bien eee oe ae ae ae te ree | One Kier OF ORS Ee Oka ial eGOt SOT LOT 601 IIT et ST Lit 611 t bal AS £3 8 L8 68 J é*| PART II. ENVIRONMENTAL CONDITIONS, 93 fe hes ae INTRODUCTION. The physiological point of view has been constantly held before us, as has been said, in planning and carrying out the complicated com- parisons and correlations with which the present publication deals. Part I shows how the numerous vegetation features employed by us were derived, emphasis being placed on the physiological character- istics of the plants considered. Part II deals with the principles and methods by which the climatic features that we have usedwere selected, and shows how the requisite numerical computations were made and how the maps were prepared. This selection had to be based, as has already been indicated, upon two different kinds of circumstances: the physiological importance of the climatic features (as these are known to influence plant activity in general) and the availability of climatic data suited to our purposes. These matters will be set forth under three general headings: (I) general influence of the environment on plant life; (II) chief environmental conditions and the general nature of their effects upon plants; and (III) climatic conditions of the United States. The first two of these sections are mainly physio- logical-ecological in nature and are general in their scope; the last is mainly climatological-ecological and deals with the actual climatic data employed in our researches. . . , .. cf > ‘an: GENERAL INFLUENCE OF THE ENVIRONMENT ON PLANT LIFE, I. EXTERNAL AND INTERNAL CONDITIONS AND PLANT ACTIVITY. The behavior of any plant is said to be controlled by the surround- ing conditions, variations in these being the stimuli, or causes, which produce in the internal, physiological complex of the organism various responses or effects. Such responses are, however, quite as dependent upon the nature of the responding organism as upon the nature of the stimuli. With the same set of environmental conditions different plants behave differently, merely because their internal conditions differ, and with unlike environmental complexes plants of the same form exhibit quite different behaviors. The behavior of plants in general thus depends upon two interacting sets of conditions, the one set being external and the other internal. The latter set of conditions makes up, of course, the nature of the plant and serves to define it physiologically. These internal factors determine the ability of the organism to respond to exposure to any given constant external condition, or to any given change in any condition, and they also determine the extent of such responses. A plant might be rigorously defined by means of these powers or capabilities to respond to stimuli, and it is some of these powers that are, indeed, unconsciously used by taxonomists in their descriptions; but, although certain groups of the bacteria are now described by conscious reference to their physiolog- ical properties, the physiological nature of the taxonomic description in botany may be said hardly to be generally recognized as yet. As a plant develops, its internal conditions pass through a series of more or less profound alterations, and the different developmental phases of the same plant often show greater divergency in response to the same environmental complex than do the corresponding phases of distinct plant-forms. Thus, environmental conditions that are favor- able to seed germination or to vegetative growth may be markedly unfavorable to the production of flowers or fruit. It follows that for the best growth and reproduction of many forms the external conditions must vary from phase to phase as growth proceeds. This is one of a number of considerations that make for great difficulty in the incep- tion of any satisfactory quantitative study of the relation of external conditions to the characters of individual plants and of vegetation in general. A second consideration that enormously complicates our problem is this, that the response, or effect of the external system upon the organism, is definitely dependent upon the duration of the component 97 98 ENVIRONMENTAL CONDITIONS. conditions that make up the environment. Considerable time is required for most responses, and a momentary alteration in the envi- ronment may often pass without apparent effect upon the plant. Thus, outside of the time factor, the necessary and sufficient conditions for the production of those changes in manner of growth that are termed etiolation are present every night, but these conditions are not effective over a long enough period of time to result in visible responses. In the study of any external factor or complex of factors it is logically necessary and inevitable that the time element enter seriously into consideration. Physiologists have found it advantageous to analyze the environ- ment into its component conditions or factors. While some factors have so far received but a minimum of attention, a large amount of reliable information is already at hand bearing upon the effects pro- duced by the action, over various time intervals, of different intensi- ties of heat, light, oxygen-supply, etc. The method of such deter- minations has been to hold all factors but one as nearly constant as possible, and to cause various selected intensities of this one factor to register their effects upon the plant, in the form of alterations in growth or other activity. But this study of the simple component factors of the environment is only the learning of the alphabet, and the task of really reading the book of plant phenomena in the light of cause and effect still rests with the future. We are already well aware, in a general way, that the responses brought about in the organism by a certain quality, intensity, and duration of any external factor are totally dependent upon the nature of the other concomitant factors which are comprised in the environmental system. For example, a given increase in the rate of water-supply may fail to produce any marked acceleration of growth in certain forms existing under excessive drought conditions, but if the increase in soil-moisture be accompanied by a decrease in the evaporating power of the air, growth response may be immediate and definite enough.! Again, the agriculturist is well aware that with many soils an increase of the nitrate content is without full effect unless other salts are simultaneously added. In such cases the result of these several increases together is not generally a simple summation of the results obtained by the single additions separately. While a large amount of laboratory experimentation of the most refined physical sort will be required before we shall even approach an adequate knowledge of the influence of single conditions upon plants, the far more difficult study of the complex environmental system of which these single conditions are always components has already begun to attract attention. It seems safe to predict that the line of work thus started will rapidly gain in prominence, and it is 1 Livingston, B. E., Evaporation and Plant Development, Plant World, 10: 269-276, 1907. INFLUENCE OF ENVIRONMENT ON PLANT LIFE. 99 conceivable that plant physiology may eventually work out the principles whereby the behavior of plants may be one day explained, predicted, and controlled. This is, of course, the hope of ecology and of the developing science of plant-culture. It is our aim in the present chapter to present merely a tentative outline of the general and more superficial relations that apparently obtain between the plant and its environmental conditions. No attempt is here made to make our consideration logically complete, either from the standpoint of physical causation or from that of the plant kingdom in general; attention is mainly turned upon some of the most obvious physiological considera- tions and upon the behavior of ordinary vascular land-plants. Il. THEORY OF PHYSIOLOGICAL LIMITS. In an etiological study of plant distribution, either natural or artificial, the conception of physiological limits must hold a very prom- inent place. We understand by this term the extremes of intensity, etc., in a given factor which a certain plant can withstand. Starting from the optimum intensity of heat for any plant, for example, we may reduce the temperature till death ensues, thus attaining the minimum temperature limit for life. If the temperature be increased sufficiently above the optimum, another death-point is reached, the temperature maximum. The plant is thus able to retain life only under temperature conditions that fall within these physiological limits. With some other factors a similar pair of limits can be deter- mined; with many, however, only a single limit exists. For example, a submerged aquatic possesses a definite minimum rate of water- supply, but it is impossible to produce death or injury by increasing this rate even to its physical limit, as by surrounding the entire plant with water. It is obvious that in such a case there exists no maximum limit for life. Cases where there is a well-marked maximum but no minimum are also frequent, most toxic substances furnishing examples of this. Plants live normally in utter absence of these substances; they also live normally in their presence, so long as the amount sup- plied does not surpass a fixed maximum limit. Life is able to proceed, then, in any particular plant, only so long as the external conditions do not surpass the physiological limits for the life processes of the form considered. In different plants and in different developmental phases or stages of the same plant these limits may be very different, so that an environmental complex that inhibits life in one phase or form may allow healthy activity in another. It is mainly in accord with this generalization that distinct climatic areas are characterized by corresponding types of vegetation, and the principle is therefore probably of primary importance in the study of plant distribution. 100 ENVIRONMENTAL CONDITIONS. The generalization just mentioned is very greatly complicated, as has been indicated, by the variations and fluctuations in the internal physiological conditions as development proceeds. The limits for life are often very different in the various developmental phases of the same form. Thus, mature seeds of many temperate and boreal annuals show temperature minima far below the freezing-point of water, while the vegetative phases of the same forms may succumb to the first frost. Winter buds of northern deciduous trees possess high powers of resistance to low temperatures, while summer buds of the same plants may not bear temperatures as low as the centigrade zero. Besides the variations in limits of growth and life in different phases of the same form, it must be remembered that, in any phase or at any time in the life of the organism, there are a number of different processes going on, such as photosynthesis, respiration, digestion, secretion, and the like, each of which has its physiological limits, and the limits for one process are frequently not at all the same as for another. In general, the pair of limits that characterize these simpler processes, which together make up the vegetative or reproductive activ- ities of a plant, are much less widely divergent than those for the mere retention of vitality itself. By retention of vitality we probably mean the occurrence of the life processes at their lowest intensity, an inten- sity that is just sufficient to maintain life, though this expression may be taken in a general way to denote simply the power of initiating the various more vigorous and obvious processes when conditions become right. Seeds retain their power to start the germination processes for long periods of time under conditions that preclude germination itself. Again, with increasing scarcity of water or lowering of temper- ature, the growth processes of all plants are sooner or later brought to a standstill, long before death ensues. Moisture conditions that are optimal for vegetative growth frequently prevent the production of fruit, so that gardeners make it a practice, with the coming of the flowering-time in many plants, to diminish the water-supply. That the factor of time enters into the determination of physio- logical limits is obvious. Many plants are able to survive a short period with the soil about their roots in a saturated condition, but succumb to a longer period of exposure to a saturated soil. Numerous forms retain their vitality through long periods of drought, when the soil is nearly air-dry, but if the dry period is sufficiently prolonged death is the inevitable result. As has been mentioned, general growth is not noticeably affected by the regularly recurring nocturnal period without illumination, but etiolation becomes marked, and various other pathological conditions are induced when ordinary plants are kept in continuous darkness for but a few days. In phys- ical terms this means merely that the effects of any set of external conditions upon the plant are always cumulative and are exaggerated, in one way or another, with the lapse of time. INFLUENCE OF ENVIRONMENT ON PLANT LIFE. 101 A combination of the time factor and that of intensity is, frequently if not always, the effective condition which determines the success or failure of plants in nature. With a somewhat rapid alternation of favorable and unfavorable conditions, where the unfavorable factors for any short period do not in themselves at first produce death, the organism may generally lose its power of resistance and finally go to the ground. In regions where such rapid fluctuations in the intensity of external conditions occur, the natural vegetation must necessarily comprise only those forms which can bear this sort of fluctuation. Alpine plants are reputed to be especially resistant to the great daily ranges of temperature that occur in their habitats, and many plants, such as lichens and certain liverworts and club-mosses, exhibit a high power of resistance to repeated wetting and drying-out. The limits for life, growth, or reproductive activity (the latter a sort of growth) define the resisting power of an organism in respect to the particular condition considered, and, by maintaining the quality and intensity of the other environmental factors and causing the one in question to vary, the range between the limits for that one may be more or less approximately determined. But, with two or more factors varying at the same time, the problem of physiological limits becomes much more difficult. The study of the behavior of plants when several factors are simultaneously in a state of change has, as we have pointed out, only just begun. We may be sure, however, that the resisting power of a plant to any single condition will prove to be markedly influenced by other concomitant conditions. The antagonistic action of certain salts, such as those of calcium and magnesium, in the work of Loew, Osterhout, and others, is a case in point, as is also the well-known fact that, by a degree of desiccation that does not surpass the death-limit, the power of many organisms to withstand both high and low temperatures is markedly increased. The common experiment comparing the effect of high and low temper- atures upon dry and moist seeds is a clear illustration of the latter case; within limits, the less water a seed contains, the more freezing or heating it can bear without losing its vitality . III. RELATION OF PLANT DISTRIBUTION TO THE PHYSIOLOGICAL LIMITS OF THE VARIOUS DEVELOPMENTAL PHASES. Each plant-form, each developmental phase, and each physiological process exhibits a minimum or a maximum, or both, for each of the environmental factors, these limits depending, of course, on the other conditions that prevail within and without the plant. Whenever an environmental factor falls below the minimum for life, in either quality, intensity, or duration, the annihilation of the organism of course ensues. A like result follows any increase above the life maximum. It is only with each of the environmental conditions falling between 102 ENVIRONMENTAL CONDITIONS. its respective life-limits, under the given set of relations between the other conditions, that a plant can exist at all. But the mere existence of a given plant-form, its mere retention of vitality, is not sufficient to give it a permanent place in the vegetation of a given region; each plant must pass through various developmental stages, must come to maturity, and must reproduce. Since the intensity lim- its for the retention of life do not approach each other so closely as do those for growth and reproduction, it is easy to understand that the duration of the different intensities or qualities of certain factors must determine whether or not a given form may come periodically to maturity in any region. Though the lower temperature limit for life in seeds, bulbs, rhizomes, and the like, and in resistant perennials, is not attained in the temperate and boreal winter, yet the temperature conditions for growth and the production of fruit obtain only in the summer season. Similarly, the moisture conditions in a desert fall, for the greater portion of the year, below the minimum for the growth of many desert shrubs, these producing new leaves and flowers only in and immediately following the rainy seasons. The same is true of root and bulb perennials and of those annuals which succeed in the desert. It is thus seen that, in regions characterized by an alterna- tion of seasons of plant activity and of dormancy, the lengths of the seasons of activity must determine whether or not the plant repro- duces adequately; and since adequate reproduction is essential to the maintenance of the form in the given region, this length of season must determine whether that form succeeds or fails. While a mature plant, or a portion thereof, may exist in a relatively inactive condition for a long period of time, in an environment whose factors lie without the limits for most forms of activity (but within those for the retention of life), the resisting power thus evidenced is usually of but a low order when compared with that exhibited by ripe seeds or spores. From a physiological point of view such bodies represent merely a certain phase in the development of the plant, a phase in which the life processes are even more in abeyance than during the dormant periods of the mature form. An annual may play a very important réle in the vegetation of a region, although during the greater It is of course to be borne in mind, in this connection, that an alteration in one environmental condition may result in death simply by causing one or more of the vital processes to be so greatly accelerated or retarded that the other given external conditions, although they have not been changed, become fatal. Thus, while a given rate of water-supply may be sufficient for life and growth under a low evaporating power of the air, an increase in the evaporation-rate, unaccompanied by a corresponding increase in the rate of supply, may result in death. Had the rate of water-supply been adequately increased as the transpiration-rate rose, such a plant might have survived. It is frequently said that in such a case death is due to a condition which was not altered. This simply means that internal conditions have been changed, so that an environ- mental factor heretofore favorable to life becomes unfavorable, without itself changing. It is in this connection that the ‘law of the minimum”’ of agriculturists has been developed. (See: for example, E. J. Russell, Soil conditions and plant growth, Third ed., London, 1917, chap. II. Also see: F. F. Blackman, Optima and limiting factors, Ann. Bot. 19: 281-295, 1905.) INFLUENCE OF ENVIRONMENT ON PLANT LIFE. 103 portion of the year the environmental conditions surpass the limits for both growth and life in the plant itself; the conditions in their adverse period do not surpass the limits for the retention of life in the seed. During a comparatively short period the environment may allow germination, growth, and the production of more seeds, and this short growing-season, together with the bridging of the adverse period by dormant seeds, constitute the conditions that are necessary and essential in order that such a plant may maintain a permanent place in the vegetation of its region. External conditions frequently surpass even the life-limits of many seeds; in such cases it must, of course, follow that the plant can not become a permanent part of the natural vegetation. This is probably true of the majority of cultivated plants that do not volunteer in the second season. A tropical plant, such as the castor-bean, may make a luxuriant growth in the temperate summer, but its seeds must be pro- tected through the winter and sown each spring. In general, the natural plants of the temperate and frigid regions must necessarily experience a longer or shorter period, usually each year, with proper conditions for growth and reproduction, and they exist through the adverse periods in some dormant phase. Not only must the conditions of the active period lie within the limits for growth and reproduction, but the period itself must be of adequate length, otherwise the necessary amount of growth could not take place, and fruit would not be matured. The principle that a plant, to become a part of the permanent vegetation of a locality, must have an adequate growlng-season and must not meet its death during the remainder of the year, must be regarded as fundamental to the study of all problems dealing with the study of plant behavior under natural conditions. This principle is commonly accepted, though perhaps seldom formu- lated. With regard to any geographical area or region, we may conceive that all plants may succeed therein, for which the physiological limits for life and those for growth and reproduction do not approach each other more closely than do the extremes of the physical conditions in the respective seasons for the given region. This view has led to a form of analogy which may be termed the sieve conception of environ- ments. According to this, we may regard the physical conditions of the surroundings as resembling a sieve or screen, with meshes of a certain magnitude, through which, as we may imagine, will pass only those successful forms which withstand the most adverse conditions of the environment. The analogy is but roughly applicable; to make it more so we may suppose that the size of the meshes in our screen is continually changing throughout the year, while the size of the imaginary particles. which are to be screened are also undergoing continuous change with the advance of the organism from phase to phase of its development. With the progress of the season the con- 104 ENVIRONMENTAL CONDITIONS. ditions change and the powers of resistance also vary. Thus modified, the sieve analogy becomes unwieldy and does not aid us very much in our thinking. Since the behavior of a plant is nothing but the summation of the behaviors of its active parts, and since all higher plants exhibit, at any given phase in their growth, various gradations in the activities of their different tissues, it follows that any adequate consideration of the physiological limits of plant activity as a whole must be ex- ceedingly difficult. It is therefore impossible, in the present state of our knowledge, to treat the question of complex limits quantitatively. The best that may be done in the discussions which follow is to attempt to bring together a series of confessedly incomplete and exceedingly inadequate treatments of the main environmental factors and their general mode of action upon ordinary autotrophic land-plants. IV. GENETIC CONTINUITY OF PROTOPLASM AND ITS CYCLIC ACTIVI- TIES, IN CONNECTION WITH PROBLEMS OF DISTRIBUTION. In the preceding sections of this chapter the general terms of the problem of plant distribution have been presented in the words of the present-day physiology of plant organs. There can be little doubt that the day of this organic physiology is about to pass. It has been, of necessity, mainly descriptive and has not concerned itself prima- rily with the details of the actual causes of plant response and their mode of action, but there is already a strong tendency to turn attention from the description of plant organs and their responses to the physical causation of these, a development of physiology which bids fair to place this branch of science on the same quantitative etiological basis as that upon which physics and chemistry are now working. From the current literature of plant distribution and of ecology in general it is suggested that many workers in this field have so far failed to realize the present status of the physiology which lies at the base of all ecological facts. Ecology, which was at first regarded as a purely descriptive study, a mere cataloging of relatively superficial descrip- tions of phenomena and a classification of these, was an outgrowth of taxonomy. But it has advanced with more rapidity, perhaps, than any other branch of science, and it has already accumulated enough descriptions so that a beginning at least in the study of cause and effect has been made. Such a study must, by the very nature of its subject- matter, take account of all the contributions so far made by physiology towards an etiology of plant phenomena in their broader aspects. For a long time the physiology of organs must be the basis of ecological considerations, and it is with this in mind that we have taken the principles of organic physiology as the basis of our discussions. We have consciously avoided such ideas as that of purposeful adaptation and other teleological conceptions, still too common in botanical writ- ing—with what success the reader will best be able to judge—and have INFLUENCE OF ENVIRONMENT ON PLANT LIFE. 105 aimed to place our treatment of plant distribution on a basis as free from anthropomorphic conceptions as is that of physiography, a science with which plant geography must always be closely related. But the modern trend of physiology, with its application of the methods and findings of physico-chemistry and its tendency to seek explanations of all phenomena in the properties of matter and energy, offers to the study of distribution at least one conception which goes far to simplify the logic of physiological limits and their modes of variation. The present section will deal with this conception. From the standpoint of general physiology, the reproductive activ- ities are to be regarded as a special form of growth. The protoplasm of any species is a continuously existing thing, to be likened, perhaps, to a river, in which the material particles are ever changing and of which the form, course, activities, etc., show a continuous variation. The channel of our river moves from place to place within the limits of its valley; the river accomplishes much work of excavation and the like at certain seasons of the year; at other seasons it is inactive and, in the arid regions, often disappears from sight completely, flowing only underground. In the latter condition we may perhaps speak of it as dormant. Following this analogy, the living substance of any species may be thought of as continually existent, but varying widely in situation, amount, activity, etc., with the ever-fluctuating physical conditions within and without its mass. Thus the conditions for the success or failure of any species in any region or habitat are that its protoplasm be indefinitely maintained in that area, and that the various cyclic phases of physiological activity follow one another in a certain order. By such a conception we are enabled to generalize without the complication incident to the special consideration of the reproductive activities. Scrutinized in this manner, the individuals of a given plant-form are seen in somewhat the same light as are the buds of an indefinitely growing perennial, such as a tree. These buds are continuously dying and being formed, but the system of growing-points which make up the tree possesses form, size, etc., and maintains itself throughout years. Growth here consists partly in the approximate replacement of parts which have been destroyed by the action of adverse condi- tions, and partly in an actual increase in the number of growing-points comprised within the entire system. By this sort of generalization we may bring all plant-forms into the same broad category, as far as the general influence of external con- ditions is concerned. The protoplasm of any species, in any region, passes through variously active and dormant phases with the march of the seasons, the success of a species requiring that there be, at least, no progressive and permanent decrease in the number of growing- points or individual groups of active cells. But there may occur, and usually do occur—outside the tropics at any rate—great periodic fluctuations in the number and activity of these biotic units. 106 ENVIRONMENTAL CONDITIONS. f Plant protoplasm may be said to pass through alternating phases or stages of diastole and of systole. In the condition of diastole it is very active, increasing rapidly in amount and extending and altering its configuration to a remarkable degree. These changes are due directly to numerous physico-chemical processes and energy transformations which, during the period of this phase, are quite violently active. In the condition of systole, on the other hand, the various characteristic life processes are at a low ebb, some of them being apparently alto- gether abated. The mass and extent of the protoplasm falls off more or less markedly, in the death of many parts which were previously . the seats of vigorous activity, and in many cases the whole organism practically fails to exhibit any form of life at all. This rhythmic pulsation is of course immediately due to internal conditions, but the latter are, in turn, to be causally related either to changes in the envi- ronment or to effects of a constant environment summed or integrated by the organism. While the point of view just suggested is not at all new, and has been of great service to some students of heredity, we are not aware that it has been resorted to in studies of the influence of external con- ditions upon the maintenance of plant population in a distributional sense. It seems to promise such utility as a logical tool that we venture, in the following paragraphs, to outline the behavior of some of the main plant types in terms of this conception. Attention may first be directed to the case of a perennial which propagates vegetatively, omitting for the present any question as to whether seeds are produced. An excellent example of this is seen in several forms of much-branched cylindrical opuntias, as the various ‘“cholla’’ cactus-forms of the North American Southwest. By the action of various agencies, such as wind and animals, short branches are easily broken off from these plants, and are widely distributed by the operation of the same agencies and by that of flood-water. Under favorable conditions these fragments possess the property of taking root and forming new plants. Such may be regarded as the simplest form of species maintenance, and it is of course the rule among the lowest forms of plants. We have here to do with the mature plant at all times, the only complication in vegetative phases lying in the fact that roots are produced and proceed with their characteristic activ- ities under the peculiar conditions offered by the dispersal of the joints. With the coming of each favorable season, mainly defined by condi- tions of moisture and temperature, growth in size occurs in numerous branches, some of the latter being portions of larger plants, while others lie singly upon the ground. By the action of adverse conditions many branches are destroyed, but enough survive apparently to maintain the status of the species in the vegetation of its area. During the vear great fluctuations occur in the quality and intensity of the environ- mental factors; periods of extreme drought and heat, periods of drought and cold, periods of abundant moisture and either high or low tem- let a INFLUENCE OF ENVIRONMENT ON PLANT LIFE. 107 peratures, follow one another in unending succession. In certain periods foliage is produced and growth proceeds with rapidity; in others the leaves disappear and growth is retarded or checked; in still others nearly all activities cease and the branches remain dormant till the return of conditions favorable to growth. In a manner similar to the above, we may consider all perennial forms that maintain themselves or spread by vegetative propagation, as in the case of those with underground branches, rhizomes, stolons, divid- ing-bulbs, and the like. Ina form like Solomon’s seal (Polygonatum), vegetative propagation is very important; a single rhizome may give rise to numerous new growing-points (by means of branching), which eventu- ally become separated from the parent rhizome with the decay of the connecting portions, so that, if time suffices, a considerable area may be occupied without other activity than this purely vegetative one. Of course bulbs, tubers, and the like are to be regarded as plant phases which are characterized by dormancy and highly resistant properties, for which the physiological limits are widely separated. There occur all gradations in stem and bud vitality between the bulb or tuber and the great deciduous or evergreen tree, all agreeing in the essential point that with the coming of adverse conditions they undergo a check in their activities. The deciduous forms lose a large proportion of all of their vegetative organs. All these forms remain alive but dormant till the return of the growing-season. A second example may be taken from those tropical forms which reproduce by seeds, but in which vivipary vaults the resting-period usually so characteristic of the seed. A history of the activities in the mangrove, for instance, might run somewhat as follows: In the mature phase of this plant there are manifested cell activities which result in the dormancy of many cells. Some of the dormant cells, the eggs, are capable of resuming growth in size under certain conditions, the main condition being the entrance of protoplasmic material from another cell; that is, the occurrence of fertilization. If fertilization takes place, and this is to be looked upon as merely one of the environ- mental changes which act upon the plant in its cycle of developmental phases, then the ovum develops into an embryo and continues growth without any marked pause or resting-stage, forming ultimately a new plant. External conditions must furnish stimuli by changing in quality or intensity at various times in this development, one of the most special of which is the falling of the germinated seed into the mud below. We see in such forms all the usual phenomena of production by seed, but the pronounced dormant phase of the majority of seeds is omitted. However, unless the embryo root reaches the mud of the substratum (which signifies pronounced environmental change) the cycle of growth is checked. A third example may be chosen from among the plants which reproduce through seeds, but in which parthenogenesis bridges the interruption which usually precedes the formation of the embryo 108 ENVIRONMENTAL CONDITIONS. from the mature egg. Here no external change is apparently required to induce the ovum to proceed to the embryonic phase, but the embryo finally reaches a resting-stage, where growth activities are checked. Other alterations in the internal conditions of the dormant phase which we term a seed often require a prolongation of the resting- period, but, in any event, before active growth in size is again manifest definite changes in the external complex are required; the seed must absorb water to a certain degree, the temperature and rate of oxygen- supply must fall within the limits for germination, and various other conditions must be fulfilled in order that the embryo may emerge from its dormant state. The dormant phase of the mature egg is omitted or very much reduced in parthenogenesis, that of the seed in vivipary. The periods of rest, or of internal conditions adverse to growth, often coincide with periods of adverse external conditions, and the dormant tissue usually possesses high powers of resistance to the latter. This is a consider- ation the importance of which, in general climatic behavior and distribution of plants, can hardly be overestimated. Our fourth and last example is taken from the great majority of plants, where fertilization is necessary and a more or less prolonged period of dormancy intervenes between the maturation of the seed and germination. The annual plant perhaps illustrates this sort of rhyth- mic activity in its simplest form. Germination occurs in the spring, when temperature, moisture-supply, etc., are favorable for this kind of growth. Later, the various developmental phases follow each other with more or less pronounced alterations in the external conditions, and when seeds have been matured the parent plant dies. This final death may occur because of the action of internal conditions, perhaps connected with the ripening of the seed, or because of the action of external factors, such as drought or frost. But the dormant phase represented by the seeds is highly resistant, and these bodies carry the living protoplasm forward through the winter of adverse condi- tions to the beginning of a new cycle of activity. The general conception outlined above may be expressed briefly to the effect that each particular sort of plant protoplasm (form, species) is indefinitely perennial, ever passing through repeated cycles, ever changing in internal nature from one developmental phase to another, growing, fragmenting (as in reproduction of all sorts), resting in a dormant condition, and always again taking up the endlessly repeating series. Of course our conception of the repetition here involved must be broad enough to include such alterations from cycle to cycle (variation, mutation, etc.) as the study of evolution demands for the origin of new forms from old. This mode of contem- plating plant activities should be as valuable in physiology and ecology as has been the conception of the alternation of generations in the descriptions of the consecutive steps in plant phylogeny. _ CHIEF ENVIRONMENTAL CONDITIONS AND THE GENERAL NATURE OF THEIR EFFECTS UPON PLANTS. I. GENERAL CLASSIFICATION OF ENVIRONMENTAL FACTORS. The environmental conditions that are commonly most potent in the determination of plant development, and that therefore appear most important in distribution, may be classified under the following headings: (1) Moisture conditions; (2) temperature conditions; (3) light conditions; (4) chemical conditions; (5) mechanical conditions. The present section will be devoted to a brief summary of the nature and effects of these factors as they vary in quality, intensity, and dura- tion. No attempt is made to denote more by the order of items in the above list than a very general estimate of the relative importance of the various factors as they are usually operative in limiting plant distribution. There is, of course, nothing to be gained in a discussion of the relative importance of a number of factors, all of which are necessary in order that a given phenomenon may occur. Such con- sideration were as bootless as a discussion of the relative importance of the hub, spokes, felloes, and tire of a wheel. The reader is there- fore requested to make nothing of our order of arrangement. Students of ecological plant distribution have usually classified these sets of conditions according to their origin or source, rather than according to their mode of physically affecting the plant. Thus, the literature contains many references to climatic and edaphic condi- tions, physical and biotic ones, and the like. Such groupings seem, however, not to have led much farther than to the mere description and arbitrary classification of distribution conditions, and, since by their very nature they point to the causes of the factors imme- diately involved rather than to the real nature of these factors or their effects upon the plant, they promise little for our present purpose. Thus, shade produced by a natural rock arch or overhanging cliff may be impossible of physical or physiological differentiation from that produced by a tree; yet the former is said to be a physical factor and the latter a biotic one. Again, the mechanical relation of the physical separation of plant and soil, together with the accompanying ruptures and lesions of the plant tissues, might arise equally well from the action of animals (a biotic factor) and from that of wind or torrential water (undoubtedly physical factors). In a study of the ultimate causes that bring the proximate, effective factors into being, such classification has its value, but in such studies we have assuredly left our field of plant distribution for the adjoining one of climatology, physiography, and the rest. In the beginning it appears more promis- 109 110 ENVIRONMENTAL CONDITIONS. ing, because simpler and logically more direct, to attend strictly to the various factors as they actually affect the plant, leaving the analysis of the sources of these factors to other studies, perhaps at a later day. We shall here consider only the proximate determining conditions in plant behavior and distribution, merely mentioning a few points bear- ing on the more remote determination of these controlling conditions, which are available from researches in climatology and other fields. Since every effect upon the plant must be supposed to be directly traceable to conditions that previously prevailed within and without the organism, and since we make no attempt here to analyze the various physiological processes that constitute the varied plant re- sponses, it has been deemed best in these considerations to regard as an external condition or factor any status of the processes of the ex- ternal world that directly influences changes within the plant. Many logical difficulties arise here, as is usual when any attempt is made to subdivide a continuous series into regions, and the only satisfactory method of procedure is to subdivide the series into arbitrary portions, being sure to define the limits chosen. Thus, for ecological purposes it seems quite as undesirable at the present time to enter into the exceedingly complex physiological considerations involved in a study of the details of plant activities as these are controlled’ by conditions as it is to take up in detail the more remote causes which bring the various effective conditions into existence. This logical difficulty arises, of course, from the fact that the plant is not an independent system, but is perfectly continuous with the universe about it; the classification, into external and internal, of the conditions determin- ing chemical and physical changes within the plant, is at best but a subjective affair with the mind that classifies. Whether a given con- dition is to be taken as external or internal will always depend largely upon the previous experience and point of view of the observer, upon his internal conditions. For the present needs it seems desirable to base our arbitrary definition of the controlling factors upon the spatial limits of the plant-body as ordinarily considered. Thus we define as effective external conditions all phases of universal progress outside the plant-body which directly affect the latter in such a manner as to produce alterations in the chemical, physical, and physiological processes that occur within. An example may illustrate this. The passing of a given region of the earth’s surface from shadow into sunlight at dawn is not a condition immediately effective upon plant processes, nor is the influx of radiant energy to the surfaces of objects in the vicinity of the organism. The plant-body is first affected in this case when there occurs an increased rate of transmission of light or heat energy through the periphery (or a decreased loss of heat, which amounts to the same thing as an increased income), so that some substance actually a part of the plant-body becomes lighted or warmed. CHIEF ENVIRONMENTAL CONDITIONS. PEE It is only by a logical short-cut, not always tenable, that we may say that the rising of the sun produces such responses as the opening of stomata, the eastward bending of flower-heads, the assumption of the day position by nyctitropic leaves, and the like. From the logical standpoint, attention is here to be confined to changes in the rate of inward or outward transfer of the various forms of matter and energy between the surroundings and the plant-body itself. It must be confessed that, while most existing types of vegetation have been rather carefully, and in some cases perhaps even somewhat quantitatively, described, yet there appears so far in the literature scarcely anything of a fundamental nature upon the external factors and their modes of action in determining plant distribution.! It is thus largely upon quantitative studies of the intensity, duration, etc., of the external conditions and upon the true physiological inter- pretation of these that the future conquests of this branch of ecology must depend. It therefore seems desirable, even at the risk of ap- pearing to “carry coals to Newcastle,’ to venture the following physiological discussion before taking up such meager contributions as can be brought together in regard to the geographical distribution of the different environmental complexes of the United States, as these may be related to the distribution of the various vegetational types. The different categories of environmental conditions will be con- sidered in the order of the preceding list, and attention wil be turned briefly to the various modes in which these are effective to bring the different plant responses about. In the interest of clear presentation, it will now and again be advantageous to overstep our logical limits in the other direction, and to touch upon some of the relations of the more remote conditions that, in their turn, influence or control the proximate and immediately effective environmental conditions. I]. MOISTURE. 1. WATER REQUIREMENT WITHIN THE PLANT. The very complex moisture-relations to which plants are subjected are to be considered as of the utmost importance in the great majority of distributional problems. These may be physiologically best studied from two standpoints—that of the requirement for water and that of the water-supply—since these two factors determine by their inter- action the moisture conditions of the plant. They may be taken up in order. Since every active cell is filled with water, it follows that there must occur constantly, or with but brief interruptions, a movement of water 1Serious beginnings in the direction of physiologically experimental investigation of the relations holding between plant-growth in the open and the controlling conditions of the environ- ment have been made by a very few workers. In this connection, see McLean’s studies of the control of seedling soy-beans by the conditions of the Maryland summer climate (McLean, F. T., A preliminary study of climatic conditions in Maryland, as related to the growth of soy- bean seedlings, Physiol. Res. 2: 129-208, 1917). References to earlier literature are there given. See also: Hildebrandt, F. M., A physiological study of the climatic conditions of Maryland, as related to plant growth. Physiol. Res. 2: 341-405, 1921. 112 ENVIRONMENTAL CONDITIONS. into all enlarging portions of the plant-body. This demand for water, due to growth, is usually quite insignificant in degree, but it is never- theless real, and the absence or inadequacy of the water-supply to growing cells must always act as a check upon enlargement. Also, since water is destroyed in the process of photosynthesis in green plants, there must be a continuous influx of moisture into all photosynthetizing cells. If this supply is cut off, the formation of carbohydrates must cease in a short time. With insufficient water- supply, growth ceases before photosynthesis, but both processes must soon be brought to a standstill. The water demand occasioned by photosynthesis is perhaps usually more pronounced than that for enlargement, yet the rate of influx thus brought about is far too small in amount to permit of measurement by simple methods. Also, water disappears when hydrolytic decompositions occur, so that such processes as the digestion of starch-are essentially drying processes. On the whole, these water requirements may be safely assumed to be only of relatively slight importance in comparison with that of transpiration, which is next considered. The fact that all plant tissues contain water, and that no cuticular or other covering is absolutely impervious to this liquid—many leaf- cuticles, etc., being rather freely permeable to water—makes it logically follow that there must ever be a more or less pronounced evaporation of water from all plant surfaces that are exposed to the outer air. This superficial evaporation through externally exposed membranes makes up the so-called cuticular transpiration, which’ varies in amount in different forms, depending upon structure, and is often of great importance in determining the need of the plant for water. The water lost by cuticular transpiration is replaced from more deeply-lying tissues, according to the principles of diffusion and of imbibition, and sooner or later there must occur an inward move- ment of water from some region without the plant-body, or else death must ensue. But, while cuticular transpiration is a very real and almost con- stant source of water requirement, it is of relatively little account in comparison with stomatal transpiration. The presence of mem- branes of high moisture-content within the leaves—the walls of the mesophyll, ete.—which are in direct connection with the external air through the stomata, makes continuous evaporation from these internal tissues an inevitable condition, unless, indeed, the foliar surface be covered with a film of water or of a solution of higher vapor- tension than that which occurs within the tissues. The internal atmosphere is maintained more or less nearly in moisture equilibrium with the wet membranes that bound it, and ordinary diffusion through the stomatal pores constantly removes water-vapor to the outer air. CHIEF ENVIRONMENTAL CONDITIONS. 148 This water-loss, constituting stomatal transpiration, usually occurs at a rate much higher than that evidenced by cuticular transpiration. The ubiquity of this form of water requirement and its generally great magnitude render it the dominating physiological moisture condition for most plant-forms growing in the open air. However it may be removed, the water-content of an active tissue that is being depleted is always replenished at a greater or less rate from other tissues in the vicinity or from without the body. If such renewal of the water-content fails for a considerable time, partial or complete loss of activity must result. In plants without storage- organs the usual transpiration-rate can continue but a short time without entrance of water from the outside. Where storage-tissues are present the external supply may, of course, be cut off for a longer time. Since transpiration is the most important factor in determining the need for an external water-supply to the plant, it will be necessary to consider here some of the conditions that influence this process. Its rate is dependent upon three conditions: (1) the structure and condi- tion of the leaves or other transpiring parts; (2) the evaporating power of the air; (8) the intensity and quality of illumination. As is well known, there occur in different plants, under identical external condi- tions, great differences in the rate of transpiration per unit of surface. Certain structures, such as waxy and hairy coverings, palisade tissue, and other features, make cuticular transpiration markedly less pro- nounced than it might otherwise be, and many stomatal characters similarly influence the rate of stomatal water-loss. Many anatomical characters are known to become permanently altered by age, of course with reference to the nature of the environmental complex, as where cuticular thickening may increase or fail to do so, according to the age and surroundings of the plant. Such physiological responses are apparently dependent upon the rate of movement of transpiration- water (Pfeffer-Ewart, Plant Physiology, 2: 121). In regions where the evaporating power of the air rises rapidly during the growing- season, plants with a highly developed cuticular response of the form just mentioned may be expected to survive longer than others with a less-marked response of this kind. Stomatal movements, of opening and closing, may also be of considerable importance in certain cases, though this whole question is sadly in need of a more thoroughgoing physical investigation than it has yet received. 1Concerning the quantitative aspect of differences in the power of plant leaves to retain water and thus retard water-loss, see the following papers: Livingston, B. E., The relation of desert plants to soil-moisture and to evaporation, Carnegie Inst. Wash. Pub. 50, 1906.— Idem, The resistance offered*by leaves to transpirational water-loss, Plant World 16: 1-35, 1913.—Livingston, B. E., and A. H. Estabrook, Observations on the degree of stomatal movement in certain plants, Bull. Torr. Bot. Club 39: 15-22, 1912.—Bakke, A. L., Studies on the transpiring power of plants as indicated by the method of standardized hygrometric paper. Jour. Ecol. 2: 145-173, 1914. 114 ENVIRONMENTAL CONDITIONS. The amount of water in the transpiring organs appears also to be important in determining the rate of water-loss. Thus, transpiration is frequently checked in the daytime, with no apparent wilting and no closure of stomata, but with a marked fall in foliar water-content, while the evaporating power of the air is maintained or even increases in magnitude. Second only to the nature and condition of the leaves, ete., the evaporating power of the air exerts an enormous controlling influence upon the rate of water-loss from plants. For any particular plant- form it appears to be by far the most potent of all the climatic factors affective above the soil-surface. The evaporating power of the air is a compound factor, dependent itself upon three other factors, as commonly considered—temperature, humidity, and wind velocity. Its resultant effects are the summation of the partial effects brought about through the influence of these three factors upon the vaporiza- tion of water and the removal of the water-vapor from the evaporat- ing surface. Evaporation, as a climatic factor, will be more thoroughly considered in another place. The intensity of the sunlight and its quality are also potent factors in the determination of transpiration from plants. While a certain small proportion of the energy of the solar rays is made potential and entrapped in the plant by the photosynthetic process, by far the greater portion of that which is neither reflected nor transmitted becomes potential in the water-vapor that escapes from the plant by transpiration. Thus, intense sunlight with a good proportion of the longer waves is markedly effective to increase the transpiration-rate of plant organs whereon it falls. The color and structure of plant organs have also to do with this, through the influence these exert on reflection and transmission. With a smaller proportion of the greater wave-lengths or with less intensity the effect is not so marked.? Ecologists have classified plants, as to their ability to withstand different degrees of light intensity, into those which thrive best in shade, in bright sunshine, etc., and have given to these groups Greek names, but inasmuch as there are all possible gradations in this power of withstanding sunlight, and since there is as yet so little information of a quantitative nature bearing upon these matters, it seems advisable here merely to emphasize the fact that one of the most important influences of sunlight on plants is upon the rate of water-loss, and, therefore, upon the water-requirement. Another form of water-loss from certain plants is the active excre- tion of liquid moisture from nectaries and other superficial glands, such as water-pores, etc. The process by which this is brought about 1 Livingston, B. E., and W. H. Brown, Relation of the daily march of transpiration to variations in the water-content of foliage leaves, Bot. Gaz., 53: 311-330, 1912.—Shreve, Edith B., The daily march of transpiration in a desert perennial, Carnegie Inst. Wash. Pub. No. 194, 1914. 2 Livingston, B. E., Light intensity and transpiration, Bot. Gaz., 52: 418-438, 1911. — iii ss. CHIEF ENVIRONMENTAL CONDITIONS. £15 is not at all understood, but we may be sure that the water-loss thus occasioned is of no great general importance in the distribution of plant-forms, especially since the greatest excretion of water, as in guttation, usually occurs at times when the transpiration-rate is low and the supply of water within the tissues is relatively great. In- deed, one of the common teleological conceptions bearing upon the so-called regulation of water-loss by plants is that these organisms actively and purposefully force water out of their tissues whenever they have been compelled by external eircumstances to absorb more of the liquid than they want. To summarize the preceding paragraphs, the entrance of water into the ordinary active plant is essential to its activity: (1) because it is necessary for the enlargement of water-saturated cells; (2) because it is destroyed in photosynthesis, etc.; (3) because it is continually being lost by cuticular and stomatal transpiration and by excretion. 2. SUPPLY OF WATER TO THE PLANT. As has been emphasized above, the transpiring tissues of a plant must receive water from elsewhere, otherwise they would soon become wilted and collapse, and other active tissues must hkewise receive water, though frequently in much smaller amounts. There are several sources from which this water may come. Water-storage tissues and dying cells, or cells passing into a dormant stage, may furnish more or less water to other tissues, according to the form of plant considered and its phase of development. A relatively very small amount of water, fixed previously by photosynthesis, must be set free in the tissues by the activity of respiration, and this may become available for growing cells or may be again transformed by photosynthesis. Also, the reverse of hydrolytic decomposition (such as the synthesis of starch from glucose) results in the chemical formation of some water. For the transpiration of ordinary plants the latter sources are surely inadequate.’ During rains, and when the temperature of the foliage falls below the dew-point of the surrounding air, the external surfaces of the plant become wet, and a considerable amount of moisture may enter the plant-body through the cuticle and even through stomatal open- ings. This source of water is especially important only for certain forms, such as mosses, liverworts, and plants of similar water-rela- tions. With heavy cuticle, trichome coverings, etc., very little water 1 Fitting suggests that respiration water may be important in this connection in certain desert tubers, etc. (See Fitting, Hans, Die Wasserversorgung und die osmostischen Druckverhiltnisse der Wistenpflanzen, Zeitschr. Bot. 3: 209-275, 1911.—Livingston, B. E., The relation of the osmotic pressure of the cell-sap in plants to arid habitats, Plant World 14: 153-164, 1911.) The latter is in part a review of Fitting’s paper. An excellent discussion and the most valuable ex- perimental treatment yet available of the amount and importance of respiration water in plants and animals is the following: Babcock, 8S. M., Metabolic water: its production and rdle in vital phenomena, Wisconsin Agric. Exp. Sta. Bull. 22, 1912. 116 ENVIRONMENTAL CONDITIONS. can find its way into the tissues by these channels, and it follows that those plants that transpire the least must exhibit the smallest amount of leaf absorption. The main source of water-supply to the majority of plants is of course the soil or other substratum in which the plant is rooted. We may consider in greater detail the first and the last of the sources just mentioned. In relatively large plants the diffusion of water from non-active to active parts is often of great importance. Thus, as a tissue dies its contained moisture may pass into other portions and there support growth and other activities. A familiar example of this is exhibited by bulbs, rhizomes, etc., that produce leaves, shoots, and even flowers and fruits without the influx of any water from without. Water from the bulb moves gradually into the more active portion and supplies the moisture for growth and transpiration; here the so-called storage- tissue plays the same réle of water source as does the moist sub- stratum in the case of ordinary rooted and absorbing plants. In many instances water-bearing tissues may lose much of their water during the growth period of the plant and may still retain vitality and the power to absorb, so that at another season, when the external water- supply is greater, such tissues may receive water in larger amounts from the substratum and so return to their original turgid condition. In many cacti, fleshy euphorbias, and the like, all of the water for transpiration—a relatively small but nevertheless important amount— and even for reproduction may be derived from the quiescent stem parenchyma for long periods of time. Many water-storage plants of the desert can retain vitality and maintain their reduced transpira- tion for several years after they have been removed from the soil and are thus able to absorb no water from without.! Such isolated plants often effect new growth and ripen fruits with the return of the proper season, the conditions that bring about re- newed activity in such cases being probably mainly those of tempera- ture. Of course, these forms must eventually succumb to lack of water, as must any other form when deprived of a water-supply, but the interesting point here is simply that they may withstand the absolute lack of a water-supply from without for exceedingly long periods of time. To most plants, a root system or its analog is essential throughout the actively transpiring phases of its development. As has just been implied, such a system must likewise be present a part of the time, at long intervals perhaps, even in the most extreme water-storage forms. Through these water-absorbing organs the moisture of the substratum finds its way to the tissues of the plant. This water often traverses long distances of stem, etc., and it is thus seen that the rate at 1MacDougal, D. T., E. R. Long, and J. G. Brown, End results of desiccation and respira- tion in succulent plants, Physiol. Res., 1: 289-325, 1915. — CHIEF ENVIRONMENTAL CONDITIONS. 117 which the transpiring portions of a non-storage plant may receive water depends upon several conditions: the rate at which water may move in through to the absorbing surfaces, the nature and condition of the roots, and the nature and condition of the water-conducting tissues. In most cases where growth ceases or wilting occurs, the inadequacy in the water-supply seems to arise, not from the attainment of the physiological maximum of absorption and transmission, but from a greater or less drying-out of the soil, whereby it fails to transfer water to the absorbing roots at a rate adequate to supply the demand of absorption. Quantitative information bearing upon the relations between soil-moisture and plant absorption and transpiration is not yet available, and our consideration of this exceedingly important subject must be very brief and very tentative.! It is certain that, with diminished supply of soil-moisture and with other conditions remaining unchanged, transpiration in any plant must be decreased in amount, also that this diminution in the trans- piration-rate does not progress parallel to the continuous drying of the soil, so that it ultimately comes about that the supply fails to equal the demand, transpiration becomes greater than absorption, the non- storage plant ceases to grow, and wilting or even partial or total death ensues. Such a decrease in the rate of movement of water to the roots does not necessarily mean any considerable fall in the average percentage of moisture present in the soil, but gives evidence merely of the fact that the movement of water through the soil-films and into the roots has become less rapid. Under such conditions the soil immediately surrounding the absorbing portions of a root-system becomes drier than that at a greater distance, and the movement of water into the drier layer is too slow to keep the surface of the root adequately moist. This matter of the possible rate of water transfer from soil to root, fundamental as it is, is greatly in need of thorough investiga- tion. The subject has been opened by Livingston and Hawkins and by Livingston and Pulling in the papers cited above. This should prove a wonderfully productive field, both for scientific ecology and for agriculture when serious attention is at length turned to it. Great differences in the water-relations of plants in different habitats are secondarily occasioned by the nature and exposure of the soil in which they are rooted. Surface drainage often conducts the water of precipitation away before it can penetrate the soil to an adequate degree, and underground drainage frequently depletes the moisture- supply of porous soils almost as fast as water enters from a shower. Evaporation removes water rapidly from some soils and but slowly from 1JIn this connection see Livingston, B. E., and Lon A. Hawkins, The water-relation between plant and soil, Carnegie Inst. Wash. Pub. No. 204: 3-48, 1915.—Pulling, H. E., and B. E. Liv- ingston, The water-supplying power of the soil as indicated by atmometers, Ibid. 204: 49-84, 1915. Also see: Livingston, B. E., and Riichiro Koketsu, The water-supplying power of the soil as re- lated to the wilting of plants, Soil Science, 9: 469-485, 1920. 118 ENVIRONMENTAL CONDITIONS. others, and the supply remaining to a plant after a period of dry weather depends very largely upon this factor. Fine-grained soils resist both evaporation and subdrainage, but they also resist water- absorption by plants, while coarser ones give up their water more readily! so that in a region frequently visited by drought the upland vegetation is, in general, most highly developed on the heavier soils. As to the internal conditions that influence the rate of water-supply to the transpiring parts of the plant, there appears to be, for any plant at any time, a maximum rate at which water can enter the roots and pass through the vessels. This rate seems to depend upon the extent of the root-system and upon the condition of the absorbing portions of the roots as well‘as upon that of the conducting tissues in general. It is probably never for very long periods that this maximum rate is attained in moist weather; most plants at such times do not appear to transpire at a rate that exceeds their maximum rate of ab- sorption and conduction. Evidence has been obtained in the arid regions, however, and this probably holds also for dry periods in humid areas, that this maximum rate may frequently be reached when the rate of oe is ereniely increased through high evaporating power of the air.” In such cases growth ceases even with an ample supply of soil- moisture and the plant remains quiescent, without other sign of in- jury, till a lower evaporation-rate allows absorption and transmission again to surpass the rate of water-loss. Thus, in the spring dry season at Tucson, Arizona, morning-glory plants attained a few leaves and then rested without growth until the higher humidity of the summer season arrived, although the soil in which they were rooted was kept continuously at or somewhat above its optimum water-content by irrigation. When the evaporation-rate had fallen markedly, with the coming of the cloudy and more humid summer rainy season, these plants resumed their growth in the usual manner.’ The ability of a plant to absorb and conduct water is, of course, an internal condition, which depends upon many things. Naturally, the more extensive is the absorbing surface of the roots the more 1 Of several soils with approximately the same chemical composition, but differing in the size of their particles, that with the finest particles gives ordinarily the most luxurious vegetation. The physical and physiological reasons for these phenomena have apparently not been taken up in detail. In this connection see Livingston, B. E., and G. H. Jensen, An experiment on the relation of soil physics to plant growth, Bot. Gaz., 38: 67-71, 1904. On the capillary move- ment of water in natural soils, see: Pulling H. E., The rate of water movement in aerated soils, Soil Science, 4: 239-268, 1917. 2 Livingston and Brown (1912): Brown, W. H., The relation of evaporation to the water- content of the soil at the time of wilting, Plant World, 15: 121-134, 1912.—Briggs, L. J., and H. L. Shantz, The wilting coefficient for different plants and its indirect determination, U.S. Dept. Agric., Bur. Plant Ind. Bull. 230, 1912. Other citations of this work are given in the two fol- lowing papers: Caldwell, J. 8., The relation of environmental conditions to the phenomenon of permanent wilting in plants, Physiol. Res., 1: 1-56. 1913.—Shive, J. W., and B. E. Livingston, The relation of atmospheric evaporating power to soil-moisture content at permanent wilting in plants, Plant World, 17: 81-121, 1914. 3 Livingston, 1907. a CHIEF ENVIRONMENTAL CONDITIONS. 119 rapidly may moisture enter, providing, of course, that the maximum rate of supply of soil-moisture to this surface is not surpassed. Also, the rate of absorption must be markedly affected by the condition of the absorbing membranes and the cells adjacent to them. If these tissues are pathologically modified, as by the presence of poisons, even a large extent of root surface may fail to allow as much water entrance as might occur through a smaller root-system in a healthy condition.! The condition of the vessels in the stem, etc., whether well or poorly developed, whether the lumina are large or small, and whether cross- walls are frequent or not, is an important factor in determining the maximum rate of water conduction with a given pressure gradient. It will be remembered in this connection that the primary deleterious effect of certain fungus growths within the vessels is due to a simple stopping of these passages. In such cases the plant might suffer from lack of water, although its roots possessed an adequate power of ab- sorption and were in a soil of adequate water-supplying power. Of course, the causes of the internal conditions above mentioned are to be sought in previously effective external and internal condi- tions, as the effects of which any present status of affairs must be considered; but this phase of environmental influence lies far beyond the matters with which we are here concerned. 3. RELATIONS BETWEEN WATER-REQUIREMENT AND WATER-SUPPLY. From the above consideration of the water-requirement and water- . supply of plants it is clear that growth and other activities are not dependent upon either of these factors alone, but depend upon the relation that holds between them. It is this relation which gives the clue to all physiological and ecological problems concerning mois- ture. So long as water moves into any tissue as rapidly as it is re- moved, that tissue may maintain itself in a quiescent state; so long as the possible rate of influx surpasses the actual rate of loss, the tissue may increase in size and carry on any processes requiring the fixation or destruction of water; and whenever the supply falls below the de- mand (7. e., whenever the demand exceeds the supply), growth and many other activities must cease. If the latter condition continues long, partial or total death must follow, or at least the more or less complete entrance of the organism into a state of dormancy. The effect upon the plant is the same, whether the physiological lack of water be brought about through an increase in the demand, through a decrease in the supply, or through both of these acting together. 1 Livingston, B. E., Note on the relation between the growth of roots and of tops in wheat, Bot. Gaz., 41: 139-143, 1906.—Livingston, B. E., J. C. Britton, and F. E. Reid, Studies on the properties of an unproductive soil, U. S. Dept. Agric., Bur. Soils Bull. 28, 1905.—Livingston, B. E., Further studies on the properties of unproductive soils, U. S. Dept. Agric., Bur. Soils Bull. 36, 1907. 120 ENVIRONMENTAL CONDITIONS. Higher evaporation-rate or increased solar intensity may raise the rate of transpiration in any plant until it surpasses the possible rate of supply to the transpiring parts. On the other hand, the drying- out of the soil or a pathological condition of the absorbing or conduct- ing system of the plant may reduce the rate of entrance or transmis- sion of moisture until the transpiring tissues suffer from dryness. The effect of this physiological drought, however caused, is a gradual loss of water and hence of turgor, which results finally in plasmolysis and wilting. Under such conditions tissue enlargement must cease before plasmolysis is accomplished, and can not begin again until a certain amount of turgidity has been regained. As long as the ratio between the rate of possible water-supply and the rate of water-demand in any tissue or organ is greater than unity, growth may occur. When this water ratio falls to unity growth must soon cease, though the organ may retain its form and vitality. When the water ratio becomes less than unity, incipient drying occurs and plasmolysis must soon follow if the ratio continues less than unity. Whether plasmolysis and wilting result in the death of the tissue in- volved depends upon the extent to which the ratio falls below unity and upon the length of the period during which this condition obtains.! Of course, it must be remembered that the matter here brought forward is very much complicated by the free interchange of water by various parts of the plant itself; the wilting of a certain tissue may not denote anything out of the ordinary in the plant as a whole, for the normal process of development often ,includes many reversals in growth. Thus, a tuber grows for a long time and then loses its water and other contents, while the entire plant of which such tuber is a part may be said to be continually advancing through its development phases. With the continuation of a drought period most plants die only by degrees; the lower and older leaves are apt to succumb first, and it is only after a somewhat protracted dry period that total death of an individual occurs. Even in such cases the existence of seeds usually carries the vital substance forward to the next favorable season. The withering and falling away of a few of the older leaves often acts as an automatic removal of the drought conditions, for such a decrease in the transpiring surface may so diminish the transpiration-rate as to prevent further wilting. The same result is frequently brought about by a temporary lowering of the evaporating power of the air or of the light intensity. The tendency to wilt, which is manifest in most plants on dry, sunny afternoons, though no actual wilting may occur, is regularly checked by the coming on of night with its conse- quent lowering of the evaporation-rate, and also, sometimes at least, through closure of the stomata. The water ratio of transpiring organs 1 Livingston, B. E., Incipient drying in plants, Science, n.s., 35: 394-395, 1912.—Caldwell 13. CHIEF ENVIRONMENTAL CONDITIONS. 121 thus falls during bright days and rises again at night. In the arid regions it appears that this night period of recovery is of very great importance. Many plants that are normally very resistant to drought conditions as they occur would probably succumb com- pletely on the second day if the night period of recovery of the water ratio were omitted. A shower of rain affects both terms of the water ratio; it increases soil-moisture and decreases evaporation, while incipient drying or partial plasmolysis may sometimes be almost immediately corrected through actual absorption of moisture through leaf surfaces wetted by rain. In the majority of ordinary plants the water of transpiration passes with comparative directness from the absorbing surfaces of the root- system to the transpiring surfaces of the foliage. Stored water is here of little general importance. Outside of the arid regions such plants appear to absorb and transmit moisture from a moist or wet soil with sufficiently great rapidity to prevent any serious wilting, even with the highest transpiration-rates; that is, the maximum possible rate of absorption is seldom inadequate. But, with a soil that is becoming dry, there comes a time when the actual rate of ab- sorption fails to keep the water ratio above unity, and in such cases wilting soon occurs. If a plant wilts for this cause it may be made to revive by mere addition of water to the soil about its roots. How- ever, if the maximum possible rate of conduction is at fault (which depends, as has been seen, upon the structure and condition of the roots, vessels, etc.), such treatment will fail to produce a complete return to the usual condition. (Caldwell 1913.) Before the problem of the quantitative aspects of wilting and of general plant behavior with regard to moisture may be seriously ap- proached, the study of soil physics and of water absorption, conduc- tion, and transpiration, must furnish us with means of determining with fair accuracy the terms of the water ratio. The study of soil transmission and plant absorption have been strangely neglected by students of plant physiology. That of transpiration and the condi- tions controlling it has progressed somewhat further, but much remains to be determined. No field of plant physiology promises greater conquests than this one of the water-relations, either from the stand- point of pure science or from that of a rational plant-culture. (Living- ston and Hawkins 1915; Pulling and Livingston 1915, also Pulling 1917. When a plant wilts from lack of soil-moisture it is well known that the soil about its roots is not dry, but always contains a considerable amount of water. This residual water, left after the roots have ceased to absorb, has been called ‘‘non-available.’’ Under a given set of con- ditions this moisture-content appears to be constant for any plant and for any soil, but the conditions upon which the magnitude of the resid- ual soil-moisture content depend are much more complex than has usually been thought (Shive and Livingston 1914). 122 ENVIRONMENTAL CONDITIONS. As the soil adjoining the absorbing membranes becomes drier, the surface tension of the capillary films about its particles increases until it finally equals or surpasses the imbibition attraction for moisture exerted by the exposed walls of the absorbing-cells. These capillary phenomena are the main factor in the attraction of the soil for water, and it is this capillary force against which the forces that produce water-entrance into plant roots must operate. It therefore appears that, at the time when absorption ceases, we may expect to find the vapor-tension of the exposed root-membranes just balanced by that of the soil solution. The amount of water remaining in different soils, with different plants, has been determined by various workers, and it has been taken to vary with the nature of the plant and with that of the soil. It is lower in sandy soils than in heavier ones, depending thus upon the specific attraction of the soil for water, a variable which depends largely upon the size of the soil particles. Non-available soil-moisture has often been treated briefly and summarily in texts and monographs as a soil constant. This it assuredly is not,’ for with a given plant and a given soil, this factor may be made to vary within wide limits, according to the status of the other conditions. It depends, indeed, for any soil and any plant, upon the transpiration-rate for the period during which wilting occurs. The higher the rate of water-loss from a plant the more water will there be in the soil about its roots when permanent wilting occurs. Plants in a moist room remove more water from the soil in which they are potted before permanent wilting occurs ‘than do other similar ones in a dry room or in the open. This state of affairs might have been inferred from the principles already brought out, that the drier the soil becomes the less rapidly will it conduct moisture to the roots of a plant, and that when the transpira- tion-rate surpasses that of intake, there must be a tendency toward wilting. The residual moisture content of a soil, with reference to a certain plant when wilting occurs, is simply the amount of water which that soil contains when the rate of water absorption and conduction to the foliage have been, for an adequate period, less than the rate of loss from the leaves. The length of the period of lag which elapses between the time when the rate of foliar water-supply first falls below that of transpiration and the time when permanent wilting ensues 1The best general treatise on soils and their relation to plants is, so far as we are aware, Mitscherlich, E. A., Bodenkunde fiir Land und Fortswirte, Berlin, 1913. The work ean not be too highly commended as, in general, a logically and physically sound treatise on this, one of the most difficult of biological subjects. On the general phenomena of capillarity and the complex principles upon which these depend the reader may be referred to Freundlich, H. Kapillarchemie, Leipzig, 1909. 2 Livingston, B. E., Present problems of soil physics as related to plant activities, Amer. Nat., 46: 294-301, 1912.—Briggs and Shantz 1912, Brown 1912, Caldwell 1913.—Shive and Livingston 1914. CHIEF ENVIRONMENTAL CONDITIONS. 123 must be a function of the internal water-conditions of the plant-body and of the difference between the rate of supply to the roots and the rate of transpiration. It will be seen at once that a statement of the residual water-content for any soil and plant is meaningless without a statement of the transpiration-rate at the time the determination was made, or, at least, the statement of some measure of the condi tions that determine transpiration. It is apparently quite possible to define these conditions with some precision by means of measure- ments of the evaporating power of the air. Aside from actual dryness of the soil, another condition produces the same effect upon the plant. This is included in what has been termed by Schimper physiological dryness. This condition exists where a plant apparently suffers from drought and yet is rooted in a moist or even wet soil. The optimally moist soil of the experiment with morning-glory described on page 119, might be said to be physiologically dry for that plant under those conditions of transpira- tion. The term more commonly connotes those cases where the plant suffers from lack of water, due either to some pathological condition of the roots or conducting organs or to a too high physical concentration (osmotic pressure) of the soil solution. In either case the symptoms are those produced by a dry soil, but the actual amount of moisture present in the soil may still be relatively very high, or the soil may even be completely saturated. The best known cases of adverse osmotic conditions in the soil solution are those of the so-called “‘alkali’’ soils, where the salt-con- tent is usually high, although the component salts are not highly toxic. In such soils ordinary plants suffer from lack of water, ap- parently not because water movement into the roots is checked through increased external capillary resistance, as in a soil that is actually nearly dry, nor because of toxic effects, but merely because of the high osmotic pressure of the soil.solution itself, which may result in plasmolysis of the superficial root-cells and the consequent derange- ment of the absorbing mechanism. Plants that are characterized by an unusually high osmotic pressure in their absorbing organs seem to succeed in such soils.? Those instances of physiological dryness which are produced by injury or by a pathological condition of the absorbing or conducting system need not here be treated in detail; it needs only to be re- marked that any condition leading to an inadequate power of absorp- tion or conduction may bring about a correspondingly inadequate water-supply and, in general, may result in the same symptoms as those produced by soils of low moisture-content or of low power of water delivery. 1 See in this regard Fitting, 1911, and the remarks on this paper by Livingston, 1911. 124 ENVIRONMENTAL CONDITIONS. The factor of duration is exceedingly important in the moisture- relations of plants. While cessation of enlargement must immediately ensue with incipient plasmolysis of a growing tissue, a partially plas- molyzed tissue may. retain vitality for a long time and may imme- diately recover with the return of an adequate water-supply, provided that the process of desiccation has not progréssed too far. Thus, many plants complete their growth in seasons and in regions where wilting occurs for several hours daily, this being corrected and positive growth being accomplished during the cooler or more moist hours of the day. For such forms a change in the rhythm of fluctuation of the water ratio might prevent maturation or reproduction, although the fraction of the total period of growth represented by the total period of wilting might remain unchanged. Also, the after-effect of adverse conditions being, as it seems, generally more pronounced than that of favorable ones, rapid fluctuations between adequate and inadequate water ratios frequently result in much less growth than would have occurred in a continuous period of favorable conditions, although the latter were of no greater duration than the total of all the short favorable periods really experienced by the plant in the first case. This factor of fluctuation or variation in the environment is especially difficult to consider at the present time; it is mentioned here only to throw emphasis on an important phase of the duration factor which will need careful investigation in the future. With regard to the quantitative aspect of the moisture limits which plants are able to withstand, very little information is available. Since the activities of the plant as a whole are the summation of the activities of its various parts, we must regard the primary moisture condition that is effective in the control of plant activity as simply the water ratio obtaining in the active tissues. But practically nothing has so far been done with this dynamic aspect of the water- relation. The determinations that are available bear simply upon the amount of desiccation which various forms or organs may bear. Many of the scattered observations on this point are presented in Ewart’s translation of Pfeffer’s Plant Physiology under the heading “Desiccation.’’ To obtain further information bearing upon the point in which we are at present interested, the whole viewpoint needs to be somewhat different from that heretofore employed. The moisture-contents will need to be uniformly calculated to comparable terms, such as to the basis of dry weight or natural volume, and the different regions of the bodies of higher plants will have to be separately considered. In connection with the foregoing discussion of the relation between the rates of entrance of water into the plant and those of its exit, Woodward’s' conception of the water requirement of plants should YWoodward, J., Some thoughts and experiments concerning vegetation, Phil. Trans. Roy. Soc. London, 21: 193-227, 1699. eS eee CHIEF ENVIRONMENTAL CONDITIONS. 125 receive some attention. The water requirement, in this special sense, denotes the ratio of the total water-loss by transpiration to the yield of plant material for any given period of time, usually for the entire growing-season. This conception has recently received renewed attention and very thorough study at the hands of Briggs and Shantz,! who have employed it as a physiological criterion by which to compare the relative drought resistance of agricultural plants. If several different plant-forms be grown under the same set of climatic conditions, it is found that the different forms differ in their water requirements; the amount of water required to produce unit weight of crop is greater in one case than in another. In such a case the plant with the lower water requirement is the one giving the larger crop with the smaller amount of water, and it is obvious that this criterion must be very valuable in the study of agricultural con- ditions in arid and semiarid regions. It is also clear that such com- parisons between plant-forms are always made with reference to some given set of climatic conditions; if one form has a lower water re- quirement than another for one climate it does not follow that the same relation must hold for the same two forms in another climate. Thus, relative water requirements need always to be stated with reference to a particular environmental complex, which must be de- fined as precisely as is possible. Of course the water requirement of a given plant-form may also be employed as a criterion by which dif- ferent sets of environmental conditions may be compared, the physio- logical properties of this plant-form being the standard of measure- ment in such a case. This whole matter promises much for both agricultural science and the ecology of uncultivated plants, and it may be predicted that water requirement will assume greater impor- tance in discussions of plant water-relations, as this field becomes more thoroughly investigated. The foregoing general and incomplete treatment of the subject of the influence of water conditions upon plants may suffice for the present. It is to be hoped that the future may furnish well-collected and well-related data on these questions and that some of the experi- mentation to be carried out in the future may be more adequate to the purposes of ecology and agriculture than is much of the hap- hazard experimentation so far predominating in the literature. 1 Briggs, L. J., and H. L. Shantz, The water requirement of plants, II, A review of the literature, U. S. Dept. Agric., Bur. Plant Ind. Bull. 285, 1913. This is a very complete and valuable annotated bibliography of the subject and includes discussion of and references to the writers’ own work. In this connection see also Shive, J. W., A study of physiological balance in nutrient media, Physiol. Res., 1: 327-397, 1915. P. 379. 126 ENVIRONMENTAL CONDITIONS. Ill. TEMPERATURE. 1. TEMPERATURE REQUIREMENT WITHIN THE PLANT. One of the fundamental conditions that have to be fulfilled in order that life processes may go forward is that the body of the organism must possess a temperature lying between certain limits; the tem- perature of the living cells must be neither too high nor too low. If the temperature rises beyond the maximum temperature limit for life, or if it falls below the corresponding minimum, death must follow. In this consideration, which is one of the most clearly established principles of physiological science, it is to be borne in mind that the numerous processes, or material and energy transformations, that make up life are partly chemical in their nature and partly physical. All processes that result in an alteration in the kind of matter within the plant are chemical. Here belong photosynthesis in green plants, all the various kinds of chemosynthesis, and all processes of oxidation and reduction, of hydration and dehydration in the chemical sense, of polymerization and hydrolysis, etc. On the other hand, all proc- esses that result merely in a change of state of the matter within the plant-body are physical. These latter do not usually receive so much attention at the hands of physiologists as do the others, and they are probably not as well known, but they are certainly no less important. As examples of such physical changes may be men- tioned such processes as coagulation or precipitation of substances out of solution or suspension, the various possible alterations in the viscosity of liquids, and even the transformations that may occur between the solid, liquid, and gaseous states of matter. It is fre- quently true of physiological phenomena that the chemical and physical processes are so closely related that it is impossible to relegate a material change to either category alone. In this connection it may be recalled how modern researches along the border-line between physics and chemistry are tending more and more to erase this line and to prove it to be quite an arbitrary demarcation. All of the innumerable processes, physical and chemical, that occur in the living plant must be thought of as having their temperature limits, just as has the grand summation of these processes. In so far as physiological studies have gone in this connection, it appears that each component process possesses temperature limits more or less different from those of others, and also different from those of the grand summation. Thus, with falling temperature growth in size is checked when a certain minimum temperature is reached, at lower minima, cell-division and photosynthesis are also checked, and at a still lower minimum respiration ceases and death ensues. It follows from this that general plant activity can not proceed at any tempera- a == ee CHIEF ENVIRONMENTAL CONDITIONS. 127 ture that might prevent the occurrence of any of the simpler physical and chemical changes essential tothe make-up of this general activity. According to the kinetic theory of matter an alteration in the tem- perature of any body is to be considered as a change in the rate of motion of its component particles and, consequently, as a gain or loss of kinetic energy by the latter. The degree to which this energy of motion is possessed by the molecules, etc., of a mixture—that is, its temperature—is to be regarded as the proximate condition determin- ing the nature of the transformations that occur therein. Therefore every temperature change must be regarded as affecting a more or less marked alteration in the velocity of each of the many physical and chemical processes that make up the life activity of a plant. It is possible, therefore, to proceed a step farther than we have gone in our generalization above. Not only is it true that the prime essential temperature condition for general vitality is that a tempera- ture must obtain under which all the necessary component processes may occur, but the temperature must be such that these component processes may go on with adequate velocities. They must not proceed with too high nor with too low rates; otherwise death must occur. Thus photosynthesis, for example, might occur in a plant at a certain temperature, but the rapidity of the formation of carbohydrates might at the same time not be great enough to make up for the loss entailed by respiration, growth, etc., at that temperature. As has been mentioned in another place, however, it is quite possible for an organism to survive a brief period of exposure to a condition to which it would succumb with a more prolonged exposure. It is clear, then, that the temperature limits usually given for plants are not definite and quantitative measures of the limiting conditions for vital activity unless they are taken in connection With the length of time during which the organism is subjected to these temperatures. The question of the duration of temperature conditions in connection with the establishment of physiological limits has received attention from Blackman,’ from Miss Matthaei,? and from Lehenbauer,’ and is worthy of still further study. From the foregoing paragraph we may formulate the following statement of a general and fundamental principle regarding the rela- tion between temperature and vital activity. The temperature of the living plant-body must not remain for more than a maximum time period at any temperature which, if longer continued, would cause any essential physical or chemical process of the general life activity to surpass the minimum or maximum limit of its velocity. 1 Blackman, (1905)—Idem, The metabolism of the plant considered as a catalytic reaction, Science, n.s., 28: 628-636, 1908. 2 Matthaei, Gabrielle L. C., Experimental researches on vegetable assimilation and respiration. III. On the effect of temperature on carbon dioxide assimilation Phil. Trans. Roy. Soc. London, B. 197: 47-105, 1904. 3’Lehenbauer, P. A., Growth of maize seedlings in relation to temperature, Physiol. Res., 1: 247-288, 1914. 128 ENVIRONMENTAL CONDITIONS. The temperature limits for vigorous activity, as these are usually given, are very different for different plants. The lowest minima are somewhat below 0° C., while the highest maxima are above 60° C. For the retention of life in dormant phases the range is, of course, much greater than for vigorous activity. Dry seeds can endure, for long periods, temperatures far below the freezing-point of water and far above the boiling-point. It needs to be emphasized that the water- content of a tissue is highly important in determining what may be its mininum and maximum temperature and the duration of such temperatures that it may survive. 2. RELATION OF TEMPERATURE WITHIN THE PLANT TO CONDITIONS OF THE ENVIRONMENT. Material changes, whether physical or chemical, generally result in the warming or cooling of the medium; some heat is generally produced or else disappears with each material alteration. Wher- ever material processes are going forward in the plant, heat must be continuously supplied, or else it must be as continuously removed; otherwise the temperature of the body must fall or rise and a cor- responding alteration in the processes themselves must ensue. In the case of a living plant the many physical and chemical processes of its life must, of course, influence one another, in velocity and direc- tion in various ways, among which the mutual effect of heat absorp- tion and liberation must be important. Thus the heat set free by respiration may be of primary importance in maintaining a possible temperature for cell growth, and the absorption of heat by transpira- tion is probably often the prime condition that prevents a too great rise in tissue temperature. If there were no outward or inward passage of heat through the periphery of the plant, it is obvious that life could be possible only so long as this complicated interplay of the heat effects of the various physiological processes were automatically so limited that no essential process might be too greatly altered by temperature change. But, just as the moisture conditions of the plant-body are usually much more influenced by water changes between it and its surroundings than by the generally insignificant destruction and formation of water within the tissues, so also the internal temperature conditions usually depend mainly upon heat exchanges with the exterior, and the internal absorption and liberation of heat which has just been considered are of prime importance only in relatively few cases, if they ever are at all in nature. The temperature of the plant tends closely to follow that of its environment; roots can seldom possess a temperature markedly dif- ferent from that of the surrounding soil, and stems and leaves are never very much warmer or cooler than the air that bathes them. If the vital processes result, at any time, in the liberation of heat, a : : | CHIEF ENVIRONMENTAL CONDITIONS. 129 marked rise in temperature is prevented by outward conduction and radiation, and the body-temperature remains automatically at about the same point as that of the material surroundings. On the other hand, if the summed result of physiological changes be a disappearance of heat within the cells, then any considerable fall in tissue-temperature is automatically adjusted at an incipient stage by intake of heat from without. The only exceptions to this rule that are worthy of mention here are the cooling effect of transpiration, whereby the temperature of foliage is sometimes as much as a few centigrade degrees below that of the air, and the heating effect of sunshine, whereby leaves are some- times a few degrees warmer than their surroundings. From thermodynamics we may be sure that the outward and inward conduction of heat are automatically self-controlling, heat not being conducted from a cooler to a warmer body. Thus an atmosphere at a given temperature will never, by heat conduction, render a plant either warmer or cooler than the air itself. But it is possible for a plant, or other body, to receive heat by radiation from an imme- diate environment the temperature of which is much lower than its own. Conversely, for a time at least, it may radiate heat into an immediate environment having a higher temperature. The first case is very important in many instances, as in the absorption of sunlight by green leaves, which will be considered under the topic “Light.” The second is of much less frequent occurrence, but may sometimes be important in the cooling of leaves on clear nights when radiation is rapid. As in the case of the entrance and exit of water, the nature and condition of the plant surfaces exert a considerable influence upon the possible rate of entrance or exit of heat, either by radiation or conduc- tion. The effect of a more or less thorough insulation of the plant- body would, of course, be the introduction of a correspondingly pro- nounced lag in the temperature changes of the plant, as far as these are due to radiation or to conduction to or from the exterior. Thus, after a temperature change in the surroundings, some time may elapse before uniform temperatures within and without again prevail. In the case of roots this feature is probably of but little importance, although the results of secondary growth often produce on old organs of this sort a layer of cork and other modified cells of sufficient thick- ness, so that a considerable retardation of heat conduction no doubt ensues. Heat radiation appears to be of little or no importance in subterranean organs. The aerial portions of the plant exhibit more considerable effects of the retardation of heat transmission. This is especially notable in many buds, in the stems of the larger plants, such as trees, and in densely hairy leaves. The bark of the cork oak is familiar to every- 130 ENVIRONMENTAL CONDITIONS. one as a substance with a low power of heat conduction, and the hairy buds and younger leaves of such forms as mullein (Verbascwm thapsus) are known to assume the temperature of their surroundings, after a rapid temperature change in the latter, with a distinctly sensible lag. One of the main internal conditions affecting the rate of outward and inward heat conduction is that of moisture-content, which, as we have seen, is also of great importance in the determination of the rates of intake and outgo of water. The rate of absorption or elimination of radiant heat is affected also by the nature of the plant periphery; rough surfaces radiate and absorb a greater proportion of energy rays than do smooth ones; plaited and folded surfaces radiate and absorb much less than plain ones of equal area; the color of the tissue is highly important in this connection, and the exposure of the surfaces considered, with reference to the earth’s surface and to the sky, is also of primary importance. Most of the heat radiation and absorption by plants occurs in the direction toward and from the sky, and by far the greater portion of the radiant energy absorbed is from direct sunshine. This latter feature has been emphasized in our discussion of transpiration and will be touched upon again in connection with the treatment of light. Of course, it is to be remembered that the effect of heat radiation in producing a body-temperature higher or lower than that of the sur- roundings is soon limited by the increased rates of both radiation and conduction in the opposite direction, so that very great differences between outside and inside temperature are not to be expected. The heating effect of direct sunshine upon green, transpiring leaves is limited, not only by outward conduction and radiation, but also by the cooling effect of transpiration. Even in intense sunshine the temperature of turgid, rapidly transpiring leaves is frequently or usually below that of the surrounding air.’ The main generalization in connection with the temperature rela- tion is simply that the temperature of the plant is never very different from that of its immediate surroundings. The important effect of the different rates of heat exchange between plant and environment is practically confined to the determination of the temperatures of leaves and other similarly exposed parts when under the direct rays of the sun, and to the production of a more or less pronounced lag in the tissue temperature changes brought about by great and rapid alteration in the environmental temperature. 1 Shreve, 1914. CHIEF ENVIRONMENTAL CONDITIONS. 131 3. THE DURATION ASPECT OF THE TEMPERATURE RELATION. The velocities of the many physical and chemical changes that compose vital activity as a whole, depend, as has been seen, upon the temperature. It follows that the outcome of each separate process, and that of the entire complex, is largely determined by the degree of temperature and by the length of the time period during which any temperature has obtained. The accomplishment of the entire plant, as measured by the amount of its growth, for example, might be the summation of the various accomplishments of the component processes during the given time period, which latter are simply the integrations of the various process velocities with respect to time. Since we are very far from being able satisfactorily to separate the component processes that go to make up the activity of any plant, it is necessary here only to call attention to the above proposition as representing a general principle the details of which must occupy many minds in the future. For the present the physiological ecologist can do nothing but consider the varying velocities of certain broad, complex processes such as growth, crop production, and the like. But our generaliza- tion is of use at least in this, that it enables us to lay out the field for future study, with a probability of satisfactory results that might otherwise be absent. We have seen that physical and chemical processes are intimately commingled in the intricate complex which we call life. To this con- ception may be appended the additional one that many of the physical properties, probably all of them in the final reckoning, depend upon chemical changes which have previously occurred. It is quite im- possible for a physical change to occur in the substance of a plant unless the various materials involved be present, and these may be assumed, in general, to have resulted from chemical changes. Since we know already that chemical action is exceedingly important in most plant phenomena, the last statement allows us tentatively to state that this action may probably be found to have the controlling in- fluence in general life activities! If this be true, the general activi- ties of the plant should follow more or less accurately the principles of chemical action. The relation of chemical processes to temperature have been much worked upon and the law of Van’t Hoff and Arrhenius has been developed in this connection. It states that for each rise in temperature over a range amounting to 10 centigrade degrees, there is a doubling or tripling of the reaction velocity. Often the coefficient is 2 or a little more or less (PH; =P+Hs, 1.2; C;H30, . C.H;-+-NaOH, 1.89)?. Usually it is between 2 and 3 (C,H,ONH,Ag, 2.12; C,H;C10». 1The whole matter here brought forward has been given somewhat more thorough considera- tion than is needed here, in the following paper: Livingston, B. E., and G. J. Livingston, Tem- perature Coefficients in Plant Geography and Climatology, Bot. Gaz., 56: 349-375, 1913. 2Van’t Hoff, J. H., Lectures on theoretical and physical chemistry, translated by R. A. Lehfeldt. London, no date (author’s preface date 1898), part 1, p. 228. 132 ENVIRONMENTAL CONDITIONS. Ag, 2.55), sometimes it is over 3 (NaOC,H;+CHsI, 3.34)”, and it may be still higher. Since the temperature of the plant follows so closely that of the surroundings, it will be safe to consider these two temperatures as identical for our present purpose. Of course, we shall expect to find that the principle of Van’t Hoff and Arrhenius may be applied to plant phenomena only between certain limits of temperature. It is perfectly clear that the generalization can hold only so long as all or nearly all of the component or partial processes are progressing according to this principle. In purely chemical processes there are always a minimum and a maximum, beyond which this principle of Van’t Hoff and Arrhenius no longer expresses the relation of temperature to velocity. A somewhat extensive literature already exists regarding the applica- tion of this principle to physiological phenomena. We may mention the main results with plants. Clausen! determined the velocity of carbon-dioxide excretion from seedlings and buds at different tempera- tures. He found that the rate somewhat more than doubled for each temperature rise of 10° C., to an upper limit about 40° C. Miss Matthaei? studied the effect of temperature on the evolution of the same gas from leaves in darkness, and also on its fixation by leaves in light, showing that the Van’t Hoff-Arrhenius principle holds here in a very satisfactory manner. Blackman’ has presented a very good state- ment of this entire problem, especially in regard to plants, and his con- cluding sentences are worthy of quotation here. He writes: “To me it seems impossible to avoid regarding the fundamental processes of anabolism, katabolism, and growth as slow chemical reactions catalytically accelerated by protoplasm and inevitably accelerated by temperature. This soon follows if we once admit that the atoms and molecules concerned possess the same essential properties during their brief sojourn in the living nexus as they do before and after.” On the whole, it seems allowable to conclude that the majority of the elementary chemical processes of living things proceed according to the general principle of Van’t Hoff and Arrhenius, and that such processes exhibit temperature coefficients, within the ordinary limits of environmental temperature of from 2.0 to 2.5. When, however, these elementary or component processes are combined into such a complex resultant as growth, it does not necessarily follow that the temperature coefficient of the complex process must be the same as that of its components. Russell‘ states that ‘the effect of temperature on the rate of growth of a plant is in no wise like its effect in accelerat- ing chemical change,” citing Bialoblocki® to support this view. aa ee a a ee 1 Clausen, H., Beitrie zur Kenntnis der Athmung der Gewiichse und des pflanzlichen Stoff- wechsels, Landw. Jahrb., 19: 893-930, 1890. 2 Matthaei, 1904. 3 Blackman, 1908. 4 Russell, E. J., Soil conditions and plant growth, London, 1917. 5 Bialoblocki, J., Ueber den Einfluss der Bodenwiirme auf die Entwicklung einiger Cultur- pflanzen, Landw. Versuchsstat., 13: 424-472, 1870. CHIEF ENVIRONMENTAL CONDITIONS. 133 The last-named author studied the influence of temperature upon the rate of growth of barley, and his results seem to show (see Russell’s graph, page 21) that the temperature coefficient here varies markedly with the temperature itself. Since later workers have failed to record the same conclusion, it seems that Russell may perhaps give to Bialoblocki’s results too conclusive a weight. It is clear that in some cases, at least, the operation of the law of the minimum! may interfere in such experimentation, thus precluding the full acceleration affect of a given rise in temperature. As Livingston and Livingston (1913) have pointed out, ‘it seems highly probable that complex vital processes such as growth may frequently fail, under natural condi- tions, to exhibit the chemical temperature coefficient. In some of these cases proper alterations in other environmental factors might disclose the otherwise masked coefficient, in other cases the limita- tions might be internal, as in the nature of the protoplasmic mixture, and the obscuring of the coefficient might persist in spite of any attempt at external adjustment.” In favor of the supposition that growth-rates of plants do show a temperature coefficient of 2.0 or above, may be mentioned the experi- mental studies of Price,” who determined the temperature coefficients for the opening of flower-buds on cut twigs of the plum, peach, apple, and other fruits. The time period required for resting buds to pro- duce flowers was shown to be reduced about one-half for each rise in temperature of 10° C. Lehenbauer’s® extensive study of the relation of growth rate, in shoots of maize seedlings, to maintained tempera- ture shows much more clearly than had ever been done before how important the duration factor is in determining the effect of tempera- ture on growth. As regards the temperature coefficient, he found that this has a value of from 2.40 to 1.88 for a range of temperature from 20° to 32° C., the seedlings being exposed to the given temperature for 12 hours. For temperatures below 20° the coefficient has higher values (for the decade from 12° to 22° its value is 6.56) and for tem- peratures above 32° the coefficient is much lower (for the decade from 33° to 43° it is 0.06). Our present knowledge of this whole matter leads to the idea, as Livingston and Livingston have stated, ‘‘that there are many cases in which growth-rates and other complex processes in plants and animals exhibit temperature coefficients of about 2.0, and that in other cases this same coefficient is probably operative but is obscured by the limiting effect of some other environmental condition.” These authors also point out that temperature coefficients of other orders of magnitude than that given may be expected, both for elementary and 1Blackman, F. F. (1908), Idem, (1905).—Mitscherlich, E. A., Das Gesetz des Minimums und das Gesetz des abnehmenden Bodenertrages, Landw. Jahrb. 38: 537-552, 1909.—Idem, Ueber das Gesetz des Minimums und die sich aus diesem ergebenden Schlussfolgerungen, Landw. Versuchs- stat, 75: 231-263, 1911. 2 Price, H. L., The application of meteorological data in the study of physiological constants. Ann. Rept. Virginia Agric. Exp. Sta., 1909-10: 206-212, 1911. 3 Lehenbauer, 1914. La.A 134 ENVIRONMENTAL CONDITIONS. for complex life processes. The processes upon which growth imme- diately depends are wholly or mainly physical, as has been stated above, and these in turn depend upon chemical phenomena. Thus, the formation. of cell walls is surely a physical process (precipitation, coagulation, etc.), but it is conditioned by such chemical processes as the formation of cellulose from water-soluble carbohydrates. Under ‘such conditions it is reasonable to expect such physical processes to exhibit chemical temperature coefficients, this being, again, an in- stance of the operation of the law of the minimum. The true physical temperature coefficient of cell-wall formation may never be evidenced, since the rate of chemical formation of wall constituents at the periphery of the protoplasmic mass may never be sufficient to allow their solidification at the maximum rate for any given temperature. Nevertheless, some physical phenomena that have to do with vital processes show, independently of chemical phenomena, temperature coefficients that rather closely approach a value of 2.0. The authors last mentioned call attention to the fact that such is the case with the vapor-tension of water between 4° and 34° C. It appears that we have here a general principle that seems to hold with rather satisfactory approximation for a number of different physiological processes in different organisms, for a considerable range of temperatures such as is frequently met with in nature. This problem of the temperature coefficient for physiological processes is by far the most important temperature question now awaiting in- vestigation. Its solution for a large number of plant-forms and for a large number of developmental phases should do much for climatic plant geography and for agriculture.! We shall return to this matter in another place. Aside from the simple matter of amount and duration of the tem- perature of the environment, it is rather widely held that alterations in temperature, if these are of great magnitude and if they occur rapidly and frequently, are in themselves a potent cause for a change, ‘often a retardation, in the rate of plant growth. Frequent changes _ of temperature seem, per se, to act as a stimulus upon some plants and to bring about a different form of development from that which might occur under more stationary conditions of temperature. Nevertheless, Price (1911) has tested this last proposition in the case of the flower-buds of peach and plum and finds that ‘‘a sudden drop of temperature to some point below 50° F. results in the cessation of all development, but that normal development is resumed immedi- ately when favorable temperature conditions are restored, t.e., that the retardation of development by cold is altogether temporary and directly proportional to the time during which the low temperature prevails.” It is obvious that this matter is in need of a throughgoing investigation. 1Certain general aspects of the temperature relations of organisms are well brought out in the following: Fawcett, H. S., The temperature relations of growth in certain parasitic fungi. Univ. Calif. Pub. Agric. Sci. 4: 183-232, 1921. es CHIEF ENVIRONMENTAL CONDITIONS. 135 IV. LIGHT. 1. GENERAL NATURE OF LIGHT. The relations of water and temperature to plants, which have already been considered, involve variations only in intensity, there being no qualitative differences involved; the amount of moisture and the degree of temperature in the plant-body are all that need to be specified in order that these conditions be defined. Light, however, may vary not only in amount, that is, in intensity, but also in quality, and herein lies a most serious complication. Furthermore, making matters still more difficult, it is impossible, excepting on purely arbi- trary grounds, to distinguish the radiant energy that we term light from radiant heat on the one hand and from the ultra-violet rays on the other. It thus appears that the term “light” itself is nothing more than an arbitrary term, denoting a range of different sorts of radiant energy, the range being characterized by certain wave-lengths. This range is extended on either side, beyond the arbitrarily limited region, by still other wave-lengths which are not included under the term “light.” Light is usually understood to mean radiant energy that is capable of affecting the human eye, having a range, then, of wave-lengths from about 400 to about 750 millionths of a millimeter. Thesun’s spectrum, however, extends to wave-lengths of about 293 uu, where the opacity of the earth’s atmosphere to these ultra-violet rays brings its range to a rather abrupt limit.!. Radiant energy with wave-lengths greater than about 750 uu, and extending beyond 2,400 uy (Nutting, 1912, page 202) are termed heat. The study of the characteristics of radiant energy has been facilitated in certain aspects, and perhaps retarded in others, by the fact that all these various radiations may be mostly transformed, on being allowed to fall upon a blackened surface, into the molecular vibrations of matter. It has thus come about that the intensity of all these forms of radiant energy is usually measured by converting them into molecular heat and by determining the temperature acquired by the heated body. It is thus that the intensity of light, or other radiant form of energy transfer, is commonly measured and described in terms of calories received per square centimeter of the cross-section of the impinging beam per unit of time. The quality of light is defined by its range of wave-length and by the relative intensities for the different portions of the range. The range of wave-length may be determined through the use of a properly constructed spectroscope. The plant never receives light of just a single wave-length; it always receives a mixture of wave-lengths with a more or less broad range. Since light is most easily perceived by us 1 Nutting, P. G., Outlines of applied optics, Philadelphia, 1912, p. 2. 136 ENVIRONMENTAL CONDITIONS. through its physiological effect upon our eyes, the range of any given light mixture is usually thought of in terms of the spectral colors, which are simply names of certain physiological responses of the human organism to light of various wave-lengths and intensities. Thus, we may state that a certain light mixture ranges from red to green, for example, is particularly intense in the green, etc. It is highly desirable, however, that biological measurements of light quality be made in terms of wave-lengths, for such definition does not depend upon the eye.' The only practical way to describe light conditions that is so far available is arbitrarily to divide the spectral range of wave-lengths into smaller ranges, and to state the intensities of these smaller ranges in terms of their respective equivalent heat intensities. Certain of the well-known Fraunhofer lines of the sun’s spectrum may con- veniently be used in this arbitrary subdivision. 2. EFFECT OF LIGHT UPON PLANTS. We have already seen that radiant heat is effective upon plants in controlling their temperature. Light also has the same effect, in so far as it is absorbed by the plant-body and converted into molecular vibrations of thermal nature. Where this effect is alone to be con- sidered it is not directly necessary to analyze the impinging waves into their spectral groups or component ranges of wave-lengths; it is only requisite to determine the total heating effect produced by the ab- sorbed portion of the total impinging radiation. Since, however, any given surface, as of a plant, absorbs the different wave-lengths in different amounts, the study of light qualities may become essential even in this connection. The heating effect of light upon plants is only incidental in our present discussion; absorbed radiation is seldom if ever of primary importance in determining the temperature of plant parts. A second kind of effect produced by light upon plants is a morpho- genic one and does not seem to depend upon the heating of the tissues. This is, in all probability, a photochemical effect, but the very diffi- cult question thus raised still awaits investigation. Here we need to consider, perhaps, the retardation of growth in length, apparently due to the action of light upon most cylindrical plant-parts and the corresponding acceleration of enlargement in most dorsiventral organs. It is usually supposed that ordinarily plants would not assume their usual form without this action of light; the characteristic, much elongated stems and greatly dwarfed leaves of etiolated plants are an example of the effect of lack of light, though the moisture relation 1 Watson and Yerkes’s valuable monograph on light measurement for biological purposes should be referred to in this connection: Watson, J. B., and R. M. Yerkes, Methods of studying vision in animals, Behavior Monographs, Serial No. 2, 1910. See also: Pulling, H. E., Sunlight and its measurement, Plant World 22: 151-171, 187-209, 1919. CHIEF ENVIRONMENTAL CONDITIONS. 137 surely plays an important réle here. Likewise, the development of certain tissues, as of leaf-palisade, are apparently often dependent upon the quality, intensity, and direction of luminous rays, and the relative positions assumed by most ordinary leaves are largely due to the asymmetrical effects of light as such. The “leaf mosaics” of plant ecology are considered as due to the operation of this condition. Also, the positions assumed by many stems and other parts are primarily due to light conditions. Such morphogenic activities are directed, not by light in general, but only by radiant energy of certain ranges of wave-length. Certain intensities within these ranges are necessary for the usual development ‘of ordinary plants. A third effect of light, and the one that is most fundamentally im- portant for all terrestrial life, is the photochemical process called photosynthesis. It is only by the action of a certain range of wave- lengths of radiant energy, within certain limits of intensity, that the production of carbohydrates from water and carbon dioxid may occur in chlorophyll-bearing cells. Fundamental as is the photosynthetic process, the conditions determining its velocity have hardly begun to be studied quantitatively, and this statement is especially true with regard to the light relation. In order to begin a study of the relation of plants to light it will be necessary first to possess some suitable method by which the light conditions may be measured. It is essen- tial that both quality and intensity of the impinging light be de- termined for the different hours of the day and for the different days of the growing-season. The present apparent difficulty of obtaining such measurements of the environmental conditions is surpassed only by its fundamental importance to plant physiology and by its practical bearing upon the problems of ecology and agriculture. 3. DURATION ASPECT OF LIGHT RELATION OF ORDINARY PLANTS. It is obvious that the result, the amount of material change, pro- duced by any of the physiological processes that are dependent upon light must be determined by the duration of the process as well as by the nature of the light conditions determining its velocity. It seems highly probable, also, that mere fluctuation of the light condi- tions, as between daylight and darkness, may have a more or less definite effect upon the development of plants. We are certain that many physiological rhythms depend primarily upon this sort of fluctua- tions. As we have seen, however, the feature of duration is usually the last one to be carefully considered in the case of any environmental condition, and we need not be surprised to note that very little indeed has been accomplished in this direction regarding light influence upon plants. Until we are able to measure and control light conditions it must be quite hopeless to attempt any but the most superficial con- sideration of this aspect of the general problem of plant control. 138 ENVIRONMENTAL CONDITIONS. V. CHEMICAL CONDITIONS. 1, REQUIREMENT OF MATERIAL WITHIN THE PLANT. At the beginning of this treatment of the main categories of environ- mental conditions we discussed the water-relation of plants. Water was given a place by itself in our series because its importance in the organism, as has been seen, appears to depend more upon its solvent powers and power to be imbibed in the plant colloids than upon its chemical influence. But it has been pointed out that water also acts chemically in the plant, being one of the two substances chemically transformed in photosynthesis and likewise one of the two products of the process of respiration. It is probably chemically important in other ways, certainly playing an essential part in many processes of hydration polymerization, hydrolysis, etc. Of course there are innumerable other substances, besides water, that take part chemically in plant activities. We are apt to think first of the three great groups of compounds that have been called foods—the carbohydrates, fats, and proteins. These are apparently all essential to vital activity and even to the mere retention of life in the most dormant phases. Besides these there are a large number of substances of a more or less complex nature that are sometimes con- sidered as foods and sometimes not. Here niay be mentioned gluco- sides, alkaloids, various lipoids like the cholesterins, phytostearins, etc. Also, substances of importance in certain of the simpler com- ponent processes of vital activity, like chlorophyll; the various enzymes—still of questionable nature—etc., may be classified here. Finally, in order that life may occur, there must be in the tissues a number of inorganic salts and their ionized products. These are not classified as foods by physiologists, although conservative agri- culturists are still prone to speak of them as “‘plant-foods.”’ They might better be termed auxiliary substances until some such time as the word ‘‘foods” may be dropped from physiology. The condi- tions in different protoplasms, in different plants, in different de- velopmental phases of the same plant and different parts of the same individual, are, however, so extremely varied that there seems little ultimate value in attempting to classify the various materials that are essential to plant activity. It is certainly far simpler, and prob- ably as satisfactory in every way, to consider merely the material conditions of life, classifying the different substances on purely chemical grounds. We need here merely emphasize the well-known point that one of the prime conditions for organic life is the presence in the organism of innumerable kinds of chemical compounds. Since all vital activity must be regarded as material change of some sort, it is clear that the quantitative and qualitative relations between these many substances must be continually changing; the substances CHIEF ENVIRONMENTAL CONDITIONS. 139 of the cell are always tending toward chemical and physical equilib- rium. Such equilibrium is, of course, never really attained, even in the case of the dormant phases of plants, like seeds and spores. Thus, as long as life exists there is always in progress a more or less pro- nounced interchange of materials between the organism and its surroundings. Oxygen, for example, disappears in the process of normal respiration and carbon dioxid is produced. If no material exchange were possible, if the system of the plant were not continuous with that of the universe about it, then this process must shortly come to a standstill; an equilibrium between the internal diffusion tensions of oxygen and carbon dioxid must be reached, and no further oxidation might occur. Under existing conditions, however, a fall in the dif- fusion tension of oxygen within the plant-body immediately creates a diffusion gradient between the interior and exterior, and the gas finds its way in from the outside. Conversely, the mere occurrence of the process of respiration sets up an outward diffusion of carbon dioxid. It appears probable that, other conditions remaining constant, every substance might prove to have its maximum concentration, or its maximum and minimum, below or between which life is possible. We must expect, however, that the concentrations of other substances, as well as light and temperature conditions, will be found to alter these limits for any given substance. It hardly needs to be mentioned here that variations in the concentrations of non-aqueous materials accompany alterations in water-content. 2. MATERIAL EXCHANGES BETWEEN THE PLANT AND ITS SURROUNDINGS. From the preceding paragraph it is to be inferred that the importance of the chemical environment, in determining the nature of plant growth, etc., and in limiting the kinds of plants that can exist in any given habitat, is definitely dependent upon the generalization that diffusion tends always to bring the plant-body and the surrounding media into concentration equilibrium. Two groups of conditions militate more or less against the attainment of this equilibrium: (1) The degree of permeability of the plant periphery to the diffusing materials, and (2) the rate of their transformation within the plant or of their removal from or supply to the immediate environment. _ Thus, if the air about a plant should contain ether-vapor, for example, the diffusion gradient would insure an inward diffusion of the ether until the vapor-pressure of the ether solution within the plant-body just equaled its partial pressure in the surroundings. If the plant epidermis were readily permeable to the poison it is clear that death must soon ensue, and a lower permeability could only postpone, but could not prevent, this result. It is probable that no substance exists to which the plant periphery is absolutely impermeable, though there are many that penetrate only very slowly. 140 ENVIRONMENTAL CONDITIONS. If the inwardly diffusing substance be altered chemically upon reach- ing the interior of the plant, and if this process of alteration be capable of removing it as rapidly as it enters, it is clear that equilibrium be- tween interior and exterior, or even any considerable solution con- centration within the plant, can not be reached. Similarly, if some poison be produced within the tissues, as organic acids in the case of certain roots growing under low oxygen pressure,’ and if the epidermal tissues be adequately permeable to this substance, then the concen- tration that is obtained within the cells must be determined by the possible rate of removal of the poison from the immediate surround- ings. A substance diffusing from roots may diffuse away through soil- moisture films, it may be absorbed by the solid-liquid surfaces of the soil, or it may be oxidized or otherwise transformed; but it is ob- viously essential that the poison be removed from the soil solution in the immediate neighborhood of the excreting roots; otherwise the rate of outward passage must be lowered, and the consequent rise of the internal concentration of this particular substance (supposing the process of its formation to continue at the original rate) might soon bring about a general upsetting of all the physiological processes so that death might finally ensue. If plant activity depends upon the absolute and relative concentra- tions of various substances within the body, then it is clear that variations in the concentrations of these substances in the environ- ment must be accompanied by more or less profound alterations in the physiological processes. We thus arrive at the well-known proposition that the concentration or diffusion tension of the various substances in the environment is of prime importance in determining how any plant may develop and, indeed, whether it may exist at all in a given habitat. It is immaterial whether the environmental con- centration of a substance at any time be the result of causes acting wholly without the plant or of internal processes; the end-result must be the same in either case. Thus, the lactic-acid organism of souring milk is checked or killed by the accumulation of its own excretions just as truly as though the acid content of the medium had arisen solely from external causes. It thus emerges that the chemical relation, unlike those of water and temperature but like that of light, must always be considered not only with reference to intensity but also in regard to quality. Just as there are many different wave-lengths of light that influence the plant differently, so there are innumerable chemical compounds, all differing qualitatively in their effect upon plants. Moreover, the effect of each one of these compounds varies not only with its own POA PR SE SS SEA, PR ee ee ee 1Stoklasa, J., and A. Ernest, Die chemische Charakter der Wurzelausscheidung verschie- denartiger Kulturpflaazen, Jahrb. wiss. Bot., 46: 52-102, 1908. CHIEF ENVIRONMENTAL CONDITIONS. 141 concentration (intensity), but also with that of many others. The general problem of the chemical relation of plants is, therefore, an exceedingly complex one, so complex, indeed, that the problem of the water or temperature relation becomes, by comparison, a very simple matter. Upon the chemical relation of plants depends, in large measure, our agricultural practice, and it is instructive to bear in mind the a priori considerations of the above paragraphs when perusing the current writings upon such questions as that regarding the use of fertilizers, for example. It is to be hoped that the succeeding developments of plant ecology and of agricultural theory may be characterized by greater catholicity of perception than has prevailed in the past, and that the forthcoming literature may be burdened with less of that familiar type of argument by which a single one out of many inter- related conditions is enthusiastically proclaimed as the real and only cause of some particular physiological phenomenon. There is much promise for the future in the study of chemical rela- tions, however. The qualities of the chemical environment of plants can already be quite readily determined; the identification of chemical compounds is no longer a general source of serious difficulty, and we are beginning to see some light in the darkness of our prolonged endeavors to determine the intensities (concentrations, diffusion ten- sions) of the various substances with which we have to deal. The interdependence of the influences exerted by the various chemical compounds occurring in the environment has recently attracted much attention, and this bids fair to be an important way by which agri- cultural theory may at length become physiological. Salt antagon- isms—the influence of the presence of a certain concentration of one salt upon the effect produced upon the plant by a certain concentra- tion of another—were first brought into prominence by Loew’, and are attracting much attention at the present time.” 3. CHEMICAL ENVIRONMENT IN NATURE. In a discussion of environmental conditions, Livingston’ has sug- gested that perhaps the simplest and most obvious classification of these promises most at the present time, and he divides these condi- tions as’a whole into those that are effective above the soil surface and 1 Loew, O., Die Bedeutung der Kalk-Magnesiazalze in der Landwirtzchaft, Landw. Versuchs- stat, 41: 467-475, 1892.—Loew, O., and D. W. May, The relation of lime and magnesia to plant growth, U.S. Dept. Agric., Bur. Plant Ind. Bull. 1, 1901. 2 Osterhout, W. J. V., On the importance of physiologically balanced solutions for plants,(T, fresh water and terrestrial plants, Bot. Gaz. 44: 259-292, 1907.—Tottingham, W. E., A quantitative chemical and physiological study of nutrient solutions for plant cultures, Physiol. Res. 1: 133-245, 1914 (this paper contains many literature references).—Shive, 1905b. 3 Livingston, B. E., Present problems of physiological plant ecology., Am. Nat., 43: 369- 378, 1909. The same paper, with some omissions and modifications, appeared under the same title in Plant World 12: 41-46, 1909. 142 ENVIRONMENTAL CONDITIONS. those that are effective below. All of our categories of external condi- tions are effective both above and below the soil surface, excepting light alone (as far as we now know), but it seems especially profitable to consider this classification with reference to chemical conditions. The chemical conditions above the soil surface are characterized by a striking and almost complete uniformity and symmetry. The air of different regions and of different habitats comprises practically always the same gases, and these, with the exception of water-vapor, occur with but little variation in their partial pressures. Of the sub- stances influencing plants, other than water, carbon dioxid exhibits the greatest variation, but even this variation appears to be, compara- tively speaking, of but little account. We may safely conclude that few plants in nature are ever appreciably influenced by variations or differences in the quality or intensity of the chemical environment above the soil surface.t Small supplies of ammonia and inorganic salts may reach the plant from its aerial environment, but with these generally insignificant phenomena of absorption we need not deal here. When we turn our attention to the soil, we find a very different state of affairs. Every soil differs chemically from every other soil, the soil solution varying between wide limits in the nature and amount of solutes present.2 At one extreme of the series are thoroughly washed sands, in which are almost no dissolved material; at the other extreme are alkali soils, which are highly impregnated with soluble inorganic salts. In the middle region between these extremes, in most ordinary soils, it appears that the quality and concentration of the soil solution are without very great differences as far as inorganic compounds are concerned, but that these ordinary soils show, very great differences in the kinds and amounts of organic matter present.® In spite of the great amount of work that has been devoted to the problems of the soil, the whole question remains as one that has hardly been really touched in a way to be of any present aid in problems of plant distribution. Of course, our general knowledge of the paucity of soluble matter in a few sands and of the superabundance of certain compounds in alkali soils is of definite value in this regard; but even here the strictly quantitative aspect of our problems remains wholly for the future to develop. 1Tf atmospheric ionization should prove an important chemical feature influencing plants in nature, and if this varies from place to place and from season to season, then this statement may require modification in this regard. See report of Spoehr’s work in MacDougal, D. T., Annual Report of the Director of the Department of Botanical Research, Carnegie Inst. Wash. Year Book No. 13, 87-88, 1915. ? Cameron, F. K., The soil solution, the nutrient medium for plant growth, Easton, Pennsyl- vania, 1911. 3 Livingston, Britton, and Reid, 1905.—Livingston, 19076, Schreiner, O., Organic compounds and fertilizer action, U. S. Dept. Agric., Bur. Soils Bull. 77, 1911.—Schreiner, O., and E. C. Lath- rop, Dihydroxystearic acid in good and poor soils, Jour. Amer. Chem. Soc., 33: 1412-1417, 1911. Also numerous other papers from the Bureau of Soils, U. S. Department of Agriculture, deal with this matter. CHIEF ENVIRONMENTAL CONDITIONS. 143 4, DURATION ASPECT OF CHEMICAL CONDITIONS. Since the chemical environment of the plant is effective through con- trolling the internal chemical conditions, and since such control is manifested by outward and inward diffusion of material, it follows that any given change in environment may produce the response of changed activity in the plant only after the lapse of an adequate time period. Diffusion of material through water requires considerable time in every instance. Also, the physiological processes of the plant can produce material transformations only in proportion to the length of time during which they are operative at their different velocities, these velocities being in part controlled by internal chemical conditions. The study of this feature of the chemical relation has just begun, and it surely demands much attention. What may be the effect upon the final result of a plant’s activity, of frequent fluctuations in the chemical nature and intensity of the surroundings, we are unable as yet to surmise. VI. MECHANICAL CONDITIONS. 1. GENERAL CONSIDERATIONS. All environmental conditions that are effective to influence plant activity through pressure of material en masse are to be classified as mechanical. From this point of view it matters not how the pres- sure may have originated or whether actual molar motion be produced. We should thus consider the flattened root which is confined within a rock-cleft, the one-sided development of trees growing in a wind- swept mountain pass where the direction of air-movement is predomi- nantly the same, the deformed branches, etc., produced by a heavy fall of snow, and the fantastic forms often exhibited by shrubs such as the hawthorns when continually browsed by animals, as all due to mechanical conditions. A mass pressure applied from without may merely hinder expansion of tissues, it may tend to compress certain parts or organs, or it may actually bring about a tearing or cutting of the tissues. One special form of mechanical pressure, which is of basic im- portance in plant growth, is definitely due to external conditions, but is first developed within the plant-body. We refer to the asym- metrical pressures produced in tissues and cells by the action of gravi- tation. Here an influence, still practically unknown excepting in its most general aspects, not a simple pressure of body upon body nor a diffusion of material, nor yet any form of energy transfer that is apparently at all immediately related to light, heat, and electricity, reaches from the external world through the periphery of the plant and largely controls certain forms of cell activity. In this we may be fairly certain that the material condition within the organism, 144 ENVIRONMENTAL CONDITIONS. which is proximately or immediately responsible for the peculiar influence of gravitation, is pressure asymmetrically developed. That this asymmetrical internal pressure that results when the position of a plant is altered with references to the earth’s center of inertia first produces a molar movement of certain portions of the cell-contents, and that this pressure and movement, with the new configuration of protoplasmic particles when gravitational equilibrium is again estab- lished, is the cause of the altered cell activities known to be produced by such change in the position of a plant, is the logical supposition which has been developed into what of theory we as yet possess in this general connection. Since the variations in the intensity of the gravi- tational influence that occur over the surface of the earth are quite negligible when considered with reference to the effect of this factor on plant development, it is obvious that gravitation does not require a thorough consideration at the hands of the ecologist or agriculturist. Here is one external factor, at least, which is practically identical in all habitats, as far as plant control is concerned. 2. DESTRUCTIVE INFLUENCES OF MECHANICAL CONDITIONS. It seems probable that the pressures developed when roots grow against or between rocks and other objects that they can not pene- trate may sometimes be a considerable factor in determining the success or failure of individual plants; but it is not at all likely that this consideration is important in the distribution of plants among different habitats. The only forms of mechanical influence from without that appear to be generally important to plant distribution and to agriculture are those due to (1) wind, (2) water, and (3) animals. In relatively few cases soil-movements, such as landslides, the caving of bluffs, ete., need to be brought directly into account in explaining the vegetation of habitats of limited extent. Ice-movements, as at the lower ends of some glaciers and at the margins of streams and lakes, are often the source of plant destruction in such places. With the mechanically destructive action of animals may be mentioned a somewhat similar action of other plants or plant parts, but here rela- tions other than mechanical are also frequently to be considered. The action of wind in differentiating the vegetation of different plant habitats has often been dwelt upon in ecological literature. It must be remembered, however, that this action is at least twofold; air-move- ment not only exerts a deforming or breaking pressure upon the plant- body, but it also profoundly affects the water-relation through increas- ing the evaporating power of the air, as this is effective both upon plant and soil. In connection with wind influence may be mentioned the cutting action of blown sand, really a factor that should be con- sidered with that of soil-movement (which includes the influence of rolling stones), and with that of ice, as above mentioned. CHIEF ENVIRONMENTAL CONDITIONS. 145 The direct mechanical effect of flowing water is familiar to everyone, and it must be accounted of great importance in determining the nature of the vegetation of many stream margins, as well as of streams themselves, of intermittently flooded stream-channels in the arid regions, and of sea and lake beaches. Other factors undoubtedly are coeffective in such cases, however. The influence of animals upon vegetation has not been much em- phasized in plant ecology, but it is undoubtedly of considerable impor- tance in many cases, as when seeds are thus mechanically destroyed in such large numbers that the establishment or spread of a species is rendered practically impossible. This factor in distribution is usually operative only on certain developmental phases of the plant; often the seedling stage is preeminently in danger of destruction by animals. In agriculture and horticulture—practical ecology under more or less artificial conditions—the influence of animals is of prime importance and has perforce received much attention. Fences, traps, scarecrows, insecticides, and even trespass warnings are material evidences of the importance ascribed to the direct mechanical influence of animals upon the plant population. For the most part, methods have been readily devised for more or less thoroughly removing this source of danger to cultivated plants. Among the different groups of animals, insects have probably been the least easily combated, and much attention is still being devoted to this important destructive factor. It will be an interesting and important chapter of agricultural ecology when the geographical distribution of various forms of animals is correlated with that of the regions where the various crops may be successfully grown. Livingston’ has mentioned the apparent importance of animals in de- termining the very existence of irrigated seedlings in the dry season at Tucson, and we have often made observations in that same region that suggest a rather important relation between certain plant-forms and animal activity. It seems probable that the destructive action of animals is relatively more important in arid regions than in most others. 3. FAVORABLE INFLUENCES OF MECHANICAL CONDITIONS. Wind, water, animals, etc., as is well known, frequently accelerate the spread of plants throughout large areas. In the majority of these cases it is a dormant phase, as of seeds, that is moved about. It is also true, however, that fragments of the plant-body other than seeds may be torn or broken away, being removed to another locality and there continuing development. Such is often the case with willow twigs, which float downstream and find lodgment and conditions for growth in a muddy bank. The movement of cactus branches in the arid regions of the American Southwest has been mentioned above. 1 Livingston (1906b), page 58. 146 ENVIRONMENTAL CONDITIONS. Books on ecology may be consulted for many instances of mechanical influences that increase the geographic range of the activities of plants, these being usually and curiously described as adaptations by which plants ‘“‘have come to be”’ fitted to growth in certain habitats, rather than as environmental adaptations by which the habitats have become fitted to support certain kinds of plants! Practically none of these considerations, however, pertain logically to a study of the proximate or immediate conditions controlling plant activities; these mechanical agencies of transport are of only secondary interest; they may be said to be only causes of causes. Thus the immediate external conditions usually considered as causing the germination of a seed must be the entrance of water, of oxygen, and of heat, and the reason for the occurrence of these immediate condi- tions is to be sought in the preceding mechanical transport of the seed. Of course such secondary causes are of great importance, and it is often practically impossible to approach nearer than these to the real seat of the external control of plant processes. While superficial and merely qualitative studies upon such influences have been fre- quent, the deeper-going quantitative and comparative work upon them remains almost entirely for the future. VII. INTERRELATIONS OF THE ENVIRONMENTAL CONDITIONS. The present section is appended here merely to emphasize a feature of the discussion of environments that has already received some at- tention at several points in the foregoing pages, namely, that external influences are seldom or never singly effective upon plants. Three considerations in this connection require a short treatment: (1) The more remote conditions of the external world, in bringing about the occurrence of any given influence upon plants, usually inaugurate other influences at the same time. Thus, with an increase in the amount of soil-moisture, the permeability of the soil to oxygen, the concentration of the soil solution in salts, ete., and the power of the soil to retain or give up heat, are more or less profoundly altered. With an increase in the intensity of impinging light comes also an increased income of heat to the foliage, and consequent alterations in aerial convection currents about the plant. (2) The same external condition usually influences the velocity of more than one of the elementary component processes of the organ- ism. Thus, phosynthesis, respiration, digestion, excretion, secretion, growth, etc., are all greatly influenced by such fundamental environ- mental relations as those of water, temperature, light, ete. (3) Since every elementary physiological process is thoroughly bound up with many other concomitant processes, it follows that an external change that alters only one process directly may indirectly be the cause of alteration in many others. If the secretion process, CHIEF ENVIRONMENTAL CONDITIONS. 147 for example, by which cell-walls are thickened or modified, be increased in velocity, this internal change must directly or indirectly alter the concentration and chemical nature of the solutions of the affected tissues. The formation of cork, cuticle, etc., profoundly alters the transpiring power of aerial plant surfaces, and similar modifications in roots must produce a changed absorbing power for solutes as well as for water. It is thus emphasized how difficult and arbitrary must be any attempt sharply to distinguish external from internal conditions, and how practically impossible it is at the present time logically to analyze the latter so as to begin to attain quantitative information concerning the various relations that have been roughly and crudely outlined in this chapter. Our reason for submitting this unsatisfactory treatment of the general subject of plant-relations is that the fundamental im- portance of these is not only theoretically but practically very great, and it seems time that a systematic beginning were made in some of the directions suggested by the foregoing incomplete analysis. If our treatment stimulates quantitative and comparative studies of plant environments, so that the present publication may soon be looked upon as useless and quite out of date, our aim will have been realized. The difficulty involved in really scientific studies of plant-relations ought not to be a legitimate reason for their omission and for the continuation of the pioneer sort of qualitative descriptions and teleo- logical interpretations, which appear to belong rather in the realm of ‘‘nature-study’’ and natural mythology than in that of true science. There are, however, already many ecological and agricultural studies on record, wherein the more logical point of view of the more ad- vanced physical sciences is given prominence, and the future of this aspect of biology seems to be assured. VIII. EXPERIMENTAL DETERMINATION OF RELATIONS BETWEEN PLANT ACTIVITY AND ENVIRONMENTAL CONDITIONS. It is perhaps not out of place here to devote some space to a con- sideration of the general character of the methods which must be employed in the more accurate determination of the relations with which the present chapter has had superficially to deal. As in all such cases, the only possible method of procedure is the experimental, and the experiments must be carried out with all the foresight and logical planning that characterize the work of the modern physical or chemical laboratory. The importance of this line of inquiry can not be overestimated; it is to be regarded as quite indispensable to the scientific advancement not only of ecological knowledge but of that most essential of all human activities, agricultural practice, and its pursuit is surely well worth the time, energy, and money that it would require. 148 ENVIRONMENTAL CONDITIONS. The experimentation needed is exceedingly complicated and ex- pensive, at least from the present standpoint of biological science, but would probably not prove particularly difficult in competent hands. A special laboratory is of course required—not a series of office rooms, nor the mere contents of an architectural exterior, but a carefully planned and elaborately and logically equipped building or series of buildings, with the requisite greenhouses, cellars, constant-tempera- ture rooms and the like. The main requirements of the work here contemplated are a variety of controlled conditional complexes, under which plants may be grown. Many of the methods of such control have still to be devised, but enough has been accomplished so that ultimate success may be regarded as assured. The moisture conditions of soil and air can be controlled with comparatively little trouble, as can also those of temperature. The control of chemical conditions offers a field for the exercise of ingenuity, and that of light and electric conditions will require still more attention. Many investigators in plant physiology have been able to control, in more or less satisfactory ways, one or two, rarely three or four, of the influential conditions, but no plant has ever yet been studied with even approximate control of all the influential conditions of its surround- ings. Since the influence of any condition is determined by the others, it is clear that, for any true appreciation of the relations be- tween plant and enviroment, all of the influential conditions must be quantitatively known. The suggestion here put forward, that thoroughgoing quantitative studies on the relations between environmental conditions and plant development are to be regarded as the only logical basis for a truly scientific ecology and agriculture, and that such studies are not possible without the elaborate facilities of a specially constructed laboratory, was largely included in a plea for a climatic laboratory made by A. P. de Candolle as early as 1855 in his Geographie Botanique Raisonée. Apparently the idea has never borne fruit. In 1891 Abbe’ repeated and indorsed the suggestion of de Candolle. The utter lack of apprecia- tion with which the arduous work of Abbe was received, in bringing together what he could in a limited time, of the literature bearing upon the relation of agricultural crops to climatic conditions, is to be estimated from the mere fact that his summary lay unpublished for 14 years and was at length brought out, in apparently perfunctory form, only in 1905! 1 Abbe, Cleveland. A first report on the relations between climates and crops, U. 8. Dept. Agric., Weather Bur. Bull. 36, 1905. See especially p. 23 et seq. THE CLIMATIC CONDITIONS OF THE UNITED STATES. I. INTRODUCTORY. From the last paragraph of the preceding chapter it is clear that no adequate description of the environmental conditions that obtain in any area is even to be attempted for a long time. In the present chapter will be brought together merely the results of certain studies which we have been able to carry out upon a very few conditions, and upon large areas. Some of the conditions studied do not directly affect plant life at all, it being usually impossible as yet to obtain quantitative information upon the subjects most pertinent to our general line of inquiry. Only in a single case have we attempted actually to obtain measurements of an environmental factor de novo; for the rest we have simply made use of data already collected. As is clear from the preceding analyses, to obtain the kinds of information most needed for such a study as the present methods, will have to be employed which are as yet quite unknown; adequate procedures re- main to be devised. Nevertheless, so great is the inertia of routine that there is little hope that the trend of observational work will alter very profoundly in the near future, and, as has been stated, we have deemed it advisable to make what use is now possible of the informa- tion at hand, with the hope that the very inadequacy of our whole presentation may itself be a potent stimulus toward the acquirement, in the future, of the kind of climatic observations upon which alone anything like a scientific ecology or agriculture may eventually be founded. Of the five main groups of external conditions which influence plant activities, discussed in outline in Chapter II, we shall consider here, and very inadequately, only the first three—moisture, temperature, and light. For the subjects of chemical and mechanical conditions no information that is at present available can be brought to bear upon the problem of plant distribution in a broad way. It seems probable, indeed, that the distribution of vegetation types is only rarely determined by any of the last-named conditions, though the detailed distribution of many species in any relatively small area is probably often related to chemical and mechanical influences. The information so far accumulated upon environmental conditions has not been obtained primarily with reference to plant activities; it has been brought together mainly in the interest of meteorology, clima- tology, and weather prediction. Therefore it is impossible at present generally to select for study those conditions that directly affect the plant. We have been forced, in the main, to study conditions or factors that are more or less remote causes of the immediate conditions 149 150 ENVIRONMENTAL CONDITIONS. influencing plants. This is not always the case, however, for the air temperature of the climatologist and meteorologist is the temperature condition of the aerial environment of organisms, and the evaporating power of the air is a factor that directly affects the rate of water-loss from the aerial parts of plants and animals. Both of these immediate, and thus truly environmental, conditions we have been able to con- sider to some extent. On the other hand, such climatic factors as rainfall, humidity, vapor-tension of water, wind velocity, and duration of sunshine are all recognized climatic factors, concerning the distribu- tion of the various intensities of which many data have been accumu- lated, but which have no direct influence upon plant activities. These climatic factors are very important, however, and often exert a con- trolling influence upon the more directly effective environmental conditions. Thus, the partial pressure of water-vapor in the air and the rate of air-movement influence the evaporating power of the sur- roundings. Rainfall greatly influences soil-moisture, and hence the ability of the soil to supply water to root surfaces, but it does not determine this environmental condition, for other factors, such as the physical nature of the soil, its exposure, subterranean water-flow, etc., must be taken into consideration in this connection. It therefore became necessary not to restrict our studies to immediately effective conditions, but to consider in most cases the more remote climatic factors which meteorology and climatology have placed at our disposal. The subterranean environment of plants has not, as yet, been studied in any way at all adequate to the present purpose, and our knowledge of the relation of this to plant distribution is still in the first stages of the purely observational phase. It has therefore been impossible for us to devote serious attention to this exceedingly im- portant category of environmental conditions. Nevertheless, on account of a general similarity of the prevailing soils of most of the broad vegetational areas of the United States, our studies of the rela- tion between the available measurements of the aerial conditions in connection with vegetational distribution are not as unsatisfactory as they might otherwise be. In the majority of the great vegetational areas with which we have to deal, the prevailing soil is a clay or a clay loam, with usually a rather deep-lying subterranean water- table, and with a more or less pronounced admixture of organic matter, and the prevailing vegetational types are, in the majority of cases, found upon this character of soil. Exceptions to this generaliza- tion are swamps and marshes on the one hand and sandy regions on the other. The general vegetational type of broad marshlands appeais to be about the same for a great range of aerial conditions; thus, these bear the same physiological types of plants under the climatic con- ditions of Baja California as under the very different ones of the southeastern coast. Also, the physiological characters of many plants CLIMATIC CONDITIONS OF THE UNITED STATES. 151 of the sand dunes of the Atlantic coast, of the Great Lake region, and of the Southwest are very similar. Furthermore, the pine forest of the Southeast is characteristic only of sandy soils and has the same physio- logical character as have the sandy pine plains of New Jersey, Michigan, Wisconsin, etc. The heavier soils of these regions all bear a very different type of vegetation. It has long been the practice of ecological and agricultural writers, in discussing any given region, to present rather elaborate tabulations of meteorological data as a sort of description of the region con- sidered, and then to turn to the discussion of the vegetational phe- nomena in hand, usually without any attempt to correlate the two sets of descriptive data. There seems to have been no doubt that there as some sort of relation between vegetation and the usual sets of climatological observations, but each author has contented himself with presenting the results of such observations, apparently with a faith that someone in the future might be able to interpret them. It has appeared to us high time that some serious attempt were set on foot to develop promising methods for such interpretations. It has therefore been necessary, throughout our studies of environ- ment, to choose and devise methods for handling the climatic data that are at hand, so as to derive from them as much information as possible about their probable influence upon plants. In many cases this choice of method has been based mainly upon general physio- logical theory rather than upon actual knowledge, since, as has been pointed out, adequate results of actual tests of the influence of climatic conditions upon plants are not to be looked for until very special facilities for this work have become available. One of the ideal aims that we have held in view during the years which these studies have occupied is the ultimate attainment of what may be termed environ- mental formulas, which might express the minimum, optimum, and maximum for each of the effective environmental conditions for any given plant-form or vegetation type. It is clear that the time is not yet ripe for the establishment of more than tentative and general suggestions in this direction, but several such suggestions will appear in following pages. It is here to be emphasized, however, that such formulas are the legitimate end of such investigations as these.! In the following sections we shall bring out the results that we have been able to obtain by various methods of treatment of the available climatological data. Most of these data have been obtained by the U.S. Weather Bureau for other purposes, commercial and political, as well as meteorological, as it appears. Our sources will be given in their proper places. 1 The idea of such formulas is not new; it has received attention, but from a different stand- point, from some of the phenologists. Simple formulas by which two factors are combined to give a single climatic index have been brought forward by Transeau, by Shreve, and by Livingston. These will be considered in another place. 152 ENVIRONMENTAL CONDITIONS. The present treatment is a purely geographical and climatological study, wherein it is sought to determine the approximate distribution of different climatic types in the United States. Since the aim of Part III of this publication will be to study the comparative distribu- tion of vegetation types and climatic areas, it has seemed desirable to prepare many of our climatic charts upon a base-map showing by different patterns the distribution of the great general vegetational areas of the country. The discussion of the climatic ranges of the several vegetational types will be reserved for Part III. The climatological charts themselves have been prepared, as far as possible, directly from the particular set of data involved, in the same manner as is the common practice in climatological work in general. The numerical data were first placed upon a copy of the Relief Map of the United States, of the U. 8. Geological Survey, (17 by 272 inches, hypsometrically colored and also furnished with contours for altitude), the numbers being written near the positions of the respective stations. Then the isoclimatic lines were sketched in in pencil and the map laid away for a time. At a later date it was worked over a second time and changes made that seemed to bring the lines nearer toward expressing the probable truth. In this revision the topography of the country was constantly scrutinized and the contour-lines of the base-map were allowed to influence the course of the isoclimatic lines in many instances, especially where the stations for which data were at hand were too far apart to show the true directions of these lines. The preparation of charts of this sort is at best largely a matter of guesswork; information is not available for the plotting of climatic details. For meteorological purposes it is usually quite undesirable to have charts showing such details, but for our purposes they were quite essential. It thus be- came evident early in the present studies that the method commonly used in the drawing of meteorological charts—of carrying isoclimatic lines from plain to plain directly over high mountain ranges, with little or no attention to altitude—should not be resorted to here; at least it should not be used on so grand a scale as is common in meteorological work. Furthermore, the degree of approximation to the actual truth is surely often greatly increased by a due regard to topography, rather than by an almost blind following of inadequate climatic data modified only by a desire for lines as smooth as possible. If we are aiming at as true a picture of natural conditions as is at- tainable, it is obviously more undesirable to pass a given line over a locality where we are absolutely certain it should not pass than to draw it through some other area where it surely does pass and wherein only its proper placing is questionable. Thus, if there be given two stations with the same climatic datum, on either side of a range of mountains, it may be taken as certain that the conventional joining wes CLIMATIC CONDITIONS OF THE UNITED STATES. 153 of these two points on the chart, by means of a line passing directly over the mountains, is absolutely certain to be wrong. On the other hand, whether to bend the connecting-line to right or left in passing around the range may not be apparent at all from the data of other stations, so that it might be drawn either way. Whichever way the truth might require the line to be turned, it seems that either way is a better approximation than the method of directly connecting the two datum-points, which we may be sure is incorrect. Usually other climatic factors, known to be related to the one receiving atten- tion, may be used to throw the weight of probability in one direction or the other. In our charts we have followed this method of rational guessing and have tried not to pass isoclimatic lines through points where we are practically certain they do not belong. We have also attempted to interpret the various climatic features by as nearly similar criteria as are possible in the nature of the different cases, and have tried to correlate our guesses in regard to related climatic features. The future may be expected to show egregious errors in many instances, but the discovery of such errors may lead to progress. After the revision of the charts, by means of the topographic con- tours and hypsometric coloring of the base-map, they were again laid aside and later were carefully scrutinized and corrected where appar- ently necessary, with reference to one another and to a large relief- map of the United States.' Only after these repeated studies and revisions had been accomplished were the climatological charts traced upon the generalized vegetation maps, or whatever base maps were requisite, and the lines inked in. The charts resulting from the above-described methods of procedure are characterized by very irregular lines in the western part. While we are convinced that many of these western isoclimatic lines are quite probably wrongly drawn (the data at hand are so unsatis- factory and the available stations are of such inadequate number and distribution), yet we think that an attempt to interpret, as far as pos- sible, the scanty information that is available should advance our knowledge of this important subject of climatological zonation more than would be possible from smoother lines drawn mainly without reference to topography, or from the complete omission of any attempt at a chart of these complex western regions. As has been pointed out above, the drawing of isoclimatic lines is frequently a matter of selecting the most probable of several directions and positions, all of which are possible from the standpoint of the limited. climatic data. Even with an excellent series of data, no two workers would place a given line in exactly the same position through- 1 The “ Relief map of the United States,’’ Atlas School Supply Co., Chicago, on which altitude is actually expressed by magnified relief. 154 ENVIRONMENTAL CONDITIONS. out its course. This possible difference in the interpretations of two students is the more emphasized, the more complex are the climatic characters of the regions dealt with, and the less adequate are the data available. We have therefore become convinced that, for the sort of subject that is here involved, it is quite wide of the mark for a writer to transform his series of data into charted lines and to publish merely the chart. It is quite essential that the data themselves, on which the chart is based, be placed in the hands of those who are interested. In the climatic studies which follow, we have been careful to point out just how each series of data have been derived and have uniformly presented these data by means of tables. The positions of the stations for which data were employed are generally shown upon the charts by small circles. In the following discussions, the different conditions or climatic features will be treated serially, under the three sectional headings, ‘““Temperature,’’ Moisture,’”’ and ‘Light,’ each one of which will be subdivided. II]. TEMPERATURE CONDITIONS. 1. DURATION OF TEMPERATURE CONDITIONS. (A) PRELIMINARY CONSIDERATIONS. Probably the most important environmental condition in the de- termination of plant distribution is the length of season or seasons, in each year, during which growth may occur. These are seasons, or time periods, during which every one of the factors of the surround- ings exists in an intensity or quality such that growth activities can go forward. If a single factor were effective beyond the limits for growth—in its quality or intensity—then this must suffice to throw the plant into a dormant phase, in spite of the fact that other factors might still be favorable to growth activities. Directly or indirectly, the ebb and flow of the environmental condi- tions affecting organic life are dependent upon astronomical causes, and the annual rhythm so commonly manifest in plant activities, as far as this is due to alterations in the surroundings, may be traced finally to the movement of the obliquely placed earth along its orbit and to the resulting procession of the equinoxes. This rhythm is always, then, either itself a temperature rhythm or else it is more or less directly connected with a temperature rhythm. This fact that the seasonal temperature fluctuations stand in a casual relation to the fluctuations in the other environmental condi- tions has given us logical reason for basing many of our climatic studies upon the duration aspect of the temperature factor. To this must be added the practical reason that temperature fluctuations, in the United States as well as elsewhere, have been much more thoroughly CLIMATIC CONDITIONS OF THE UNITED STATES. 155 studied than have the seasonal changes occurring in any other climatic factor. Still another consideration to be mentioned in this connection is this, that the most advanced modern civilizations of the world have developed in humid temperate regions, where temperature changes seem to be actually the immediate causes of the annual rhythm in plant activities, and hence the temperature-relation in its general aspect is more familiar and more useful to most of us than might be any other. Practically only in arid regions do moisture conditions play an important and direct réle in determining the march of the natural vegetational seasons, and the light-relation as such is perhaps never important in this connection with outdoor vegetation. By greenhouse culture in temperate regions the summer temperature season is prolonged throughout the winter, and here alone is it notice- able that plants often suffer, during the early winter months, appar- ently for lack of light. In the open, however, whenever temperatures favorable to growth prevail, the light intensity is much greater than it is in winter in our greenhouses. Since natural fluctuations in temperature are usually characterized by being gradual and continuous, it is quite impossible, even for a single year and for a single plant, to determine sharply what are the time limits of the growing-season. It is much more difficult to fix seasonal limits in general; the only method at all possible here is that of averages. Thanks to the elaborate routine observations continued through many years by the United States Signal Service and its successor, the United States Weather Bureau, a vast accumulation of temperature data are at hand for a large number of stations in the United States. From these it is possible to determine averages and means, of various types and by various procedures, which may be taken as fairly representative of the average conditions of the country throughout a series of years. It is these temperature data, and the results of various mathematical treatments of these, upon which we base our temperature considerations. Data bearing upon plant activities, sufficiently detailed to be at all comparable to the temperature data just mentioned, are totally lacking, and must remain so long after the much-needed laboratories for the study of environmental relations shall have become available. While the discouraging character of this state of affairs is pronounced enough, nevertheless it need not cause us to refrain altogether from attempts to relate growth activities of plants to temperature conditions. Considerable preliminary information, and perhaps some that will later prove to be of a deeper-going sort, may be obtained if we _ merely approximate the temperature limits of general plant growth by a simple inspection of the knowledge which is available at present. Everyone who has dealt with plants at all, from the standpoint of the temperature relation, is convinced that the occurrence of a ‘‘killing 156 ENVIRONMENTAL CONDITIONS. frost.” (in the agricultural sense) practically marks the end of the active season for the vast majority of plants. The average dates of occur- rence of the last killing frost in spring and of the first in autumn should furnish us with a valuable index to the approximate length of the temperature season of general plant activity at any given station. It is of course well known that the growth activities of many plant- forms are checked and that death of all but dormant phases frequently ensues with temperatures far above those requisite for the recurrence of a killing frost. It is just as clear, however, that many forms make a considerable growth before and after the frostless season, which we consider as limited by the average dates of occurrence of the last and first killing frosts respectively. It seems therefore safe, in default of any better means for improving our knowledge, to resort to the average length of the frostless season as the basis for the duration aspect of the temperature conditions in the United States.2 A somewhat detailed consideration of the frost- less season in the United States will comprise the next following sub- section. It will have been remarked that no attention has here been directed toward an approximation of the upper temperature limit for plant growth. This matter will receive some consideration in the sequel, but it may be remarked here that it is not nearly so easy to attain to an approximation of this general maximum as of the general mini- mum for plant growth. This is partly due to the fact that the effect of freezing is quickly manifested upon many living plants, while in-° jurious effects of high temperature are generally but slowly exhibited and therefore less readily observed. Furthermore, the response to frost is almost always a direct and unequivocal effect of the surround- ing temperature upon the organism, while with high temperatures, alterations in the transpiration-rate, and in the rate of possible water- supply to plant roots—alterations in the water-relation, in short— become quite hopelessly confused with the temperature effects. It seems probable that relatively few plants will be found that are directly prevented by high temperature from thriving anywhere in the United States. Wherever this appears to be the case, a more thorough ex- amination of the facts has usually resulted in the conclusion that the high temperature is at least primarily effective only as the more remote climatic cause of an alteration in the moisture-relation. a a a i le 2 On the employment of the average length of the frostless season in this sort of studies, see the following: Livingston, B. E., Climatic areas of the United States as related to plant growth; invitation paper read before the American Philosophical Society, Philadelphia, April 1913, Proc. Amer. Phil. Soc. 52: 257-275, 1913.—Livingston and Livingston, 1913.—Fassig, O. L., The period of safe plant growth in Maryland and Delaware, Monthly Weather Rev., 42: 152-158, 1914. Fassig calls attention to the fact that the occurrence of the last and first minimum of 32° F. furnishes as good a criterion for the determination of the average length of the growing season, and perhaps a better one, than does the occurrence of actual killing frost. CLIMATIC CONDITIONS OF THE UNITED STATES. 157 But the occurrence or non-occurrence of a given plant-form in a given region depends upon other features than length of the season of active growth. As was mentioned earlier, it frequently occurs that the limiting condition preventing the occurrence of a certain plant in any area is to be sought in the nature of the surroundings during a dormant period. The length of the season or seasons during which growth can not occur is perhaps frequently as important in plant distribution as is the duration of the growing-period itself. Also, if environmental conditions are adverse enough they may result in the destruction of plant protoplasm even in its dormant phases, and it thus becomes necessary to study the duration of extremely low tem- perature, a thing which it seems quite possible to do and to which we shall devote some space in the present section. We turn our attention now to the variations in the length of the average frostless season throughout the United States. (B) THE LENGTH OF THE PERIOD OF THE AVERAGE FROSTLESS SEASON. (TABLE 2, PLATE 34.) Although data for the determination of this exceedingly and very obviously fundamental condition of plant growth, whether it be agriculturally or ecologically considered,! have been in existence and have been increasing in volume and in value for a long time, it is apparently not until very recently that the subject has received even cursory mention in the literature. Abbe (1909), in his excellent re- view of the literature upon the relation of climates to crops, already cited, makes no reference to the length of the frostless season as a climatological feature. As has been mentioned, this work should be connected with the date of its preface (1891) rather than with that of its long-delayed publication. It is thus highly improbable that this feature of climate had entered very seriously into the considerations of workers in climatology up to about the year 1891. The publication in 1906 of Henry’s elaborate presentation of the climatological data available for the United States? put the informa- tion upon spring and autumn frosts, that had been collected up to that time in this country, into a form so that it could be made use of. In the early stages of the studies reported in the present publication, in the winter of 1909-10, Mrs. Grace J. Livingston undertook to derive an approximation of the average length of frostless season for each station for which Henry gives the requisite data. This was done by 1 “Probably no factor in the study of climate from the standpoint of the agriculturalist should be given more consideration than the average length of the growing-season. This is the key to an actual knowledge as to the possibilities of success or failure in the production of crops, since in practically all portions of the United States agricultural products are menaced by frost at some period of their growth.’’ (Day, F. C., Frost data of the United States, and length of the crop growing season, as determined from the average of the latest and earliest dates of killing frost, U. 8. Dept. Agric., Weather Bu. Bull. V, 1911.) ‘Henry, A. J., Climatology of the United States, U.S. Dept. Agric., Weather Bu. Bull. Q. 1906. 158 ENVIRONMENTAL CONDITIONS. determining the number of days intervening between the average dates of the last killing frost in spring and the first in autumn. - It was realized that these average dates for many of the stations were not based upon adequate observations, but it was thought desirable to make what use was possible of the data at hand as being the best that were available, and our studies progressed satisfactorily, using the lengths of the frostless season thus derived. About 1910! a series of frost data for the United States, much more complete than that presented by Henry, became available through the publication by the United States Weather Bureau of the 106 summaries by sections. In the following year appeared Day’s “ Frost Data of the United States” (loc. cit.,) which comprises several forms of frost charts, including one (Chart V) of the average length of the crop growing season, days. This bulletin contains no presentation of the data from which the charts were prepared and no reference to any publication wherein the student may find these,” but, through the kindness of Professor Day, we have been informed that most of the data for the preparation of the charts were taken from the summary above mentioned. Under the heading “Source of Data,” the following statements are made in Bulletin V: To secure data that would show more nearly the actual conditions that prevail in the fields, orchards, and gardens, the most extensive compilation of frost data yet undertaken by the Weather Bureau has been accomplished and the results have been spread upon the accompanying charts. The data from approximately 1,000 of our cooperative stations having the longest records, usually from 10 to 30 years, except in the most newly settled localities of the West, where records for shorter periods only are available, have been summarized, and the local con- ditions due to physical environments brought out in much greater detail than has heretofore been attempted. These charts being based upon the results of observations made in the open country and therefore not subject to the artificial conditions prevailing in the large cities where the regular stations of the Bureau are mainly located, differ from any that have appeared in the past in that areas having peculiar climatic features not heretofore shown on such charts are now clearly set forth. Chart V of Bulletin V shows isoclimatic lines, for the country east of the one hundred and third meridian, ‘‘the average length of the crop growing season, days, being the number of days between the average date of the last killing frost in spring and the average date of the first killing frost inautumn.” The distance between any two adjacent lines 1 Summary of the climatological data for the United States, by Sections, U. 8. Dept. Agric., Weather Bur. This elaborate presentation of “all the available data as they siand”” (reprint of section 1, page 1, introductory remarks) comprises 106 separate pamphlets, all of them without any date of publication. They include, for the most part, data for periods extending through 1908 or 1909.) 2On page 4 of Bulletin V1l we find the somewhat unsatisfactory bit of information which follows: “The chart showing the average length in days of the crop growing season was prepared from a somewhat different list of stations than was used for the charts of average dates of frosts, hence an actual determination of the length of the season from charts 1 and m1 might differ a few days from the date shown on chart v.’’ But the data are not given for any of the charts. Se Oe a ee ee gre nO De CLIMATIC CONDITIONS OF THE UNITED STATES. 159 represents a variation of 10 days in the average length of the frostless season. West of the one-hundred-and-third meridian the stations are so few, so poorly located, and the periods through which observations have been accumulated are in many cases so short, that it was not deemed advisable to continue the chart farther west. Here, however, the data themselves are placed upon the map. | The chart just described is the first one of the length of the frost- less season to be published for the United States, and its appearance is to be considered as marking a very great step in advance in the climatology of the country. It should be of great value to agricul- turists and ecologists, and it is to be hoped that the future may wit- ness more uniformity and completeness in the keeping of the records from which a more perfect chart of this feature, not unaccompanied by an adequate tabulation of the source data, may evenually be obtained. Since our own studies have been so largely based uy on the average length of the frostless season as the duration factor for many climatic conditions, the bare chart (as presented by Day) has been of com- paratively little value in our work. After the publication of the Summary by Sections, Mrs. Livingston made a recalculation of most of the great mass of derived data that had already been prepared, using the new average lengths of the frostless season obtained from the average dates of the summary. The result exhibited some con- siderable modifications in our derived data. Since the information of the summary is later and much more complete than that given by Henry, we have adopted the data from the latter source as the basis of much of our work, using those from Henry’s Bulletin Q only for a few stations for which the summary fails to give frost data and for which these are furnished by the other source. In table 2 are presented all the frost data that have been used in our studies. In the first column are given the names of the stations, alphabetically arranged under each State, the States being also alphabetically arranged. An n after the station name denotes that the observations were made in the vicinity of the place marked. In the second column are given the altitudes of the stations (in feet) so far as these have been available. The third and fourth columns, respectively, contain the average data of the last killing frost in spring and of the first killing frost in autumn. These data are quoted directly from the Summary by Sections (the number of the section in which the station occurs being given in parentheses directly after the station name) excepting in relatively few cases (indicated by an H after the station name in the first column), where the average dates have been obtained from Henry’s Climatology of the United States. In a few 1These altitudes have been obtained from several sources and we have been unable to verify all of them. If there are errors in some cases they are probably but slight ones. 1 60 PLATE 34 Lies A ASS Lye “ae BS Reis Ve AS \7: iS ae CAS Eo \ SOG Gnd 0G “Wy ag ° a SESS SSN xy i / “AN I SS NA) : rast ai Wwe Oe Sef Tepe VAS Nese ONL An a os TR a os A ck SY ZaiN : Ss cee Save: ae Ze Y ee 4 ZO es vn we SN < \\\ i oe ay ue ee ss ee Oe Ms : a SG. oe Dt si BSN Zs LZ Yip ey SS eS = Sv < ak cdi Ss ay es Pa SS ee ye Fo ea The very broad, full lines are the vegetational boundaries of plate 2. Lengths (days) of the average frostless season (data from table 2). CLIMATIC CONDITIONS OF THE UNITED STATES. 161 cases frosts are so infrequent that no average dates can yet be obtained. In such cases the words “‘very infrequent”? are entered in these columns. ‘The frostless season should be considered as approximately the entire year in these cases. Similarly, for a few stations the words ‘‘possible throughout year’’ indicate that the data at hand do not indicate any reliable average dates of last and first killing frosts and that the average frostless season is either very short or absent. For such stations the duration factor which we are considering should be regarded as practically nil. The fifth column of table 2 gives the average length of the frostless season (in days) for each station, this being derived from the dates of the third and fourth columns, not employing the first date, but including the last. TABLE 2.—Frost data and length of average frostless season for 1808 stations in the United States. (Plate 34.) No. of Average date of — Length of Station. Altitude. years ‘of [=> A ee record. Last frost | First frost nOStess 2 : : season. in spring. jin autumn. Alabama: feet. days. PAMITTIBLON (82) Be «2 6") ‘Oct. "23 200 MIObDUNEHS2) coc Fei cob kee 57 38 Feb. 24 | Nov. 30 279 Montgomery (82)......... 240 37 Mar. 10 | Nov. 8 243 Mmcantar(S1))2 264s ne 857 15 Apr. Oa Oct.) 1b 188 Sipelriea (82). o.c6 60 sacs on 817 30 Mar. 17 | Nov. 9 237 SATS )Vorts ssa ka kinda emcee 400 7 Mar. 10 | Nov. 13 248 THEVA DDLO! (SZ) ati cieeiaee + ailcca ave © oe 6.08: 20 Mar. 14 | Nov. 10 241 Sasa esEra vices been (Eb) tt crcse lee cevchi tame sie eee aa ce ail sisrerateeiaite ms Mar. 21 | Nov. 12 236 RBC VIELS (2) choo core oc oa! aw es cee erste es 8 Mar. 24 | Nov. 6 227 peottsboro (81) ..:........ 652 25 Apr. 10°) Oct. 28 201 SE CP) a aes Ci tee 147 34 Mar. 13 | Nov. 8 240 Talladega (82)............ 554 20 Mar. 26 | Oct. 30 218 Thomasville (81).......... 385 18 Mar. 15 | Nov. 8 238 fuusealoosa (SL)... 5s. os 230 28 Mar. 23 | Nov. 6 228 Union Springs (82)........ 216 22 Mar. 9 | Nov. 18 254 Wmiontown) (82) .-.). <..s/«.0. 273 23 Mar. 15 | Nov. 8 238 Valley Head (82)......... 1,031 24 Apr. 5) | ‘Oct: 20 198 @Where the frost dates have been obtained from the Summary by Sections the number of the section in which any station occurs is placed in parentheses directly after the station name. Where the dates have been obtained from Henry’s Climatology of the United States this fact is shown by H in parentheses after the stationname. The letter nin parentheses after a station name means that the data were obtained from a location in the immediate vicinity of the station named. 162 ENVIRONMENTAL CONDITIONS. TABLE 2.—Frost data and length of average frostless season for 1803 stations in the United States. (Plate 34.)—Continued. Average date of — Length of Noxtob? |S ot> eet _ Se eeree Station. Altitude. sore. 4 Last frost Fir st frost se In spring. jin autumn. Arizona: feet. days. Arizona Canal Co. Dam (3) 1,372 10 Feb. 15 | Dec. 18 296 Benson i(S) fvesets ceicekisies 3,523 7 Mar. 20} Nov. 10 235 Casa Grande (3).......... 1,396 5 Mar. 5] Nov. 22 262 Columbia (4) .2.0.'./...0's + <2 1,900 8 Mar. 20] Nov. 22 247 Congress'\(4) ce 20: Adlcsaneecoce Feb. 19 | Dee. 25 309 Paso Robles (14).......... SOO! sy olde secs Apr. 1] Nov. 5 218 (Peachland! (16))s..-... 68.4.0 11?) 0 ae Gere, 5 cote Apr. 14 | Nov. 21 221 Placerville (14).20.. 6 tec. ccc TS20) 4 a cca ete Mar. 15 | Dec. 31 291 Porterville (12 )ict. woe seas AGT ai A iliararats ete ane Feb... 7. | Dec.’ 310 306 ROW By (EE) oc erence sistas AGO)” yt oetcnPosteeys Jan. 26] Dec. 3 311 ed Blige (LS) sedan oe kts SOV: View sincere eee Mar. 27 | Dec. 16 264 Redding (15) suites 2c eee BOL” | Vac seis Feb. 20]| Nov. 27 280 Reaglancgs CED) cw.) a sere oheae LSD2h ss ose. .aiee bei Feb. 19 | Dec. 12 296 Sacramento (15).......... GA pol orca tate ok Feb. 16] Nov. 15 272 Salinas UA). oid calc vc nels BOO OV ide a. eieta sa Feb. 18 | Dec. 2 287 BIBT ATO OCU ick a bs otots wibnons 256 u Wales aceite Mar. 31] Dec. 20 292 San Francisco (14)........ 207 pede icceheene Jan.. 25 | Dec. 16 319 San Jacinto (14).......... T6650, 5 4 | ston aeons Mar. 27 | Nov. 20 238 SE gel fot: 7oWy QI: 9 eS eee ae Obra dling cee ee Feb. 61] Nov. 27 294 San Leandro (14)......... BOW Ga teiae ci Mar. 9] Dee. 8 274 San Luis Obispo (14)...... COT ee Sree cnet Mar. 3] Nov. 18 260 San Miguel (14).......... GEO. il ebars petees Feb. 22 | Nov. 13 264 Santa Barbara (14)....... LEO! pec wees Jan. 19 | Dec. 13 328 Santa Clara (14).......... OO%) Wuileosar wares be Feb. 27] Nov. 25 271 Santa ‘Crus (14)... ake. es BO Ua sa eee Sas Mar. 10] Dec. 9 274 Santa Margarita (14)...... MOG) ole ere i ehaatn es Feb. 15 | Nov. 25 283 Santa Paula (14).......... BOO.) lavs e saab Jan. 23 | Dec. 18 329 Santa Rosa (16).......... LSU.) « ile delaras Apr. 24] Dec. 10 230 BUSsON I CLBYs dten:s es 2s aes 2) ee RI a otc Mar. 19 | Oct. 12 207 Soledad) GUS) vais Sisckels a bo BS yo llak ovis eaten Feb, ..9.) Deavis 301 SLOKeV (CLA). Sek wast sch ms PU vit shins och ia ae Feb. 23] Dec. 3 283 Bite lo) eis se een clave MOLE AY opis wie cee berate Feb. 13] Oct. 13 242 Susanville (15)........... ME EUS, Thane Mawel etE May 10] Sept. 22 135 *ROHBONE DIS (L4) 55 diane baci Oi OG4S slic hake Mar. 17] Nov. 28 256 PLE ROY CLG). tediew wrehhaew tet BAS lan eee Feb. 17 | Nov. 18 274 SDUGLO;CES) pcp anacauie cee Be. . tlei veaeis Mar. 7] Nov. 15 253 CLIMATIC CONDITIONS OF THE UNITED STATES. 165 TABLE 2.—Frost data and length of average frostless season for 1808 stations in the United States. (Plate 34.)—Continued. No. of Average date of— Length of Station. Altitude. years of |—————_-_-___- average record. | Last frost | First frost pany in spring. | in autumn. aoe. California—Continued: feet. days. Privat (UG) ees. sce does G20i 6 Caer. oe Apr. 14] Nov. 1 201 Kipper Take i(16) 7 .2.: 32/2. ES SOOn — Wicaatpatee cece Mar. 30] Nov. 13 228 Upper Mattole (16)....... 7 7: Sn eens ies ee Mar. 7] Nov. 18 256 i Valley Springs (14)........ Git Pee Ree oa Apr. 12] Dec. 17 249 Wanalin (14) oo). 5 68. oe sae ee os Mar. 13 | Nov. 27 259 Watsonville (14).......... Dey re Oe lee Seer Mar. 8] Nov. 38 240 Westpomt (14))o..5 os. ace. Zragoo) les castes: Apr. 16] Nov. 8 206 Wallon lS icc cre-s < linac aero ee. May 12] Sept. 27 138 Grand Junction (9)....... 4,608 16 Apr. 18 | Oct. 18 183 Grand Valley (9)......... 5,089 14 May 10] Sept. 29 142 ierecleva(S)ices ot csice es ose 4,639 16 May 6] Oct. 1 148 MPstIN AECL) once as: srelsiersiena love SS400e, Ae ct as May 17 | Sept. 24 130 Peete (7) fens ectce ose DSTI Say ee et May 20 } Oct. 2 135 CURA CORES GeO neao Sener, SSO Lt Lalisers tees oie May 1 |} Oct. 1 153 POLY OG 1CS) soraeis seicie aiste te 3,745 9 May 91] Sept. 22 136 ISTE CA) caictics ss ees wee GF59Gi 0) (IS career cers May 15 | Sept. 25 133 OSEFHASLENC A, )). vars Any a(Octaie» acters SRO Hikes craeitae.« Apr 274 Oet.-vers 159 Las Animas (7)........... SR SOON wanes cities May 2 | Oct. 2 153 OAV ees saicnre ate aks sib 6,190 9 June 16] Sept. 6 82 ERO Y(S) x oe Wess BRAS 4,380 16 May 2] Sept. 29 150 Long’s Peak (n) (8)....... 8,700 9 June 26] Sept. 2 68 PUTATCOS CD) iq cess oicivle uss «eee 6,960 10 June 91] Sept. 17 100 INVCOKER (9) ise a0 ore oS els 6,183 14 June 12] Sept. 12 92 Montrose (9) io5<.664 sre: 5,811 10 May 16 |} Oct. 2 142 mroraime: (S):<2 oak coe oes ot 7,750 12 June 15 | Sept. 13 90 Lepare EO) age Hee ca eecoe 6,500 13 June 12] Sept. 3 83 SLOTS) cscs, oS /che. «2 cles 5,694 8 May 5] Oct. 3 151 HE MION CL) teoe ees cero: BUSA, Pal eiers deers ck Apr. 27 | Oct. a 163 Rocky Ford (7)........... AP Mid Pl Miser yaa ener: May 2] Oct. 3 154 parttiache: (EH) s-slees aiios MENCAOMAL Yh llereta erties eee May 24 | Sept. 17 116 SUNOS GE See eee E GROSD PO Aer o Neate sts May 381 | Sept. 20 112 eoesan urs) (BH) .f....5s.0.05 Ti MOA IEC Meier eee creo June 9 {| Sept. 11 94 Banta Clara (7)).00. 02) (ofan) Oia | atte ect June 31] Sept. 24 113 Sih (CO) Re ee ee eee 5,441 12 May 21 | Sept. 27 129 SiraT MOak (8))o. Gen ees oe CLIMATIC CONDITIONS OF THE UNITED STATES. 167 TaBLE 2.—Frost data and length of average frostless season for 1803 stations in the United States. (Plate 34.)—Continued. Mio OF Average date of— Length of Station. Altitude. years:of 2 | ae. record. Last frost | First frost eee in spring. |in autumn. BEASOE. Georgia: feet. days. Adairsville (85)........... 772 17 Apr. 5] Oct. 27 205 PUISATII(GD) oacie si a'cieie eto 2 230 10 Mar. 6] Nov. 11 250 Milanahsa| (85)! 2.5 see ere 293 10 Mar. 2) Noy. 17 260 AaNericus; (SH)'; .!65 os. ost 362 14 Mar. 8] Nov. 13 250 ACRES) (SG) sents ss sth es oe 694 12 Mar. 27] Nov. 8 226 PAtlantal(Sd) do ccccss ss eee 1,218 18 Mar. 23] Nov. 3 225 Aapiuista (S8S6)is2 obo cee 180 18 Mar. 24] Nov. 7 228 Bainbridge (85)........... 119 6 Mar. 15 | Nov. 11 241 lalcelya(S5) ef ec oe a ole ts 300 15 Mar. 15 | Nov. 14 244 Wamaici(S6) i062. oss 613 10 Mar. 25 | Nov. 3 220 MeATTOlEOR (Sayin. eo oe clots oc wave ste, 6 4 Apr. 41] Oct. 29 208 Wlayton (86): 2.2.0). ie? 2,100 16 Apr; 15 |(Octz 319 187 Wolumbus (85). ic. ose. c css 262 12 Mar. 6] Nov. 17 256 Wovineton (86)... 5... 60% 800 14 Mar. 31] Nov. 8 222 Dahlonega (86)........... 1,519 18 Apr af Octet 13k 207 rire leyatSO) seers. hia poe lle ms he claa ss 12 Mar. 22| Nov. 6 229 Eastman (86)............ 361 8 Feb. 26] Nov. 11 258 PELLORLOMN (SG) ser = cles ole lle eneleie et ec ola c 6 Mar. 19} Nov. 4 230 Hiberton: (86) v2 o.e5.. . 25k 710 18 Mar. 26] Nov.. 8 227 Horsyth (S86) e255. ssa. os 735 9 Mar. 24] Oct. 31 221 Port Games: (85) ..6....55 2 166 13 Mar. 13} Nov. 9 241 Gainesville (86)........... 1,254 14 Mar. 31}; Nov. 3 217 SPISVINE(SO)E Se... 52 eS 1,052 a7 Mar. 31] Nov. 3 217 PPCH DUBE (S5) ac) to ee eal ce «ec bee sc 10 Apres 2) Oct.0130 211 Greensboro (86).......... 598 ve Apr.’ 1.| Nove 21 214 Aerittin (SO) eee ee ete sees 975 9 Mar. 20] Nov. 4 229 farrison: (86) )i'/5... 0s. eres 245 10 Mar. 23] Nov. 10 232 Hawkinsville (86)......... 235 14 Mar. 17} Nov. 12 240 Mess) CED). Meese oe es ee NOOE A iss 10 ees Mar. 17 |.Nov. 20 248 most Mountain (S5)/.. 0. se lc coe et dees 9 Apr. 3 | Nov. 4 215 Louisville (86)............ 259 17 Mar. 15} Nov. 10 240 Humpkin (85)).2...2.. 06.3 650 15 Mar. 17] Nov. 11 239 MTAGON ASO) foek 6c 68s 6 ltd 370 12 Mar. 20] Nov. 13 238 Marshallville (85)......... 500 alee Mar. 19} Nov. 6 232 Milledgeville (86)......... 276 13 Mar. 22] Nov. 5 228 Monticello (86)........... 800 11 Mar. 17 | Nov. 13 241 Morgan (85) fos)... 55 <5 te. 337 16 Mar. 16] Nov. 6 235 Wewnan (85) '.2..< 6... es 959 13 Mar. 25 | Nov. 8 228 Point. Peter (86) ...... fe. 1,000 18 Apr. 11] Nov.- 1 214 Penis (85) Sees eos Set 365 17 Mar. 16] Nov. 9 238 uaitman: (S5)Ssi... $255 6 eet 173 14 Mar. 11 | Nov. 13 247 itamsey (85) te.) 22.5 6.8? 1,363 14 Apr, 14 | Octs--18 187 Bignie (GD). ate. oak 52 576 17 Mar. 31] Nov. 1 215 Bt rarys:(S6) 0) 22 257.4. 20 ‘i Mar. 6] Nov. 24 263 Savannah (86)............ 65 18 Mar. 91] Nov. 27 263 Statesboro (86)........... 253 9 Mar. 91] Nov. 138 249 Malbotton (85)... 2... o.. 750 16 Mar. 25] Nov. 9 229 Tallapoosa (85)........... 1,150 10 Apr. 7 | Oct. 30 206 Thomasville (85).......... 273 12 Mar. 6] Nov. 18 257 Walona (86). .5.26.02%.655. 12 9 Mar. 11] Nov. 17 251 Waycross (86)............ 131 17 Mar. 11 | Nov. 16 250 West Point (85)..: 2%...) 620 10 Mar. 20] Nov. 2 227 Idaho: American Falls (22)....... 4,341 15 May 27 | Sept. 8 104 Blackfoot) (BH) 5... 6.52 A BOeoh - alistere cclett tera ats May 29} Sept. 12 106 168 ENVIRONMENTAL CONDITIONS. TABLE 2.—Frost data and length of average frostless season for 1803 stations in the United States. (Plate 34.)—Continued. No. of Average date of— | Length of Station. Altitude. years of |————;_——_ pier record. | Last frost | First frost —_ season in spring. jin autumn. Idaho—Continued: feet. days Bose (22) cote pete ws ate 2,770 10 Apr. 28] Oct. 22 177 Burnside (22) -\.. .. «ss 5,500 9 June 10] Sept. 9 91 Chesterfield (H).......... BAGO! (is ais meters ah July 21 | Aug. 10 20 Coeur d’Alene (21)........- 2,157 6 Apr. 31 | Oct 1 154 Inftrpet=. eK C4 Dads Seta oo. ob plano Soto” 9 July 9] Aug. 17 39 Garnet (22) see ls sates ost 2,000 9 Apr. 17} Oct. 20 186 Grangeville (21).......... 3,500 a May 19] Sept. 25 129 Idaho Falls (22).........- 4,742 4 May 22 | Sept. 12 113 ellopr (21) iery..ce stevie ste 2,330 4 May 14 | Sept. 25 134 ake (22))e-< rcistetsstevlenstare 6,700 11 June 25 | Aug. 20 56 Makeview (2 ister cele = ee 2,250 12 Apr. 29 | Oct. 9 163 Mancdore (22)he cesses 5,300 4 May 18] Aug. 16 90 MP wIstOnN (2) ee. stiees serie 757 8 Apr 8 | Oct. 27 202 Lost River (22)....2..2. 65 5,700 10 June 9] Sept. 1 84 Martin (22))Gere cd cere = 5,600 4 June 26 | Aug. 24 59 Milner (22). 30. cc. wee 4,097 5 May 19 | Oct 4 138 Moscow (21)- 22... are 2,748 16 May 8] Oct. 10 155 Withee (Ciba Gude aan op at 2,750 15 June 21] Sept. 19 109 Oakley (22)e cin. ace ccin 4,191 15 May 31 | Sept. 12 104 Ola (22) Pe aeeran races 3,100 9 May 25] Oct. 4 132 Orofinot(21) Meee she sot 1,027 4 May 18] Oct. 12 147 (Paris! (22) '. sete st= clelereverers 5,946 13 June 14] Sept. 3 81 Payettes(22) i. st .cee - 2,159 15 May 11 | Sept. 29 141 Pocatello (22) het oe ar i> see 4,483 9 Apr. 20] Oct. 12 175 Pallocke(21)sarterevcteiee cues tie 2,050 12 Apr. 26] Oct. 12 169 Porth (Zh) eres feces. ote es 1,665 16 May 14 | Sept. 14 129 Priest) River (21)... a. < «ses 2,078 6 May 29 | Sept. 22 116 Roosevelt (21)..........-- 7,250 6 June 29 | Aug. 21 53 St. Maries (21)..........- 2,263 12 May 8 | Sept. 14 129 Stroh | C48 SAI craig oat bos 4,040 3 May 26] Sept. 7 104 Soldier (22),......-.--++: 5,140 1l June 27] Aug. 19 53 Swan Valley (22)......... 5 434 8 June 28} Aug. 15 48 VERNON (22) sete vis wieie sletere|e slelsyeln eal slelets 1l June 14] Aug. 29 76 OV ATTAIN CLL) ig ciclels cosa al etarees 5,350 4 June 12] July 23 41 Weston (22)\s fe. bc eis eletee 4,610 1l June 2/1] Sept. 9 99 Illinois: PATINIONG(OG) « o stere.o tee ane ee 531 15 Apr. 14] Oct. 21 190 FAIGGO( G4) xs oe o's ewle aie 738 8 Apr. 29] Oct. 13 167 Alexander (65)..........- 670 15 Apr. 24 | Oct. 6 165 FATItIOOU (O4) cass oi 0's eo 861 ch May 4 | Oct. 5 154 WRGHEOW (DA) diver wc wes evete 830 14 Apr. 29] Oct. 5 159 PRUINOTE (G2) occ cio sts wise awe 687 22 May 6 | Oct. 6 153 Lo tveh or hl (610) JAS SER CR EET UCC 598 7 Apr. 21] Oct. 21 183 Bloomington (65)......... 840 16 Apr. 27 | Oct. 9 165 Bushnell (65). <....52..0.0 662 14 Apr. 25] Oct. 14 172 COAIOI(OG): vette aeres vino es 359 38 Mar. 30] Oct. 28 212 Cambridge (64)..........- 824 14 Apr. 22] Oct. 10 171 arinvie (OO). .cs sss b's 663 17 Apr. 22] Oct. 11 172 Charleston (65)........... 720 16 Apr. 24 | Oct. 6 165 Ghicawe (64) ii. ans esr ss 824 38 Apr. 16] Oct. 15 182 Contsburg (65) 55.5. 26.00% 738 13 Apr. 24] Oct. 14 173 Cobden (66) < 495 12 Apr. 16 | Oct. 16 183 Biloran(GO))<.cccers ccicls s = eels 495 16 Apr: 18) Oct. O1t 176 Rerivst (04) 928k 3. o's 508 842 16 Apr. 29] Oct. 10 164 ; Golconda (66)..........--- 500 20 Apr. 6] Oct. 26 203 Greenville (66)............ 635 21 Apr: 45) Oct.) 15 183 Griggsville (65)......-.... 650 19 Apres 21 | Oct: aal7 179 Efaliweays (66)lo.t2 2 ons oe 569 11 Apr: | =5.}) Oct: ) 26 204 Pa vanal(O5)icc-\-feiceesis ove 475 15 Apr. 23 | Oct. 14 174 Leics a 1(72) apogee Spoor 500 20 May 5] Oct. a 155 EMI SbOro!(G5)\ss.0.00 oes ke = 675 14 Apr. 22] Oct. 15 176 Tela, (GSAS os oppo 541 15 Apr. 26 | Oct. 9 166 Kishwaukee (64).......... 730 13 May 41] Oct. 1 150 Minoxvaille (GD) t6 0schrs- 32's 775 14 Apres) 2on|OctssaaS 171 LaGrange (64)............ 657 16 Mayaw oly |SOcta i. tal 163 Mablarpe (65)ico:.<. 5. sss 698 14 Apr: 24. }) Oct; 7'5 164 Weanark: (64)). seme 3 oes . oes 883 21 May 61] Oct. 1 148 Salle sO) cat. Bra. ictetalell Rael, Sc Melos 4 Apr (25 |) Oct 313 168 HeINCOINWGD))sciieteceas es a,22 482 16 Apr +27 /|\.Oct. +10 166 Martinsville (65).......... 630 16 Apr: 4225|Octs 14 2 Neartinton (65)).).......6-: 633 17 Apr. 29 | Sept. 30 154 Mascoutah (65)..........- 425 11 Apr. 22] Oct. 14 175 McLeansboro (66)........ 462 12 Apry 17 Oct, Ae 180 MiaMOBIG(GD) soe a. oo. ostee 745 13 Apr | +27, 1'Oct: 8 164 Monmouth (@F)...--..... 784 15 Apr. 289) Oct) 710 165 BVIOTTIRON! oie 486 15 Apr. 21 | Oct. a6 178 WitawaG4)) ce. .ds sheets 500 19 Apr. 26] Oct. 8 165 Palestine: (66)\s.% «2... sc 500 18 Apr: 18i-|) Oct: al2 Uy if HsETIse OE) /cscee else] sere © esi ae 692 13 Apr: 22) Oct. 16 Vif IS(Go)ias Hee vices Se sides 600 16 Apr 22.) Oct» 419 180 Peoria (G5) cin ae oo ele-es sro 609 53 Apr. 15) |/Oct.. 18 186 Blon(G5) seen cea sale 700 21 Apr. 28 | Sept. 30 155 BPONTIAGCH(OD))jorcteres «0 vs svete 546 6 May 1) Oct. 14 166 Rrantowle(G5)icck sl. acess cise 768 17 Apr. 26 | Oct. 7 164 Evcinhwalle: (65))sc.c0.0<0 2 5s 670 17 ADEM con Octen be 172 te lon (6G) iets vc: ete «1-2 fone 459 12 Apr. 12:1 Oct. 16 187 Springfield (65)........... 609 29 Apr... 18) | Oct, 17 182 SErCRbOR (G4)eank. 2 cies sciew 616 16 Apr. 30 | Oct. 8 161 Sycamore (64)............ 855 16 May 41] Oct. 1 150 Maiden) (66). sea shace eet 500 16 Apr, 13} Oct;. 14 184 msi) (G4)isce. si. s1e- « oles 798 14 Apr, 28.) Oct: 14 169 METTONGGG) ees «6 oie ss cine 515 8 Apr. .2o |) Oct: 7 167 WValnit (64) 20.5.2 66s fies 717 16 Apr. 24 | Oct. 8 167 Winnebago (64)........... 900 20 May ia2 tsOrt.) m2 153 Indiana: PAIGEESON, (OF) sic. 2 «0's 1+ «0.0% 892 14 Apr «lor Oct... £1 169 PRT OIA (BE) as oe 2 ateles & dite 1,052 15 Apr. 30 | Oct. 8 161 PRINIUTTN (OU) roc res's os .o-2 6 ee 874 13 May 31] Sept. 29 149 Bloomington (68)......... 800 12 Apr. 19] Oct. 20 184 OT (OM lies cae seal 835 13 May a7. |- Oct." <2 148 Butlerville (68)........... 767 16 Apri a. 25. |-Oct., oll 169 Collegeville (67).......... 662 9 May 3 | Oct. 7 157 Molimbus’ (68). 5-0... - 726 632 16 Apr. 26 | Oct. 6 163 i —_—": 170 ENVIRONMENTAL CONDITIONS. TABLE 2.—Frost data and length of average frostless season for 1803 stations in the United States. (Plate 34.)—Continued. Average date of— Length of average frostless season. No. of Station. Altitude. years of >| record. | Last frost | First frost in spring. jin autumn. (ORS ie eee Ce eee ee ren Indiana—Continued: Connersville (H) Apr. 27 | Oct. 3 Delphi (67) May 3 | Sept. 30 Evansville (68) Apr. 7 | Oct. Farmersburg (68) Apr. 22! Oct. Farmland (67) Apr. 25 | Oct. Fort Wayne (67) May 2] Oct. Greenfield (68) Apr. 21 | Oct. Greensburg (68) Apr. 22 | Oct. Hammond (67) Apr. 27 | Oct. Huntington (67) May 4] Oct. Indianapolis (68) Apr. 16 | Oct. Jeffersonville (68) Apr. Oct. Kokomo (67) Apr. Oct. Lafayette (67) Apr. Oct. Laporte (67) May Oct. Logansport (67) Apr. Oct. Marengo (68) Apr. Oct. Marion (67) May Oct. Mauzy (68) May Oct. Moore’s Hill (68) May Oct. Mount Vernon (68) Apr. Oct. Northfield (67) May Oct. Paoli (68) Apr. Oct. Princeton (68) Apr. Oct. Richmond (68) May Oct. Rockville (68) Apr. Oct. Rome (68) Apr. Oct. Scottsburg (68) Apr. Oct. Seymour (68) Apr. Oct. South Bend (67) ; May Oct. Terre Haute (68) Apr. Oct. Veedersburg (67) May Oct. Apr. Oct. Vincennes (68) Apr. Oct. Washington (68) Apr. Oct. Worthington (68) Apr. Iowa: Algona (54) Apr. May Amana (H) Apr. Atlantic (H) May Belle Plaine (H).......... May Bonaparte (H) Apr. Carroll (H) May Cedar Rapids (54) Apr. Charles City (54) May Clarinda (54) Apr. Clinton (H) Apr. Corning (H) Apr. Corydon (54) May Davenport (54) Apr. Decorah (54) May Des Moines (54) Apr. Dubuque (54) Apr. Elkader (H) a ee ee) OAK WONUNIMHAOOCAWAN _ to NoOoOrReE NO hoe to bo NR eee ee RPOOCOrRANNWOAWNWNO as a a. CLIMATIC CONDITIONS OF THE UNITED STATES. 171 TABLE 2.—Frost data and length of average frostless season for 1803 stations in the United States. (Plate 34.)—Continued. Wa: of Average date of— Length of Station. Altitude. years of |_—————_—______- chi record. Last frost | First frost rostless in spring. jin autumn. en Iowa—Continued: feet. days. HMayette: (H) .sctec. sees vie OOS ber ile ose May 8 | Sept. 18 133 Horest City (54)). 2025... 2%: 1,226 16 May 5 | Sept. 26 144 fercenfeld (Hoes. ce os. TSO) Shei abier ak: Apr. 29 | Oct. 9 163 Grundy Center (H)....... O00 N.C iicoteee tc May 3 | Sept. 26 146 Hampton (54)).\02 v + ote Evansville (76)........... Greensburg (76).......... Hopkinsville (76)......... EDV INUOUL CTO) tare ae itiels Steels sien sens oe Leitchfield (76)........... (exington'(75)s% i.» lw MATOttOH(7O) sex ss vo hsv c end owisville (76)... «002+. cee Maysville (75)i... beicces sce Middlesboro (75)......... Mount Sterling (75)....... Owensboro (76)........... IPACUIGHINULO) bay caciearciche RICH MCG Ce Diane vis os ee PAGO Gi aad etek eeaie chs. crane aiRitll k aik, sant Beet ee Shelby City (75).......... Shelbyville (76)........... Williamsburg (75)......... Louisiana: Abbeville (45)............ CLIMATIC CONDITIONS OF THE UNITED STATES. 173 Tas LE 2.—Frost data and length of average frostless season for 1803 stations in the United States. (Plate 34.)—Continued. No. of Average date of— Length of Station. Altitude. VERLSOLAsy— aS a See record. Last frost | First frost pera in spring. jin autumn. Louisiana—Continued: feet. days mlexangria (46). sas «sieve 77 19 Mar. 8 | Nov. 10 247 pArntte: (45) )-) setae, <0. 24-2 0s )eue 130 18 Mar. 13 | Nov. 10 242 Baton Rouge (45)......... 35 15 Mar. 1 | Nov. 19 263 Burrwood (45))2.... 0603.55 1 14 Heb: «8. | Dee: 175 300 Wathoun’(46))2 oh )5.2i< od 08): 180 15 Mar. 15 | Nov. 9 239 @Wameron (45))2. 6.560065. 6 15 Feb. 22 | Nov. 26 BAHT Cheneyville (45).......... 67 18 Mar. 1]| Nov. 8 252 MimtoniG5) .eeb esas . arciec, ccc eo es Gas esi 16 May 16 | Sept. 15 122 PGR ULIO(D) 1 oan A age ec 5 May 22 | Sept. 15 116 Gardiner (106)........... 163 16 May 6 | Oct. 1 148 Hiewiston (106).4 5. 22. «<-.- 185 24 May 5] Oct. 2 150 Mayfield (106)........... 1,000 18 May 17 | Sept. 22 128 Millinocket (106)......... 386 6 May 16 | Sept. 20 127 North Bridgton (106)..... 450 15 May 15 | Sept. 15 123 ASrorig | (LOG) 2k . 62s ties a2 NMancelons (62)". .... os.6.o5r. Mramistec:(G2)'s5.)<.< bese Marquette (61)........... Menominee (61).......... Muskegon (62)........... Newberry (61)... 2. 5.6.)s2: MEET (OD) )-.02 22s + aise cieiais CL SSavEny (83) Port Huron (63))........ 2; Saginaw, W. S. (63)....... Brlonace (Ol)... 5 ves. cle RaerOSepa(G2)ic..). vc--s siete Barna (G2) ois /ee)0 ses, «200 Sault Ste. Marie (61)...... Thomaston (61).......... Traverse City (62)........ Wiaseni (G2) =. 0.666 bos sh S. Whitefish Point (61)...... Minnesota: Bird Island (55).......... Collegeville (56).......... Brookston (D7) i.j-.. stres. 5 LOTT Glo) ae eee Fairmont (n) (55)......... Fergus Falls (55)......... Grand Meadow (56)....... Lake Winnibigoshish (57).. ong Prairie) (55)... 2.555% feaverne) CED) 235,566.50 ..0 2% MPRRTNCL 2529) sos ete ohece, 6) ovis. 65.0 Cer G9) a Minneapolis (56).......... Moorhead: (57)):..°. = ss. . 2% Lainey EG) Vee eee (it ee oro) 0 397) ee ae New London (55)......... New Ulm (55)........ ; PAT RApICS (B76: 0.60.3 6058 Pine River Dam (56)...... eed Wane (56)ii:. 5s. soot Rolling Green (H)........ Str Charles (56): 2. 02%...2 8) bo Ww DONNANOUNWO OD OM DO = Average date of— Last frost | First frost in spring. jin autumn. May. (iti Octi7 12 May 18 | Sept. 18 May 11 | Oct. 5 May 17 | Sept. 27 May 16 |} Oct. 4 May 13 |} Oct. 1 May 10] Sept. 15 May 8 |} Oct. tl June 7| Sept. 5 June 31] Sept. 15 May 23 | Sept. 14 May 41] Oct. 7 May 41] Oct. 9 May 24] Sept. 28 May 13 | Oct. 3 May 15 | Oct. 2 May 16 | Oct. 6 May 5 | Sept. 29 May 29 | Sept. 11 May 7] Oct. 15) May 28 | Sept. 13 May 7 | Oct. 9 May 9 | Oct. 1 May 11 | Sept. 28 Apr. Lil Octet May 13 | Oct. 2 May 14 | Sept. 29 June 8] Sept. 1 May 17 |} Oct. 5 May 41 Oct. 11 May 17] Oct. 10 May 9] Sept. 25 May 7 | Sept. 28 May 15 | Sept. 22 May 4 | Oct. 3 May 6 | Oct. 3 May 11 | Sept. 22 May 16 | Sept. 25 May 22 | Sept. 24 May 18 | Sept. 23 May 11 | Sept. 19 May 12 | Sept. 25 May 14 | Sept. 21 Apr. 29 | Oct. hi May 13 | Sept. 22 May 12 | Sept. 25 June 3] Sept. 9 May 9 | Sept. 30 May 6 | Sept. 27 May 20] Sept. 19 May 18] Sept. 20 May 4] Oct. 11 May 4} Oct. 3 May 16 | Sept. 27 Length of average frostless season. days. 164 123 147 133 141 141 128 152 92 104 114 156 158 127 143 140 143 147 105 151 108 155 145 140 178 142 138 85 141 160 146 139 144 130 152 150 134 132 125 128 131 136 130 161 132 136 98 144 144 122 125 160 152 134 176 ENVIRONMENTAL CONDITIONS. TABLE 2.—Frost data and length of average frostless season for 1803 stations in the United States. (Plate 34.)—Continued. ee Average date of— Length of Station. Altitude. years of Pir record. | Last frost | First frost pianist in spring. jin autumn. ia cis Minnesota—Continued: feet. days. St. vPavli(G6) esc. aes Sate 848 36 Apr. 27 | Oct. 3 159 St: Peter|(56)- 2%... ns = ns 840 14 May 17 | Sept. 27 133 St. Vincent Pembine (57).. 789 14 June 3 | Sept. 14 103 Sandy Lake Dam (57)..... 1,234 17 May 21 | Sept. 23 125 Shakopee (56)........... 750 14 May 12 | Sept. 25 136 TV ELACD Es cimeteleieis trail aga 1,350 ia June 5 | Sept. 13 100 Wabasha: (DS) ikintec\\cis- > sake 681 14 May 1 | Oct. 5 157 Winnebago (56)........-- 1,100 il May 6 | Sept. 28 145 Worthington (55)......... 979 15 May 7 | Sept. 23 139 Mississippi: VANIStiTI CUO) eakiae stes os bt iene 200 15 Mar. 30 | Oct. 29 213 Batesville (Eset. we. eet PZ Nile oe start tokske Mar. 24 | Oct. 24 214 Biloxi (SO). scene eici: cies erences 24 18 Feb. 25 | Nov. 28 276 Booneville (79)......-.+-- 504 15 Mar. 29 | Nov. 1 217 Brookhaven (80).......--- 500 16 Mar. 18 | Nov. 4 231 Ganton(S0)seicey crags 228 16 Mar. 19 | Nov. 4 230 Columbus C(O )ee seeks ote 250 14 Mar. 27} Oct. 31 218 Crystal Springs (80)....... 468 16 Mar. 24} Oct. 31 221 Greenville (10) ee onal aere 126 18 Mar. 18 | Oct. 31 227 Greenwood (79).......--- - 140 14 Mar. 19} Oct. 27 222 Hattiesburg (80).......-.- 154 16 Mar. 11 | Nov. 9 243 Holly Springs (79).....-.- 600 14 Mar. 28 | Nov. 2 219 ealkcesvilles(SO)s acters snckelal= otebelslatetescrora 16 Mar. 5] Nov. 10 250 Mowisvalle: (ZO) iene oes shane 561 16 Mar. 26 | Oct. 30 218 Magnolia (80).......----- 415 15 Mar. 15 | Nov. 9 239 Meridian: (SO) ices «..cnk fe) 375 20 Mar. 17] Nov. 2 230 Natchez, (80)\...--.2<-ss0: 206 16 Mar. 9] Nov. 12 248 Palo vAltio! CE) eas oi: ain Carer SOO Mh ccisre ares Mar. 27 | Nov. 4 222 Pearlington (80)........-- 10 16 Mar. 1] Nov. 24 268 Pontotoc: O)ines > seivel- cle 475 18 Mar. 28 | Oct. 28 214 Vicksburg (80)i2 200.255 247 38 Mar. 6] Nov. 13 252 Water Valley (79)......... 300 17 Mar. 27 | Oct. sl 218 Waynesboro (80)......--. 191 17 Mar. 20} Nov. 4 229 Woodville (80).........-- 560 16 Mar. 12 | Nov. 14 247 MWiazOO Gilby (OU) ais cin sisiebels 116 16 Mar. 28 | Nov. 2 219 Missouri: Appleton (49) ...--0..eccelecsee ee eceee 18 Apr. 20 | Oct. 19 182 ART OIONUCD Lis see ie cit data siete alle sees meted 12 Apr. 22 | Oct. 12 173 HOLL iecreniets ne wg ae Slave 1,000 8 Apr. 17 | Oct. 15 181 Bethanyvaol ees ks ase se 881 16 Apr. 26] Oct. 8 165 Birchtreo: (50)ic. kee. s nee 1,200 16 Apr. 17] Oct. 16 182 Brunswick (51)........... 652 20 Apr. 17 | Oct. 19 185 Caruthersville (50)........ 860 13 Apr. 5} Oct. -22 199 Molde bes (BD) een sie crarose Wathe 784 20 Apr. 18] Oct. 14 179 Darksville (51)........... 826 17 Apr. 23] Oct. 14 174 PGRN TEN) erecta ts vie. crave. 3twiaxllie Sis ule wie: 6: > Seve 10 Apr. 16] Oct. 13 180 WIOSOUOUDU) sie siowesc ness 498 7 Apr. 21 | Oct. 12 174 airport: (BSL )iaes «© s:c' © av 535 12 May _1 | Oct. 8 160 Wavette SL). cs/ceses s cies 725 24 Apr. 21 | Oct. 14 176 BEOM BL) hates ls cin's oars 818 14 Apr. 21 | Oct. 8 170 Gallatin (GD) ivice sy tals <0 803 17 Apr. 17 | Oct. 19 185 GZan6 (HO) a cvanke es cun void els wees pe sisieis 6 Apr. 23 | Oct. 15 175 COMIDACGD) unin a so ain cs Wed 700 15 Apr. 29 | Oct. 9 164 Grant: City’ (G1)... a2 6 een 1,130 12 Apr. 26 | Oct. 13 170 Greenville (60)... sec ecards ane ceccees 12 Apr. 16 | Oct. 9 176 * CLIMATIC CONDITIONS OF THE UNITED STATES. 177 TABLE 2.—Frost data and length of average frostless season for 1803 stations in the United States. (Plate 34.)—Continued. Weck Average date of— Length of Station. Altitude. Years ol eee cee Fees record. | Last frost | First frost | “TC8“©S$ in spring. /in autumn. pass Missouri—Continued: feet. days. Hannibal (65)io2....0 27 «<< es 534 17 Apr, 16) Oct. 16 183 Harrisonville (49)......... 912 19 Apr: 19))| Oct. 26 190 Houston (50) hess. Ghee stecas 1,280 17 Apr. 25 | Oct. 9 167 Arontoni(a0) ees cce ee. ocd 925 21 Apr;, 28! | Oct. 3 158 Sackson? (50) 22.5.2: alesis 2 458 17 Apr. 22 | Oct. 11 172 Jefferson City (51)........ 628 16 Apr. 19] Oct. 15 179 Joplin (49) ).ssh: hoses 979 6 Apr. 15] Oct. 19 187 Kansas City (51)......... 963 20 Apr. 10 | Oct. 23 196 Koshkonong (50)......... 911 9 Apr. 15 | Oct. 22 190 Mamar (4Q)ioo. 085.606). 005)5 964 18 Apr 18albOct. a la 182 Hebanon (49))......052...2. 1,265 18 Apr. 14 |hOct.. Lz, 186 exanpion! (O))ee cect ee oe ee 813 16 Apr 7 NOetas 7 183 MEE DErGy (DL) ccc a Fie + avers 864 18 Apr. 20 | Oct. 8 171 Mouintany (Ol)es.5.a< de5.5 500 11 Apres 1225|) Oct le 173 EACOM (CO) ioe oe 881 8 Apr. 20] Oct. 16 179 Marble Hill (50).......... 420 16 Apr. 19} Oct. 13 177 Marshall (49)............. 779 16 Apr. 19) Oct. 11 175 Maryville (51))...35.0..-.04- 1,160 15 Apr. 21 | Oct. 7 169 NEEXACO (DL) Secs c1e:ds as oi 797 30 Apr. 16] Oct. 16 183 RVTSERIATECOL!) Stigere sy Siete swe 622 9 Apr 1215) Octaer te 174 Mineralspring: (49) 25.2. S25]: lea o faces © 12 Apr. 14] Oct. 16 185 ogi OE) ee ae, || 10 Apr. 19 | Oct. 5 169 Mountain Grove (49)...... 1,490 8 Apr. 23 | Oct. 14 174 Mt. Vernon (49).......... 1,480 16 Apr. 25 | Oct. 8 166 IWeoshoi (49) o..152 016 21h. ose 1,023 17 Apr. 24>] Oets 15 174 Wevadar(49) ... 02 cd ao ancl 860 1? Apr. 19] Oct. 14 178 New Madrid (50)......... 285 5 Apr, 24 | Oct. 20 179 @alfield'(50)):...2.:..550...06° 793 17 Apr. 16)Octrr 22 189 (CG beak (50) ie ae a ee ee 1,246 Me Apr. 16} Oct. 18 185 MPEP GRC L) she ete alls Sais: eae 1,013 19 Apr. 25 | Oct. 14 172 Poplar Bluff (50) .......... 344 9 Apres, og Octal 193 Princeton (Ol) posses: «snc 1,026 18 Apr. 24 | Oct. 9 168 vehmond: (1)! 45 .0.s1h< Sere 824 7 Apr: 7 | Oct. 24 200 ESD MLANCSO) arc alacials. fecs-<. ser 1,092 13 Apr 20 |) Oct. 15 178 St. Joseph (51)).-3 oc. 5 356 =. 825 8 Apr. 24 | Oct. 17 176 Rob aylrO018 (GB) oie co:5 0 al oe ores 568 35 Apr: 3 | Oct. 27 207 Sedalia (49)).:cciels occ siis se seis 889 14 Apr. 19 | Oct. 19 183 “EGETCGTTE CLD) RG AO IEEE | | 80 Bice oe eae 12 Apr. 19 | Oct. 11 175 Shelbinay (51) xcs. oc oi ese 781 8 Apr. 24 | Oct. 10 169 Rikestom (50). joi ose). eee 328 14 Apr. 10 | Oct. 18 191 Springfield (49)........... 1,350 20 Apr. 14 | Oct. 18 187 Steffenville (51)........... 576 14 Apr. 24] Oct. 10 169 DIE CE ACL) rcwnto oie So0i oe bets 1,000 15 Apr, ‘1 |! Oct 45 187 Mirenton (51) s.1o.6-o06 5 oe 812 13 Apr. 20 | Oct. 9 172 Wnionville:(61)./........ 5. 1,072 15 Apr. 25 | Oct... 13 171 Warrensburg (49)......... 883 16 Apr: 18) j7Octs 17 182 Warrenton (51))....:..../.- 865 18 Apr; 21°} Oct; 15 177 Wheatland (49)........... 920 13 Apr. 18 | Oct. 8 173 ATI EOT (AD) ors 5 s:<1-cielens Siecle shouela cititie 0% 17 Apr, 21), |) Oct. 18 180 Montana: PRRRELN( DO) is osc. c els aie oben ates 5,200 10 June 25 | Aug. 24 60 Agricultural College (27)... 4,700 8 May 28] Sept. 7 102 MANACONGA (28)). oe 2 se. wale 5,300 8 June 13 | Sept. 4 83 LEGO 2) | 4,071 10 June 6] Aug. 25 80 i415 | oh) C-22) Ae eee 4,461 4 June 19 | Aug. 11 53 178 Montana—Continued: ENVIRONMENTAL CONDITIONS. TABLE 2.—Frost data and length of average frostless season for 1803 stations in the United States. (Plate 34.)—Continued. Mascal Average date of— Length of average frostless Altitude. Wey | == record. | Last frost | First frost in spring. Jin autumn. j—— ee | | Station. Banigan(26)\ cicaaiees wares - Boulder (27)... 5 cme oe i Busby. (26) pccss cco cc ctllt siuesteeetents ; BiUtten(27)\..cit so eckesine a ee 5 Canyon Ferry (27)........ : Cascade (29) sc. «dec achs o @hester(Z0)e- sence oer é Chinook (30) 532.12. 00 ae : CWhoutest\(29). 0. ..556.2-6 3 Clear Creelo(29) oe aseeelicu cote oe ; Columbia Falls (28)....... y Copper (27) oasssate see ally wee Hee ke wes 4 June 11} Aug. 3 53 Crow Agency (26)...... 27 May 15 | Sept. 26 134 Culbertson (30)........... 7 May 30] Sept. 5 98 Gut Bank.(29)te doses oe a June 14] Aug. 29 76 Dayton (28). wccccecu sen 6 May 17 | Sept. 15 121 Decker, (26) cic retee oace 5 May 23 | Sept. 7 107 Deer Lodge (28).......... 8 June 16] Sept. 6 82 Dallons(27) sas- es tere oe 9 June 9 | Sept. 1 84 East Anaconda (28)....... 4 June 1 | Sept. 14 105 Pkalaka (26)asse wnt cette this che. oe i May 22 | Sept. 22 123 Fallon (26) .esoesemees coe 4 May 22 | Sept. 29 130 Porsyth)(26) eek sc csee 3} May 9 | Sept. 29 143 Fort Benton (29)......... 9 May 16 | Sept. 30 137 Wortine, (28) ers .cc seas ee 3 May 31] Aug. 6 67 Hore: Logan: (27s. tivis ee 12 June 16] Aug. 30 75 Hort: Shaw (29)\). .22. 2000 2 May 10] Sept. 16 129 Glassow (30) ..5 .. fee 14 May 22 | Sept. 12 113 Glendive: (30). . 2 kaos Ae 16 May 12 | Sept. 22 133 Sold Bute.9)5 .\. sts saliva cies tebe ees 3 May 22] Sept. 1 102 Great Falls (29).......... 18 May 7 | Sept. 16 132 Eramiulton:(28)ie. .).ce> cee 7 May 12 | Sept. 24 135 Harlowton) (27) scicccbs oa 15 June 6 | Sept. 4 90 evra. (20) cette. ss tela xen 11 May 15 | Sept. 14 122 Helens (OZ)... iar eeece ek AUN LOY? (2G6)iantes cues ste DOLGAMA BU) ane wa Meee alien suc aes Mansel (28)ee. wee. vee Lewistown (29). ... ee 3;96b. | es chee es May 25] Oct. 4 132 Gardnerville (12)......... A S30 -ilt.c ameeeere « June 15 | Sept. 23 100 Gevperi(@2): sotick Sosse wah isbe lis Galerie cl alerail everest 6 June 23 | Sept. 3 72 Lewer’s Ranch (12)....... C262 Oe escstenete tare May 26] Sept. 28 125 op rent (1:2) vied tok cree vee E5700). Biss aehiras Apr. 14] Nov. 6 206 ovelogk (12) ..aa. sce SONG. Wiewenar aes May 22] Sept. 22 123 Palmetto (12)ee. «23. Sek 6,500). [vc eesti May 30] Sept. 21 114 Potts (12).. bee. 8088; “dis ees June 16] Oct. 2{ 108 * Quinn Riv. Ranch 12) sated, A860). | |idswaehtdawa June 19 | Sept. 6 79 Reno (12).. - » tah A BSB) dame May 16] Sept. 31 138 Tecoma (12). tg OE rey 4,812) ° lin seine May 28 | Sept. 14 109 Winnemucca (12)......... 4,344 Rw aaineweg May 15 | Sept. 23 131 New Hampshire: Bethlehem (105).......... 1,470 18 May 22} Sept. 19 120 Clonsora ClO) eo vets os eG 350 38 May 7 | Sept. 30 146 UPHAM CE ites ss se aes es OR Nec chee ae May 8 | Oct. 3 148 HIAnGverClOB) ihc, «nt 603 24 May 18 | Sept. 25 130 ieene (LOG) Saks. ol. cet 506 16 May 16 | Sept. 20 127 NeshaaiGLOayes’ Jc.8 es. .44 125 24 May 5| Sept. 10 128 Piymputn CLO

, :jtcu ae 498 (H) 38 May 1] Oct. 19 171 [Rome Chl). cateoock ce ae Sere 250M odlies Acaeceeats May 10 | Sept. 30 143 Saranac Lake (104)....... 1,620 16 May 24 | Sept. 13 112 Setauket (104)............ 40 20 Apr. 11] Nov. 8 211 Bo. Canisteo OL) i, ce .c:stemisie ete tecactettare 10 May 21 | Sept. 20 122 Syracuse (102)............ 579 6 Apr. 28 | Oct. 16 171 Watertown (102).......... 737 9 May 91] Oct. 11 155 Wedgewood (103)......... 1,430 9 May 11] Oct. 10 152 West Point (104)......... 167 9 Apr. 20] Oct. 17 180 North Carolina: Asheville 7S)ie co. ose s ote 2,255 6 Apr. 20] Oct. 13 176 BenUfore (OL Neca il. gen snc 10 8 Mar. 15 | Dec 8 268 Brewers (89)............- 1,950 5 Apr. 21 | Oct. 11 173 Caroleen (89) ..........55- 806 9 Apr. 17 | Oct. 27 183 Chapel Hill (90).......... 500 25 Apr. 8 | Oct. 30 205 Charlotte (SG) s<5.5056 svc 773 30 Mar. 29 | Nov. 4 220 Madentan (OL) cbc. naele<.s oe 30 15 Apr. .3]| Nov. 2 213 Fayetteville (90).......... 170 13 Apr. 4{WNov. 7 217 Goldsboro (90)........... 102 12 Ape: . 6 iv NGvs is 210 Greensboro (90).......... 843 15 Apr (7 {Ook a6 201 PRG GtGN ee (OL): ecw uscsy salve 11 33 Feb. 28 | Nov. 11 256 Henderson (91)........... 490 15 Apr. 7 | Oct. 31 207 Highlands (Ei scwsns. sen BS SIs) AL steam May 5 |} Oct. 7 155 Horse Cove (78).......... 2,800 17 Apr. 20 | Oct. 22 185 FAIOSTON WOO) cehwes esi cs 46 s Apr. 7 | Oct. 29 205 IGNOM (SU) tes wae eek stk 1,186 28 Apr. 18 | Oct. 18 183 PABNWIS CVS ones tices sar 3,800 13 May 3 | Sept. 30 150 Ratiloton (OL) hes acs cae 380 14 Apr. 8 | Oct. 27 202 10 | Oct. 29 202 Loumbure (G2) .5 ss. .hes sa 375 17 Apr. CLIMATIC CONDITIONS OF THE UNITED STATES. 183 TABLE 2.—Frost data and length of average frostless season for 1803 stations in the United States. (Plate 34.)—Continued. Average date of— Length of average frostless season. No. of Altitude. QO) SS record. | Last frost | First frost in spring. jin autumn. ——— | — |_| _ Station. North Carolina—Continued: Lumberton (90)........... Mianten) (91))s3y328,ceeavoein MT ariOn4(S9) oct aieke <. 4 5i0 Mocksville (90)........... Moncure (90)............ TORO: (89) eo acisooabio « cleus Morganton (89).......... NEGATE: (OO) ciohs:c.cfa% <) 015) 04 Mt. Pleasant (89)......... Nashville (O12) 23... 28.0:s'2a:6% Wewbern: (91) ii). 6/15 «sk. Pakride) (90) 25. «245 55 Gisye PAtbErson\(S9).as.<<:2 sls. Syed tts poro (90) saq ss... 6 ansr0i- tench (90), secs. aces «oper eeamseur(90)).c1. <, Stee te Goal Harbor (81)... : . <<< Devil’s Lake (32)......... PRokanson (81)... May 8 | Sept. 15 130 AVVRLIISTORIE (bs chs ere ict e store 1,872 30 May 18 | Sept. 14 119 Willow City (81).......... 1,471 17 May 30 Sept. 11 104 Ohio: Acron(69))cs nee ae ciete eer 1,081 18 Apr. 27] Oct. 9 165 Bangorville (72).......... 1,380 22 May 6}! Oct. 3 150 Bellefontaine (70)......... 1,276 15 Apr. 28 | Oct. T 162 Bowling Green (69)....... 670 17 May 10] Oct. 2 145 Bucyrus (69)... 6210.12 1,000 15 May 91] Oct. 3 147 Cambridge (72). 3). 4.0%...) 803 16 May 5 | Sept. 28 146 Camp Dennison (70)...... 570 16 Apr. -25"| Oct. “it 169 Canal Dover (72)....... 5: 884 15 May 8 | Oct. 2 147 @antony(69)/ fe eas: cf etors's ots 1,065 18 Apr. 27 | Oct. 5 161 Cincinatti! (70))e0 2.03 31 628 37 Apr. 14 | Oct: 265 194 Gircleyille!(71) eo eeccices er 694 14 Apr. 28 | Oct. 5 160 Glarksville (70)=. 2+ -. oat 1,010 16 Apr. -25-| Oct. 7 165 Cleveland!(69)°: 5.05.25. 762 38 Apr. 16 | Oct. 31 198 Goalton (71)ec er ci atereietet 718 14 May 3] Oct. 4 154 Golumbus (Gl) tects oe 918 31 Apr: 16+| Oct: “A7 184 ID ygrorn, (ff U)iegeaeousoms oki 790 15 Apr. 27 | Oct. 10 166 Wefiance:(69))-.ua. ssn ste 712 15 May 7 | Sept. 29 145 Welaware (il )iiek acts sce cre 927 13 May 3 | Oct. 3 153 Wemids((2) sac cre acc ee cee 1,325 18 May 3) Oct. 10 160 Hindlay#(69)i2. «....2005 25 776 18 May 3] Oct. 4 154 Garrettsville (69)......... 1,005 22 May 18 | Sept. 29 134 Granville (72) ).cc) Ase cxocmeteeed ae May 21 | Sept. 18 120 HLOSEHUGN (GS) san eaccs eo 2,600 14 May 10 | Sept. 25 138 Sioux Malis (34) :.:.......- 1,400 18 May 12] Sept. 19 130 Bearish (83)... 2 ol soe oar 3,647 18 May 9 | Sept. 27 141 abrondall, (kL) 3. wcicie ae ices A ATR f Este ape stets May 6] Sept. 23 140 PY AICHON (4). cc cectear a new 1,234 33 May 2] Oct. 38 154 Tennessee: Benton is) iy ssw a ctieteteee 880 15 Apr. 17 | Oct. 19 185 PSOMIVER (2 0) so 2a x ste cies ere 450 14 Apr. 2] Oct. 28 209 Bierdstown (H)....;.05..2..¢ 1,026) 9) Hseuaetecs Apr. 11 | Oct. 20 192 CRYUNAUE CGT) cea als aienion & 500 13 Apr. - ‘7 | Oot. "28 200 Chattanooga (78)......... 808 30 Apr. 2 | Oct. 26 207 Clarksville (77)\2¢. oes. dees 520 15 Apr. 10 | Oct. 28 201 ECHL CPS) we satel cd mcis 850 13 Apr. 17 | Oct. 23 189 Elizabethton (H)......... 2 Gua le .cucttemtetee Apr. 22 | Oct. 21 182 tie Toren sl @ 42) Lia Re SR yc wr 1,850 12 Apr. 21 | Oct. 15 177 MIDPONUS Ga) coke ii tse sie 560 10 Apr. 10] Oct. 21 194 Greeneville (78)........... 1,581 14 Apr. 19] Oct. 19 183 Hohenwald (77).........: 983 14 Apr. 19 | Oct. 9 173 ROEBON HT tacit ssl. c sn-cie as 450 10 Apr. 4] Oct. 30 209 Johnsonville (77)......... 364 13 Apr. 10 | Oct. 20 193 ISTIONWILG (iG) cblecvees ce wee 977 38 Apr. 3] Oct. 28 208 NGORINCIUE 77) oss sic anes ced 770 10 Apr. 8] Oct. 20 195 INPORATINIO CLT) vodis' acen kes 409 38 Mar. 21] Oct. 31 224 Mountain City (78)....... 2,486 ll Apr. 28] Oct. 15 170 INBANVIUG' (27) cones vcpn uae 573 38 Apr. 2] Oct. 26 207 INGWPOFt CEL) sacpcmcawe an A a Re i RE Apr. 12] Oct. 30 201 Rogersville (78)..........- 1,150 16 Apr. 17 | Oct. 19 185 CLIMATIC CONDITIONS OF THE UNITED STATES. 189 TABLE 2.—Frost data and length of average frostless season for 1803 stations in the United States. (Plate 34.)—Continued. No.of Average date of— Length of Station. Altitude. years:Of 8 (=> a i ened record. | Last frost | First frost wal in spring. jin autumn. ; Tennessee—Continued: feet. days. era iS) ceieets cote «: «ais 1,410 16 Apr. 22 | Oct. 13 174 Bavannan (1))26 «ales «errs 350 8 Apr. 14:| Oct., (21 190 Shenandoah (93)........-- 937 3 Apr. 29 | Oct; «8 163 Spottsville (92)..........-- 15 17 Apr. 17] Oct. 16 182 Stanardsville (93)......... 670 12 Apr. “12: Oct.s 2b 196 Staunton (93)..... Riateitconts 1,380 17 Apr. 25] Oct. 12 170 Stephens City (94)........ 710 14 Apr: 21 | Oct) ae 176 Sunbeam (92).........--- 60 6 Apr. 11;.| Oat: > 32 203 WATS wi (OS) ier + 1 class's wie 200 16 Apr. 15 | Oct. 22 190 Wiest Broo k(OS))s 5 se ties sepuslllevctete Gass oe 4 Apr: 15. | Oct. ).26 188 Williamsburg (93)......... 74 8 Apr. 9 | Oct. 26 200 Woodstock (94)..........- 927 11 Apr. 22 | Oct. 7! 168 Wytheville (74)........... 2,293 15 Apr. 18 | Oct. 10 175 Washington: A Derdeen! (LO) sieie1- sice)> mise 162 (H) 10 Apr. (27 |;Oet.. 222 178 Belling 1619) is.jrrsy- (sol sv-teccie, oie ates 10 Apr. 22°|@ot. +221 182 G@entraliay(19)ing. one - aa 212 (H) 9 May 6] Oct. 14 161 Glearwater (lOve. pumas medic tere fear le 9 Apr. 27.) Nove 67 194 @le. Blum (20) nai. oc. cee 1 ,930 9 June 9 | Sept. 7 90 Colfax:(20)). estes ans): och. 2,300 13 May 25 | Sept. 10 108 Golvallen(20) ie caictainte lore . aes record. Last frost | First frost in spring. jin autumn. sg bara West Virginia—Continued: feet. days. Pilkhorm (EH). 2 s53.25024 TL OSSr an Alot trie Apr 245 Oct 17 176 MOLEING (G3) lce cite te ose we 1,940 10 May 18 |} Oct. 10 145 renvile(7a)sc.cescces soe 738 16 Apr. 30) |) Oct. Vs 166 EEO ON 0G) aes ope eran cee 985 16 Apr. 30 | Oct. 7 160 PERTTUUONING (A) ticle c/a ba'.s) 5) Saree 1,400 12 Apr = iid fOets 23 189 Huntington (74).......... 510 15 Apr, 19s Octstas rere ewisburg (74). 330 oS. 2,200 10 May 10} Oct. 11 154 HEGRATIGNS)/o8 tes alo aaysvclen 665 8 Apr.) 21) Oct. 24 186 Hast Creek (73) ..:./o5.0 fas 1,033 14 May 6 | Oct. 5 152 Miarlinton, (73)). ). o).).. acne 2,169 10 May 2 | Oct. 1 152 Martinsburg (94)......... EL SITINVe Ga erase tte Apr. 20] Oct. 14 177 Moorefield (94)........... OOOme sheets. oe Apr. 30] Oct. 4 157 Morgantown (73)......... 1,250 16 May 1] Oct. 13 165 New Martinsville (73)..... 634 16 Apr: 28°) Oct. 14 169 Muttallbure (74)... 2... 052% Pee Aap 16 May 1] Oct. 10 162 Parkersburg (73).......... 638 20 ADT “lo Octe. wkd 179 HAATHONSEG Ma) Me aciinias oa ate 1,662 10 May 10 | Oct. 7 150 Philippi (73) es). .es- aed 1,192 11 May 8| Oct. 9 154 PICKS CS) hoc etand sale 2,785 16 May 91] Oct. 14 158 Point Pleasant (73)....... 553 16 Apr. 20] Oct. 20 183 Powellton (M)\.\...05.. vie: GO4e Ca eee es Apr Zon Octs el2 172 sEerra Alta (73)...50. 00085 3,207 10 May 11 | Oct. 1 143 NVElisburg (EL) \.6 6c ec cit Te 225) fo Sites cosets May 3] Oct. 15 165 Wheeling (96)............ G453 1) Fin acceme ne Apr. 1d |.Octr 25 193 Wisconsin: mmmneTss (GO) 8)... 0). oo 8 1,200 18 May 22 | Sept. 27 128 Appleton (GO)... 6... ..se8 795 11 May 6 | Oct. 1 148 PESANIE COS) cies iec?srer eis a2 647 16 May 14 | Sept. 21 130 PFAETOTIN (OS) crore vise ess: secs 1,115 18 May 22 | Sept. 15 116 BplatbrCGO) ates ws wien saet 750 yy Apr.: 16 | Oct. 18 185 Brodhead (60)\5...56. 663.8 812 itil Atprs Lge Octaey lia 183 Butternut (58)........... 1,508 15 June 4] Sept. 9 97 Chilton (BO). o... 6.2 lon 860 15 May 8| Oct. 38 148 Wrandon (60). 5.60.0 2824 1,060 12 June 3] Sept. 14 103 Melavan (60)... swe oe 920 ily Apr. 20] Oct. 16 179 Dodgeville (59)........... 1,116 10 Apr. 25 | Oct. 8 166 Mawinine (58)... 02... 2 52 983 15 May 6) Sept. 24 141 BramsClaires(58))..0..56 6225 800 19 May 10 | Oct. 1 144 Miorence (60))2 0. 3.356. 8202 1,293 18 June 2] Sept. 13 103 Fond du Lac (60)......... 800 22 May 5 | Oct. 2 150 Grand Rapids (59)........ 1,021 11 May 23 | Sept. 26 126 Grantsburg (58).......... 1,095 V7 May 14 | Sept. 19 128 Green Bay (60))...2/0.6..5552% 617 23 May 3} Oct. 3 153 AM COCK | (D9))2. «5.016% «sei 1,091 18 May 18 | Sept. 28 133 efarvey: CEL) Relns cane oo cae pots tebeml i AIDS Creo Mens May 4 | Oct. 1 150 athielde (59) 0 .. secc. 5 oe. s 973 12 May 20 | Sept. 22 125 Hayward (58))....%... 205% 1,197 19 May 26 | Sept. 12 109 EMISHOTO) (59). 3 = Avice Ao oe 1,000 19 May 13 | Sept. 24 134 Koepenick (59)........... 1,683 18 June 3 | Sept. 17 106 MiaC@rosse (59)\..:...% 262 o'er 681 37 Apr. 30 | Oct. 10 163 Lake Mills (60)........... 897 19 Apr. 261Octs) 5 175 Wancaster-(59)!)../52..502. 1,070 18 Apr. 25 | Oct. 10 168 Miadison (G60) iis... thee 974 31 Apr. 22 | Oct. 18 179 Manitowoc (60)........... 616 47 May 9] Oct. 10 154 MVICUBtOM (OO) a2 2,c)s.0 are ae 882 14 May 17 |} Sept. 24 130 194 ENVIRONMENTAL CONDITIONS. TABLE 2.—Frost data and length of average frostless season for 1803 stations in the United States. (Plate 34.)—Continued. Average date of— Length of No. of Station. Altitude. years OL : i: record. | Last frost | First frost | *70SUe5* in spring. jin autumn.| 5©4598- Wisconsin—Continued: feet. days. Meadow Valley (59)....... 974 19 May 21 | Sept. 23 125 Medford (59) 29. i. 8.45. ie 1,420 19 June 3 | Sept. 12 101 Milwaukee (60)...........- 681 39 Apr. 28 | Oct. vf 162 Neillsville (59)............ 996 21 May 23 | Sept. 20 120 New London (60).......-. 762 14 May 14 | Sept. 27 136 Weonto (60)/5.07 i laos ees 590 19 May 10 | Oct. 2 145 Osceola (58) avast 806 19 May 12 | Sept. 26 137 Oshkosh'(60) es oni ee 744. 19 May 7 | Sept. 30 146 Pine River (G0) ....)5. stss2 900 15 May 13 | Sept. 28 138 Portage. (G9) heals. stersie 809 14 May. 3° |}Octy’ 4 154 Port Washington (60)..... 713 16 May 6] Oct. 12 159 Prairie du Chien (59)...... 690 22 Apr. 27°| Oct. 12 168 Prenticei(59) ee soos wee 1,551 11 June 6] Sept. 9 95 eacine’ (60) sce 2 ian sine tee 633 13 Apr; »28) | Oct... 23 168 Shawano (60)............- 796 13 May 14 | Sept. 26 135 Sheboygan (60)........... 831 12 May 8 | Oct. 11 156 Spooner (58)ina-:.,- ae i. 1,104 13 May 24 | Sept. 14 113 Stevens Point (59)........ 1,113 17 May 25 | Sept. 26 124 Valley Junction (59)...... 930 18 May 16 | Sept. 24 131 Wiroquan(59))o2 5b feature 1,412 19 Apr. 30 | Oct. °°5 158 Washburn (H)...........- ARS oy: wens he May 16] Oct. 12 149 Watertown (60).......... 824 18 Apr. 27 | Oct. - 32 167 Waukesha (60)........... 864 14 Apr. . 28 | Oct. .12 167 Waupace (60))b 0. on eee 857 13 May 20 | Sept. 27 130 Wausau (59)0s.. 2282.2 1212 14 May 30 | Sept. 22 115 Weyerhauser (58)......... 1,297 vf May 30 | Sept. 12 105 Whitehall (58)/2.7 2... ni. - 675 17 May 6] Oct. 4 151 Wyoming: Alcova-Pathfinder (24).... 5,366 11 May 21 | Sept. 19 121 I ARITN Gep) ic Heiress tree arene ay, Ailere stieenc eters May 10 | Sept. 20 133 IBtitalo (25) ie. Sas «ieee A OOD. te ih Se aes May 28 | Sept. 21 116 Cheyenne (24) 5; 0.00 asint- 6,088 38 May 21 | Sept. 17 119 Chugwater (24)........... 5, 282 9 June 1 | Sept. 15 106 Aplsark' (25) teas Mawes A SAG The ete doe May 4] Oct. 14 163 Eaton’s Ranch (25)...... A OOO calves aries ssi May 16] Oct. 5 142 Fort Laramie (24)........ 4,270 15 May 15 | Sept. 20 128 Gillette (25) 7310 eee shade AOA ally. getiee ats May 23 | Oct. + 134 MESTIPOBIC2D)) canine bitoh ciate oar (1 Oem fl Fe 1 ae ee May 31] Sept. 8 100 IW AUIE CIO NCO). or so acieaets Coase ccvalee cee May 19 | Sept. 21 125 Bartley (24) e502 ty oes 5,000 6 May 29 | Sept. 17 lll PATIO (AE) i ivisis:c iy Mite we vale I Ta eae carer May 26 | Sept. 11 108 GATING 249% :1¢ oyscie 6 «Soe 7,188 15 May 30 | Sept. 16 109 NG (BAN Ss. 23.5, «'Saxed sabe 5,007 16 May 25 | Sept. 13 lll Moororoft (25). ...¢..s08% yer (i Ti ae A oe ae May 23 | Sept. 20 120 DVLOONG 24) cidcinchac os cites 6,000 9 May 26 | Sept. 14 lll Newcastle (25)........... eR | Pe 3 oS May 22 | Sept. 20 121 PR OAN Ses ieee. wed 4,900 7 May 25 | Sept. 17 115 Pineweitiy (4)).). Poe oes 5,038 8 May 27 | Sept. 19 115 aw Mnn (Saye ee fh pee 6,744 6 June 3 | Sept. 13 102 sdalctgts (1s lt C25) eee re See «Bilis. yas eek May 21 | Sept. 24 126 Shoshone Dam (25)....... Dyaeo.!) Ehcveneass May 26 | Sept. 21 118 Thermopolis (25)......... BSB Pkt Cae May 8 | Sept. 13 128 Wheatland (24). .......%5% 4,741 9 May 11 | Sept. 24 136 Wyn00te (24). vas wacseeege 4,207 3 May 21 | Oct. 2 134 net =i = = CLIMATIC CONDITIONS OF THE UNITED STATES. 195 Most of the data of table 2 have been plotted on a map of the United States, to represent them in a graphic way. After the numerous stations had been located and the average length of their respective frostless seasons had been placed beside the points representing them on the map, lines were traced as accurately as possible through points having a common length of frostless season, a line for each increment of 20 days, beginning with 80. By this means the map was subdivided into 15 sorts of seasonal areas. The first sort has, according to our data, an average frostless season of less than 80 days, the second has a season of 80 to 100 days, and so on by steps of 20 days until the average frostless season for the fourteenth sort of areas is 320 to 340 days, and for the fifteenth, over 340 days. The only one of our stations that is unques- tionably without frost, thus having an average frostless season of 365 days, is Key West, Florida. In tracing the equiseasonal lines we have followed the data of table 2 as accurately as possible, making no attempt to smooth the lines. Topography has been allowed to exert a deciding influence in many cases where observational stations are too far apart satisfactorily to determine the positions of the lines. This is especially the case for that portion of the chart which lies west of the one hundred and fifth meridian of west longitude. In a very few cases the data of single stations have been ignored, where the length of season given in the table is obviously a marked exception for its region, thus suggesting the possibility of error or inadequacy in the data themselves. Wherever a small local area is indicated by the data from two or more stations, however, the area has been shown on the chart. It was found at once that the approximate equiseasonal lines for 20-day increments were altogether too crowded in the mountainous region of the West, when drawn upon a chart of any convenient size, and for this region all lines have been discarded in this region, excepting those for 80, 120, 180, 240, and 300 days. This method probably yresents the details for this part of the country as accurately as is to be expected from the data now at hand. The omitted lines, all drawn for the East, have been abruptly terminated wherever they would enter the more generalized portion of our chart. Plate 34 is a repre- sentation of the chart just described. For the sake of clearness, the chart has been shaded so as to fall primarily into the five classes of areas. These areas denote regions with average lengths, in days, of frostless season of (1) less than 120, (2) 120 to 180, (3) 180 to 240, (4) 240 to 300, and (5) over 300. The other lines, representing greater detail, are shown in dotted form. A comparison of plate 34 with Day’s plate V (loc. cit.) shows, at first glance, a remarkable series of differences. Closer scrutiny brings it out, however, that the apparent discrepancies are mainly due to the fact that the lines of Day’s chart have been obviously subjected to a 196 ENVIRONMENTAL CONDITIONS. very strenuous smoothing process. Since a smoothed chart is thus already in existence, we have thought it best to let plate 34 represent as nearly as possible the present status of our climatic and physio- graphic information, and, as has been pointed out, we have made no attempt to smooth our equiseasonal lines. Professor Day informs us that some scattered data other than those available for our use were included in his study of the frostless season, and it seems probable that a few discrepancies between his chart and ours may be related to this fact. At any rate, for all practical purposes, and until such time as much more complete and reliable observational data may have been obtained, we may say that these two charts are in very good agreement. So far as we are aware, no attempt, other than Day’s and our own, to prepare a chart of the average length of frostless period for the United States has yet been made. The main generalization to be derived from plate 34 is that the areas of equal frostless seasons traverse the country, roughly, in a west-east direction, being displaced, however, to the northward in the vicinity of the Pacific and Atlantic coasts and to the southward in the regions of the western and eastern mountain systems. The coastal displace- ment is especially pronounced on the Pacific, where the 300-day season reaches as far north as Washington. Here the season of 180 days seems to extend even into British Columbia. On the eastern coast the last-named season extends as far northward as Massachusetts. The northward extension of the same season is seen to be limited in the central portion of the country, approximately by the southern boundary of Iowa, while the western mountains displace its northern limit, in southern New Mexico, to about latitude 33° north, and the Appalach- ians displace it, in northern Georgia, to about latitude 35° north. Again, the 120-day season is not represented at all on either coast, but extends as far southward as latitude 35° north, in Arizona and New Mexico. The Great Lakes exhibit a tendency, as far as our data extend, to lengthen the average frostless season in their vicinity. The chart shows also a frequent tendency toward an upstream extension of any given length of average frostless season in the vicinity of the larger rivers, even where this has no obvious relation to altitude. As has already been stated, the data for the average length of the frostless season have been made the basis for many of our studies of other climatic features, on the assumption that this time period may be taken as a rough approximation of the length of the season of active growth for a large number of plant-forms. It seems probable that it is proportional to the average growing season for most plants, at any rate. CLIMATIC CONDITIONS OF THE UNITED STATES. 197 (C) LENGTH OF PERIOD OF AVERAGE FROST SEASON. By the frost season is here indicated the period of the year during which frost is apt to occur. In this season such plant-forms as are killed or thrown into the dormant state by the occurrence of freezing temperatures should not be active. While actual growth of such plants often occurs within this season of any year, in frostless periods of a few days, yet this growth is soon checked, and foliage, etc., thereby produced is usually destroyed by the recurrence of frost, so that the result of such short growing-periods is seldom to be considered as advancing the organism very much toward maturity or reproduction. It may thus be generally assumed that the average frost season for any region represents the average period of dormancy for a large number of plant forms. It is obvious that the average length of the frost season is the com- plement of the average length of the frostless period.“ Thus, from table 2, the mean length of the frost season may be obtained for any station by subtracting the number of days given for the frostless season from 365, the total number of days in the year. It is also obvious that the chart of the mean duration of the frostless season is simultaneously a chart of the mean length of the frost season. Thus, on plate 34, the area represented as having a mean length of frostless season of less than 120 days is characterized by an average period of general plant dormancy of over 245 days, ete. It seems highly probable, though there is at hand no direct informa- tion in this connection, that many plant-forms are excluded from cer- tain areas in the United States, not by the lack of an adequately long growing-season nor by killing temperatures, but by too great a duration of the dormant period. It may thus be possible that, for a given plant, a certain locality might possess a growing-season quite adequate in every way for maturation and reproduction, and yet the length of the enforced period of dormancy might be so great that death from autolysis, respiration, and the like might ensue before the return of the conditions requisite for full activity. The question thus raised can not be answered until after the accumulation of a much more thorough knowledge of the limiting conditions of plant-life than is now available. Indeed, the first prerequisite for an attack upon such questions is some such laboratory for the study of environmental relations as has attracted our attention earlierin the present publication. (D) LENGTH OF PERIOD OF HIGH NORMAL DAILY MEAN TEMPERATURES. (TABLE 3, PLATE 35.) On the supposition that high temperature may, directly or indirectly, prevent the appearance of certain plants in certain areas, or that this may be the critical requisite for the complete development of certain » nae : gn 198 ENVIRONMENTAL CONDITIONS. forms, it follows that the duration of relatively high temperature may, in some cases, be a limiting vegetational condition. It has therefore seemed worth while to attempt a cartographical study of this feature. The temperature observations that have been carried out by the Signal Service and by the United States Weather Bureau have already resulted in an enormous mass of data. From our present point of view these data are very unsatisfactory in many respects. The distribution of the stations of observation is, as has been remarked previously, exceedingly unequal, and seems to have been the result of political rather than of scientific interests. Furthermore, the exposure of the thermometers at the various stations follows no general rule; some- times the instruments are placed on the tops of high buildings, some- times near the ground; they are seldom in the open country, and are almost always subjected to whatever peculiar conditions prevail in or over cities. Nevertheless, in spite of the many quite obvious funda- mental errors which a more rational guidance might have been able to avoid, the faithful labors of the observers and interpreters of the Bureau have resulted in a very valuable mass of statistical information upon temperature conditions, and it is from this alone that informa- tion such as we require may be obtained. Especially valuable to us is the Herculean work of Professor Frank H. Bigelow, who has done what was possible to bring order and meaning out of the chaos of existing observations. In the present instance, as also in the next following, we have drawn our fundamental statistics directly from Bulletin R of the United States Weather Bureau.! In this bulletin Bigelow has presented the normal daily mean temperature for every day in the year for 177 sta- tions in the United States. Although the introductory statements of this work are none too clear as to the sources of the temperature data used in the computations, it is implied that these normal daily means are the direct outcome of a graphically mathematical treatment of the normal monthly mean temperatures as given in Bulletin § of the United States Weather Bureau.? Since, however, the list of the last- mentioned bulletin comprises but 123 stations, it is obvious that other data than these have been employed in the preparation of the daily normal means. It thus appears that the fundamental homogeneous reductions for at least 54 stations have never been published and that these have nevertheless been made use of in the preparation of the list of 177 stations which we are to employ. While there can be no rational doubt of the approximate reliability of all the data given in the first- mentioned of these two publications, it is to be regretted, where there 1 Bigelow, Frank Hagar, The daily normal temperature and the daily normal precipitation of the United States, U. 8. Dept. Agric., Weather Bur. Bull. R, 1908. 2 Bigelow, Frank H., Report on the temperatures and vapor tensions of the United States, reduced to a homogeneous system of 24 hourly observationsf{ or the 33-year interval 1873- 1905, U. S. Dept. Agric., Weather Bur. Bull. 8, 1909. a eS i a a CLIMATIC CONDITIONS OF THE UNITED STATES. 199 are so many mathematical steps between the actual observations and the finally resulting daily normal means, that so many of the funda- mental properties and characteristics of the data upon which the latter have been based still remain practically unattainable to the student of these important statistics. The enormous amount of work represented by the portion of Bulle- tin § that deals with the normal monthly mean temperatures resulted in the elimination, as far as this was possible, of the error-producing effects of variations and alterations in the hours of observation at the different stations throughout the long period of observations, as well as in the reduction of the variously derived daily means to a homo- geneous system. For an account of the ingenious methods employed in this work the reader is referred to Chapter I of Bulletin §; for not nearly all of the stations are observations for the full 33 years available, and the possible approximate reductions of the means of short-record stations to a 33-year basis were not carried out in Bulletin 8, though a method for this sort of reduction is given on page 32. Whether these reductions have been carried out for the temperature data of Bulletin R we are not informed, but it may safely be supposed that the state- ment of the normal daily means of temperature for the 177 stations dealt with in Bulletin R, approach the truth as nearly as was possible, all circumstances being considered, at the time of the preparation of the bulletin. It is interesting and worthy of remark here that the letter of trans- mittal accompanying Bulletin §, signed by Willis L. Moore, Chief, sounds a truly prophetic note in this sentence: These data, and the normals that have been deduced from them, will form the funda- mental basis for future studies on climatology and for the investigation of the relations between plant life and the thermal and hygrometric conditions that prevail in nature. We have chosen 68° F. (20° C.) as our critical normal daily mean temperature in the present connection, and have determined from the tables of Bulletin R the number of days in the year to which are ascribed normal daily means of 68° F. or above. These days are con- secutive in every case, owing to the smoothing process by which the normals have been derived. There are comparatively few stations in the United States without any such days, and for one station, Key West, Florida, this period of what most dwellers in temperate regions would probably term hot days is extended throughout the year. Scarcity of information has no doubt led us into more considerable errors in the western mountainous area than elsewhere. 200 ENVIRONMENTAL CONDITIONS. TasLe 3.—Length of period with normal daily mean temperatures of 68° F. or above, and of period with similar means of 32° F. or below, within the year. (Plates 35 and 36.) Mean of | Mean of 68°F. or | 32°F. or above. | below. Mean of | Mean of 68° F. or | 32° F. or Station. above. | below. Station. 2 | nn Oe ee Alabama: days. days. Iowa—Continued: days. days. PATIITIBLOID Sie cise Sie) s lo" 141 0 Des Mowmes 6. os iox0s « 97 105 Birmingham......... 160 0 Dubuque, A355 s.506..2 89 112 ee eee - ee 176 0 WKeokatktast vee eae 107 89 Montgomery........- 171 0 Sioux: Cityjeuc.: vieip e's a 91 119 Arizona: Kansas: MIB DBLATI ccteecutstosteters 0 92 Goncordis\..ts,. Puan BIAS? 186 0 TWIOGEO: ates cis cle 113 70 Pole cee tareever ames 211 0 "PODGMA dens. celeetstenul-s 118 74 Arkansas: WHIGHICS 74), acas ice eels 125 54 Fort Smith. 23... .1.'+< 149 0 Kentucky: Daittle Rockers sor 153 0 Lexington 1s 6.5 een we 115 0 California: TiOWIEVAMNG crocs se eke la 128 0 SER ciate teierens 0 0 Louisiana: REET es Nae evertere 150 0 New Orleans......... 192 0 Independence........ 118 0 Shreveport..:.......- Los Angeles.......... 54 0 Maine: Red! Blutis chs tadtes 140 0 IASGUDOM LE: lana oeieleleter eta Sacramento.......... 110 0 Portland: ces iaishets oie San Diego. iis cspeiels as 50 0 Maryland: San Francisco........ 0 0 Baltimore:j4: izes soc San Jose: 2. cists .siniiets 0 0 Washington, D. C..... San Luis Obispo...... 0 oO Massachusetts: Golorado !(, *h Sl OTe OO Oa eT eB ORGONIE. Si(. syeres se laters eves ealetowie geet b> CSBP OMe 6) et GE CORNERS 6.6 5/2 5) e a) aS eie DUTANTZOsS Sees ae eats Grand Junction...... es aratste tates ote ecto ee 89 70 Detrortcvae seats eats Connecticut: FISCANnADAL 2.62 aa toes Hartford.) .c6. 0c bent | (GO), 94 tl) Grand Haven... 7. as New Haven....--..0-| -@¢ \ + 84. |) Grand Rapids,.....<. Blondac: "0 a are ie HELO UGED tin recateretehceneys Jacksonville.......... 191 Marquette. ...5 66.50 SINATSDDGL eo! io05 3: sintoln ee Port PLUTON: ch.cpcste sins eeocceccccveel CUO F—| £ iW fi = RAAUY WOU s SVE s ce ee o rr Georgash 9) o> Pe ae PP lhe” VIMEGOFDORG ich sic eyeamic ELST GER sie coseis eisieie ore ols ePaul cots (Ruse ecveceseccecccecs, £06 | #$é=jW ft | SVBURAULEREEERe ee eseseaeses Shs Pataret sini tas Vicksburg. crys wae Saye ateretinhss 183 Missouri: Columbia: ce on sice oe ties PAA eee hai diates Hanvibal’x.cidwdeknan EEG b ¢ wie e's. Sie Kalignal:... dsckwustaas Springfield........2:- 109 78 Miles ‘City. < os os sen Indiana: Nebraska: TOVARSVIUG. cca vieresact 22e) | e228. th) dnoOlnei ican downass Indianapolis.........| 108 | 68 || North Platte......... acai apt alee ee el ee eG Wee IR? | Sy RIA Joa tere teen nite ae Charles City.........| 81 | 126 || Valentine,........... EIMVERDOTEG xis tis creates * CLIMATIC CONDITIONS OF THE UNITED STATES. 201 TasBie 3.—Length of period with normal daily mean temperatures of 68° F. or above, and of period with similar means of 32° F. or below, within the year. (Plates 35 and 36.) Mean of | Mean of Mean of | Mean of Station. 68° F. or | 32° F. or Station. 68°F. or | 32°F. or above. | below. above. | below. Nevada: days. days. South Dakota: days. days. RONG a ore ovata. 32 26 AUTON See ae aes 67 140 Winnemuceca......... 61 68 APICIT Grete rete chat Aadens. 85 129 New Hampshire: RAD IGNCTEY Sid ore ae as 55 121 MGONCOLG Ge; <:5icta,2/s) sor 44 113 PVC UOMAT ne a cere © 89 122 New Jersey: Tennessee: Atlantic City......... 91 2 Chattanooga......... 141 0 @WaperMiay\cx o/s. ess 101 0 BManroxyHlet owes, s.. aries 128 0 New Mexico: Memphisaceiacn cs... 153 0 ROB WCll eo cleiaiveicvens oe av 140 0 Nash valless: rrctvara acre 141 0 SPH fede ee aes eae 57 76 Texas: New York Abilene i245. «sae 163 0 PAN DANG setters seo cieks ciate 76 107 Amarilloae eens tate os 114 0 Binghamton......... 58 109 Corpus Christi....... 218 0 TAI O Ss. 45, <5 sos 64 104 IPP asOecete Scho oe 156 0 WC ANLON Ae weeciasde cei. 47 129 HortewWorthes oer. ess iis 0 RH AORS Socgoacteta cles oie 62 108 Galvestom. .o..0.<2:.- 215 0 Me WEY OLE )..c ayciere oss 94 64 iPalestines foc. sais «sar 172 0 ROR WEEO: CWetisioe’sieievates 57 107 SanvAmtonionie es... a2 197 0 Rochester: . 3.5). 2. = 63 109 Paylorssee cio wee ee 187 0 SI ACUISOM techs x eieeeye’ = 67 108 Utah: North Carolina: Modena: h4 ce ace 55 65 SABHOVINE. 2c) sie.s ces ¢ 90 0 Salt Lake City....... 87 63 ROHATIOULE Swe ciaxciee © eas 138 0 Vermont: Pat CET AS Hos o/s <6 sie ere 148 0 Burlington. ces... 32 129 TDC TET TG i ay actin sears 137 0 Northfield: << .c.. +... 0 137 Wilmington.......... 147 0 Virginia: North Dakota: @apeiHenry,...- secs: 128 0 BOEMATHAT CIC: .\.e:s.0levaiate <= 4 148 Tsynchbure o> -.<6 520 121 0 Wevils: Walkke)s 62.2 so. 30 158 Norfolk sti. ccacieetet 132 0 Williston ss. cicls od soe 51 152 Richmond os.cf. acto 133 0 Ohio: Weytheville...:..<...... 3. 85 0 MP ITICINTAGL Sf cls as aS ee . ‘Y a) > "a | 2 Pt NN “NN NN SRK SS i) PSs Y= ys A/S/ aA ALAA CX 7/7 = ANAS QD N\ NEN S : i? 6 a. 4 Ls s) _ : I ~ 7 Ses soe cs ae . ; $3 é Saas Val 1 : : aa POT ea AS, E | 9 ° ° © Oe - = aS 202 Af 869 AL ch «(Gk «(Clk COLCCd Stl lel 6a : *z 078]d st deur sty} Jo osvq OT, jo oe syuoUIeIoUy )=*(¢ UUIN{OO “0 @ 9[Ge} UIOIy BVP) MOTO IO “WT (ZTE JO soinzer1eduiey UBoUT ATep [VUIIOU MOT YIM porsed jo (sAep) syySuoT 203 SSS oe \' PLATE 36 Ss * \ j RI SRD \\ +f ~ as , 6 ee Pex SS SIS Oho) WEY , beet ny) 1G RUD ANS y Anes z= WS ‘oy a< \ ‘\ , hen c*.. < >; G SS. at Si e, 8 Ser! ay aS? G Z; ¢ sy YA LD 0) 74 [ rs A V2 pede, wy) 9 nN i ft 4 Qe ———a| VS SS -=4.=— YW AS a= = = ¢ 4 5 pa See an te RA Beh | NE TAS \S MRSS SESS y RS q an 2 Ibis i int Le Ue Ni SING J LA VAS 8 18 : 1 ee ee ee ee ee AQ (seq) Gs2M) JSa10} UdaITIIA ysaloj uaaIZIDAd d1yAydosaw wisyyON ysal10} UddI1F10A9 onAydosour u1ayyION == uoljIsuely an Aydois Ay yse10} snonpidap ~ purlsseis ysor0y D1 Aydosout ysor0y uia}seay INOS snonp19aq a 204 ENVIRONMENTAL CONDITIONS. The numbers obtained by our study are given in table 3, and the chart formed with these is reproduced as plate 35, the locations of the stations used being denoted by small circles. The equiseasonal lines exhibit the data in a graphic manner. Increments of 30 days in the length of this hot season are represented. It is noticeable on plate 35 that our lines assume a generally east- and-west direction to the eastward of the one hundredth meridian of west longitude, and a north-and-south direction in the vicinity of the Pacific ocean. Both mountain systems exhibit a tendency to displace the various climatic belts to the southward, a tendency shown still more markedly by the Pacific Ocean, and the same tendency is exhibited to a relatively slight degree by the Great Lakes. The region thus normally without the hot period here considered embraces almost the entire Pacific coast, from the vicinity of San Luis Obispo to the Canadian boundary; also the northwestern part of Minnesota, the whole of the northern peninsula of Michigan, a small adjacent part of Wisconsin, the northern extremity of the southern peninsula of Michi- gan, and the extremely northern parts of New England. This entire region is generally in high repute for summer resorts. (KE) LENGTH OF PERIOD OF LOW NORMAL DAILY MEAN TEMPERATURES. (TABLE 3, PLATE 36.) Just as the length of the hottest period may be supposed to influence the appearance or non-appearance of certain plants, so the mean length of the coldest period of the year may have its effect upon the distribution of the sarne or other forms. It seems improbable, how- ever, that a normally long period of very cold weather may be essential to the full development of any organism. This coldest season always finds the plant in a dormant condition, and it is hardly possible, on physiological grounds, that extreme cold should be directly advanta- geous to its survival, but it is to be realized that nothing is yet quanti- tatively known in this connection. On the other hand, it is quite clear, in general at least, that cold weather tends to exclude many plant- forms from the vegetation of any region where such weather may occur. Numerous plants that readily survive a single frost are com- pletely annihilated by the occurrence of several days of freezing weather. The question here raised refers mainly to the power of dormant plants to retain life under more or less persistent conditions of temperature below that of frost. As a critical normal daily temperature mean we here take 32° F. (0° C.) For each station included in the temperature tables of Bulle- tin R of the U. 8. Weather Bureau, it has been determined how many days in the year possess a normal daily mean of 32° F. or below. Owing to the averaging and smoothing process by which the daily normals have been derived, these data exhibit a regular annual march, and all ite Se ; CLIMATIC CONDITIONS OF THE UNITED STATES. 205 dates accredited with normals of 32° or below occur consecutively. Thus the period so characterized may be termed the normal season of cold weather. Our method of procedure in the present case brings out the fact that nearly one-half of the United States is without such normal periods of cold weather as are here considered. The longest period encountered is of 158 days. The numbers obtained from this study are shown in table 3, and the chart of plate 36 exhibits the equiseasonal lines derived therefrom, this chart thus exhibiting graphically the extent of the areas having different lengths of cold season as here characterized. The station locations are again indicated by small circles on the chart. Increments of 30 days have again been employed, as in the charting of the periods of hot days (plate 35). From plate 36 it is seen that the area indicated as without cold season occupies the entire country south of a line passing, approximately, from Cape May, New Jersey, to Tucson, Arizona, and is extended westward and northward to-embrace approximately the southern third of California, the western two-thirds of the remainder of that State, and the western half of Oregon and of Washington. It is in this area that the most highly reputed winter resorts occur. The area characterized by 150 or more days of normal daily means of 32° F. or below occupies only the northern third of Minnesota, the northern half of North Dakota, and a little of northeastern Montana. The eastern mountains and the Great Lakes appear to have little tendency to extend the areas of cold winter weather southward, but such an effect is noticeable in the case of the western mountains. The influence of the Pacific Ocean is conspicuous, crowding the area of cold season far back from the coast, even north of the Canadian boundary. 2. INTENSITY OF TEMPERATURE CONDITIONS. (A) PRELIMINARY CONSIDERATIONS. The physical conception of life phenomena allows us to regard the organism as a spatial system, in which a complex series of chemical and physical changes are ever in progress; during life, material and energy are always entering the system and are as unceasingly leaving it. The accomplishment of growth, maturation, reproduction, etc., of a plant is thus to be regarded, at any moment in its history, as the summation of the effects produced by the innumerable physical and chemical changes which have thus far occurred. Since all energy and material transformations are influenced to a greater or less extent by temperature, and since this influence is usually very important, it follows that, if other conditions were but constant, or if they always varied in the same manner, the state of a plant at any moment might be treated almost as a direct function of the various aerial temperature conditions to which it has béen subjected in the past. Of course the 206 ENVIRONMENTAL CONDITIONS. other conditions do not always vary in the same way and they are never constant, not even as related to criteria without the plant-body. Even though they were constant for any plant, progressive alteration within the organism would assuredly produce great variations in the relations of the external world to internal conditions as criteria. It is therefore quite hopeless to contemplate an accurate causal interpretation of plant states merely on the basis of integrations of temperature effects. Nevertheless, the same incubus of hopelessness overshadows similar attempts along lines of approach based upon other external factors, and since we are sure that no single criterion alone will lead very defi- nitely toward the solution of our problem, progress may be sought only by treating the different factors separately and studying the results, after which artificial and natural combinations of factors may be attacked. Furthermore, the study of the temperature conditions of plant environments appears promising in this particular, namely, that temperature not only directly influences plant activities, but also influences all of the other effective environmental conditions to a greater or less degree. There appear to be, in general, two possible criteria for comparing the temperature intensities of several different localities. By one of these the comparison is made of the extremes, merely, of the yearly maximal and minimal temperatures for the several stations. Thus the duration factor is completely left out of account. It is, however, possible to consider and compare maxima and minima, not for the entire year, but for shorter seasons, and these seasons may be of different length at the different stations. The difference between the maximum and minimum temperatures may then be obtained, giving the range of normal temperature for each particular season and place to be considered. The last-named function of the temperature condi- tions does take some account of the duration factor (since the season is described in each case), and may, in certain instances, have an impor- tant relation to plant activities. By the second general criterion the intensity conditions are summed or integrated, in some manner, through a given length of time, and the duration factor is thus seen to play a direct and important part in this procedure. Mean temperatures for long periods of time are not apt to be of value in studies of the relations between plant activities and the environ- ment; seasonal or yearly means, which comprise such an important part of the usual meteorological reports, seem never to have given any real promise in this direction. Such means do not take account of the duration factor; the summed daily means are divided by the number of days considered, so that either this number of days must be the same for all localities compared, or else the numbers obtained can bear no relation to the corresponding extent of plant growth and other activities. Thus, a long growing-season with a given mean tempera- ee CLIMATIC CONDITIONS OF THE UNITED STATES. 207 ture is not at all the same, as far as vegetation is concerned, as a short season with the same mean. In studies regarding the relations between temperature intensity and plant development it is necessary to measure and compare the relative effectiveness of the temperature conditions at one station with that of the conditions at another; it is not the temperature of the atmos- phere or of the plant that primarily interests us, but it is the possible effect which the various degrees of temperature may have in controlling plant activity. To make the following subsection clear we shall digress at this point to explain the various concepts and the terminology which will here be employed, thus presenting a tentative discussion of the various sorts of temperature indices that may be used in vegetational or other dynamically applied climatology. The word ‘‘temperature” is used with a variety of different mean- ings, some of which are very vague. Temperature and heat-content are frequently confused, but the heat-content of a body is only one of the conditions that determine its temperature; the heat capacity or specific heat of the matter in question and the amount (mass) of matter considered being also influential in determining the temperature. The definition of temperature involves difficulties unless based on the kinetic theory of matter, in which case the temperature of a body is considered as a measure of the mean kinetic energy of its particles. Temperature is often said to be the relative measure of the sensible heat of the body in question, of its hotness or coldness, this conception being based on the heat-sense of human beings. The latter definition must assume that the matter considered does not change its.state (as solid, liquid, or gas) when the human sense-organ is applied to it as a measuring instrument, but the heat-conductivity of the material is important in this connection. Thus, a block of steel and a similar one of wood seem to have different temperatures by the criterion of sense, although they may be of quite the same temperature as determined by a thermometer. The commonest way of measuring temperature is to state it in terms of the relative volume assumed by a given mass of some standard substance (such as air, mercury, alcohol, etc.) when that mass of substance is in heat equilibrium with the body whose tempera- ture is to be determined, this equilibrium being attained when heat does not migrate in either direction between the standard mass and the body under consideration. Thus air-temperature is measured in terms of the relative volume assumed by the liquid in a thermometer when this liquid neither gives heat to the air nor receives heat from the air. In short, the thermometer liquid is allowed to come to the same temperature as the air and then this temperature is stated in terms of the volume occupied by this liquid under these conditions. Since the mass of the thermometer liquid is constant for any instru- ment, the different volumes assumed at various temperatures may be 208 ENVIRONMENTAL CONDITIONS. indicated as a temperature scale on the thermometer-tube, and these volumetric graduations may be of any convenient magnitude. Thus, centigrade, Fahrenheit, etc., degrees are the respective increments in the volume of the thermometer liquid corresponding to equal incre- ments in temperature rise. The term ‘‘temperature” often means temperature reading (on some thermometer scale), and it is also used in a self-evident adjectival sense, meaning pertaining to temperature. In order to apply construc- tive reasoning to temperature conditions it is necessary to be somewhat more explicit than is usually the case, so that the terms we shall use require definition. Temperature readings or measurements are simply numbers that represent comparative temperatures, and they may therefore be con- sidered as conventional indices of temperature. The numbers of the Fahrenheit and centigrade scales are merely these indices expressed in two different kinds of units. Thus, the normal daily means of Bulletin R of the United States Weather Bureau are to be regarded as averages of a number of means for the given date, these means themselves being averages of a series of temperature indices representing the different temperatures encountered in the air throughout the day. According to this terminology, all climatological studies of temperature conditions thus start with temperature indices. These indices tell nothing about the possible effects of the temperatures represented, as these may accelerate or retard any process; they have reference only to the state of molecular motion of the particular body whose temperature is considered. For the problems before us, as has been mentioned, the various degrees of temperature effectiveness upon plant activities must be measured and compared, rather than temperatures themselves, and it thus follows that we require indices of temperature efficiency. These are to be derived from the indices of temperature, with due regard to the nature of the process to be studied, and various attempts have been made to obtain from temperature indices these other indices that are to be measures of the plant-producing power, etc., of given atmos- pheric temperatures. We shall deal below with the various methods that have been tried. The relative applicability of these methods is to be determined only empirically, but certain a priori considerations need to enter into the critical discussion of this relatively new and very important subject. According to the way in which indices of tempera- ture efficiency are derived from temperature indices, we may have four distinct classes of the former, which will now be taken up separately. (1) Direcr INptces oF TempprRaAtuRE Erriciency FoR PLANT GROWTH. Direct efficiency indices are obtained directly from the corresponding temperature indices, the numerical values being the same in both cases CLIMATIC CONDITIONS OF THE UNITED STATES. 209 This method of derivation assumes that the rate of plant growth varies proportionally to the environmental temperature. Any thermometer scale may be used and a set of efficiency indices thus obtained may be transferred from one scale to another by arithmetical treatment. The assumption here is exemplified as follows. If a plant grows 2 units per time period at 2° C. it should grow 10 units per time period at 10° C., 25 units at 25° C., ete. While direct indices of temperature efficiency (on the absolute thermometer scale) are of great value in studying simple physical processes, such as the expansion of gases, they do not promise much in connection with the study of physiological processes, and need not be seriously considered in our practical applications. (2) RemarInpeR Inpices oF TeMPERATURE ErFiciIENcy For PLianr Growru. The derivation of what we shall here term remainder indices of tem- perature efficiency is but little more complicated than that of direct indices. A constant difference between the temperature indices and the corresponding efficiency indices is assumed (or derived from experi- ment), and this difference is subtracted from every temperature index, thus giving the required efficiency indices. It will be seen that this method virtually does nothing but alter the position of the zero of the thermometer scale, after which alteration it employs direct efficiency indices as these have been defined above. Thus, the rate of plant growth at 40° F. may be considered as unity and it may be assumed that this rate becomes 2 at 41°, 10 at 49°, 50 at 89°, etc., the constant difference above mentioned being here 39. In the phenological studies that have employed this sort of efficiency indices it has been assumed that if the plant does not attain the particular growth-rate that is taken as unity, it does not grow at all; that is, with a temperature of 39.5°, for example, no growth is supposed to occur, when unit rate of growth would be obtained with a temperature of 40°. It is clear that the method of direct indices of efficiency is a special case of that of remainder indices, the constant difference being reduced to zero in the case of direct derivation. The integration of temperature data by these remainder indices has received the attention of workers in phenology for many years and a large amount of literature bears upon this subject. For a review and citations of the earlier phenological studies, the reader is referred to Abbe’s Relations between Climates and Crops, already mentioned. Because of their close relation to our special field of study, Merriam’s researches upon the zonation of temperature conditions in the United States must be considered here. The conclusions arrived at by this author have been largely adopted by plant and animal geographers in this country, and Merriam’s zonal terminology has come into very general use, despite the exceedingly tentative nature of the data on which this is based. 210 ENVIRONMENTAL CONDITIONS. Merriam’s work' and that of his colleagues of the Bureau of Biologi- cal Survey of the U. 8. Department of Agriculture constitute by far the most thorough study that has yet been brought forth of the rela- tions of plant and animal distribution to temperature conditions in the United States, and Merriam’s temperature integrations have furnished, for two decades, practically the only available information in this regard. In his most complete account of this arduous work of integra- tion, Merriam writes: Several years ago I endeavored to show that the distribution of terrestrial animals and plants is governed by the temperature of the period of growth and reproductive activity, not by the temperature of the whole year; but how to measure the temperature concerned was not then worked out. * * * At one time I believed that the mean temperature of the actual period of reproductive activity in each locality was the factor needed, but such means are almost impossible to obtain, and subsequent study has convinced me that the real temperature control may be better expressed by other data. * * * If it is true that the same stage of vegetation is attained in different years when the sum of the mean daily temperature reaches the same value, it is obvious that the physiological constant of a species must be the total quantity of heat or sum of positive temperatures required by that species to complete its cycle of development and reproduction. * * * I am not aware that an attempt has been made to correlate the facts thus obtained with the boundaries of the life zone. * * * If the computation can be transferred from the species to the zone it inhabits—if a zone constant can be substituted for a species constant—the problem will be well nigh solved. This I have attempted to do. In conformity with the usage of botanists, a minimum temperature of 6° C. (43° F. [42.8° F., see footnote, p. 231])? has been assumed as marking the inception of the period of physiological activity in plants and of reproductive activity in animals. The effective temperature or degrees of normal mean daily heat in excess of this minimum has been added together for each station, beginning when the normal mean daily temperature rises higher than 6° C. in spring and continuing until it falls to the same point at the end of the season. The sums thus obtained have been platted on a large scale map of the United States, and isotherms have been run which are found to con- form in a most gratifying manner to the northern boundaries of the several life zones. * * * While the available data are not so numerous as might be desired, the stations in many 1 Merriam (1894.) A very much abbreviated statement of the results embodied in the above paper was published as Part III of the same author’s Life Zones and Crop Zones of the United States, U.S. Dept. Agric., Div. Biol. Survey. Bull. 10, 1898. This latter involves but two pages (54, 55) and does not include the climatic map. Nothing approaching an adequate presentation of the data upon which these important studies are based has, as far as we are aware, ever appeared. It can not be too strongly emphasized that work of this sort is deprived of by far the greater part of its possible usefulness in building up our knowledge whenever a derived chart is published without the station data upon which it is based. It appears that most writers who have dealt with climatic charts have considered these as an end rather than as a means. Such charts are simply broad and necessarily very general presentations of the facts or observations upon which they are constructed, and can accomplish little more for the student of plant distribution or of agriculture than to inform him where in the given region to look for stations with certain climatic characteristics. As soon, however, as his interest is thus aroused he requires the station data, and if these are not at hand, further quantitative studies therewith are effectually precluded. We will not suppose that this common suppression of basic data is to be related at all to any desire on the part of writers to veil the exact methods of their procedure in the preparation of charts; we suppose rather that the suppression here in view has usually arisen from lack of facilities for publication or from lack of time and energy requisite for the preparation of tables or for the placing of the numerical data upon the published charts. It is hardly conceivable that a writer who has derived important generalization from a mass of figures should not appreciate the probability that the same figures may be utilized, in the same or in different ways, by other students of the subject. Had Merriam’s publications included a list of stations each with its numerical climatic indices, the latter might have been put to many other uses than the mere preparation of the simple charts. 2 As the chart was published, however, the minimum here referred to was 0° C. (32° F.). See Merriam’s note, Science, n.s., 9: 116, 1899. CLIMATIC CONDITIONS OF THE UNITED STATES. 211 instances being too far apart, still enough are at hand to justify the belief that animals and plants are restricted in northward distribution by the total quantity of heat during the season of growth and reproduction. Merriam’s chart (1894, plate 12) of the summations just described is here reproduced in its essentials, as our plate 37, for purposes of com- parison, and because of its pioneer nature and present scarcity. From this map it is seen that the warmest zone of the United States, as here indicated, is characterized by a summation temperature of 26,000 on the Fahrenheit scale (14,500 on the centigrade scale), and that this zone is restricted to the lower Colorado Valley, the extreme southern portion of Texas, and the southern half of the Florida peninsula. The zone characterized by temperature summations below 10,000, F. (5,500 C.) occupies, in general, the highest portion of the Cascade and Sierra Nevada Mountains, the Rocky Mountains, northern Minnesota, a little of northern Wisconsin, the northern half of Michigan, and the northern half of Maine. The isoclimatic lines for 11,500 F. (6,400 C.), and 18,000 F. (10,000 C.) are seen to have a west-east trend, but are more or less markedly displaced southward by the western and eastern mountains and northward by the Pacific and Atlantic oceans. Our own work with remainder indices will be presented farther on. (3) ExponentTIAL INDICES OF TEMPERATURE EFFICIENCY FOR PLANT GROWTH. As has been stated in our earlier discussions, it appears that some possibility of advance lies in studying climatic temperature conditions with reference to the chemical principle of Van’t Hoff and Arrhenius, and a first attempt in this direction has been made by Livingston and Livingston in the paper already cited. This principle states that chemical reaction velocity usually about doubles (or somewhat more than doubles) for each rise in temperature of 10° C., or of 18° F. It is to be understood that the principle of Van’t Hoff and Arrhenius is applicable, even in purely chemical problems, only within certain temperature limits, and it is sufficiently clear that the same general sort of limitation must influence its applicability in physiology and ecology. Weare primarily interested here in growth processes and their rates, and, obviously, there may occur natural temperatures either above the maximum or below the minimum for growth, so that here are unquestionable limits for the application of the principle just stated. Furthermore, the principle itself supposes that the process considered increases its velocity with each rise of temperature between the limits of applicability, and we are well aware that increasing tem- perature is not accompanied by increased growth-rate throughout the range from the minimum to the maximum for growth in plants. An optimum temperature can always be found above which the pre- viously increasing growth-rate begins to decrease. Thus the applica- bility of the Van’t Hoff-Arrhenius principle to organic growth phe- oi ENVIRONMENTAL CONDITIONS. nomena as a whole can not be maintained beyond the limits set by the minimum and optimum temperatures for growth. However, these physiological constants are not the same for different plant-forms, and they may also be assumed to vary with other conditions within and without the plant. It therefore seemed desirable, in an attempt to apply this law of temperature coefficients to the climatological delimit- ation of geographical areas, to choose such limiting temperatures as should give promise of merely approximating the physiological limits for a large number of plant-forms. In this we follow Livingston and Livingston, who give a more complete discussion of this whole question than is required here. The authors just mentioned calculated their indices of temperature efficiency on the basis of the supposition that general plant activity occurs at unity rate when the daily mean temperature is 40° F., and that this rate is doubled with each rise of 18° F. in the daily mean. Thus, with a daily mean temperature of 58° F. the rate becomes 2.0, with a mean of 76° F. it becomes 4.0, etc. It thus becomes clear that the relation here assumed between any index of temperature efficiency and the corresponding index of temperature itself is an exponential one, expressed by the equation I = 2s, where J is the index of efficiency and t is the corresponding index of temperature. From this equation, by substituting the various values of ¢, Livingston and Livingston prepared a table of the efficiency index values corresponding to the temperature indices from 40° to 99° F. This table is reproduced as our table 4. Of course, constants other than 2, 18, and 40 might be employed, thus giving other values to the indices of efficiency; these indices, as has been stated, are based on the supposition that plant Taste 4.—Exponential indices of temperature efficiency for plant growth, based on a coeffi- cient of 2.0 for each rise in temperature of 18° above 40° F., for each temperature from 41° to 100° F. © >) = © My © © © 2 eas 2 on 4/3251 2.) 325] 2.| 385] 4.) S38 2.| 588 B82] “seg eel nw sg eM So x BS Sul xs ae | ess] 68) ess 83) 858] 83) sas] so] oss BS | See so] Saal 821 4a9 | 22/488 | 25 Ci a — 3) = SI £o |e $e |e $e |e iota I 2 il oF ey °F sly 41 1.03938 53 1.6493 65 2.6192 77 4.1572 89 6.5972 42 1.0802 54 1.7412 66 2.7212 78 4.3206 90 6.8566 43 1.1226 55 1.7815 67 2.8284 79 4.4902 91 7.1258 Ad 1.1666 56 1.8512 68 2.9391 SO 4.6662 92 7.4048 45 1.2123 57 1.9240 69 3.0545 81 4.8490 93 7.6960 46 1.2599 58 2.0000 70 3.1748 82 5.0396 94 8.0000 47 1.3096 59 2.0786 71 3.2986 83 5.2384 95 8.3144 48 1.3603 60 2.1603 72 3.4283 S84 5.4424 96 8.6412 49 1.4142 61 2.2451 73 3.5629 85 5.6568 97 8.9804 50 1.4696 62 2.3331 74 3.7024 86 5.8782 98 9.3329 51 1.5273 63 2.4245 75 3.8480 87 6.1090 99 9.6980 52 1.5874 64 2.5198 76 4.0000 bate 6.3496 100 + |10.0792 1The temperature efficiency index is here assumed to be doubled for each rise of 10°C. (18° F.) above 40° F., at which temperature the index of efficiency is taken to be unity. CLIMATIC CONDITIONS OF THE UNITED STATES. 213 activity in general doubles with each rise in temperature above 40° F., at which temperature the rate of development is considered as unity. (4) PuysrotocicaL InpicES OF TEMPERATURE EFFICIENCY FOR PLANT GRowTH. All three methods so far considered for deriving temperature effi- ciency indices from temperature indices assume that the rates of plant activity increase continuously as the temperature rises. If these various series of efficiency index values be plotted as ordinates on graphs whose abscissas are the temperature indices, then the graphs for direct and remainder indices both take the form of straight lines with an upward slope of 45°. The graph for exponential indices, on the other hand, has the form of a curved line slightly concave upward, which much more nearly approaches being horizontal in the region of climatic temperatures than do the other graphs. In other words, within the range of temperature indices encountered in climatology, the calculated efficiency index increases much more rapidly with rise in temperature for the direct and remainder methods of calculation than it does for the exponential method, as adopted by Livingston and Livingston. None of these graphs, however, shows a maximum, and we know that the true graph of temperature efficiency for plant growth must possess two points where the ordinate is zero and must have a maximum somewhere between these points. This maximum of the graph has a relatively large ordinate and its abscissa is the index of the optimum temperature for growth. It thus follows that none of the methods so far discussed can possibly furnish true indices of tem- perature efficiency for plant activities, excepting, as has been empha- sized, within certain limits of temperature range. If a perfectly satis- factory method of calculating efficiency indices from temperature indices is to be devised, it must be of such nature that both low and high temperature values will give efficiency indices of zero, and inter- mediate temperature values must give indices whose graph shows a well-defined maximum. Livingston’ has attempted to obtain efficiency indices from the variations in plant growth-rate experimentally determined for different temperatures. As has been said, the best study of the actual relations between temperature and growth is that of Lehenbauer, and Liv- ingston has employed the results of that writer in this connection. He considered the average hourly growth-rates of shoots of maize seed- lings exposed to the same temperature for a 12-hour period, the tem- peratures included in Lehenbauer’s study ranging, by increments of one degree, from 12° to 43° C. Lehenbauer’s curve, plotted with growth-rates as ordinates and temperature indices as abscissas, was first smoothed by the use of a flexible spline, so as to give a generalized Livingston, B. E., Physiological temperature indices for the study of plant growth in relation to climatic conditions, Physiol. Res. 1: 399-420, 1916. 214 ENVIRONMENTAL CONDITIONS. curve, which was continued at either end to cut the axis of abscissas at 2° and 48° C. The ordinates corresponding to the various abscissal temperature values were then actually measured. Finally, all the new ordinates thus derived were expressed in terms of the ordinates for 40° F. considered as unity. The resulting series of values are Livy- He presents ingston’s physiological indices of temperature efficiency. a table showing these values for each single degree of temperature from 36° to 118° F., and that table is here reproduced as our table 5. Taste 5.—Physiological indices: of temperature efficiency for growth, based on Lehenbauer’s 12-hour exposures with maize seedlings. Centigrade scale. Fahrenheit scale. Fahrenheit scale. Degrees. Index. Degrees. Index. Degrees. 111 82 222 83 342 84 85 86 87 ee PNOWDDORPWNH KKH OO to to co © ow o- 0. 0. 0. 0. 1 A 1. 2. 2. 3. 3. 4, 4, 5. 6. sf 8. 9. Index. 106. 110. 115. 118. 120. .222 122. 122. .667 .667 444 .333 .333 -000 121 CORR NN WON 889 778 000 111 000 000 333 .000 CLIMATIC CONDITIONS OF THE UNITED STATES. 215 This physiological method of deriving temperature efficiency indices is of course empirical, but it is not more so than either the direct or the remainder method, and it has what seems to be a better experimental foundation than has either of these. Furthermore, Livingston’s physiological indices appear to be much more in accord with what is actually observed in the case of growing plants than are the exponen- tial indices of Livingston and Livingston, and it may be said that we have at last obtained a method for estimating temperature efficiency that is applicable throughout the entire range of possible temperatures, the graph of these indices showing approximately the same form as Lehenbauer’s graph of actual growth-rates. Of course it is desirable that this set of relations be eventually broadened so as to include other plants than maize and other developmental phases than that of the seedling, and when this is accomplished the efficiency index values for general use will probably need to be altered; it would be surprising if indices for seedling maize plants should prove to be representative for plants in general. Enough data are now at hand, however, for a some- what rational beginning, and we regard these physiological indices of Livingston as the most promising of all the different kinds of indices of temperature efficiency that have been proposed. We turn now to the employment of these four different kinds of temperature efficiency indices in our study of the climatic conditions of the United States. In the application of temperature efficiency indices to climatological study we have followed phenological workers in summing these indices throughout a certain period, the resulting summation being supposed to represent, in the form of a single number, the efficiency value of that period, and for the station in question, as far as temperature is concerned. We have used Bigelow’s normal daily mean temperatures (Bulletin R of the United States Weather Bureau) as our daily temperature indices, with which all our computa- tions begin. For the time-period we have here again used the period of the average frostless season. The procedure has been (1) to find the efficiency index corresponding to the temperature index for each day of the average frostless season, at each station, and (2) to add these daily efficiency indices all together to give the seasonal efficiency index for the station in question. This method allows the length of the average frostless season, as well as the values of the daily efficiency indices throughout that season, to take part in the control of the final value which represents the seasonal temperature efficiency for growth. Thus, a long season with relatively low temperatures may give a higher summation index than does a shorter season with higher daily mean temperatures. The use of Bigelow’s normal daily means of tempera- ture as original data should give to these studies an exceedingly broad and general character, and may be supposed to produce some approxi- mation to the average conditions of temperature for many years. 216 ENVIRONMENTAL CONDITIONS. (B) SUMMATIONS OF DIRECT INDICES OF TEMPERATURE EFFICIENCY FOR PERIOD OF AVERAGE FROSTLESS SEASON. As has been said, direct indices of temperature efficiency offer little promise for our present purpose. We therefore present here only the numerical values (in the third column of table 6, summations above 0° F.) and do not reproduce the climatic chart based upon these values. This chart shows practically the same climatic zonation as is shown by Merriam’s chart (our plate 37) and by our charts based on the remain- der indices, the numerical values given to the various isoclimatic lines being of course different in each case. (C) SUMMATIONS OF REMAINDER INDICES OF TEMPERATURE EFFICIENCY FOR PERIOD OF AVERAGE FROSTLESS SEASON. (TABLE 6, PLATE 338.) Remainder indices of temperature efficiency have been derived in the present study by the use of three different values for the constant difference, 32, 39 and 50. It will be remembered that the efficiency index is here taken to be equal to the corresponding temperature index minus the constant difference. Thus we have subtracted 32, 39, or 50, as the case might be, from each of the daily normal means given in Bulletin R, for the period of the average frostless season, and have then summed the daily normal mean efficiency indices thus obtained for each station considered. Columns 4, 5, and 6 of table 6 present the results of these three kinds of summations, above 32°, above 39°, and above 50° F. The method here followed is the same as that employed by Livingston and Livingston, in their summations above 39° F., and our final results for that series of summations are the same as those presented on their chart of ‘‘direct summations,” figure 1 in the paper already cited. Our summations above 32° F. are derived by the method actually employed by Merriam, but his failure to publish the data used in making his chart (our plate 37) make detailed comparison impossible. CLIMATIC CONDITIONS OF THE UNITED STATES. 217 TaBLE 6.—Summations of normal daily mean remainder indices of temperature efficiency for plant growth, for the period of the average frostless season. (Plate 38.) [The daily indices are derived by subtracting 0, 32, 39, or 50 from the values of the normal daily mean temperature on the Fahrenheit scale.] Direct summation of normal daily mean temperatures for period of average frostless season. Length of Station. frostless season. Above Above Above Above BPS 39° F. 14,882 PES EENNNN AI oars ' a, 15! s)sieie/soeseve's 231 16,660 9,268 7,651 5,341 MYLO ULE ay. cecyady cues tc stat creh sane ays. 279 19,756 10,828 8,875 6,085 INT ONGPOMIELY: 5 \ “ie 189 13,086 7,038 5,715 3,825 Nilo thi: By See Ae Ae ee 194 13,660 7,452 6,094 4,154 | Kentucky: 1 UPS atel'g 0) Nate ye potas LR Pn CeCe 187 12,880 6,896 5,587 3,717 MSOUIS VILL Gs G sioarax kieis atepeteceigeer > 196 13,791 7,519 6,147 4,187 Louisiana: New Orleans) eaescisae ceo 310 21,971 1,251 9,881 . 6,781 Shreveport -4.'- 21-16 -ieee 1 252 18 , 228 10,164 8,400 5,880 Maine: ; ABL DONG sa eyacre ciecele ata age sae 167 2,119 3,775 2,606 936 L{syydtcnete cor wnaiyechatt sh ayo 157 2,699 4,675 3,576 2,006 Maryland: i Baltimore i.perciciens ce stele ionciste- 213 14,281 6,816 5,974 3,844 Washineton, D:) Cvs scjesn > 197 13,385 7,081 5,702 3,732 ‘ Massachusetts: ‘ BORtOM « ceiaiaieescetieineianslage ince 185 11,630 5,710 4,415 2,565 Waritueketias cusiissemeias eee ais 209 12,411 5,723 4,260 2,170 ] Michigan: ‘ ) FN ays Te) PERT Cod OTE 137 8,330 3,946 2,987 1,617 DI GHOIte «cect ets sie sete ees 164 10,738 5,490 4,342 2,702 PBOADIAL sata siete jet oer ese7eis 140 8,536 4,056 3,076 1,676 ASrarid ELavGN ve sis eliienst ayes s 167 10,481 5,137 3,968 2,298 Grand Rapides is seuss isis 164 10,745 5,497 4,349 2,709 Honehton ye ono 2 0 aap 4.6 152 8,979 4,115 3,051 1,531 RAT UGE Le sne'a.c o's = ele eat oral 140 8,367 3,887 2,907 1,507 7 ort Euuronk aeas' isyics aie 6 155 O77. 4,817 3,732 2,182 4 Bault mie: WASEIO som ps sie anre 138 7,959 3,543 2,577 1,197 Minnesota: DUN. > cities ose: eee 68 > 152 8,973 4,109 3,045 1,525 MGOrheadiuisaig 55 26 caiman 132 8,499 4,275 3,351 2,031 LAMAN, Adel nds Scab 159 10,342 5,254 4,141 2,551 ; Mississippi: WiC FE ee ares, Sear er b eee 230 16,424 9,064 7,454 5,154 , IV LOMB UIR I ils co, wiha' nein diners atnw 252 18,032 9,968 8,204 5,684 ' Missouri: | Colm DIB sy, 4 basis k eee wie 179 12,487 6,759 5,506 3,716 Jab Net et oy. Ubgete aL Reng ae ie ae edene 183 12,701 6,845 5,564 3,734 TEADBAS CACY a cena es cpa ote vs 196 13,362 6,090 5,718 3,758 Bt. Toms. was aes ek pias phe 207 14,395 7,771 6,322 4,252 Brrnghel diag. 4...m. = 298 21,898 12,362 10,276 7,296 IGP ASO: coe atcarctels paste se 236 16,819 9 , 267 7,615 5,255 Horé Wortley. 6 «sche serch cts re 261 18,816 10,464 8,637 6,027 Galvestonwes.n. ce Aaa ne ns 331 23,538 12,946 10,629 7,319 Palestine paces Bardens 245 17 ,690 9,850 8,135 5,685 SanvAtOnlO!, non. cle steerer ete oe 276 20,119 11,287 9,355 6,595 | TAVIOVAG ek she ote ee aap ale 254 18,656 10,528 8,750 6,210 . Utah: | IMG d enn e Se ii efiice. «tos erarsieetans 130 8,451 4,291 3,391 2,091 Balt Walkke:Guty's cite oe cs ond crete 182 12,078 6,254 4,980 3,160 Vermont: Burlington x s,. s.1cccete 143 9,000 4,424 3,423 1,993 Northfield ses. %)..: 2 eee es 126 7,812 3,780 2,898 1,638 Virginia: HyNnChDure ys. t.csstne ce eew on 201 13,759 Waar, 5,920 3,910 IN Orfolle. ase cyaise Cerone Se eeeks 230 15,691 8,331 6,721 4,421 IGHMONG Ses Liiele sh-clne eta oars 215 14,942 8,062 6,557 4,407 Wiythevillosy ss chitin ae. « 175 11,544 5,944 4,719 2,969 Washington: WNOxth ELS. . s,0:5.2 al testis 316 16,180 6,068 3,856 696 DGALUOL sents sence eeer es 246 13,770 5,898 4,176 1,716 Bppkane . ete is sicin ncleta tees: 202 11,992 5,528 4,114 2,094 Watoosh, Inland,’ «+ sitejah iors 271 13,623 4,951 3,054 344 Walla Walla. .t. 5.00. dee. dee 216 13,819 6,907 5,395 3,235 West Virginia: LOINC Sane Gree je 145 9,521 4,881 3,866 2,416 PAU KOVSDUPR. sacs’ wir cages 179 12,195 6,467 5,214 3,424 Wisconsin: Gineen AV. 0 ok <6 ome eet ov 153 9,663 4,767 3,696 2,166 TUAUSPORBE on bids dix bug a horateintns 163 10,707 5,491 4,350 2,720 INTECHBOM as. xtis 6 wha tetaimayes 179 11,370 5,642 4,389 2,599 RVULECVEATUICENG S) 0.6, 0:Sis ip: Wistar yao) acne 162 10,191 5,007 3,873 2,253 Wyoming: i WHOVENNO RE ities bk cebiedss ois 119 7,528 3,720 2,887 1,697 EBD GY ss iceaewtars'a enn stolen pinly 108 6,911 3,455 2,699 1,619 PLATE 37 ‘z oyeyd st oseq oy, “spetpuny quosoidod sonyea [BOMOUNN “WeLIIoy 10438 ‘VoA OY} IOJ (“ .ZE PAOG”) AoUATOLYO a1njze1ddU19} JO SIdIPUI JOpUTeUTOI JO sUOT}wMIUING (WSeq) (S9M) uor}Isuely 48910} U991F19A9 48910} UVVIB19A9 I 4se10j Snonpioep -purssei5 on Aydosaur urIyIION o13Aydosour UIOYIION F. g WU Yy Yi} _ ysor0y snonpideq purysseiy } 4 4A¢+ Ry WIN QQ) ‘ \ \} : ib SWS SAN SS { Sy AS Vd». xX NY NYS SN WIS SOOT t SY a y 2 AN ‘ ey k < AR FAT GANAS SAAR Ne AN SAN VSS NN Nah RN N NYEY ESS x \ NSA LSS EMSS NENYSSS S SNL RR NSA RS SS a SWS ' PSOE QI : b SNA! RAR cas 7 id eit Ei lecri i (os ALIN GAiiizee® Si he | NOR tl L/ ae eel eS 2 eae Nh ol ; r oh NY Je AOR Ae 5 A) ery [Aa aa ae ooh oft GL 18 €8 98 of8 68 16 66 06 46 66 JOT COT SOI 80 oh 9 ath “BL 222 PLATE 38 'z oyR]d st osvq oY, ‘spuvsnoyy Juoserdos sanyea yBoLOUIN “(9 a[q¥y WO] BYVP) UOSwaS SsoT}SOIJ OFVIOAT Jo potted 10J (“yf .6g PAO’) AoUTOYJo oANyvIOdUIO} JO SOOTpUT JopUrBUIOI Jo SUOTFBMUMING S371W 009 WS AN . N \\ Zt jet, AJ 7 Ce 4 = ~ Wk Ly Ys | " 16 0&6 86 ‘ ol6 ol6 66 66 TOL — 38910} u9913 1909 — dNAydosaw wiay oN rr = : of = Ol seq) : rea BEET [S| LY 7 Or es eee OS) PL. ig oe BG res 4X PX / 2 y 4? 3 S01 ceri) 38910} u991BI9A9 on Aydosaw wiayON 4se10j 2 Aydosour Us9}SB9Y4INOS Alt 38910} 90181949 mAydosB Ay ofl uonisues ‘Z O78[d slasvq ey], “Spaipuny yuososdes sonyea [eoweuNN *(z WUINTOd ‘7 91qB} Woy BYP) TOStaS 8S9]}80.1 ODIOAT JO potrod 10} (AzTUN jo onywa xopul SuLAvY sv pounsse “J OF) AousToYge o1nyvsoduIe} Jo soorput [eIyWOUOdxe Jo suoNeUMUING uor}IsURy 38010} snonpis9p -purjssei5 ysar0y 94 Aydosour ANE us9}seayNos NY ° KA Say AS \ y \ \ AN \\ ‘ x TED h + ‘ \ 7 ?) ) \ o> VANAS \ Se ¢ SANS \ AAW . e YS N mn AVA \ WA SS OA ‘\\ : ‘ ad N 3 WRN neXt SS o \ \' Sos K) \ SAN RRA NY .. vey Was RO Ah es \\ ey Pot \ . ‘e by. 7 z a fo N E 7 \\ 2 ae =) Sat Ssh a ) y NY G) Fie " = f ZA game YL 5 [\"\ —— = 9 5 4 fae) SS oe is wl ae ) Lib iS AN 100 107? Ub 10d LOK 28 97>. 05 08” Oly 6G B77 abe So Sls Br ot eo ‘ = 224 PLATE 40 y = io LE ZL 4 y, . . MEA t f Y Z; Z aN SAN \X \\\ 2S % = V3 AW v N GG. al TS Si A NN XA NES WS SS te WEY ea se ES \ . SAN present thousands. Narrow full lines divide map into five different] Broad full lines are the vegetational boundaries of plate 2. ZR ay, es er. Dh KA; LR shaded areas. Smaller subdivisions are shown by dotted lines. leas season (data from table 7, column 4), Numerical values re Yoz Summations of physiological indices of temperature efficienc &, | CLIMATIC CONDITIONS OF THE UNITED STATES. 225 Charts were made for each of these four series of seasonal efficiency indices, but they all agree in the general delineation of the climatic zones and only the one for summations above 39° F. is here presented, in plate 38'. This chart differs from the corresponding one of Living- ston in a few details, but the two are practically identical. The increments of seasonal temperature efficiency indices shown on the chart of plate 38 are each 1,000 in the East, and the numbers placed upon the isoclimatic lines denote thousands. These values may be reduced to the corresponding ones based on the centigrade thermometer scale by the use of the familiar factor 5/9, the starting-point for our summations being 39° F., or 3.9° C. The lines of this chart are seen to have a generally west-east direction east of the Rocky Mountains, several of them being southwardly displaced by the Appalachians. The western mountains produce a very great southward displacement, and another considerable dis- placement of some lines, in the same direction, appears due to the immediate vicinity of the Pacific Ocean. It is interesting to note that the area having an index of 7,000 or less extends southward on the California coast nearly to the parallel of latitude 33° north, while the same area on the Atlantic coast extends southward only to about 35° north latitude. Most of the country appears to be characterized by these seasonal indices of temperature efficiency having values between 3,000 and 10,000. The region where these indices are less than 3,000 seems to occupy northern New England, northeastern Michigan, northern Minnesota and North Dakota, western Montana, central Wyoming, and the Rocky Mountain system. The region having indices above 10,000 appears to occupy the valleys of the Gila and lower Colorado Rivers, a narrow strip of the Gulf coast of Texas, and the southern half of the peninsula of Florida. 800.6 9.31 14,565 1.95 18.2 Wicksburg....%: -> 892.6 9.19 16,194 1.97 18.1 Missouri: Columbia. ...-4..- 583.8 9.43 10,241 1.86 17.6 Hemnibal). ss e? 585.4 9.50 10,189 1.83 17.4 Kansas City...... 612.8 9.33 10,368 1.81 16.9 SteUiouis)... 050% 677 .6 9.33 11,868 1.88 17.5 Springfield. ...... 588.8 9.53 10,031 1.79 Pra Montana: FIRVTCS piaics eee 311.8 9.79 4,036 1.32 12.9 Helena wc stk 331.0 9.57 3,710 a 11.2 Kalispelle.s, 145.5 297 .2 9.50 2,827 1.00 9.5 Miles City....... 402.2 9.64 6,253 1.61 15.6 Nebraska: Tancolni ssa ..08 Sis 538.9 9.51 9,062 1.77 16.8 North Platte..... 446.7 9.64 7,192 1.67 16.1 Omaha joc. oe. 534.7 9.52 9,087 1.78 17.0 Valentine? ..°..;./. 394.5 9.69 6,393 1.67 16.2 Nevada: Renos ie.cetist ess 338.5 9.65 4,134 1.27 12.2 Winnemucca..... 364.7 9.68 5,463 1.55 15.1 New Hampshire: Concord o0. tes. 368.0 9.74 4,724 1.32 12.8 New Jersey: Atlantic City..... 544.2 9.69 7,878 1.50 14.5 Cape May....... 533.5 9.57 8,417 1.65 15.8 New Mexico: LOR Wellin 2, see 678.4 9.52 12,448 1.93 18.4 Santa Fe.... 442.5 9.53 5,350 1.30 12.0 New York: ID ANIY.s. 6 sos fe 5.0% 466.3 9.61 6,633 1.49 14.3 Binghamton...... 404.4 9.71 5,399 1.38 13.3 TSTCCALOR Merc icin nolets 433.8 9.59 5,761 1.39 13.3 (OE C70) Te Ina ih 358.5 9.80 4,713 1.34 13.1 INGW! WOK: ..f5 0% 554.4 9.44 8,104 1.55 14.6 MOB WOEHG bus cc es > 6 430.3 9.59 5,524 1.34 12.8. Rochester........ 454.6 9.65 5.807 1.38 13.3 Syracuse......... 440.4 9.65 6,022 1.43 13.7 North Carolina: Asheville......... 495.4 9.75 7,504 1.55 15.1 Charlotte........ 717.5 9.39 12,554 1.86 17.5 AATHGIAB bccn cee tins 804.9 9.52 13,771 1.80 Lf, PRONGUEH ay o-X. sien 4/0 700.3 9.39 12,329 1.88 17.6 Wilmington...... 769.0 9.46 13,561 1.87 17.6 North Dakota: Biasmarok......0ss. 3 342.4 8.47 4,792 1.65 14.0 Devils Lake...... 301.2 9.76 3,754 1.28 12.5 WVULIStON Gy vs ei ses 320.5 9.81 4,508 1.46 14.1 CLIMATIC CONDITIONS OF THE UNITED STATES. 229 TasLe 7.—Summation of normal daily indices of temperature efficiency for plant growth, for the period of the average frostless season. (Plates 39 and 40.)—Continued. tea alte = o 1 Ge oH ' acq [aces | Ss¢ | seg | =23 eeu) ete te ees g83 aes “Ss - ~V~ nev aeegiisead s=)o= 82 |8e-2 | 888 eis | B88 i pe e255 gas Bose Bae Station. #26 Eee eer 283 aSad a 5 Ret'g o Ben 6 25 &.2 3 7s Ba Sues caait it Fev re eed na ee eae th an ke aed off | Sae2s| gag | “ees | - ane 3 age ely a Ora >a > oo ica) fa Ay loa oe Ohio: @incinnati...cie. 620.2 9.43 10,725 1.83 1/38 Cleveland........ 508.9 9.48 (healt 1.48 14.1 Columbus: + = S253: 543.9 9.56 8,799 1.70 16e2 BanGusley ssc: 524.8 9.31 7,766 1.59 14.8 Moledons.ae wae ss 489.5 9.62 7,508 1.59 as Oklahoma: Oklahoma }).....).)2 <0: 729.9 9.29 13,098 1.93 17.9 Oregon: Baker City....... 290.7 9.68 3,116 Weil 10.7 Portland 33.3205. 4: 502.9 9.18 4,780 1.04 9.4 Roseburg......... 429.3 9.61 4,465 1.09 10.4 Pennsylvania: TONE ars clei 491.2 9.50 6,741 1.45 UB 76 SATTISDUTE. «5; <.,- 546.4 9.59 8,367 1.60 15.3 Philadelphia...... 591.7 9.46 9,397 1.68 15.9 Pittsburg: <..5... 532.9 9.61 8,659 1.69 16.3 Se@rantOW....2 tare! 460.8 9.64 6,463 1.46 14.0 Rhode Island: Block Island...... 481.7 9.22 5 , 447 123 Liss Providence....... 500.9 9.49 7,241 1.52 14.5 South Carolina: Charleston....... 946.4 9.20 16,874 1.80 iW fons: RO OMMIIIDIA: = ctai.<.5 6 823.3 9.21 15,140 2.00 18.4 South Dakota: HEMET GOHY ees Sievels «haces 369.8 9.82 5,604 1.54 15.2 1 ET e ee See: 461.8 9.57 7,566 1:72 16.4 Rapid @itys.. 5.5 370.1 9.65 5,059 PAZ 13.6 Manktoms... << si... 464.4 9.61 7,616 heeft 16.4 Tennessee: Chattanooga..... 692.5 9.43 12,395 1.90 17.9 iKnoxvilles.c.5s.. 643.6 9.50 10,886 1.78 16.9 Memphis......... 788.0 8.85 14,392 2.06 18.5 Washville... 0.05. ; 710.9 9-32 12,886 1.95 18.1 Texas: PASILENI Oi 5 ci01 suas is 873.6 9.14 15,937 2.00 18.2 PATHATINON <2 = 3.54 ss 598.9 9.65 10,668 pe} 17.8 Corpus Christi.... 1 tSie9 9.08 21,392 2.08 18.8 IBGE ABO sh sc cis aisle» 826.8 9.21 15,043 1.98 18.2 Hort Worth... .-. 961.0 8.99 17 ,652 1.88 18.4 Galveston ..\./s 5.25 1,175.2 9.04 21,163 1.99 18.0 IPSLESUINIGs,. 8 ga & eess eas a g8oO% 325 Sa ones Em. 36% ssc aie eile B¢os Or eve Station. a & ei Sau g ae 8 3 a3 5 3 23% |283'-ag| 253 | sean | seed g 3 Stn2 3 LABs ee °Ssa a8 $ oso ac | S852 OPES eas f=) > = na ond a = ua & SaaS ES Beas SB2Z5 Sazs ea) foe Ay foe iow Virginia: Lynchburg....... 626.2 9.45 10,631 1.80 17.0 Nortolle i. ia debs es 718.0 9.36 12,194 1.81 17.0 Richmond........ 702.8 9.33 12,305 1.88 17.5 Wytheville....... 487.0 9.69 7,313 1.55 15.0 Washington: North Head...... 496.3 TSTLE 2,693 0.70 5.4 Beatle... 0 it. ccceck 462.2 9.04 3,692 0.88 8.0 Spokane: ....... 446.3 9.22 5,059 123 11.3 Tatoosh Island... 407.8 7.49 1,947 0.65 4.8 Walla Walla...... 573.3 9.41 8,378 1.55 14.6 West Virginia: IKI eee ote revi 395.2 9.78 5,685 1.47 14.3 Parkersburg...... 543.9 9.59 9,009 1.73 16.6 Wisconsin: Green Bay....... 382.2 9.67 4,942 1.34 12.9 LaCrosse... S25) 449.9 9.67 6,705 1.54 14.9 Madison: 7. stew 460.9 9.52 6,434 1.47 14.0 Milwaukee....... 403.2 9.61 5,261 1.36 13.1 Wyoming: Cheyenne........ 295.3 9.78 3,640 1.26 12.3 ander. cies = 275.5 9.80 3,548 1.31 12.9 It is seen at once that the values given in the first column of table 7 are much smaller than are the corresponding ones of table 6. Living- ston and Livingston made a study of the ratios obtained by dividing each exponential seasonal index by the corresponding remainder index, derived by using the constant difference of 39, and the resulting ratio values are here reproduced in the third column of table 7. The chart of our plate 39 presents the climatic zonation exhibited by the summations of efficiency indices derived by the application of the exponential method (Van’t Hoff-Arrhenius principle of chemical reaction velocity), and it is essentially the same as the second chart (fig. 2) of Livingston and Livingston. In a general way, the two charts of plates 38 and 39 agree in the positions and directions of the isoclimatie lines, but they differ in a number of details. A somewhat thorough comparison of these two charts has been made by Livingston and Livingston, using the ratio of the value of one index to that of the other (column 3 of our table 7), and they present a chart of these ratios as their third figure, which we do not here reproduce. As has been pointed out, these authors con- “ NT CLIMATIC CONDITIONS OF THE UNITED STATES. 231 clude from their study of these two methods of estimating temperature effectiveness that the method that derives efficiency indices by sub- tracting 39 from each daily mean temperature index gives, ‘‘in a broadly general way, and for most of the area of the United States, nearly the same climatic zones”’ (loc. cit., p. 375) as those given by summations of temperature efficiencies based on the chemical coeffi- cient of 2.0. Nevertheless, these authors point out that ‘‘the similarity between the results derived by these two methods of temperature integration is, however, only superficial and roughly approximate. The ratios of direct summation (above 39° F.) to chemical efficiency summation, range in magnitude, for the mean frostless season in the United States, from a minimum of 7.49 to a maximum of 10.44.” Their chart (fig. 3) shows clearly that these ratio values (column 3 of our table 7) are to be considered as some sort of climatic measure. The marginal regions of the United States are frequently characterized by low ratio values and the two main mountain systems seem to have high ratio values. For most of the area of the country the ratio of the summation index derived by the method of subtraction, to the index derived from the chemical coefficient, has a value of about 9.5, and the assumption of this as a constant ratio between the two indices does not introduce very large errors for most of the area with which we are dealing. The feature of these chemical efficiency indices that should attract our attention, however, is their relative values; according to the funda- mental assumptions upon which these efficiency indices are based, these values should be proportional to the amounts of plant accom- plishment within the frostless season, at the corresponding localities. Thus, referring to plate 40, if plant production in the region of East- port, Maine, has a value of 300 for the average frostless season at that station, that in the vicinity of Jacksonville, Florida, should have a value of 1,000 for the frostless season there. The extreme range of this seasonal temperature efficiency, as shown by the chart of plate 39 and by table 7, column 2, is from 276 (Sault Ste. Marie, Michigan) to 1,538 (Key West, Florida), or from unity to about 5.6. By the remainder indices (plate 38, table 6), the corresponding range is from 3,543 to 17,077, or from unity to about 4.8. It is thus brought out that, while one method of deriving efficiency indices would lead us to expect only 4.8 times as much plant activity at Key West as at Sault Ste. Marie, the other would lead us to expect this ratio to have the value 5.6. Since the physiological indices of temperature efficiency promise to be much more valuable in climatological study than either of the kinds of indices so far applied in our study, we need not here enter further into this comparison. : 232 ENVIRONMENTAL CONDITIONS. (E) SUMMATIONS OF PHYSIOLOGICAL INDICES OF TEMPERATURE EFFI- CIENCY FOR PERIOD OF AVERAGE FROSTLESS SEASON. (TABLE 7, PLATE 40.) The physiological indices here employed, as indeed the summations themselves, are reproduced from Livingston’s paper (1916, 1) already cited. For each normal daily mean within the period of the average frostless season the corresponding physiological index was obtained from Livingston’s tabulation (our table 5, Fahrenheit scale), and all the indices thus obtained were summed for each station considered. The seasonal physiological indices of temperature efficiency thus obtained are reproduced in column 4 of our table 7. In the same table are also given the ratios of the physiological seasonal index to the corresponding remainder index (above 39° F.; column 5) and to the exponential seasonal index (column 6). The geographical distribution of the seasonal indices of temperature efficiency, physiologically derived, are shown on the chart of our plate 40, the lines of which are reproduced from Livingston’s paper (1916, 1). As that writer states, the general delineation of climatic zones is here much the same as in the case of the other two kinds of summations. The lines again show a general west-east trend, and are again displaced southward in the vicinity of the oceans (especially on the west) and of the mountain systems. A cursory glance at these three charts of tem- perature efficiency summations for the period of the average frostless season (plates 38, 39, and 40) shows them to be so generally similar that one might almost serve for either of the other two, as far as the forms of the various climatic zones is concerned. Which method of derivation of the efficiency indices is used seems not to be of great importance in the general seasonal result. As far as present knowledge goes, then, one method appears to be as satisfactory as either of the others in this respect. The authors of these methods have discussed some of the main features wherein these three charts differ in detail, and we do not need to enter deeply into this matter here; but the following points may receive brief mention. The actual values are much lower in the case of the exponential indices (plate 39) than in either of the other cases (plates 40 and 41). Furthermore, the values obtained by the remainder method are generally, but not always, somewhat smaller than those derived from the Lehenbauer measurements for maize growth. It is not possible, however, to reduce the values of one of these three series to those of another, by employing any constant ratio, as is shown by the variations in each of the three sets of ratio values given in table 7. For convenience, we may represent the summation by the remainder or difference method (above 39° F.) by D, that by the exponential or chemical method by C, and that by the physiological method (based on growth of maize seedlings) by G. We find (table 7) that the aver- —— | | = rd CLIMATIC CONDITIONS OF THE UNITED STATES. 235 age value of D/C is 9.44 and that this value ranges from 7.49 to 10.44. Similarly, the average value of G/D is 1.60, the range being from 0.65 to 2.20, and the average value of G/C is 15.0, the range being from 4.8 to 20.0. While the first ratio value D/C shows a geographic varia- tion that is not apparently related to temperature conditions (Living- ston and Livingston, 1913), both of the other ratio values, G/D and G/C, exhibit a variation that is obviously related to temperature, and the charts of these values (not here reproduced) appear very much alike and also very similar to the charts of the summations themselves, as far as the direction of zonation is concerned. The main differences between these three summation charts, that require attention at the present time, have to do with the relative magnitudes of the summation or seasonal efficiencies indicated for the various stations. By the remainder method (table 6, plate 38) the seasonal temperature efficiency of southern Florida is about four times as great as that of middle New England. By the exponential method this ratio appears to be about 3.7 (table 7, plate 39), and by the physiological method it is 6.1 (table 7, plate 40). Which of these three ratios most nearly expresses the actual relation between the seasonal temperature efficiency for middle New England and that for southern Florida can not be determined without much more knowledge than is at present available. The ratios just given show clearly that the physio- logical method indicates a much greater range of seasonal temperature efficiency throughout the country than is indicated by either of the other methods, but whether this greater range is also represented by corresponding differences in plant growth must be left an open question for the present. As has been stated, we follow Livingston in deeming it highly probable, for various theoretical reasons, some of which have been expressed above, that the physiological method of obtaining efficiency summations for temperature will prove of more service than either of the others. Attention should finally be called to the fact that the chart of our plate 40 brings out five zones or provinces of temperature efficiency for plant growth. These zones are somewhat similar to those shown on plate 34, but the present chart is of course much less detailed. (F) ABSOLUTE TEMPERATURE MAXIMA. The absolute maximum temperatures as given in the Summary by Sections were placed upon a chart and isotherms were drawn for 100° and 110° F. Most of the area of the United States was thus shown to lie between these two lines. It is a remarkable fact, and one that emphasizes the importance of the duration factor in climatology, that there is, on the whole, so little variation between the highest tempera- tures on record throughout the country. The following list of stations and their highest observed temperatures (according to the Summary) are given here merely as an illustration of the fact just mentioned: 234 ENVIRONMENTAL CONDITIONS. Hl oF, Houghton, Mich.......... 103 Vicksburg, Miss.......... 101 Ishpeming, Mich.......... 98 Natchez, Miss............ 105 Escanaba, Mich.......... 100 New Orleans, La.......... 102 Billings; Mont......2.+ s - 112 Galveston, Tex... 5sheJed 98 Havre, VEO. etek nos 108 Silver City, N. Mex....... 103 RICE POULT sty ia radar toes 103 Tampa les sores «oath foie 100 Cairo woe ces na tenn SLO IY BF ec vg) OW Ey 5 OR SOR 96 The area characterized by maxima of less than 100° F. comprises northwestern Washington and coastal points on the Pacific north of about the fortieth parallel of north latitude, central Idaho, western Montana and Wyoming, northern Minnesota, northwestern Wis- consin, and the entire northern peninsula of Michigan, some of the western and not so much of the eastern margin of the southern penin- sula of the last-named State, eastern Ohio, the northern half of Pennsyl- vania, the Appalachian area south of about the thirty-eighth parallel, all of New York, the western half of New England, Atlantic coastal stations from southern Maine to North Carolina, the southern third of Florida, and a few coastal stations on the Gulf of Mexico. Besides the area thus described, there are several restricted areas of maxima below 100°, the main ones of which occupy the regions of the Sierra Nevada and of the Rocky Mountains. It is thus clear that only a very small portion of the United States has been characterized, during the periods of record, by maxima below 100° F. The area characterized by maxima of 110° or above comprises local- ized sections, the main ones of which are as follows: (1) the southern third of California, the great Sacramento-San Joaquin Valley, and Arizona south of the great plateau; (2) western Texas and the Rio Grande region; (3) southwestern North Dakota, northwestern and southeastern South Dakota; (4) western Kansas, south central Ne- braska and northwestern Oklahoma; (5) northeastern Arkansas, south- eastern Missouri, southwestern Illinois, and a little of northeastern Missouri, and southeastern Iowa. It seems clear that the variation in absolute maxima throughout the country is so slight that this criterion can be of no practical use for our present purpose. Of course, it is patent that the lengths of the periods of observation are very unequal for the different stations, and it seems probable that, with much longer periods, the absolute maxima will approach about b10° F. for approximately the whole country. (G) ABSOLUTE TEMPERATURE MINIMA. (PLATES 41 AND 42.) A chart of the absolute minima of temperature (as these are given in the Summary by Sections) was prepared in the same manner as was that of the absolute maxima. This chart brings out some rather definite climatic relations, and appears to be valuable for our purpose; it is therefore here reproduced as plate 41. The Fahrenheit intervals have been so chosen in the making of this chart that they correspond _— -- 20-7 PLATE 41 Zou =, o ret) s\ ida ts ihe 5° , Si S s SERS at ° t Bic ol = ~~] i Ti Cal 95° 93° of 89° 87 a3 LS ¢ ‘Sy (ae te od Yj y Vig y ~ zs Yj iG 7A S “4 aa YU Ds SOc ee ). Numerical data are degrees F. Narrow lines divide the map into five differently shaded areas. Broad lines are the vegetational boundaries of plate 2. Hees ai SS des ae < x6 Ps > (aio & Tey Ay Tt ms ae Kc Se ae oH : i= SS RS) OF: ey iy. es SOZDS — SR J ; S ON = t GNX \Y '\ Svar. a, When Pe a ~ Sie Le fae = fa AG 7 ee eG Se WN YE «< Pe eN) wn SS Wes aa | Absolute minimal temperatures (as far as records show EOS oy? KS SES Di Sue gy } Cs ©, ~~ °o9 ioe] — _ ‘Z eyed st asuq OY, *\[ SeardOp oIv VIWP [VoLTOUIN ofS wW S37UN 009 \Y \\ AN \\ Sy S 236 AR \\S qaseq) ysa10j 99181949 anAydosaus Us9Y4ON OSM 38010} U99. ony IAD ON eS ee eee eee a CLIMATIC CONDITIONS OF THE UNITED STATES. P50 to centigrade intervals of 10°, between the extremes of 0° and —40°. Thus, there appear 5 lines, representing 32° F. (0° C.), 14° F. (—10° C.), —4° F. (—20° C.), —22° F. (—30° C.) and —40° F. (—40° C.). No attempt has been made to smooth the lines; they represent, as nearly as possible, the actual data given in the Summary by Sections, the topography being also taken into account, as usual, in the placing of the lines. The five different zones are shown by different patterns in plate 41. There is only one station with a value of over 32° F. (Key West, Florida), so that no temperature province for values above 32° is shown. Plate 41 shows that the isoclimatic lines based on this criterion have generally the usual east-and-west trend of temperature lines. Here they are markedly displaced to the northward in the vicinity of either ocean. Southward displacement by the mountains is also more or less pronounced. The regions with absolute minima above 14° F. contain most of the popular winter resorts. The United States Weather Bureau chart showing the lowest tem- perature ever observed! is here reproduced as plate 42, for comparison with our plate 41. The increments represented by the lines are each 10° F. It is seen that this chart agrees with ours in its main points, but that its lines have been subjected to an effective smoothing, so that they are much more regular than those of plate 41. (H) AVERAGE DAILY NORMAL TEMPERATURE FOR COLDEST 14 DAYS OF YEAR. (TABLE 8, PLATE 43.) Since the absolute minima of temperature do not furnish an indica- tion of the intensity of cold wswally encountered at the various stations, it seems desirable to employ some normal temperature mean that may represent this. We have chosen for this purpose the average of the normal daily means for the 14 days having the lowest normal daily means, as given in Bulletin R of the United States Weather Bureau. It is to be noted that this 14-day period does not include the same days for the different stations, so that this climatic feature may be expected to be somewhat different from the mean temperature of some uniform period, such as the first two weeks of January, etc. The nature of the normal daily means of Bulletin R is such that it is impossible always to select 14 days as representing the lowest values. Thus, for Anniston, Alabama, the normal daily mean is 42° F. for all days from December 27 to January 24, and the average of any 14 of these 29 days remains 42°. In table 8, which gives these averages, the first and last dates of the period considered are given for each station. Wherever the period includes more than 14 days the normal daily mean is constant (and the same as the average given) for the entire period. When a 14-day period includes several values of the normal daily mean, the day repre- 1U. S. Weather Bureau, Chart of lowest temperatures ever observed. (To and including 1914.—Letter from Professor C. F. Marvin). 238 ENVIRONMENTAL CONDITIONS. senting (as nearly as possible) the middle of the shorter period having the minimum value is taken as the seventh or eighth day of the 14- day period, by which method the position of this period in the calendar is determined closely enough. The data given in the last column of table 8 are represented by the chart of plate 43, on which the isotherms are shown for increments of 5° F., from 0° to 60°. The climatic zonation is here seen to be generally similar to that of the other temperature charts, but there are differences of detail. TABLE 8.—Average normal daily temperatures for the coldest 14 days of the year. (Plate 43.) [The period is frequently more than 14 days in length. Where this is true the daily normal is constant throughout the period given.] Station. Period. 5 Station. Period. Alabama: Die Illinois: 3 Anniston. %)./:". .).... Dec. 27 to Jan. 24 42 CRIT On ein eis cteserereee Dec. 30 to Feb. 3 35 Birmingham...... Jan. 1 to Jan. 22 45 CHICAO = <.cisisiee wat Jan. 17 to Feb. 4 23 Mobilei seca: Jan. 2 to Jan. 15 49 Mai Sales. sown ck. Jan. 14 to Jan. 27 21 Montgomery...... Dec. 30 to Jan. 14 47 IPEOTIS 5: eB acon obs Jan. 3to Feb. 2 23 Arizona: Springfield........ Jan. 9 to Jan. 29 26 JUN teen Sept = Dec. 28 to Jan. 14 26 Indiana: IPhOSnIN 2 cas meee Dec. 31 to Jan. 13 50 Evansville........ Jan. 3 to Jan. 24 32 Vumanntecsvaseke Dec. 27 to Jan. 16 54 Indianapolis......} Jan. 4 to Jan. 28 28 Arkansas: Iowa: Fort Smith....... Jan. 3 to Jan. 25 38 Charles City...... Jan. 7 to Jan. 26 1l Little Rock....... Jan. 4 to Jan. 17 40 Davenport.....:.. Jan. 13 to Jan. 26 20 California: Des Moines....... Jan. 8 to Jan. 26 20 Wurekialis;.oacaecies Jan. 26 to Feb. 8 46 Dubuque... ccs. Jan. 5 to Jan. 26 18 ITORNOS, aye 2c aontes Dec. 24 to Jan. 20 45 Keoki .'. + aise ais Jan. 10 to Jan. 23 23 Independence..... Dec. 24 to Jan. 19 40 Sioux (Citvigesne scl Jan. 11 to Jan. 24 15 Los Angeles....... Jan. 4to Feb. 5 53 Kansas: PLCC UBL. ate eater Dec. 31 to Jan. 13 45 Concordia... e.5 4 Jan. 6 to Jan. 25 24 Sacramento....... Dec. 21 to Jan. 17 45 DOAPGS eles w5.ces Jan. 2 to Jan. 23 27 San Diego........ Dec. 29 to Feb. 12 |. 54 Topeka isis. ds sre sists Jan. 9 to Jan. 21 25 San Francisco..... Dec. 30 to Jan. 16 49 Wichits...c.c.cniccsus Jan. 10 to Jan. 23 29 San Jose... .cc.es Dec. 28 to Jan. 22 48 Kentucky: San Luis Obispo...} Dec. 29 to Jan. 31 51 Lexington.....>... Dec. 31 to Jan. 31 33 Colorado: Louisville......... Jan. 7to Feb. 2 34 DO GINY GM sreth seen e.a,0 are Jan. 8toFeb. 3° 29 Louisiana: IBsiig har oy. ye eae Jan. 1to Jan. 20 24 New Orleans...... Dec. 23 to Jan. 27 53 Grand Junction...} Jan. 2 to Jan. 19 24 Shreveport........ Dec. 28 to Jan. 25 46 PLS s cite atstaie ous Jan. 3 to Jan. 31 29 Maine: Connecticut: Mast port .%s si tenraiar Jan. 5to Feb. 9 20 Hartford .. s - Jan VianctOm oiertere ecss Jan Tennessee: Chattanooga...... Jan. Knoxyineseccciine: Jan Memphis. sacs oe. Jan INashvaillevs. ces 2 Jan Texas: Atbilen@2s ose aso Jan (Amarlloee. sss. Jan Corpus Christi Jan HISPason tees cee Dec Hort Worthe....%. Jan Galveston..c.. 10to Jan: 23 26 1 to Jan. 28 38 . 11 to Feb. 27 41 1 to Jan. 25 33 2 to Jan. 31 29 7 to Jan. 27 31 . 13 to Jan. 26 14 7 to Feb. 1 15 9 to Jan. 26 16 . 13 to Jan. 26 19 . 14to Feb. 9 25 . 26 to Jan. 20 17 PLATE 43 240 ee ee ‘zg aynid stosuq oJ, *,] Sooadop ov sonyBA [voLoMAN N a a ee head *(g o]qey WOAy BYBP) BOA Jo SAUP HT SOP] IOJ soinjesoduro4 Aprep [eul10” aABIOAY mai) 66 = TOL “ag qaseg) — 38910} uaas319A9 i dn Aydosau ———*- == yse203 21yAydosaur Usa}seayINoS ‘4 iO? 4 sy Ibe: i S) — < pees “Way io ak ee ed pot | Sloss IZ e oh lL ° ° ° L8 68 16 86 .€6 46 66 JOT COT SOL LOL GOL TIT SIE GIT LIL .61E .I2t St Ser 241 PLATE 44 *z ayeid St aseq OY], ‘Jf SeoIZop o1v SONTVA [BOOUINN' *(WIBIIIOJA] 10938) IBA OY} JO SYOOM XIS 4S0}40Y OY} 10J Sonzeroduoy [euIIOU Weay] QS9A\) 3SqJ0} UaaiBIAAg yse10j In Aydosaw Wia}seayjNos 444747 4 CCts7 ALAZA 44/ \S \N N \ A AY SN cae =) ’ rw IW Z : : a SS SN xf > Ue . ‘ L S Tia | TAN ‘il Ss ¢ =a) 8', | FOO Yh is 3 it we \ iN SEDO SSS R roo LW tig" » *, & S i ea e ey . S = ASRS | all i Nl See ego 2, _ fale (ee VIALE TN ol 69 al ak Pp /ery 74 ob L oS 88 §8 48 68 J6 86 96 er ————— i ut NU y) lA = eS le ee te 2.“ i ee ee 242 ENVIRONMENTAL CONDITIONS. (I) MERRIAM’S MEAN NORMAL TEMPERATURE FOR HOTTEST SIX WEEKS OF YEAR. (PLATE 44.) In the same paper (1894) from which we have already made extracts, Merriam calls attention to the fact that, while his summation indices (our plate 37) appear to furnish satisfactory criteria for relating tem- perature conditions to the northward limits of species distribution, yet these do not seem at all satisfactory in connection with the southward extension of northern forms. This author writes (1894, p. 233): It is evident * * * that the southward range of Boreal species * * * is regulated by some cause other than the total quantity of heat [7. e., his summation indices]. This cause was believed to be the mean temperature of the hottest part of the year, for it is reasonable to suppose that Boreal species in ranging southward will encounter, sooner or later, a degree of heat they are unable to endure. * * * For experimental purposes, and without attempting unnecessary refinement, the mean normal temperature of the 6 hottest consecu- tive weeks of summer was arbitrarily chosen and platted on a large contour map of the United States, as in the case of the total quantity of heat. We here reproduce in its essentials, as our plate 44, the chart thus obtained—Merriam’s (1894) plate 13— because of its scarcity and of its interest in connection with our own studies. The marked differences between this chart and that of our plate 37 (also reproduced from Merriam) are practically confined to the Pacific Slope. East of the Sierra Nevada, Cascade, and San Bernardino Ranges the zone with a normal for the hottest 6 weeks of above 79° F. (26° C.) corresponds well with that of the Merriam summation above 18,000 (F.) or 10,000 (C.); the zone characterized by a 6-weeks normal of from 72° F. (22° C.) to 79° F. (26° C.) corresponds with that having a summation of from 11,500 (F.) or 6,300 (C.) to 18,000 (F.) or 10,000 (C.); the zone with a 6-weeks normal of from 64° F. (18° C.) to 72° F. (22° C.) corre- sponds to that with a summation from 10,000 (F.) or 5,500 (C.), to 11,500 (F.) or 6,400 (C.) ; and a similar correspondence is noted between the zone having a 6-weeks normal below 64° F. (18° C.) and that with a summation of less than 10,000 (F.) or 5,500 (C.). On the Pacific Slope, however, no such series of comparisons can be instituted. While the coldest zone of the summation chart does not appear at all on the Pacific Slope of the United States, the zone of the 6-weeks normals, which corresponds to this elsewhere, occupies the whole coast as far south as Los Angeles. Furthermore, the next to the coldest zone of normals extends much farther westward and southward in the region under discussion than does the corresponding zone of summations; the former occupies the coastal area west of the San Bernardino and San Jacinto Mountains, south of Los Angeles. Merriam has drawn im- portant conclusions from these differences, bearing upon the delimitation of his life-zones, a matter which will receive some attention in Part III of the present publication. (J) NORMAL MEAN ANNUAL TEMPERATURE. (PLATE 45.) The normal mean annual temperature is commonly employed by climatologists for comparing climatic temperature intensities, and it % o7vid stoseqoyy, “WZ SeoiSop or SoNyvA [eoTIoUN N ‘(nveINg J9q}VEA\ “g "A 109378) somyesoduiay [enuue uvour peuI0 Ny 801 086 86 oL6 e C) AGE Gseq) (S2M) 4so10j u : 991BI9AA | ST [omlhse,. scmoltlen,” “latte” Maite, = ob ie 1 SHIN uia}saMyON, ) <= AOS : ~purjssei5, i = ° ‘J KARIN, xsi - ar i \ SN Yann: = UJ9}se9) \ SS AN i: Y % . ; IL pueyssess) WAZ ; A j LA44AtAA 4A0707; IEA, AXXy : ae vesaq AAAAAAAS 1asaq-tuiag Set \ ’ a MULL ANY oy : NYY i w RETNA ws \ \ > 7 \Y ee A . W's XS E U7 ’ ' RA VFA ; Xo . 2 zy Wa PS ny % —_ en \ NY GG ~ PS ANN KON a 4 B54 f \ A ‘ me a ANAS, 7 S a oe ; “ bo st , : : : Y }, Ff Peek slat a sy 4 Gite Saat othe ] agg a Zz 1otS > 4 c i iedetet see CRTs Re tn oi Va WINONA FZ FA ee ena ‘ 0 4 i AWS Sh) 4 As Wy ry f POULLNEGES : fe io Ps fA‘ os iy . : os /, * ty AA A H > Oe wis = ¥ WZ oi YQ SS NS D A \ eae * HBS NS \' , RAN YSSN SLY tiytyyyy: oh oan “nie N \N \ SS Sas =A i ‘ Ni i —| al in | ll : i \ we A SS A rae a i ce Nee IX Pam) Pa Oe Z-~ off $69 lL «SL GL CLL” ~-GL . _ lS 88 $8 28 68 16 6 6 oo PAS ee cae 244 ENVIRONMENTAL CONDITIONS. is added to our list of temperature features for this reason more than for any other. Our chart for this (plate 45) is reproduced from that of the United States Weather Bureau.’ It requires no special comment, excepting that we have represented four of the lines as full, so as to bring out the general temperature zonation according to this criterion. 3, CONCLUSIONS FROM THE STUDY OF TEMPERATURE CONDITIONS. The most obvious generalization to be drawn from our temperature studies, as represented by plates 34 to 45, is patent to everyone, namely, that the temperature zonation of the United States has a pre- dominantly west-east direction. Latitude is of course the controlling geographical feature that brings this about, and the values of the various forms of temperature indices increase toward the south and decrease toward the north. Modifying features are the mountain systems and the oceans. The isoclimatic lines bend northward near the Atlantic and Pacific coasts. Also, they generally bend southward on either side of each of the three main mountain systems. On plate 34 the different patterns represent the country as divided into 5 climatic zones or provinces, accoiding to temperature conditions, and Merriam’s chart for summation indices above 32° F. (our plate 37) shows a similar convention. Of course, any number of zones might be considered, but it is perhaps most useful to follow Merriam in this matter if a few definite zones are required. These 5 temperature provinces do not, however, need to be given names of the sort used by the author just mentioned, and if special names were requisite they should be climatically descriptive; they should of course not be named after geographic areas. We therefore suggest, in this connection, that a 5-zonal arrangement for temperature conditions will probably prove satisfactory, these being subdivisions of the larger temperate zone of geographers, and that these 5 temperature provinces of the United States may be termed simply and directly: very warm, warm, medium, cool, and very cool. It might be as well for scientific purposes to number these provinces serially, but such a procedure would not be satisfactory in non-technical discussions. The simple, descriptive terminology here suggested is clearly understood by everyone, while such terms as upper and lower Austral (Merriam) apparently fail to be understood by many who actually employ them. It should also be noted that the area of the United States does not include all of the north temperate zone, and our suggested terms leave opportunity for other subdivisions lying north and south of our group of 5. Thus, south of the very warm temperate temperature province may be one called hot and still another called very hot, while north of the very cool temperate province may be two more temperate subdivisions, the cold and the very cold. 1U. S. Weather Bureau, Chart of normal annual temperature. (To and including 1914.— Letter from Professor C. F. Marvin.) CLIMATIC CONDITIONS OF THE UNITED STATES. 245 Probably the most generally useful of our charts of the temperature conditions is the one for the length of the period of the average frostless season, and the five temperature provinces just mentioned are indicated on that chart (plate 34). In terms of that particular temperature index, the relation between our simple names and the index values is shown in table 9. For other temperature indices the values would TABLE 9. Length of period of average frostless season. Temperature prov- inces of temperate zone in United States. days. Below 120 120 to 180 180 to 240 240 to 300 Above 300 of course be entirely different, but the general zonation for these other indices may be generally comparable to that of plate 34, if proper limiting values are chosen. In the case of the temperature summation indices obtained by the physiological method (plate 40 and fig. 1), for example, the zones noted in table 10, roughly comparable to those ae pi ri Fig. 1.—Temperature zonation, according to physiological summations for period of average frostless season. Temperature efficiency provinces: very warm, more than 20; warm, 12.5 to 20; medium, 7.5 to 12.5; cool, 2.5 to 7.5; uery cool, less than 2.5. Numerical values represent thousands. (See also plate 40.) 246 ENVIRONMENTAL CONDITIONS. shown on plate 34, may be distinguished. These temperature prov- inces are shown on figure 1, reproduced from plate 40, for ready reference here. TABLE 10. Physiological Temperature prov- summation in- inces of temperate dices of tem- zone in United perature effi- States. ciency for plant growth. thousands. Of course, it is not expected that any two charts, based upon different forms of climatic indices, will agree as to details. Thus, the Pacific coastal region is seen to be mostly included in the cool province on plate 40, or figure 1, while it lies mostly within the medium province on plate 34. What particular form of temperature index, or what com- bination of such indices, will be found most valuable in distinguishing the climatic zones for studies of dynamic plant geography remains to be determined, but it may be safely predicted that no single form of index will be sufficient for the purpose of ecological and agricultural climatology. It is unfortunate that Merriam’s zones (our plate 37) and the unsatisfactory terminology that goes with them should have been allowed to become stereotyped; climatic temperature conditions have many dimensions and the useful comparison of climates requires the employment of many more than a single one of these. III. MOISTURE CONDITIONS. 1. INTRODUCTORY. As has been emphasized earlier in the present part (page 120), the moisture condition immediately effective to control plant activity is the water-content of the particular cells and tissues involved, and if it were possible to study the duration and intensity aspects of this condi- tion such a study ought to be fundamental for the ecological relations with which we have to deal. Asin the case of the temperature relation, however, it is impossible to make any progress at present by attacking the problem in this ideally logical manner; here also it is necessary to consider less immediate conditions and to pass to what is considered as the external environment, without even attempting at present to inquire, in more than a very superficial way, concerning the nature of the internal water-relations which directly determine plant phenomena. Our analysis of the matter before us proceeds somewhat as follows: | CLIMATIC CONDITIONS OF THE UNITED STATES. 2At Vital activity is influenced by internal moisture-conditions that mainly remain to be studied by physiological science. We are sure that these internal conditions are largely dependent upon external water-relations, and our task is to find ways of measuring and defining the latter as they exist in nature, in such a way as to render our description of the moisture conditions of the environment valuable to those interested in geographical distribution. It has been already noted that such a procedure is rather simple in the case of the tempera- ture-relation, for the immediate and internal temperature conditions effective in the control of plant activity are closely paralleled at all times by the more remote conditions of the environmental temperature; there is usually no great lag between the march of the external, or ecological, and that of the internal, or physiological, temperature con- ditions. This is, of course, simple because heat migrates with com- parative readiness either into or away from the plant and hence equilib- rium in this regard, between plants and their surroundings, is seldom very far from being attained. Similarly, the environmental moisture conditions are also effective to control the immediate, internal moisture conditions, through the relative rates of the entrance and exit of water. In this case we have to deal with material instead of energy, but the general relations are the same. Therefore, it is with those conditions of the environment that may influence the rates of entrance and exit of water that the present section has to deal. A plant may suffer from lack of water, (1) because of too slow a rate of entrance of this sub- stance into its body during some previous time period, (2) because of too rapid a rate of exit, or (3) because these two conditions have been simultaneously effective. The environmental conditions to be here considered for the area of the United States will be presented under the two captions, the supply of water to the plant, and the removal of water from the plant. 2. SUPPLY OF WATER TO PLANT. (A) PRELIMINARY CONSIDERATIONS. A discussion of the power of the surroundings to supply water to plants should begin (for ordinary plants) with the power of the soil to supply moisture to roots; it makes no difference, for this primary in- quiry, what conditions may determine this power, for the only thing directly affecting the plant in this connection is this power itself.) Since, however, ways and means for comparing the water-supplying power of the soil at various times and places are still to be perfected, 1 Livingston has emphasized the erying need for methods of measuring and comparing the powers of soil to supply moisture to unit absorbing surface, and he and his co-workers have suggested three methods for the quantitative measurement of this power, all of which appear promising in this direction, but little has yet been done of a positive character. See, in this connection, the following papers: Livingston, 1906, b.—Jdem, 1909.—IJdem, 1912, b.—Living- stcen and Hawkins, 1915.—Pulling and Livingston, 1915. 248 ENVIRONMENTAL CONDITIONS. it is obviously impossible to deal here with this fundamental factor. We turn, therefore, to the conditions next in order of remoteness from the plant itself, and recognize at once that the water-supplying power of the soil is determined by its water-content and its physical make-up. Charts can not yet be made, however, to represent the mean water- content of the soil throughout any considerable area, and the charts now in existence,! of physical soil properties, can be of no quantitative value in such discussions as the present, until the corresponding mois- ture-contents (with their seasonal fluctuations) may be similarly represented. Our inquiry is thus forced back once more to a considera- tion of the factors determining the soil-moisture content. These factors are (1) precipitation, (2) superficial supply by overflow, superficial drainage, and subterranean supply and run-off, and (3) removal of water from the soil by plant-absorption and by direct evaporation. In the first of these tertiary conditions influencing the supply of moisture to vegetation we have, finally, a well-recognized climatic factor that has been measured and recorded, in a way, for many years throughout the area of the United States. It is impossible at the present time, however, to make any quantitatively comparative use of what little information is at hand regarding the second set of factors just mentioned;? this information is still far too general and qualitative to be of service in an inquiry such as the present. With the third set of conditions above mentioned (plant absorption and direct evaporation) we shall have to deal in the following subsection, for the same environ- mental conditions that control the removal of water from the plant are effective to determine plant absorption—in a great measure, at least—and loss of soil-moisture by evaporation into the air. While precipitation is thus clearly seen to be in no sense a direct or immediate condition influencing water-supply to plants, it is very frequently a condition that may be roughly related to plant activity, as is well recognized by everyone; the mean annual rainfall of a given area has long been regarded as of great value in estimating the possi- bility of plant growth in such an area.® As in the case of temperature, precipitation and evaporation should be considered as they affect plants, rather than as they affect any given 1 See, in this connection, the numerous soil surveys of the Bureau of Soils of the U. S. Depart- ment of Agriculture. 2 The reader interested in underground waters will find numerous bits of still unrelated informa- tion in the series of Water Supply and Irrigation Papers published by the U. 8. Geological Survey. Especially interesting is also the following paper: McGee, W J, Wells and subsoil water, U. 8. Dept. Agric., Bur. Soils Bull. 92, 1913. 3 Of course it is obvious enough that this proposition holds only with certain restrictions, as, for example, where the subterranean water-table is considerably below the soil surface. Thus, the cat-tail (7'ypha) or tule swamps in the vicinity of springs in the Salton Basin of California have the same ecological aspect as have similar marshes near the Atlantic seaboard, though a comparison of the precipitation data for these two regions utterly fails to show any reasons for expecting such similarity. The sand-dunes of the Salton Basin and those of the Lake Michigan shores, on the other hand, show differences in vegetational aspect which may clearly be related to differences in rainfall between these portions of the continent. LL CLIMATIC CONDITIONS OF THE UNITED STATES. 249 rain-gage or atmometer tank, but these conditions have just begun to attract attention in connection with the quantitatively dynamic aspect of the study of plant activities, and we are not able to go nearly so far with their treatment as is possible with temperature. Work such as that of Képpen and of Lehenbauer for temperature influence upon plants is greatly needed for the corresponding influence of the moisture conditions, but this sort of work has not yet been attempted, even if it may have occurred to anyone. When such work is accom- plished (which will be possible only with good equipment for the general control of environmental conditions), then it will be time to consider efficiency indices of the moisture conditions in somewhat the same way as we have attempted to deal with the suggested indices of temperature efficiency. (B) PRECIPITATION. (1) Inrropucrory. Since rainfall is so remote from being the immediate environmental condition controlling the water-supply to plants in nature, the measure- ment of this climatic condition must not be expected to show very definite relations to plant activity or distribution. As has been indi- cated, we employ rainfall data not because they are desirable, but because they are the nearest approach to what is desirable that the present state of our knowledge affords. In this case, as in that of tem- perature, we usually employ the length of the average frostless season as our duration factor. Since the effect of precipitation is markedly cumulative, we have also tentatively established a second annual period, which may prove to be more satisfactory for this condition than is the length of the average frostless season. This period is obtained by adding to the average frostless season, at its beginning, a period of 30 days. By this scheme the rain falling during the last 30 days of the frost season is considered as pertaining to the following frostless season. Thus the snow and rain of March is frequently very influential in determining the kind of plant growth that can occur in the following month, especially if the latter is comparatively without precipitation. The length of this added period is taken as 30 days quite gratuitously; perhaps it should be longer or shorter and it prob- ably should have different lengths, according to other climatic condi- tions, for different localities. At any rate, it has seemed desirable to make test of this modification. In the discussions that follow we shall let P represent the normal total precipitation for the period of the average frostless season, while z will represent the corresponding nor- mal total precipitation for the longer period just described. It is obviously not to the point at all to employ the summed pre- cipitation for a portion of the year as a measure of the water-supplying power of the environment available for plant growth during that period. Rather is it requisite to study the average water-supplying 250 ENVIRONMENTAL CONDITIONS. power of the surroundings throughout the period in question. A usable index of this is obtained by dividing the quantity P or 7 by the number of days in the average frostless season. This procedure is logically no better here than the average temperature for the frostless season would be in the case of temperature relations, but, as has been said, lack of knowledge prevents as logical a treatment of the moisture- relation as is now possible in this aspect of the other case. Rainfall is universally measured in terms of depth units, which denote volume or weight units per unit of horizontal surface. The position of the horizontal surface of reference is assumed to be at the level of the soil surface. Raising this surface a few meters above the soil has no considerable influence upon the readings in most regions, though it would be undesirable to place the rain-gage funnels at any very great distance above the ground in an arid country. In such a country a considerable amount of rain might frequently be recorded on a gage supported a few hundred meters above the ground, while a gage directly beneath, at the ground-level, might remain quite dry; the rain-drops often evaporate as they descend. Of course, the opposite is sometimes true in a very moist region, where the drops may increase in size as they fall. Our precipitation data are in terms of inches of depth, since inches are still employed in the tables of the United States Weather Bureau publications, from which we derive our original values. All such data may of course be readily converted into metric values, where more universally comparable numbers are desired. As a basis for our computations we have again had recourse to Bige- low’s tables of normal daily values (Bulletin R of the U. 8. Weather Bureau). These tables present the results of an elaborate treatment of the observation data in the United States, resulting in a precipita- tion value for each day in the year for each station considered. Thus the normal precipitation is given for each day of the year and for each station in the list. Ifa ‘‘normal”’ year, in this sense, ever occurred, then the actual precipitation for each day in the year, for any station, would be the value given in Bigelow’s table. We have treated these normal daily precipitation values in somewhat the same manner as was followed in handling the normal daily means of temperature given by Bigelow in the same publication. By the use of these tables it is possible to study the comparative lengths of what may be called normal drought periods and normal rainy periods, as will be brought out below. All of our computations involve both duration and intensity factors, as will also appear in the discussions that follow. (2) Norman Mean Darty PrecrIPITATION FoR PERIOD OF AVERAGE FROSTLESS ’ /?p SEASON (= ; (TABLE 11, Puate 46, AND Fia. 2.) i The mean daily rainfall for the period of the frostless season should be a general measure of aridity in a certain sense, and we have obtained jt ttt, i Ta 251 PLATE 46 %, > VW 89: LD —— Bec 69 *% o98]d SI aseq OY], SOOUI Ul ov BVP [BoIIoUIN NY oA8 o&8 ho | Nae *oel \ | IS AS y Y.2G WX ABEKa th: Aah Ku vob L o8h o6L o8 088 098 oL8 , By Oe ig. as 68 AG (seq) ysaloj u9a1819A9 d1yAydosau usz3yjJON SESS NN ee CLINGS? COL NOFE LLAACEL, Ze A ~ Ags ‘ Y soe, SO ~ \ NN \ SiS Bae ¥ SHON \ SSX ri ee. \) 3) Nt ‘soourAoid wor}ezId19eI1d BAY OJUT dvUI oY} OprAIp SouTT *(@ WUINOO ‘TT equ} WOT] BYBP) WOSvas SsaT}SO1J OSBIOAG JO potsod 10} suOTyeyIdioa1d JeuIOU ATE (IS9M) 4S910J ua013.19A9 dAydosour uldy}ON, OK = yseroy o1yAydosour u19a}SeayjNoS Py 7 w oh ae ~ BOLL QOL atl coll “ysol0y snonpiseq AN GIT Ia oSET, ysal0J U901319A9 on AydoisA4y oAlE 611 uorjIsuey 38010} snonproep = pueyssei5) puejsser5 il poet iil WH ns =] eS IN eget ob 252 ENVIRONMENTAL CONDITIONS. indices for this by summing the normal daily precipitation values within the period of the average frostless season (Bulletin R, U. 8. Weather Bureau) and then dividing the result by the number of days represented in the period. These averages may be represented by the symbol =, where P is the total rainfall for the average frostless period and S is the number of days in that period. The values of this ratio, for the stations considered, are given in the third column of table 11, the values of P being placed in the second column. The precipita- tion data for a few of the stations listed are not from Bulletin R: they are either from Bulletin Q (in which case the station name is followed by an H in parentheses), or they are from the Summary by Sections (in which case the name is followed by an S and the number of the section of the Summary in which it is listed,in parentheses). Table 11 also includes evaporation data, and where two stations are given for the same data, the first (not in parentheses) is the one to which the precipitation data refer. The second one (in parentheses) is the one referred to by the evaporation data. Where the precipitation data are not derived from Bulletin R (those marked H or §), the total precipitation for the period of the average frostless season was approximated by calculation from the normal monthly precipitation data as given by Henry (Bulletin Q) or in the Summary by Sections. To accomplish this approximation the monthly normals for all whole months included in the period of the average frostless season were added together, and to this sum were added a fractional part of the next preceding and of the next following monthly normal, these fractional parts being, in each case, that part of the value for the whole month that is represented by the number of days of that month included in the frostless season. Thus, if the average frost- less season extends from June 5 to September 6 and if the monthly normal precipitation values for the months involved are a, b, c, and d, then the approximate total precipitation for the period of the frostless season is 25 avs satbte+ = d=P Here S, the number of days in the average frostless season is 25+31+- 31+6=93, and P_5/6a+b+c+1/5 d ie , 93 ee i Ee a ee —_— Ce ae CLIMATIC CONDITIONS OF THE UNITED STATES. 253 Taste 11.—Precipitation and evaporation data for the period of the average frostless season. (Plates 46, 57, and 58.) 3 58 32 re SB eit oe Vea ea | Poe : RA gS i sees Sh Moisture ratios ae 3 Pr gs = 52 for period of & a3 2 as Am Bm os = ar) 568 Ss om °o83 Ro | 36 2 fO Be a5 a > a oOo Sh Sm 3 © 3 a —_ © 8 © 5 i al BS | gee readies cle eden be Bey leone Wh eet & & a & Ap | Fen] 858 ad ace = 3 2 A © nes =| oR $5 | €8% | $346 230 | 3398 = a a - = Alabama: inches. | inches. | inches. anch. inches. | inches. BAASSES EASED «io 4-c)o vise as, 0p 24.43 0.122 30.11 OTTO ioe creer sepolletae s.eysrare fave eke cs evetei[ ecieuaeisyenete Binmngham....:..-.. 28.41 0.123 33.46 (Os: Lisa 1 sions Bel (Pores pcha i Boke eee [Rif Svan c NMBPUC has ce ease es 47.97 0.172 53.51 0.192 35.16 0.130 1.36 lay? Montgomery.......... 31.88 0.131 ST ake 0.155 41.90 0.172 0.75 0.90 Arizona: Fort Apache (S3)?.... 9.82 0.063 10.57 0.068 36.74 0.236 O27 0.29 Fort Grant (S3)....... 9.98 0.042 10.85 0.045 78.91 0.330 0.13 0.14 Dinar GAAS eee GY 0.020 6.35 eect ores gee lorena ceeteccrereecell ore cetera Preseath (54)... .52 65s 7.48 0.057 8.10 0.062 29.72 0.227 0.25 0327 Arkansas: MOrh SAIN... .). 2+ = 2 >. 28.16 O2122 31.29 0.136 37.01 0.161 0.76 0.85 PMO TOUR. o.. ies ves es 30.60 0.129 35.25 0.149 39.63 0.167 0.77 0.89 California: Cedarville (S15)....... bg Ah pape de PLOT |arerere eral eters a eral | Cevararete oie tare taneierecss' | a sketeretarene Te sy LEG Foy | A eS a sae O.025 sh enc ee 0.034 37.52 0.183 0.14 0.19 PIPER eins! esi stovess. 5 06, « 17325 0.070 24.10 OO Bi lisse oecarecetall ok cor srw evel axeter ett <'a)->, | ameceviexexe oe SEED Cerrone es cise cs a5 4.48 0.017 5.94 0.023 56.87 0.220 0.08 0.10 Independence......... s = ee 2.79 0.014 4.17 0.021 69.55 0.349 0.04 0.06 Moe Angeles. .........> 12.88 0.039 iy day 0.046 34.81 0.104 0.37 0.45 Laue 11.88 0.045 15.55 0.059 70.75 0.268 0.17 0:22 Baeramento....:...... 9.62 0.035 13.41 0.049 46.38 0.171 0.21 0.29 San Francisco......... 15.70 0.048 19.84 0.062 32.49 0.102 0.48 0.61 Sho SS aes eee 12.63 0.043 16.86 OR OS Girlie tare.< ctce|lcrede sero ecal [ie Siteorabeks ell hana eur sone San Luis Obispo....... 8.63 0.033 12.43 CDE | ee cp re obetct aera. sucifiever suc tepave’clltexecal smegeuche Colorado: Colorado Sps. ae Sew 10.12 0.066 176 0.077 30.48 0.199 0.33 0.39 Denver.. are rarsnay 7.49 0.049 9.82 0.064 38 .92 0.254 0.19 0.25 Montrose (S9) . Eee aate s 3.90 0.028 AT 0.034 atk 0.262 0.10 0.13 MEAG Sn sss she cure ses 7.65 0.047 9.09 CLD Seer ce cil ene eeeiaraliereererelecerell'e ota we iakens Connecticut: UT 20.28 0.123 23.88 Rae Reece aera cceererencil sy ccc) ave: cretn | ay are.aloneven WW OEIAVEN.. 5. sce es 23.55 0.131 PRI) 0.153 20.05 Ooi i Le 1 bab 7/ New London (H)...... 21.02 0.113 24.75 0.1338 20.40 0.108 iO 1.21 Florida: Jacksonville........... 45.97 0.157 49.61 0.169 39 .67 0.135 1.16 e25 SMOMEREIREENG wre falas coc, ) “ @ “7 | o,! a AA a3 a & oD tt Moisture ratios pe As 3? 2 Eo for period of 3 Sn ee ig Vie Re a7 “ s B 234 4 agen average frostless g % 3s B3r a 28 season, P/E. Be > 2° 5 5 o aa 3” Be RE 2g oo = 2 = as ah EA ym — Station. 2 o > & > ho ar) am Sm Se [de |4e2|-S | 82 | $8 ; AZ | US | Cea se | 8 ; toate =a 8 3a5 BO 2 Oo a ae ew 2 eS ae | Es | Es | Ess g i P/E | «/E ; ° 5 oa KN on 5 oO % ‘3 oo = a Aas = O05 Ao : $3 | 88%] $8 $38] gaa ; = Ss = e a | Florida—Continued: inches. | inches. | inches. | inches. | inches. | inches. ) Gece ae ePahagat } 45.341 0.146| 48.52] 0.156 | 38.51 | 0.124] 1.18 | 1.26 | Pensacola’ to: sees ae 45.01 0.158 49.51 0.174 41.59 0.146 1.08 1.19 fae ere a ay } 50.13 | 0.150 | 53.13] 0.159 | 46.35 | 0.138] 1.08 | 1.15 Georgia: F At aRtA., -sisre boots seve ne 27.48 0.122 S204 0.145 36.95 0.164 0.74 0.88 ; MAING «Sys ho setae os 29.97 | 0.131 | 34.40] 0.151] 35.21] 0.154] 0.85 0.98 IWMaeorr eins nts ) Se ee 2a lt a= Fl : , es gS = oD Sh Moisture ratios ad eee ee ake g2 for period of Sq | 38 22 ahi aS average frostles gf | 8S 8 a os season, P/E. BS 2° seeps RS aa og 2 wa Sank = 2 oo qm EO, 1 a er Station. 2S > & i = as ale Bm Shee Sag Me 2 So3 S 28 2% Ls 3 o RAs Ba Qn ete i BS Ee = 3 St 3 = = OM ° © o g ao a a bp ae A go’ | 8S fee ave ee P/E | x/E 65 Sgt | S25 © op "3 & a> 2OR ag o® S "SO ~ . i~| : g. | gba | gk ais] gis aS Sat] soe gav| Say | a = a a a | Maine: inches. | inches. | inches. inch inches. | inches. LEEDS) 701.09 9) ere eae 18.26 0.109 2123 0.127 13.97 0.084 eS Leer ard Gs a Sea 17.79 0.113 20.95 0.133 17.13 0.109 1.04 1.22 Maryland: AMEEIOTO safe sc ooo oe 26.42 0.124 30.16 0.142 34.78 0.163 0.76 0.87 Washington, D.C.....| 24.96 0.127 28.71 0.146 30.26 0.154 0.83 0.95 Massachusetts: LO Ch Ne 3 eee PALA, 0.114 24.84 0.134 22nd 0.123 0.93 1.09 IWantbucket... 6... 06s 18.99 0.091 22.74 0.109 18.40 0.088 1.03 1.24 Michigan: JUST) Gp OO IIS eRe 14.97 0.109 Wd, 0.128 14.85 0.108 1.01 1.20 IPREREOUE et ei c ois aydieye aie 16.82 0.103 19.15 0.117 24.39 0.149 0.69 0.82 TOES) 0: aes Geen 16.15 0.115 18.93 OPPS Sa As cietectee lot PAS cagecelllavaetersuc tered taeasnet lovers Grand Haven......... 15.28 0.091 iit 0.106 19.28 0.115 0.79 0.92 Grand Rapids......... 15.19 0.093 17.64 OPO | sree eta a ahoye rare gene lta. etel shal etrenel| tereterered wre te HIOUPHCON <5. o)5 6.5 5.0.0 ois 16.25 0.107 18.57 Oe TOBA S cteteetewsllhs bdceteraih ss lle, otadevenebeue Porstemistees « IDA (C5 5 14.02 0.089 16.51 0.104 17.59 0.111 0.80 0.94 Marquette... 2.52.66. 14.94 0.107 17.59 0.126 14.48 0.103 1.03 nA PA | OrugEUTON: «..s 2. ss... 14.73 0.095 16.98 0.110 18.85 0.122 0.78 0.90 Sault Ste. Marie....... 14.07 0.102 16.48 QEUTOM a oscvcaslerscalare cra erevors llateverece-etantsl iatree lotenatiehs Minnesota: WNTAD ES eels, co's oS ees 2 18.66 0.123 20.97 0.138 15.13 0.100 E23 1.39 Minneapolis.......... 20.00 0.124 PAY (UE Pewee ers eerie Mean.aara ioe (ma doc coc NAGOOTHCAG. c16 so ele ee 6% 14.59 Out 17.03 0.129 15.46 0.117 0.94 1.10 Su. LES 18.91 0.119 21.09 0..133 18.33 Oalts 1.03 Ws ih, Naito 9.54 0.093 12.05 OLIN? 10.39 0.101 0.92 1.16 Mississippi: EO TS 6 eee 30.63 0.133 Soni Qe aa ieee Ree Cll S Soe: olay ot. | ieverretepenere: [toteheertiscetrs RUACICSDUED). (oie). o.6 fo os ore 34.87 0.138 39.83 0.158 37 .34 0.148 0.93 1.07 Missouri: MBUINITIAIA cc ioc0 g 6so0 5 3 PALE SF 6 0.120 24.80 ORESO Rs eects le Bie wi etenn lleieyaxeeveteus llelietonetpels ca EMSMTATINEVAN Sc .0:556.5.5 <3 22 20 22202, 0.120 24.88 ORAS Sc coere ete lar acute biel lola: s1ecatehanewel ehafoiatelevee ee al \ 25.89 | 0.132 | 28.94] 0.148 | 30.26] 0.154| 0.86 | 0.96 Hamar (S49).......... 26.58 0.146 30.20 0.166 25.83 0.142 1.03 ab aley So. LUA ee PPA ayy) 0.112 25.83 0.129 39.29 0.195 0.58 0.66 SERMHOHCI: ..c250--6- 27.54 0.147 ole? 0.166 24.96 0.133 1.00 B25 Montana: ead ad eee 2 7.08 | 0.053! 8.96 | 0.067| 31.30 | 0.234] 0.23 | 0.29 ; > ae } 7.78 | 0.064] 9.45 077 | 20.86 | 0.171 | 0.37 | 0.45 J MES ETISU yA css ok 7s fall) 6.48 0.045 7.82 0.054 29.70 0.206 0.22 0.62 256 ENVIRONMENTAL CONDITIONS. TABLE 11.—Precipitation and evaporation data for the period of the average frostless season. (Plates 46, 57, and 58.) —Continued. Station. ealispelicvt\sc. artesian iste Lewistown (S29)...... (Fort Maginnis)....... Miles City: 23. 35 eke Poplar (S30)Eo. sneer (Poplar River)........ Nebraska: Goncords shh hcbscles aks (Manchester)......... New Jersey: Atlantic (City... s/c ccs. Wane Mayes kicedees cs New Mexico: Fort Stanton, :.5.6%..). PPRIIDAING ica cv. wesc alere Gr MOB WOHO: (AotW kaso SV THOURG et clas a's North Carolina: SABO VLLG tie x arse Wl a ae COHATIOUUG, sy 02 a tee vie IOTLENAG ahs ele biatitheeia RR Pula EY ctv ar sb win Uewie b Mia ie WV ULINIITUEGON iy Gon cle ae North Dakota: ISIDBLOK ess ci eee Total normal precipitation for period of average frostless season (P). Mean normal daily precipitation for = period of average frostless season (P/S). 2 cssess sesssosse9s 99 9O Total normal precipitation for period of average frostless season preceding 30 days (7). plus 81 12 .03 .83 .39 .O1 64 .37 .21 64 .50 .82 .39 .14 45 OL 21 AT 61 43 .20 .63 .30 24 .90 .10 41 Oo wl» co esses sesscscsS so 99 ~ period of evaporation for Total average frostless season, 1887-88 Se oo ,! XM os Moisture ratios oe for period of ee e average frostless e8 season, P/E. a3 3 n oS, ae SB 2: ee P/E x/E = 50 a gq 52 558 = inches 0.173 0.45 0.62 0.174 0.34 0.41 0.142 0.90 1.01 aperortee elisa ives eo he ; 0.159 0.82 0.92 0.171 0.60 0.73 cde elo Pe Bee BPE he, 0.122 | 0.94 | 1.09 0.084 1.30 1.50 0.290 0.23 0.27 0.293 0.18 0.20 0.134 0.84 0.94 Ae Pe APS os se ee Bee pet FS ; Se aS eee Bee ane 0.114 0.87 0.98 0.133 0.7 0.81 ees et! he ae Sb 232 fac a @ Sm Se laa Sos S 2 oo 23 ao 2 Ho £3 as A a ao £9 SB aes. ieee a6 o& gh | Bo af 2 OP aN fee ae P/E | x/E 65 Sug | 62s o & a & Ae | Poh] &a8 al ict CE aie, (es Sa Bae 4561 42= 5S | Sat] soa SaL | sak wl = a SS a Texas: inches. | inches. | inches. inch. inches. | inches. Aibilenieied e2i.c bisreies es 20.30 0.083 21 165 0.088 46.00 0.188 0.44 0.47 eae een took 17.52 | 0.088 | 18.76 | 0.094] 40.02] 0.201] 0.44 | 0.47 (Hea “"7*"}) 94.79 | 0.078 | 26.12 | 0.082] 32.41] 0.102] 0.77 | 0.81 Corpus Christi........ 22.83 0.077 Done 0.085 35.09 0.118 0.65 0.72 Hi Pasot s4 Ac) See Yt eg | 0.033 8.09 0.034 64.41 0.273 0.12 0.13 Wort’ Clank) oka... 19.52 0.072 20.31 O)..075. | soe cies aini] swe oot ee ll = alent ene ann ee Fort Ringgold (S1).... Cayenne City)..... \ 15.82 0.052 16.62 0.055 46.90 02165 0.34 0.35 FortiwWorth 2. =o 2219 0.087 24.28 OSOGS Ts ccrcestons eons PON ts Pyne sy Galveston fi.) Seek 42.82 0.129 46.50 0.140 44.06 0.133 0.97 1.05 Palestine’s'\ae. eon ae 28.55 0.117 32.18 0.131 36.25 0.148 0.79 0.89 San Antonio.......... 22.08 0.080 23.70 0.086 43.63 0.158 0.51 0.54 Mayvlor.rs ch. cee. 25.66 0.101 28.48 0 ae Ip 6A (NS Utah: IModenAat es. heres 1.99 0.015 STS e 0 0) Ml ( Pe) oe Salt Lake City........ 6.49 0.036 8.70 0.048 51.71 0.284 0.13 0.17 Vermont: Burlington...:..04....| 16.55 0.116 18.83 0.182. [oc cc ccell cane oe 5c meena enna nnn Northfield: . 3.220 14.26 0.113 16.16 0.128 12.28 0.097 1.16 1.32 Virginia: Ibynchbure,..3.. 5.7: 22. 2.| 28.88 0.124 28.50 0.142 28.68 0.148 0.87 0.99 INDOXLOMUG Saale alc ah Ae 33.69 0.146 37.79 0.164 27.26 0.119 1.24 1.39 PRIGMINOMG crate cis 26.41 0.123 30.09 Oe ©: 10 Py eI Wytheville............ 22.67 0.130 26.76 Oe ys He PPPS CEE A Washington: Gane a: } 34.94 Oil 41.17 0.130 20.18 0.064 1:% 2.04 Olympia (S19)........ 13.81 0.073 17.69 0.094 19.85 0.106 0.70 0.89 PGAUTIE SUE win skie ncics.cee 18.29 0.074 A ya | 0.088 | in diced el] cle. ome © oe clete es teen ee MBOKANOW vi". crete ec 7.69 0.088 9.14 0.045 33.80 0.167 0.23 0.27 Tatoosh Island........ 53.92 0.199 62.88 0.232 14.05 0.052 3.84 4.48 WHE Wella tet oon, 8.08 0.037 9.95 0.046 45.87 0.212 0.18 0.22 West Virginia: is os ee cet ee 19.01 0.1381 22.26 |e Rs: a re PMS Parkersburg eigenstates 21.32 0.119 24.70 Ore t-te ry PE Wisconsin: G¥een Bay iisisks tees: 26. tO 0.109 19.43 0.126 19.39 0.127 0.86 1.00 ba Crosses fs. sisee cts 20.62 0.126 22.91 0.141 21.97 0.1385 0.94 1.04 IMGQIBOISS ti, Sak hee 20.37 0.114 22.63 O. 126 [inns ce alls bn nore es] ace = & ae eee Milwaukee........... 16.63 0.1038 19.28 0.119 19.11 0.118 0.87 1.01 Wyoming: GHOVRNNG sc kesh sd can 6.33 0.054 8.57 0.073 32.48 0.275 0.20 0.26 Landers J) aa eee cio 3.87 0.031 6.27 Oss: al ees (PE I CLIMATIC CONDITIONS OF THE UNITED STATES. 259 The daily normals of precipitation for the period of the average frostless season are shown graphically by the chart of plate 46, where the isoclimatic lines represent increments of 20 thousandths of an inch. The values are seen to be high for the southeast (the maximum being 170, for Cape Hatteras, North Carolina). The lowest values occur in the arid region as this term is commonly understood (the minimum being 9, for Reno, Nevada). On this chart the lines for index values 60, 100, and 140 are represented as broader than the others, since these lines are useful in delimiting the main precipitation zones or provinces of the country. The line for value-60 extends northward from about Cape Mendocino, roughly parallelling the Pacific coast and passing into Canada near the western margin of the Rocky Mountains system. This same line reenters the United States near the eastern margin of the same mountain system, passes southward and somewhat eastward, so as to lie just east of the Rocky Mountains proper, and enters Mexico near the mouth of the Rio Grande. This line includes, within the loop thus formed, all of the region commonly called arid, and some- what more. Northwest from the area thus demarked as arid the values increase, and the lines for values 100 and 140 lie just within the United States, in the neighborhood of Tatoosh Island, Washington (which station has the value 199). East of the arid region as above defined the values of these daily normals increase slowly, and the line for value 100 passes from north to south through the eastern plains or western prairies, approximating a line drawn from Corpus Christi, Texas, to Winnipeg, Manitoba. At its northern end, however, this line bends eastward, apparently passing somewhere north of the upper Great Lakes, and then bends southward so as to reenter the United States north of Port Huron, Michigan. It crosses Michigan from east to west, bends southwest- ward to Grand Haven, Michigan, and reaches Cincinnati, Ohio. From this point it passes northward to Detroit and then follows the valley of the lower Great Lakes and St. Lawrence River, to pass again into Canada near the northern end of Lake Champlain. It apparently again bends southward and touches the Atlantic coast once more near Nantucket, Massachusetts. Between the arid region and the north- south portion of this line, and north of the portion about the Great Lakes, lies a region that may be called semiarid, the values lying between 60 and 100. The line for value 140 delimits what may be called the southeastern rainy region, which here includes southeastern Louisiana, southern Mississippi, Alabama, and Georgia, all of Florida, and the Atlantic coastal region north as far as the entrance to Chesapeake Bay. Key West, Florida, lies outside of this rainy zone. The region lying between the line for value 100 and that for value 140, including most of the eastern half of the country, may be con- 260 ENVIRONMENTAL CONDITIONS. sidered here as a semirainy region. There are thus four precipitation zones or provinces roughly marked out on this chart, which may be defined as in table 12. TABLE 12. Normal daily pre- cipitation for the Province. period of average frostless season. thousandths inch. Dry province. 4... .....> Below 60 Semidry province........ 60 to 100 Semirainy province...... 100 to 140 Rainy province.......... Above 140 The four provinces thus indicated will be repeatedly referred to in our further discussion of moisture conditions. (3) Toran NorMAu PRECIPITATION FOR Preriop oF AVERAGE FRosTLESS SEASON PLUS PRECEDING 30 Days, Divipep By Number oF Days IN AVERAGE FROSTLESS Season (7/S). (Table 11.) This rather artificial index of precipitation intensity is based, as has been mentioned, upon the consideration that some of the precipita- tion occurring just before the opening of the frostless season is still effective in the early part of that season. In many places the first few weeks of the average frostless season are normally more or less dry, and yet plants may be able to begin their activities with the advent of frostless weather, on account of soil-moisture left over from the latter part of the preceding frost season. The length of the additional period of 30 days was chosen quite arbitrarily, in an attempt to bring these considerations into the index, which we term 7. Bigelow’s precipita- tion normals (Bulletin R) were againused. The values obtained are given in column 4 of table 11, and these totals divided, in each case, by the corresponding number of days in the period of the average frost- less season (7/S) are given in column 5 of the same table. The chart obtained from these averages (7/S) shows no pronounced differences from that presented in plate 46, and it is not reproduced here. This chart is mentioned, since the method by which it was obtained is new and may be of value in the future, for special studies of certain regions. The values of 7 will be otherwise employed below. (4) Numper or NorMauiy Rartny Days tn Pertop oF AVERAGE Frosriess SEASON. (Table 13, Plate 47.) This kind of index of precipitation intensity is frequently employed by climatologists, though for other duration factors than the one here a er ae 261 PLATE 47 *Z oyeid st oseq eyy, ‘seourAoid uoryezIdroe1d oA OFUI deur oy} OprAIp sour NY (z UUIN[Od ‘ET 91qB} WOT; ByeP) MOSveS Sso]}SOIj OBBIOAG OY} JO POrrod oY} Ur ‘Your [°— Wey} e10U Jo WOTeAId1O9Id [eUIIOM YIM SXep Jo sIoquINNY qqseq) OSM 38010} u921310A0 uontsuen S010} uselBTEA9 38210} U2e1d.10A9 on AydoBAy qse10} snonpidap onéydosow urayyoN "AAydoseur usoyON Uia}SaMyION -puryssesy purjsser5 CLALLTLETS GN en a (rr ‘Ne 1% ns . ai * ‘ as Ah se A ney on Ahh 447 SACLE ND > Josaq-tuas = yasaq ‘ WON EY PES SY : . \ INS Wales dee IY \ SAN WQy xt Wf, zZ< Se LOE SSSR SE RK ee BS. RK SRR ee et ae Wo Rad ee TWN NST 7 CLG : WAAL Op j g = = SOI se XS X\S x RSS NY S: SS Y sian NU ROR WSN 7 a xS ei ©) if — Ss ec WS “4 ee NSN . WRAY WY WS N SA NSS Pe a es i Sas I ‘ i} AY = a | | it : } A \ se BSpA A wf it > ai A i ith a cf s il >: U il El ake IN IN ISSN THAN AA Ses we Y is 1A) 8; Gy 4 c oe te = i a ee E A f VA a N ‘4 A SS N we J x | i t' NSS IS} Je LES i aN |S ’ oe Dp ~ . 7 aPC lL dN] Rees ese! 2 a fen f MOL COL SOE LOT GOL TIT St SIT QATl AIT QUST. Ct 92T LOr — Y Ac FL $9 09 69 tL 2 Gh olh “GL AB 6B 88 L8 6B JMG 86 06 L6 6 a Ss He s PLATE 48 *z qed sroseq ogy, ‘seoutAoid uoreyidrooid Ay OUT dem oY} OprlAIp sour] [[N J *(g WUIN]OO ‘ET O[qe} WoL] BBP) WOSvaS Ssop}SOIJ OFVIOAG OY} JO Ported oy} UI ‘sso] 10 YOUT ['( Jo UOyeyIdroosd peuMsOM YWLM SABp JO SI9quIN Ny lll ll fll lll qseq) (SPA 38010) ugesB 90a uonsuen 39940} uaasB19A9 y8940j u9asd19A9 myAydosB hy ¥Sa10) SNonplzep ou Aydosow wsoyyoN —- IA Ydosar usa1Samy1ION *puessesy 380103 In Aydosaur 3Sa10J usayseayinoS snonptoeq pucysses5 WS AS LG 66 TOL Ol SOL LOE .60L TIT ‘zg oye[d stoseq oy, ‘sooutAoid uoryeydrooid oag OUT deur 94} aprArp souT] [NY (*F WuINIOd ‘eT 9Iqv4 WII] BCP) ‘sSo] 0 Your [‘( JO WOT}eyIdtO9Id [euNIOU YIM SXep Arp Aq poyuosoidad o1¥ VY} WOSvAS SSO]}SOIJ BVAAV JO Porsod Jo sosvyuI0I0g 86 i, Se ay . F Z : A : i (seq) (sam 48910} u9aiBIaAe uorisuen 38910} ugaIBI9A2 48910} usaisI9Aa an Aydor3 Ay ysas0y snonpioep ==. mAydosaut urayoN =. - pueyssess) ——— ete =] 7 a! eS 77 263 1] 0 Pe H, se as ql eF <“ v Nt \ 7 NY Ze ELA oe of ay i AK N PAS WYO RIS’ a - \\ %; RN a Deiox LA, Z, M4 SS he > [ACA Y . NSS SPOACS a2 WES , \X oh - WW Se LIN cP N 4 mt \ N \ XN re Be Ge ee a FS me SS SUT a ae 7 ets oe s mat pill me : are Mis a J acs ral oh | ie is. I) Ch iN y $ 7 ¥ (x77 667747 4 i 7 7 4 ee By QO 46 CK Fail Ry = LA : Raddy hel ape Spc] 89 £9 69 AL SL GL olL 6L JIB 88 $8 LB 68 o 6 66 JOE .e0l SOI LOI . q 264 ENVIRONMENTAL CONDITIONS. used. Normally rainy days are here considered (arbitrarily) as those with normal daily means (Bigelow’s table, Bull. R) of over 0.10 inch. The days are counted without reference to when they occur in the period, so that we do not touch here upon the question of rainy periods. ~ These data are given in column 2 of table 13. TasLEe 13.—Number of days in the period of the average frostless season with normal precipita- tion of more than 0.10 inch and with normal precipitation of 0.10 inch or less, the latter also expressed as percentage of the number of days in the average frostless season. 47, 48, and 49.) 3a Be nis ae Erne Station. ‘a is =] Lox (Pir a a S S ag _—- = Alabama: days. PANITNISCON: siojciert rosetta s 145 Birmingham....... 166 Mito oy pee een, key 248 Montgomery....... 177 Arizona: Phoenix. Fests cnet 00 Arkansas: Fort Smith........ 159 Woittle Rook: .4..%'... 182 California: Marea shit) dis vav-ciesogs 55 WresnO\. cccsiers «sian 00 Independence...... 00 Los Angeles........ 40 Red Bluftcsvecen 33 Sacramento........ 13 San Francisco...... 62 DHT GORG: tir fice eares 43 San Luis Obispo.... 25 Colorado: IWENVEL os oa aces 00 IPQEDIO sy ws bstecvle 0 ee 00 Connecticut: PiarifOrd cick s sic 128 New Haven........ 148 Florida: Jacksonville........ 198 WUDILER ceislea tos 3 234 iey “Wentia ies coxe 161 Pensacola.......... 226 TRANNY. vis dips a atkins 165 Georgia: SACL ANVGR Fave cla sw ates 155 JATIOUBUR. wes siete x» 160 IMIBOOM Vieheie a ora nis 144 Savane sawn s.x.3 175 Thomasville....... 229 With normal daily precipita- tion of 0.10 in. or less. age frostless season having normal daily precipitation Percentage of days in aver- of 0.10 in. or less. wo: (oo ey ~ eto IH © Station. Chicago wun ces nee hs Palle 2 es teens Springfield......... Indiana: Evansville......... Indianapolis....... Iowa: Charles City....... Davenport: <. et oo o°0 +» Sa Aa | Bobo ae Ae | BoE sf | 28 | 2823 SM (ese ede ee = = Ay = = Ay Massachusetts: days. days. p. ct. New York: days. days. p. ct. ESRIASINN Shove ae ois ws 110 75 41 IAN DANY So Ercole ts 110 67 38 Nantucket......... 55 154 74 Binghamton....... 95 63 40 Michigan: é 1B4b titel eee Be 80 93 54 JU 92 45 33 Wanton... cee oe ate 43 96 69 MBEETOM.|c/c0 22s 3's 66 98 60 MGRACA sto srcvocssr ots 015. 97 63 39 Escanaba.......... 105 35 25 News ¥orle dirs, oc srs 143 67 32 Grand Haven...... 51 116 70 Oswerouseice nes ae- 55 120 69 Grand Rapids...... 46 118 72 FLOCHEStED AS eee erel 35 136 80 HIGUPHTON. 6.56. os - 84 68 45 SVEACUSE “2s Gcae.: 98 73 43 Marquette......... ra 69 49 North Carolina: Port HUrOn. «25. . 26 129 83 Asheville erie oie oye 142 34 19 Sault Ste. Marie.... 64 74 54 @harlottese sce ee 175 45 21 Minnesota: Hatterass eheeeac ae 256 00 00 D6 eee 124 28 18 Ralershor epee. cs,c% 180 33 16 Minneapolis....... 120 41 26 Wilmington........ 189 44 19 Mioorhead......... 67 65 49 North Dakota: Su i ee 106 53 33 Bismarck sayce cea 21 108 84 Mississippi: Devils Lake....... 53 68 56 i 185 45 20 Walliston. 32 52../2..5 21 98 82 MarecksbUrre......%.-- 207 45 18 Ohio: Missouri: @incinnatisse. se 101 93 48 SIS ttre oe A 98 81 45 @Wleveland!. 2 | 2.052: 95 103 52 Wismmipal’s... 5556. 127 57 31 Columbus 7s). osc: 106 78 42 Weansas City. .....- 151 45 23 Sana iskeyis;csars cies « 89 106 54 Sis IC 115 86 43 MOlEd Ore sreteye wists 49 125 72 Springfield......... 161 26 14 Oklahoma: Montana: Oklahomans. see 90 124 58 BEPSEVEC cays <.2-,0 2 0 6 11 111 91 Oregon: LE UGE ot eee 00 144 100 Baker) City. ss. 4c): 00 127 100 Li 2 00 140 100 Portland's. aiden. 73 172 70 LUCE @ ina 3 137 98 ROSGDUER elecie se cle 1 197 100 Nebraska: Pennsylvania: LAR) FA AGS eens 131 43 25 PTI PAs ee ert Seieo 130 64 33 North Platte....... 29 122 81 ievarrisbure. 220.62. 113 83 42 RONPARSREDSUD S015) oye. s acc «0 138 32 19 Philadelphia....... 114 92 45 Walentine.is........ 72 60 46 iennclotdel anon oas 91 88 49 Nevada: SCLANtOWn cs cisnc 102 74 42 lMEl Os 336en goa 00 138 100 Rhode Island: Winnemucca....... 00 131 100 Block Island....... 142 76 35 New Hampshire: Providence........ 111 79 42 ME ONCOLG a. ce <5 066i 104 42 28 South Carolina: New Jersey: @harleston)....2.-... 206 70 25 Atlantic City...... 108 99 48 Columbiat hose sles 154 77 33 of 0.10 in. or less. it 266 ENVIRONMENTAL CONDITIONS. TaBLE 13.—Number of days in the period of the average frostless season with normal precipita- tion of more than 0.10 inch and with normal precipitation of 0.10 inch or less, the latter also expressed as percentage of the number of days in the average frostless season. (Plates 47, 48, and 49.)—Continued. Station. Station. normal daily precipitation age frostless season having of 0.10 in. or less. With normal daily precipita- tion of more than 0.10 in. With normal daily precipita- tion of 0.10 in. or less. Percentage of days in aver- With normal daily precipita- tion of more than 0.10 in. With normal daily precipita- tion of 0.10 in. or less. Percentage of days in aver- age frostless season having normal daily precipitation of 0.10 in. or less. S. Dakota—Cont'd: days. days. p. ct. Virginia: days. days. p. ct. Rapid Citys): . shea. - 28 115 81 Lynchburg........ 166 35 17 Vw Te wes est Vy RIT g AN ht IRR AEE ere eR OAs |) PURPA 0h ela ta Mt wine K ee Tennessee: Rachniond's 50: 42 160 55 26 Chattanooga....... 143 64 31 Wytheville........ 156 19 11 Knoxville; ish Hes 147 61 29 Washington: Memphis... 2 91» 161 63 28 North Head....... 150 166 53 Wash ville.i.s'ss se 158 49 24 Sextbles a acka we es 46 200 81 Texas: ’ Spokane:. Sho... 0. 00 202 100 PAE: 3/522 '- «ses 53 192 78 Tatoosh Island..... 199 72 27 ATARINO as 3c rele 57 142 71 Walla Walla....... 00 216 100 Corpus Christi. .... 39 259 87 West Virginia: Fort Worth........ 58 203 79 Parkersoure-e ce sie 120 59 33 Galveston......... 257 74 22 Wisconsin: Palestine. 3f.'. 2s. 153 92 38 Y “Green Bay jas cease 85 68 44 San Antonio....... 65 211 ae Pa Grosse side cee 135 28 17 a tee wee de wreh) CR Be 8 Re Oy | CREE 5 oe laa oom eae IVE OOONA seco eia'e is elt 00 130 100 Wyoming: Salt Lake City..... 00 182 100 @heyennew-s5 34 20s 00 118 100 Vermont: Vander 2 sic, ce nece 4 104 95 Burlington: <.)....... 108 35 25 Northfield: .:.-../.: 86 40 32 The values representing the number of normally rainy days in the period of the average frostless season were plotted in the regular way and the resulting chart is given as plate 47. This chart shows isocli- matic lines at intervals of 25 days; the total range for the country is from 284 days (New Orleans, Louisiana) to none at all (various sta- tions in the arid region). The general zonation is seen to be very similar to that shown in plate 46. The heavy lines show the four provinces above described. (5) Numper or NorMAuity Dry Days In Pertop of AVERAGE FROSTLESS SEASON. (TaBLe 13, Puare 48.) This index is the complement of the preceding one, and is derived by subtracting that from the number of days in the period, in each case. A dry day is thus one that has a normal daily mean precipitation (Bigelow, Bull. R) of 0.10 inch or less. If the former be considered as es CLIMATIC CONDITIONS OF THE UNITED STATES. 267 an index of raininess, this may be taken as an index of dryness or aridity. The values are given in column 3 of table 13 and are shown graphically on plate 48. This chart shows a total range for the country of from 294 days (Los Angeles, California) to no days (Cape Hatteras, North Carolina). The isoclimatic lines here again represent incre- ments of 25 days each, full lines being shown for the values 50, 100, and 200. In a general way, the climatic zonation of the country is similar to that of plates 46 and 47, but this chart is markedly different from the others in certain details, and of course the actual values are different. Some of these differences will be considered below. (6) PercentaGe or Days 1N Pertop or AveRAGE FRosTiess SEASON THAT ARE Dry Days (wir NorMAL PrecipIraTION or 0.10 INcH or Less). (TaBiE 13, Puare 49.) This index of precipitation intensity is obtained simply by expressing as percentage each value of the third column of table 13, in terms of the corresponding length of the period of the average frostless season. These percentages are given in column 4 of table 13. They express the relative frequency of dry days in the period. These values are shown graphically by the chart of plate 49. The total range for the country is from nil (Cape Hatteras, North Caro- lina) to 100 per cent (various stations in the arid region). The lines of the chart are drawn at intervals of 10 per cent, those for 20, 50, and 100 being full lines, and the zonation is once more similar to that of the other precipitation charts already mentioned. (7) Lenetu oF Loncest NorMAtty Rainy Periop In Pertop oF AVERAGE FROSTLESS Season. (Tasue 14, Puare 50.) In many regions the duration factor for the favorable range of moisture conditions is not as great as that for the corresponding range » of temperature conditions, and the former thus becomes the main duration factor influencing plant activities. In such cases only a portion of the period of the average frostless season is suitable for active plant growth. In southern Arizona, for example, the normal frostless season is very long (241 days at Tucson, from March 26 to November 22), but all of this period is practically without rain, excepting only a portion of the summer. The summer rainy period at Tucson extends from about July 1 to about September 15, but there is also a spring period of general plant activity, extending from the cessation of frost to about May 1. The latter period is nearly rainless, but the soil-moisture content is high, due to the residual effects of the winter precipitation. Thus there are here two periods of general plant activity within the period of the average frostless season, one from about March 26 to about May 1 (at which time the winter moisture is about dried out of the soil) and the other from about July 1 to about October 15 (when the summer moisture has largely disappeared). It 268 ENVIRONMENTAL CONDITIONS. is thus possible to consider two different periods of plant growth in this region, both of which lie within the limits set by the average frostless season, but neither of which is as long as that season. In some other regions there is but one period of general plant growth, but this is not as long as that of the average frostless season. When moisture condi- tions have been more thoroughly studied it may become possible to consider both the frostless season and that with moist soil, in deriving the duration factor for plant growth in general, but we have not found it expedient to attempt this at the present time; the relations encoun- tered are too complicated and information is too meager. Nevertheless, we have been able to derive two duration factors for precipitation, which may be superimposed upon the temperature duration factor here generally employed. These two new factors are the lengths of the longest rainy period and of the longest dry period within the period of the average frostless season. The first of these is considered here and the other will receive attention under the next- following heading. In attempting to derive an index of the normal duration of moist conditions, we have again begun our computations with the data of normal daily precipitation given by Bigelow (Bulletin R). Our pro- cedure has been quite arbitrary. In the case of each station the series of daily normals given by Bigelow has been considered as separated into a series of overlapping groups of 5 days each. Numbering the days of the period of the average frostless season consecutively, days 1 to 5 constitute the first group, days 2 to 6 constitute the second group, days 3 to 7 constitute the third, etc. The 5 daily normals for each group are averaged to give the mean daily normal precipitation for that group, and these means are set down to form a new series. Beginning with the first (in the period of the average frostless season) of these new group means, the groups are marked as rainy or dry, accordingly as the value of their means are or are not greater than 0.10 inch. Thus, a 5-day group is called rainy if its group-mean is over 0.10 inch, dry if this mean is 0.10 inch or less. If we designate the succes- sive 5-day groups throughout the normal frostless season by the alpha- bet letters, and if we follow each letter by an R or D, to denote ‘‘rainy”’ or ‘‘dry,” as the case may be, we obtain a series more or less similar to the following: AR, BR, CR, DR, ED, FD, GR, HD, ID, JR, KR, LD, MR, NR, ete. In this example the first four groups (A to D) are seen to be ‘“‘rainy.’’ Then follow two “dry” groups (E, F), after which is a single ‘“‘rainy”’ group (G), which in turn is followed by two “dry” groups (H, I), ete. Now, the last day of each ‘‘rainy”’ group is con- sidered as occurring in a normally ‘‘rainy”’ period, and the last day of each ‘‘dry”’ group is considered as within a normally ‘“‘dry”’ period, and it thus becomes possible to determine the extents of the various “rainy”? and “dry” periods thus established. If, for example,-group CLIMATIC CONDITIONS OF THE UNITED STATES. 269 A includes April 1 to 5, group B April 2 to 6, group C April 3 to 7, etc., it follows that the period April 1 to 8 (inclusive) is a ‘‘rainy”’ one; the period April 9 to 10 is “dry”; that of April 11 (a single day) is “rainy”; that of April 12 to 18 is ‘‘dry,”’ ete. The dates of beginning and ending of each normally ‘“‘rainy’”’ and normally ‘‘dry” period of the normal frostless season having been thus obtained for each station, the length of each period is noted, in days, and the lengths of the longest normally rainy period and of the longest normally dry period become the two indices desired for the station in question. The beginning and ending and the length of each normally dry period in the period of the average frostless season are given in the second column of table 14, and the corresponding dates and length of the longest normally rainy period are given in the third column. Roman numerals refer to months, the arabic numerals not in parentheses represent the days of the month, and these data are followed, in each case, by the length of the period, in parentheses. Thus, the longest normally rainy period in the period of the average frostless season for Anniston, Alabama, extends from May 18 to September 8 and includes 114 days. As has been remarked, this method of treatment is quite arbitrary, but it seems to furnish indices of normal raininess and droughtiness that may be valuable. At least, these indices are worthy of a test, and may be employed till more satisfactory ones may be devised. It should be noted that the smoothing process applied to the precipita- tion data by Bigelow (in deriving the daily normals) is here overlaid by another very efficient smoothing process of our own (the use of 5-day averages), so that the natural irregularity of precipitation is very largely obliterated, which is to be desired when normals are requisite. It may also be mentioned that the indices here set forth might have a still greater value for such work as this if the constant 0.10 inch were made somewhat smaller. Such an alteration would of course render the normally rainy periods longer and the normally dry periods shorter. The testing of such modifications may, however, be left to a later time, and to other workers, if they will take up this important phase of the climatology of precipitation. 270 ‘% oyu stosvq oy, *seoutAoid uoryeyzdwasd oAy oul duur 9} OPLArp SoUTT [NY “gE WuIN]od “FT o[qu9 Woy VyEp : WOsUaS Sso]}SOIJ JO ported Ur (7x94 9a8) porsod Lures yseBu0] AypeuLION Jo (sep) sqyZuay Sit 38910} u901B 1949 mAqdoishy UWi9}S9MYLION pecrse-sreo-er Sas hibe dab denies olll Alt =. (aseq) 4sal0j ud91319A9 SnAydosaw wiay0N Ose 38910} Usa1B 1949 aj AYydosaw wiayyi0N uon sues 38910} snonpisep - puefssess) USIZTILTAA VITTTTTL, $351W O09 3} Sia ee 4 i ¥ ne ye Q PSN $ ——7 en ~, 4sa10j I Aydosow Wia}seayjnos }Sal0} Onpiseq WY D> Ls} sn \ WOO SS REO) INNS LISS | SS) APRS NS . en) - vin RNs) x 8 RN fs. SS ‘ ANAS AE RNS \ ‘ PP ey J a J \ ~ ey 'S s* S SSS / ts 3 eat ie PA f = | re An : ise 5 ae : rh Sete Soe fares) a ay ka See 4 rf. es 7 BS a rk "hs r 3g Ber ae Mi eRe nne , Bi) {ere ith Fc Sera So ae we wn ~ ae, bed is ¢ PLATE 50 Fel 0 SR Ole r Gj ( 4% Gi, , Sw te eee YY: 4 8 = a 5 ‘J 4 ie te Kam Cae AS UG y ‘Bat PIG * <) 4, i at Pa ini Y ts A \ oS oe q ve A BIR ' Ye SS, is N hol u Pe heuteh se sane sce oR anes ol oll 6L — Me YC \ eK ak aN Ne ofS, el6 «666 TOT | ai V c&8 = G8 HII Stl Str QL GIT 18 lS 68 16 4 Jer ett <¢ ‘Z ORI Staseq ayy, ‘seourAosd uoryeyIdwaad a SoU TINA % UUINIOd “FT a[qu_ wos; eyep {uOSvas ssaT}s Spon ae a Con ee OI} 9DBIBAB JO potted Ur (4x04 908) poled Arp Ayyeu10u yso3uo] Jo (sXep) syysueyT 4s9 48010} usa1Z0A9 (SoM 3Seloy U9a1T19A9 uonisuen 3Sas0} usa1819A9 onAydosau usayyON snAuder arAydoss Ay 48910} snonpisa: : SOU UTIYTION, Wa}saMyyION ~puepesessy e ——— eceees Hala Yi, ————— fatetatntce SS - — isan — i OOO nldiBigia jf Y My 4 SJ ., 2 Ns Wer aT eae. oO y exes J ¥e10} IN Aydosaus : —— Wyr9}Sseey NOS snonpisaq > Dn. VN, See yoo fy 2 28 ‘ | NS Y ‘ = Ps ip ‘eos U - SON SSR N e+ <8 q s y \ wl PO PLATES1 SS SSNS OS a : Y f il all 6 Fay pe iS SSS an iS RY y LY S SS N aA A No , Zh Gy LLY Le WAU BA’ Z ae ages S J y oe \ 4 at ? | AZZ Rts TAD fe x Sie ) p \ } : \ A | i iit | e: .! } aa y —s Ag = “ Beeiaawis N a i Mi? A = = a oe Ry i hos ea > ee ee SS SZ ZsSs= \ AL DG oh olL 62 M8 £8 | Uh \ Ie / / J a ane | Na 272 ENVIRONMENTAL CONDITIONS. TaBLE 14.—Beginning, ending, and duration of each normally dry period and of longest normally rainy period within the period of the average frostless season. (Plates 50 and 51.) A normally “dry period”’ begins on the last day of the first 5-day period whose average normal daily precipitation is not over 0.10 inch; a normally ‘‘rainy period’’ begins on the last day of the first 5-day period whose average normal daily precipitation is greater than 0.10 inch. Months are represented by Roman and days of the month by Arabic numerals; numbers in parentheses are the duration of the respective periods, in days, these being in full-face type for the longest dry period in each case. Station. Dry periods. Longest rainy period. Alabama: Amniston).*:: <0. IV, 29 to V, 17 (19); IX, 9 to 10 (2); | V, 18 to IX, 8 (114). IX, 27 to 28 (2); IX, 30 to X, 20 (21). Birmingham..... IV, 29 to V, 18 (20); IX, 27 to X, 5 (40). .| V, 19 to IX, 26 (131). Mobile...-.0-oe. V, 13'to 17 (5);X, 10. to'26 7); XI, 17°11 V, 18 to X, 9 (145). to 20 (4). Montgomery......| IX, 1 to 9 (9); IX, 18 to X, 2 (46)...... III, 11 to VIII, 31 (174). Arizona: Phoenix... <3 1 TT 24%to. RL Sea) s ack srtteente hierdie No rainy periods. Arkansas: Fort:Smith.; o: 2. IV, 2 to 10 (9); VII, 16 to 25 (10); VIII, } IV, 11 to VII, 15 (96). 26 to IX, 6 (12); IX, 25 to 27 (3). Little Rock...... VIII, 9 to 10 (2); IX, 2 to 8 (7); X, 1 to | II, 20 to VIII, 8 (142). 29 (29). California: BMurekaes ye 2 eee Ocbe Dkelg 2 (LOa) aera fae ere era ane III, 30 to V, 4 (36). Fresnopyticn eke TERA te elt (258) ae ae eee No rainy periods. Independence... «| LU, LO to EX, 240099) ac a 2s ere cere Do. Los Angeles...... DE 2246 RE GLE RCZOO ig ht cee te ore dks I, 28:to. Tl; 21 (5)z Red Blu) Vie auto Saeed Ra 2o0)ias eh ks ote oe XI, 19 to XII, 16 (28). Sacramento...... II, 25 to III, 6 (10); III, 19 to XI, 19 (242) .| III, 7 to 18 (12). San Francisco.. PT Vato nels 18 (247) OGL see I, 26 to III, 16 (50). San Jose.. oe MEANY: toms NS (ZARA: oocee cree eat II, 7 to III, 16 (38). San Luis Obispo at VAL to PR Si (abe)i acre etn weston ke III, 4 to 31 (28). Colorado: Denveric. cle Meck 0 iGe (Lobe conte cn oilers ce ibcereee No rainy periods. Pureblaccnt see te TVS 23itoixs 17. (IGS) cea ket eo tne ee eee Do. Connecticut: Hartford........ V,/ 13 (@); VI, 16 to 26 (1): VEL, 3f to | Vi, 27 to VIM, Se 4ea New Haven...... IX, 5 (6); IX, 27 to X, 3 (7). V, 21 to 22 (2); VI, 15 to 23 (9); VIII, 31 to IX, 2 (3); IX, 27 to X, 1 (5). VI, 24 to VIII, 30 (68). Florida: Jacksonville...... It, 23 (1); TIL, 4 to 7 G);.X;. 29 to: XI, | TH, Ste %,; 28 Gaaar 4 (37). ARTY 0) ht) ey pe ee II, 15: to 17 (3); IM, 8 to 21 (4)> IV, | V, 20 to 21 10 Key Weat..ss.%%. 1 to 19 (19); IV, 21 to 28 (8); V, 18 to 10"(2)$ ACT LY to! 27 (CLT) 32, Le to. 29 (15). Xp U1 torV, 10" GS2)e-VI, 28 toevil, 2 (5); VII, 18 to 21 (4). VII, 22 to XI, 10 (112). Pensacola........ IV, 23 to V, 25 (33); IX, 20 to 21 (2); | V, 26 to IX, 19 (117). X, 10 to 16 (7); XI, 14 to 22 (9). SE SLINITIGL cb ie ace ars a II, 9 to 13 (5); III, 2 to 16 (15); III, 29 | V, 26 to X, 9 (137). to IV, 25 (58); X, 10 to 13 (4); X, 16 to 18 (3); X, 24 to I, 9 (78). Georgia: Atlanta. s.<5's 0 0% IV, 29 to V, 17 (19); IX, 9 to 10 (2); | V, 18 to IX, 8 (114). IX, 27 to 28 (2); IX, 30 to XI, 3 (35). AUGURGAL ue ite IV, 28 to 29 (2); V, 6 to 17 (12); IX, 6 | V, 18 to IX, 5 (111). to 9 (4); IX, 29 to XI, 7 (40). IMAGO. cre ra dys ee IV, 17 (1); IV, 27 to V, 18 (22); IX, 28 | V, 19 to LX, 27 (483); to XI, 13 (47). ———e CLIMATIC CONDITIONS OF THE UNITED STATES. 213 TaBLE 14.—Beginning, ending, and duration of each normally dry period and of longest normally rainy period within the period of the average frostless season. (Plates 50 and 51.)—Continued. Station. Georgia—Cont’d: Savannah........ Thomasville. .... Idaho: Lewiston........ Poeatello........ Illinois: PEORIA) feaveiees ac. Springfield....... Indiana: Evansville....... Indianapolis. .... Towa: Charles City..... Davenport......- Sioux City........ Kansas: @oncordia......- Dodge City...... Mopeka. si. ons» Wachita.......- Kentucky: Lexington....... Dry periods. IV, 9 to 11 (8); IV, 18 to 30 (13); V, 8 tor2? (20) 3X, 19) to 23 ©) Xe. 26:to XI, 27 (33) ESE CO LOE, clist (24) pata. pees alee: V5 29) 0G PX DT ETA on ters aeiete nots coche ie DV, Dito Ext 2 eo (202) as toeeene os Sete oe TVs 21 tor XL 2e (TS) i.cnerd tee cnscton nec VII, 20 to 26 (7); VIII, 8 to TX, 26 (50); X, 4 to 15 (12); X, 20 to 28 (9). IV, 18 to V, 2 (15); V, 16 to 24 (9); VII, 15 to 22 (8); VIII, 5 to 9 (15); VIII, 31 to IX, 6 (7); IX, 18 to X, 15 (28). VII, 15 to 27 (13); VIII, 5 to 20 (16); VATS 29a) TEXs 9) C2) exe 9) X, 6 to 13 (8). IV, 16 (1); VII, 8 to VIII, 14 (88); VIII, 22 to IX, 19 (29); X, 8 to 18 (11). VII; 6 to 13 (8); VII, 17 to VIII, 15 (30); VIII, 20 to IX, 13 (25); IX, 19 (1); X, 8 to 17 (10). IV, 29 to V, 3 (5); V, 14 to 24 (11); VIII, 5 to 9 (5); VIII, 27 to IX, 25 (30); X, 6 to 14 (9); X, 20 to 23 (4). VIII, 12 to 17 (6); VIII, 29 to IX, 7 (10); IX, 17 to 26 (10); X, 8 to 19 (12). VII, 27 to VIII, 8 (13); VIII, 26 to IX, 26 (32). IV, 23 to 29 (7); VII, 24 to 30 (7); VIII, 6 to 7 (2); IX, 3 to 6 (4); IX, 23 to X, 13 (21). VIII, 5 to 10 (6); VIII, 30 to IX, 10 (12); IX, 17 (1); X, 1 to 8 (8). IV, 23 to 28 (6); VIII, 8 to 22 (15); IX, 19 to 21 (3); X, 5 to 9 (5). IV, 2, to) 8'()3 VIL, 1S: toL7, (); Vill 9 to 12 (4); VIII, 24 to IX, 3 (11); X, 7 to 15 (9). VI, 12 to 14 (3); VIII, 6 to 16 (11); VIII, 28 to IX, 11 (15); IX, 20 to 27 (8). IV, 25 to VI, 5 (11); VII, 9 to 16 (8); VIII, 4 to IX, 5 (33); IX, 16 to X, 14 (29). IV, 18 to V, 14 (27); VI, 27 to VIII, 7 (42); VIII, 9 to X, 15 (68). IV, 10 to 21 (12); IV, 26 to 30 (5); VIII, 31 to IX, 6 (7); IX, 23 to 24 (2); X, 3 to 15 (13). IV, 9 to 27 (19); VII, 21 to 28 (8); VIII, 7 to 11 (5); VIII, 25 to IX, 5 (12); IX, 23 to X, 19 (27). IV, 28 to V, 8 (11); VI, 7 to 8 (2); VIII, 29 to X, 23 (56). Longest rainy period. V, 28 to X, 18 (144). TL, 7. tome 25 (238). No rainy periods. Do. Do. III, 31 to VII, 19 (111). V, 25 to VII, 14 (51). IV, 29 to VII, 14 (77). IV, 17 to VII, 7 (82). IV, 19 to VII, 5 (78). V, 25 to VIII, 4 (72). DV) 17) to) VELL aes) V, 17 to VII, 26 (71). IV, 30 to VII, 23 (85). IV, 23 to VIII, 4 (104). IV, 29 to VIII, 7 (99). IV, 9 to VII, 12 (95). VI, 15 to VIII, 5 (53). V, 6 to VII, 8 (64). V, 15 to VI, 26 (43). | VI, 1 to VIII, 30 (91). | IV, 28 to VII, 20 (84). VI, 9 to VIII, 28 (81). 274 ENVIRONMENTAL CONDITIONS. Taste 14.—Beginning, ending, and duration of each normally dry period and of longest normally rainy period within the period of the average frostless season, (Plates 50 and 51.)—Continued. OO Station. Dry periods. Longest rainy period. Kentucky—Cont'd: VOuIsvUILC. «5 s- 25 V, 13 to 18 (6); VIII, 30 to IX, 26 (28); X, 4 to 22 (19). V, 19 to VIII, 29 (103). Louisiana: New Orleans..... V, 10 to 17 (8); IX, 8 to 25 (18); XI, 1 (1).| V, 18 to IX, 7 (148). Shreveport....... VI, 19 to 22 (4); VII, 10 to 17 (8); VIII, | III, 5 to VI, 18 (106). 3 to IX, 5 (84); IX, 20 to 23 (4); IX, 28 to X, 17 (20). Maine: Mastportaesceccls WV. 9uto 1254) Vil, LO to-19 0): var, 31 to VIII, 8 (9); VIII, 27 to IX, 13 (18); IX, 26 to X, 5 (10). VI, 26 to VII, 21 (26); VIII, 30 to IX, 12 (14); X, 1 to 3 (3). VI, 20 to VII, 30 (41). Portland? ¢? 2% 4- V, 15 to VI, 25 (42). Maryland: Baltimore....... IV, 16 to 23 (8); VI, 1 (1); IX, 27 to X, 21 (25); XI, 1 to 3 (3). IV, 18 to 23 (6); IX, 3 to 8 (6); IX, 26 to X, 19 (24). VI, 2 to IX, 26 (117). Washington, D.C. IV, 24 to IX, 2 (182). Massachusetts: Boston .tise ct te: TVs 2ittore21(@)e WM, Leis Ve Shih: Vie 15" to iVILS 1985); WITT, Si to IX, 10 (11). DVs 17*to, V,,23 BO Vi;-4, to 20) (lL): VI, 22 to VIII, 4 (44); VIII, 22 to TX 133) ES 20sto 35) 1 (12) oe 19 to 24 (6); XI, 2 to 5 (4). VII, 20 to VIII, 30 (41). Nantucket....... VIII, 5 to 21 (17). Michigan: Sh ee V, 25 to 28 (4); VII, 9 to 16 (8); VII, 22 to VIII, 6 (16). V, 1. to.61(6)5 Vi al to 2 (LD) Ville 2 to 18 (17); VIII, 25 to X, 11 (48). VII, 10 to 19 (10); VIII, 20 to 31 (12)... IV, 29 to V, 6 (8); V;, 29'to VI, 1 (4); VI, 9 to VII, 2 (24); VII, 4 to 28 (25); VIII, 7 to 21 (15); VIII, 26 to LX, $ (14); X, 1 to 12)(12). V, 2 to 7 (6); VI, 1 to VII, 28 (58); VIII, 7to Lx, 8 (83) 5 ax, ator i252). VIII, 7 to IX, 28 (53). Detroit: .\i.. .). 2 V, 22 to VIII, 1 (72). BEiscanaba.’. «3 o... Grand Haven.... V, 17 to VII, 9 (54). V, 7 to 28 (22). Grand Rapids.... V, 8 to 31 (24). Houghton: 421.2: V. 25D) RAVE, SitolV Roll (Oo) eerie V, 26 to VII, 7 (48). Marquette.......| VII, 8 to 19 (12); VII, 29 to VIII, 31 (84) .| V, 16 to VII, 7 (53). Portvreuron. ...;.. V, 19 to 23 (5); VI, 14 to IX, 12 (91); | V, 24 to VI, 13 (21). TX519' to xX; 9L(21): V, 15 (1); VI, 9 to 26 (18); VII, 8 to 30 (23): VIII, 12’ to 27 (16): LX, 21 to 29 (9). Sault Ste. Marie. V, 16 to VI, 8 (24). Minnesota: POUNCE So oietene.n) ose V, 5 (1); VII, 17 to 27 (11); VIII, 28 to 31 (4). Minneapolis..... VII, 12 to 24 (13); VIII, 15 to 18 (4); EX eee tO LoC6)e Moorhead....... V, 14 to 1658); V, 26 to 31 (7); Vil, 23 to VIII, 13 (22); VIII, 28 to IX, 22 (26). Stale evsues IV, 28 to V, 8 (11); VII, 12 to 25 (14); VIII, 18 to 26 (9); IX, 23 to X, 3 (11). V, 6 to VII, 16 (72). IV, 30 to VII, 11 (73). VI, 1 to VII, 22 (52). V, 9 to VII, 11 (64). Mississippi: Meridian........ V, 18 to 19 (2); VIII, 20 to 28 (9); X, 4 to 27 (24). V, 20 to VIII, 19 (92). Ee Oe CLIMATIC CONDITIONS OF THE UNITED STATES. Zio TABLE 14.—Beginning, ending, and duration of each normally dry period and of longest normally rainy period within the period of the average frostless season. (Plates 50 and 51.)—Continued. Station. Mississippi—Cont’d: Dry periods. Longest rainy period. Vicksburg....... V, 14 to 16 (8); VIII, 20 to 27 (8);I X, | V, 17 to VIII, 19 (95). 6 tol? (7)'s2X, 3 to 21619): Missouri: Columbia.....2:. =: VII, 18 to 21 (4); VII, 28 to VIII, 17 | IV, 19 to VII, 17 (90). (21); VIII, 28 to X, 14 (48). Manmibal ja... IV, 15 to 20 (6); VI, 9 to 18 (10); VII, | IV, 21 to VI, 8 (49). 12 to 15 (4); VIII, 24 to IX, 5 (18); IX, 30 to X, 15 (16). EV, 14°) BV, 16°to 17): EX tors @) EX, 27 to: Keone) IV, 18 to VIII, 31 (136). Strvlcouisis 4 cases Vile TS atorlon (7), Vill, 5) tow EX 24 LVe 7 to, Vill on(Ons (51); X, 4 to 18 (15); X, 20 to 24 (5). Springfield... M4 toplSa Gh ) Set Oe eh Re ek ee IV, 15 to X, 3 (172). Montana: awrest. os cirse 2 V, 16 to VI, 6 (22); VI, 20 to IX, 14 (87) .| VI, 7 to 19 (13). ielelenas fh xhies NV, 8:to SEX! 25 GAs) ic2. 5. tees a Oe. No rainy periods. alispell ccs ¢00. Vi (14-40 EX OORGAO) ie ec eens ee ee Oo. Miles City....... V, 8 to VI, 6 (29); VI, 13 to IX, 24 (104).| VI, 7 to 12 (6). Nebraska: Lincoln.........-| IV, 20 to 22 (3); VIII, 26 to IX, 10 (16); | IV, 23 to VIII, 25 (125). IX, 20 to X, 10 (21). North Platte..... V, 2 to 19 (18); VI, 9 to 10 (2); VI, 25 to VII, 23 (29); VII, 29 to IX, 29 (63). V, 20 to VI, 8 (20). Omahay i's. 3). Ps TEXG 0 4a (REX Oxtoexe SCD) ee . eve DV 927, toils ls C27). Valentine........| V, 10 to 15 (6); VII, 14 to 17 (4); VII, | V, 16 to VII, 13 (59). 20 (1); VIII, 12 to IX, 18 (88). Nevada: Reno sac e eas. Ve dito EX SING38)i-t. Sees oe Ae No rainy periods. Winnemucca..... Vi Grito: Te 23N (ST) ieee fee Do. New Hampshire: Wancord .5..656 65. V, 8 to 14 (7); VI, 28 (1); VIII, 31 to | VI, 29 to VIII, 30 (63). New Jersey: Atlantic City.... Cape May....... New Mexico: IX, 13 (14); IX, 28 to 30 (8). IV, 13 to 24 (12); V, 3 to 13 (11); V, 23 to VI, 8 (17); VI, 19 to 28 (10). IV, 18 to 25 (8); V, 3 to 14 (12); V, 24 to VI, 8 (16); VI, 19 to 28 (10); VII, 9 to 12 (4); VIII, 30 to IX, 8 (10); IX, 23 to X, 6 (14); X, 15 to 20 (6). VII, 14 to VIII, 29 (47). VII, 13 to VIII, 29 (48). Santa Fe........ IV, 16 to VII, 15 (91); VII, 26 to IX, | VII, 16 to 25 (10). 19 (86). New York ALD SIN os oe sles oo 2 IV, 24 to V, 21 (28); IX, 5 to 9 (5); IX, | V, 22 to IX, 4 (106). 24 to X, 7 (14); X, 13 to 17 (5). Binghamton..... V, 3 to 15 (13); VII, 10 to 14 (5); VIII, | V, 16 to VII, 9 (55). 12\to 18 (7) ) EX, 5 to Xs 6 G2). SITE ALON 2 hoausie sic IV, 27 to V, 17 (1); Vi, 5 to 18) 4): |’ VI, 19'to VILL, 30: (73). VIII, 31 to TX, 11 (12). Cantone s..5.0).°., 3 V, 10 to 25 (16); V, 28 to 30 (8); VII, | V, 31 to VII, 11 (42). 12 to 23 (12); VII, 30 to VIII, 27 (29); IX, 3 to 14 (12); IX, 19 to 25 (7). MGW ACA (sie ohetsie e's. V, 5 to 13 (9); VIII, 11 to 18 (8); VIII, | V, 14 to VIII, 10 (89). 31 to IX, 1 (2); IX, 3 to 13 (11); IX, 16 to X47 (22): New Mork 5.2... - IV, 16 to 23 (8); V, 16 to 23 (8); IX; | V, 24 to VIII, 31 (100). 1 to 8 (8); IX, 26 to X, 4 (9). 276 ENVIRONMENTAL CONDITIONS. TaBLE 14.—Beginning, ending, and duration of each normally dry period and of longest normally rainy period within the period of the average frostless season. and 51.)—Continued. (Plates 50 Station. New York—Cont'd: OBWER0 501.2 «heb os Rochester....... Syracuse sf sch c:: North Carolina: Asheville........ Charlotte........ Hatteras) :<).54%.4. Raleiphisiee.t ae Wilmington...... North Dakota: IBISMarCl: + enh fe Columbus....... Sandusky........ ih We) C276 (op ee Oklahoma: Oklahoma....... Oregon: Baker City...... Portland: 3.554; Roseburg........ Pennsylvania: RTOS gs nreocale cate Pittsburgh....... Scranton........ Dry periods. IV, 28 to V, 25 (28); V, 28 to 30 (3); VII, 12 to 23 (12); VII, 30 to VIII, 27 (29); IX, 3 to 14 (12); IX, 19 to X, 2 (14). U V, 2 to 18 (17); V, 23 to 24 (2); V, 30 (1); VI, 5 to 18 (14); VII, 12 to 23 (12); VII, 28 to VIII, 11 (15); VIII, 18 to 26 (9); VIII, 31 to X, 19 (50). V, 29 to VI, 13 (15); VIII, 12 to 20 (9); VIII, 31 to X, 7 (38); X, 15 (1). TXS 7 to 104) 5 x, 23 tox, 13i(Zh)e 222s IV, 28 to V, 1 (4); EX, 23 to X, .17-(25); XI, 1 to 4 (4). ING drought: DErIGds 5 2G) antec oa a 6 eee HV, 25 to V,.o°C01)- 1X, Sito. 1205) + x, 2 to 7 (6). DV, bitorsi (2a) ek) told) 5) is.. 35 ee V, 12 to 29 (18); VI, 22 to IX, 17 (88).... V, 28 to VI, 7 (11); VIII, 3 to IX, 25 (54). V, 19 to 31 (13); VI, 28 to IX, 14 (79).... TV, 219) to-20 (2) DV. 27) toevV, oz) ave 18 to 23 (6); VIII, 10 to 14 (5); VIII, 31 to IX, 25 (56). IV, 17 to V, 16 (30); VI, 17 to 22 (6); VIII, 4 to 15 (12); VIII, 23 to 27 (5); EX: 21 tos, 1 Gr). IV; 17. to 22: (6)3'Vi 3 to 7-5) VI, 6 to 11 (6)3; Vile 4 (1)? VEL 26 sto. EX, 27 (83); X, 5 to 17 (13). IV, 15 to V, 8 (24); V, 14 to 24 (11); VIII, 9 to 16 (8); IX, 6 to X, 26 (51). IV, 25: to V;6 (12):,V, 19: (1); VI, 14'to 20 (7); VII, 4 to 14 (11); VII, 29 to VIII, 28 (31); IX, 2 to X, 15 (44). IV, 3 to 22 (20); VI, 16 to VII, 4 (19); VET} 29" to VIEL, 64) VIEL Site IX, 6) (17); LX; 16:to X, 2.48). Ya oy ey SEE). ccna qed ee WV, 2Astows,: Si(RGS) os. ees oo. pe ee TV) 16 to. GOS) is. ot hes eco IV, 21 to V, 13 (23); VII, 10 to 20 (11); VIII, 4 to 8 (5); VIII, 27 to IX, 7 (12). IV, 15 to V, 12 (28); VII, 12 to 20 (9); IX, 1 to 8 (8); IX, 20 to X, 6 (17); X, 19 to 23 (5). IV, 15 to V, 16 (32); V, 30 to VI, 3 (5); VI, 12 to 17 (6); VIII, 30 to IX, 9 (11); IX, 12 to 21 (10); IX, 27 to X, 7 (11). IV, 29 to V, 9 (11); V, 14 to 16 (3); VIII, 6 to 11 (6); VIII, 29 to IX, 18 (51). IV, 21 to V, 15 (25); VII, 12 to 21 (10); IX, 1 to 8 (8); IX, 20 to X, 9 (20). Longest rainy period. V, 31 to VII, 11 (42). VI, 19 to VII, 11 (23). V, 14 to VIII, 11 (90). IV, 21 to IX, 6 (139). V, 2to IX, 22 (144). II, 29 to XI, 11 (256). V, 6 to IX, 7 (125). IV, 28 to X, 31 (187). V, 30 to VI, 21 (23). VI, 8 to VIII, 2 (56). VI, 1 to 27 (27). V, 24 to VIII, 9 (78). VI, 23 to VIII, 3 (42). VII, 5 to VIII, 25 (52). V, 25 to VIII, 8 (76). V, 20 to VI, 13 (25). IV, 23 to VI, 15 (54). No rainy periods. X, 4 to XI, 16 (44). No rainy periods. V, 14 to VI, 9 (57). V, 13 to VII, 11 (60). VI, 18 to VIII, 29 (79). V, 17 to VIII, 5 (81). V, 16 to VII, 11 (57). | SZ CLIMATIC CONDITIONS OF THE UNITED STATES. 277 TaBLE 14.—Beginning, ending, and duration of each normally dry period and of longest normally rainy period within the period of the average frostless season. (Plates 50 and 51.)—Continued. Station. Dry periods. Longest rainy period. Rhode Island: Block Island..... IV, 21 to 27 (7); VI, 15 to 28 (14); VII, | IV, 28 to VI, 14 (48). 1% to 28) @'2)? VILL, 27 to LX, 6°); IX, 19 to X, 7 (19). Providence...... BV, 21 to 23 @)s Vals Oi Viol) vib) Vid; 20\to VILT, 30) 42). 15: to: Vill, 19) (35)- Vit, Sl to: Exe 10 (11); IX, 26 to X, 3 (8). South Carolina: Charleston....... III, 3 to 7 (5); IV, 2 to 6 (5); IV, 20 to | V, 21 to XII, 2 (196). V, 6 (17); V, 14 to 20 (7). Columbia..-2.. 5-| LV,15° to 15(11)= LV, 25: to Vi. Ue (23)5 |v A18 to x, 23)(129). EX 24 tole 7, (4) xe 1 to 23-13) XI, 1 to 8 (8). South Dakota: (ERINON Gs 36d .01d5: 2s VitStonl8. (GyeVi, 23°to Vid C2) Ville | Vile 4 to) VIL. 815): 9) to VILL, .2):(25)=) VILL, 15° tor 1x; 20 (37). IPIOREE SS. fae ate ek V; 3 to VI, 2)(81); VI, 7 to 20 (14): VII; | VI, 21 to VII, 4 (14). 5 to IX, 30 (88). Rapid City...... V, 7 to 30)@4); VII, 1 to LX, 26 (88)!....|V, 31 to VI; 30-81). BViamlction' so ancs.6 <'< Ville L7G) VEEL, £6 to Xe 17, (Sa) 5a | aVira.to: VIL, LG (io)e 22 to X, 3 (12). Tennessee: i Chattanooga..... V, 2 to 17 (16); VIII, 10 to 17 (8); IX, | V, 18 to VIII, 9 (84). 2hto. 22) (21). Knoxville........ V, 12 to 17 (6); IX, 5 to 14 (10); IX, 23 | V, 18 to IX, 4 (110). to X, 28 (86). Memphis........ VII, 14 to 21 (8); VIII, 10 to 11 (2); VIII, | III, 22 to VII, 13 (114). PAR Ko DG IPA BO ln oy PAO Olea. < 27 to 30 (4) Nashville........ V5, 90) 20) @L2)' Vine 70) XS, 3" ton 4a 21 toeVilk, 16" (8%): 26 (24). Texas: PANSHENIO ei ts.ce,c.ccie TE 16) to DV, 22\"(88)s TVs 28 to. Vin. 4 ev. 23 to Vi, 16)(25): E COE DV ne Seton 225 (oe Vela Lise ston exe 7 (83). IATHATTNO e's oshe's IV, 17 to V, 6 (20); V, 20 (1); VI, 16 to |} V, 21 to VI, 15 (26). VII, 15 (80); VII, 27 to VIII, 4 (9); VILLE LS "tol EXe6(23)i2 BX? to xa 1 (51). Corpus: Christi... .|) II, 22) to V, 12)(82): V; 20 to VI, 16 | 1X; 2 to X, 1:0): (28) 3 Vi 2T to EX I (6M is Xs 2) to XI, 3 (33); XI, 8 to XII, 16 (39). Paso we os 2's. TE 21 too Villy 2224). Vil, 25°-to: |) VEL, 23) to 22: (2): XI, 11 (110). Fort Worth...... IE, 9 to IV, 24 (47). Viv Ul to 27 (7): | IV, 25 to VI, 10° 7). VII, 5 to 21 (17); VII, 27 to IX, 18 (54); IX, 30 to XI, 24 (56). Galveston....... L238 tot oO aio toon (io) Vie Vill nO to eins EAN 7): 10 to 23 (14); VII, 6 to 8 (3); XII, 13 to 15 (3). Palestine... css.’ III, 28 to IV, 8 (12); VI, 17 to 20 (4); | IV, 9 to VI, 16 (69). VII, 8 to IX, 8 (638); LX, 20 (1); X, 1 to 9 (9). San Antonio..... II, 24 toIV, 17 (53); V, 18 to VI, 13 (17); | IV, 18 to V, 17 (30). VI, 15 to 27 (13); VII, 10 to VIII, 23 (45); TX, 5 to 11, (7); IX, 18 to 23 (6); IX, 29 to XI, 26 (59). 278 ENVIRONMENTAL CONDITIONS. TaBLE 14.—Beginning, ending, and duration of each normally dry period and of longest normally rainy period within the period of the average frostless season. and 51.)—Continued. (Plates 50 Station. Texas—Continued: Dry periods. Longest rainy period. Tayloriig 19) to. 12) (4) ic.o se eee III, 28 to X, 13 (200). Richmond....... LV, 23'to V;4 (12); IX,'3 to:8' (6); EX, | V, 6 to EX, 2 G2a3 30 to X, 18 (19); XI, 2 to 3 @). Wytheville....... EX (6' to: OA) eX 7 ton 0 (4) eee eee IV, 19 to IX, 5 (140). Washington: North Head..... LV; 24 to 2X;3 (163) a= 2 os ee ee ee X, 4 to XII, 22 (80). Seattloscs etic n: IEE, 22,001); TV; 10: to Xs, 1s. 8%) 5 X | Sa 2 to 22 Ge 24 to XI, 1 (9). Spokane... 2./...4)|) wees, toe 14 (O02) oe etal tae aie eens No rainy periods. Tatoosh Island... Walla Walla..... West Virginia: V, 15 to 21 (7); VII, 8 to TX, 1 G6)..... EV jf COCK, SH SIG) se .seeie ks eee ee IX, 2 to XII, 9 (99). No rainy periods. lng re eins VIII, 8 to 9 (2); VIII, 28 to IX, 6 (10); | V, 19 to VIII, 7 (81). Ex 17 to-X, 10) 24). Parkersburg..... IV, 20 to V, 9 (20); VIII, 28 to IX, 4 | V, 10 to VIII, 27 (110). (8)'s EX 17, tox 11 (2b). Wisconsin: Green Bay.......| VI, 11 (1); VI, 24 to 27 (4); VII, 13 to | V, 4 to VI, 10 (88). 26: (14)ei VINT, 15) tosis 4 2) = es, 23 to X, 3 (11). Tia Grosse . yo uns V, 1 to 4 (4); VIII, 13 to 24 (12); X, 2 | V, 5 to VIII, 12 (100). to 10 (9). = Madison. -...-7%.. IV, 23 to V; 2.(10); V, 19 to 21 (3); VIE, | V;22 to VI, 28 (Ga). 29 to VIII, 17 (20); IX, 16 to 23 (8); x 2 to 18° e). Milwaukee...... IV, 29 to V, 2 (4); V, 23 to 29 (7); VII, | V, to 30 VII, 12 (44). 13 to VIII, 21 (40); VIII, 31 to IX, 7 (8); EX..15 to 25: (11); &; 1 to 7 (7). Wyoming: Cheyenne....... Veros to ke 17) (ES) oe, re ce Shee ee No rainy periods. Teer sees. was &5 Viv ato Tk> Lh (LO Dias sik fe aa eee V, 27. to VI, 2°). The lengths of the longest normally rainy periods in the period of the average frostless season, as given in the third column of table 14, are shown graphically on the chart of plate 50, in which the isoclimatie lines represent increments of 25 days. The total range for the country is from nil to 256 (Cape Hatteras, North Carolina). All of the region west of about the one-hundred-and-first meridian has exceedingly low values, most of them being zero, excepting the strip of country border- CLIMATIC CONDITIONS OF THE UNITED STATES. 279 ing the Pacific. The droughtiness of the intermountain area is here relatively exaggerated, on account of the choice of 0.10 inch as our constant in deriving these indices; the stations here characterized as having no normally rainy periods are not to be considered as really all alike in this particular. For the detailed study of the arid regions there seems to be no doubt that a lower value than 0.10 inch will be required. The precipitation provinces of the country, as indicated by the full lines of plate 51, are similar to those shown on plates 46, 47, and 49. (8) LenetH or Loncrest NorMAtity Dry Periop In PERIOD OF AVERAGE FROSTLESS SEASON. (TasBLE 14, Puate 51.) These indices for droughtiness, corresponding to those for raininess just discussed, are given in the third column of table 14, and the method by which they were obtained has already been described. Plate 51 shows the chart constructed from these values. The isoclimatic lines are shown for intervals of 25, with full lines for the values 150, 50, and 25. The total range of this index, for the entire country, is from zero (Cape Hatteras, North Carolina) to 299 (Los Angeles, California). The meridian of 101° west longitude again forms an important demarcation, separating the more humid east from the more arid west, and some other features are like those of the other precipitation charts. In the details of the Southwest this chart has peculiarities somewhat like those of plate 48, and will be referred to below. (9) Normat ANNUAL PRECIPITATION. (PLATE 52.) Since total annual precipitation is surely of considerable value in a general estimate of the aridity or humidity of a region, we include in our series a reproduction of the chart of this feature published by Gan- nett.’ This chart has been drawn, as its author states, with consider- able reference to topography as well as to the records of about 4,000 stations, and it appears to us to be the most useful annual rainfall chart of the United States thus far published. Data for normal annual precipitation for 167 stations are given in the second column of table 15, for use in other connections, but these are not the data from which this chart was constructed. The data given in that table are taken from Bulletin R of the U.S. Weather Bureau. Table 15 also includes other data, which will be considered below. In Gannett’s chart, as here reproduced (plate 52), the isohyetal lines for 10, 30, and 50 inches are dotted and the others are full. All are drawn for increments of 10 inches. By the emphasized line for a precipitation of 30 inches the country is divided into 3 main }Gannett, Henry, Distribution of rainfall, U.S. Geol. Survey, Water Supply Paper No. 234, reprinted from report of National Conservation Commission, 1909, Washington, 1909. The chart referred to is Plate I, and is published in color. The isohyetal lines of our chart (plate 52) have been copied from Gannett’s plate I by means of a pantograph. PLATE 62 S314 Coo Gs x IW SLAG IWS x VE Qa sheke a \\ LANAKSEAN WAN SSA VA RRS >. SNS DAN ‘. ) x = WN eS ———= Wet: DE SINERY Sa Reo Le Id SI oseq oY, “seoutAoId UONEAIdIOoId oAY OFUT deur ayy OPIAIp Sou] [NY “qqouUVL 10958 Sc) ep NS KL f, Ws. Se X\ ) XS 4 Y os AISA aM ms 7 A tty J. LY 4 2Gs4 OOS, ) Uj DES WOW Sen ~ 4 SS N : = ye WwW NEN YS ‘AS RNAS ANASAN SSYEANA is 2 ral MY oe Day @, thos oe}: ‘(seyour) senywa ur UoryeyIdroeId penuuE [BULLION voutsuENn #910) Snonpisap ~purlesess 389103 M17 Aydosour wia}seayinos 4 BS CLIMATIC CONDITIONS OF THE UNITED STATES. 281 regions. The humid northwest occupies the western half of Washing- ton and of Oregon, practically the full width of northern California, and the whole of the Sierra Nevada Mountains. The arid and semi- arid central region occupies the country south and east of the zone just described, and extends eastward as far as a line drawn from the western end of Lake Superior to the Rio Grande at a point about 100 miles above its mouth. In this general description we of course neg- lect the restricted mountain areas of Idaho, Wyoming, Colorado, ete. The humid east occupies the region lying east of the line just men- tioned. It should be added that the northern half of the southern peninsula of Michigan is also to be classed as semiarid on this basis, since its normal annual precipitation is less than 30 inches; this is a restricted area. _ The arid precipitation zone is here shown as having values below 10, and it occupies the Great Basin and extends southward into Mexico from Arizona and southern California. The humid region, as above defined, is here considered as divided into two provinces by the line for value 50, thus indicating a humid and a semihumid, or a rainy and semirainy province. ‘These four precipitation provinces do not require special discussion; their general characteristics are very similar to those pointed out for plate 46. The general north and south trend of the isohyetal zones is here seen to be modified by the Gulf of Mexico and by the southern Atlantic so that the lines of the eastern portion of the humid region trend north- eastward, or even eastward, instead of southward. The mountainous regions, of course, have higher precipitation indices than lowland regions of the same latitude. In general, the zones tend toward an arrangement parallel to the two coast-lines, which is readily explain- able on meteorological grounds and which is the reason for the north- south trend noted in all charts representing moisture relations. (10) Conciusions From Stupy or PRECIPITATION CONDITIONS. The charts of precipitation conditions (plates 46 to 52) exhibit features that are markedly unlike those of the temperature charts (plates 34 to 45), as was of course to be expected. The precipitation zones have a strong general tendency to extend in a north-south direction, while those of temperature generally extend from west to east. We have found it convenient to consider the following four pre- cipitation provinces in the United States: (1) the humid (or rainy) rain- province, occupying a small area of the extreme Northwest and a larger area of the southeastern Gulf and Atlantic coasts; (2) the semihumid (or semirainy) rain-province, occupying a rather narrow strip of country southeast of the northwestern humid area and nearly all of the country east of a line drawn from Corpus Christi, Texas, to Winnipeg, Manitoba; (3) the semiarid (or semidry) rain-province, occupying a narrow strip 282 ENVIRONMENTAL CONDITIONS. east and south of the northwestern semihumid area, the plains region, roughly defined as between the mountains and the Winnipeg-Corpus Christi line, and portions of the States bordering the Great Lakes; (4) the arid (or dry) rain-province, occupying, roughly, the region west of the Rocky and Big Horn Mountains that is not included in the west- ern and northwestern portions of the other provinces. The general forms of these rain-provinces are set forth in figure 2, which is taken from plate 46. 3 111 109 1 1 ae a — amr 93 91 8 8 8 83 81 79 77 6% 4% 71 GO GT Cys Lo | 5 hee € ie 2 =) Sy ; ( x \ we AN Ny My S aN ‘id “HHA rr Pie ACN IN, aN ppt Lo 117:— 1116s 118 lll 109 107 1068 106 101 9 Fig. 2. Moisture zonation, according to precipitation indices for period of average frostless season. Precipitation provinces: Humid, more than 140; semihumid, 100 to 140; semiarid, 60 to 100; arid, less than 60. Numerical values are in the thousandth of an inch. (See also plate 46.) Plates 48 and 51 show somewhat marked departures from this generalization, consisting mainly in the westward or southwestward displacement of the arid province, so that the latter comes to include the southern half or more of the Pacific coast and little or none of the Great Basin. The other features of these two plates are somewhat different from those of the other precipitation charts, but still agree with them in a general way. The southeastern humid province appears on plate 49 as three localized areas, two of which are where they would be expected (from a study of plates 46, 47, 49, 50, and 51). The third area, however, is differently placed, occupying portions of Minnesota, Iowa, Mis- souri, Wisconsin, and Illinois. On plate 51 this southeastern humid, or rainy, region is much more extensive northward and northwestward CLIMATIC CONDITIONS OF THE UNITED STATES. 283 (than in the case of plate 46, for example), and southern Georgia and Alabama, and most of Florida are here shown as in the semiarid province. 3, REMOVAL OF WATER FROM THE PLANT. A. INTRODUCTORY. (1) GenEeRAL ConTROL OF WATER-LOSS. The external conditions! that are effective in the control of water- loss from ordinary land-plants are generally confined to the aerial environment, for water is probably seldom lost through the subterra- nean periphery of the plant-body. The water-extracting conditions of the aerial environment are more directly related to climate than are the water-supplying conditions of the subterranean surroundings. Some of these conditions have been studied by meteorologists and climatologists, and the published records of the U. 8. Weather Bureau will once more be drawn upon for climatological information wherever possible. There are just two features of the aerial environment of plants that are directly effective in controlling their rates of water-loss—the evaporating power of the air and the intensity of absorbed radiant energy. These two conditions should not be confused; one is dependent upon air-temperature, air-humidity, and velocity of air movement, and the other depends upon the quality, intensity, and duration of sunshine, which is not generally a function of the air conditions immediately about the plant. Also, these two conditions should not be confused with evaporation, which is almost always done in common parlance. The rate of evaporation from a given water-surface is deter- mined by various internal conditions (resident in or back of the sur- face) and by these two external conditions. We shall, however, still frequently employ the term ‘‘evaporation”’ as practically synonymous 1 On the internal conditions that are effective in this regard, see Livingston (1906, 2).—Idem (1913, 2). Bakke 1914.—Shreve, F., The vegetation of a desert mountain range as conditioned by climatic factors, Carnegie Inst. Wash. Pub. No. 217, 1915, and the citations given in these papers. The xerophytism, mesophytism, etc., of plant forms, as these have been roughly con- sidered by observational and classifying ecologists, are mainly based upon the appearance or structure of the aerially exposed parts and, until the recent development of the concept of transpir- ing power, or resistance to transpirational water-loss (which is quite distinct from the transpira- tion-rate itself), but little progress has been made toward the quantitative definition of plants in this regard. If the transpiring power of plants might be as well known as the shapes of their leaves and the arrangements of their floral parts, a great impetus should be given to the more permanent aspect of ecological study. With a knowledge of this power for the various plant forms should of course go a similar knowledge of water-absorbing power and water-conducting power (see especially Livingston and Hawkins (1915), in this general connection), for the xero- phytism, etc., of a plant may depend on one of these latter rather than upon transpiring power alone. The measurement of these more recondite internal conditions has not as yet been seriously attempted, and methods therefor are still to be devised. Ecological plant geography will eventually need to define its plants physiologically as well as taxonomically, and the geographical distribution of species (itself as yet attempted only in a crude way) will become of little interest without some real knowledge of the physiological qualities by which these species resist or favor the various influences of the environment. In the present part of our study we confine attention to environmental conditions. 284 ENVIRONMENTAL CONDITIONS. with atmospheric evaporating power, since this will be readily under- stood and since an attempt to avoid such practice seems somewhat pedantic at the present time. When the evaporating power of the air begins to enter seriously into climatogical studies this meaning of the word ‘‘evaporation”’ may be dropped. In short, it séems desirable to avoid clouding the main issue for the present, and we shall frequently employ the word ‘‘evaporation”’ to correspond with the word ‘‘pre- cipitation” as here used. The expression “evaporating power of the air,’ or ‘‘atmospheric evaporating power,” will also be used, however. (2) ArmospHERIC EvaporaTiING Power. This term is here used in Livingston’s! sense, meaning the tendency of the air about the plant to accelerate transpiration. The expression has been seriously opposed as an “‘inaccurate and misleading expres- sion,’ by certain members of the staff of the United States Weather Bureau. Some of the discussion that has been raised in this connection is indicated in one of Livingston’s papers (1915, 2) and in footnotes, editorial and otherwise, incorporated therein. The objection to the term, as so far brought out in the literature, seems to reside mainly in the consideration that the non-aqueous gases of the air actually hinder vaporization of water (which they do to a comparatively slight degree), so that a decrease in the amount of these gases present must increase the evaporating power of the air. It does not appear, however, that this is really an objection to the term in question, especially since the meaning is understood immediately by everyone, and since no better term seems available as yet. Doubtless some of the misunderstanding brought out in this discussion hinges on what may be meant by air. The air is a mixture of varying proportions of various gases; it always contains (in nature) nitrogen, oxygen, and a little argon, but it also contains carbon dioxide and water-vapor, and frequently numerous other gases. We see no reason for not considering these last-named gases as a part of the mixture, and it is in the sense of air as the gas mixture bathing the evaporating sur- face in question that the word has been employed by Livingston and is here employed. Now, such a gas-mixture as the air may vary in the nature and proportions of its constituents, and it may also vary in its density, or pressure. As the pressure decreases the air becomes less dense, and this purely physical change makes it possible for evapora- 1In this connection, see the following papers: Livingston (1906, 2).—Jdem, Evaporation as a climatic factor influencing vegetation, Hort. Soc. New York, Mem. 2: 43-54, 1910.—Idem, A schematic representation of the water-relations of plants, a pedagogical suggestion, Plant World 15: 214-218, 1912.—Atmometry and the porous cup atmometer, Plant World 18: 21-30, 51-74, 95-111, 143-149, 1915.—IJdem, Atmospheric influence upon evaporation and its direct measurement, Monthly Weather Rev. 43: 126-131, 1915.—Idem, A modification of the Bellani porous plate atmometer, Science, n. s., 41: 872-874, 1915.—Jdem, A single climatic index to represent both moisture and temperature conditions as related to plants, Physiol. Res. 1: 421-440, 1916.—Idem, Atmospheric units, Johns Hopkins Univ. Cire., 160-170, Mar., 1917. CLIMATIC CONDITIONS OF THE UNITED STATES. 285 tion to proceed at a higher rate than is obtained with greater pressure. It is likewise true that an alteration in the water-content of the air changes the rate at which evaporation may occur, ceteris paribus. Furthermore, a change in the temperature or velocity of movement of the air (wind) over the evaporating surface also alters the possible rate of evaporation. It is thus seen that both physical and chemical changes in the air exert an influence on the rate of evaporation from exposed liquid or solid water. That a decrease in the amount of gas present per given volume should accelerate evaporation is surely not a matter to cause misunderstanding, for we regard the mixture as air, at what- ever density it may occur. It thus appears that the evaporating power of the air increases as the density of the air decreases, and this con- sideration appears to clear up the whole difficulty above mentioned. The usual popular quibble over the conception of limits arises here, as elsewhere in physical science, when we consider the result of decreas- ing the air-pressure to zero. In such a case the air approaches, and finally should become, an absolutely empty space, without tempera- ture and without chemical nature. Such an absolute vacuum would have the highest possible evaporating power, in Livingston’s sense, and after such a condition had been reached (if it could be maintained) the rate of evaporation from exposed liquid or solid water should be controlled only by conditions resident in the liquid or solid itself. This condition is impossible of attainment, of course, so that the evaporat- ing power of the air never becomes infinite, but this consideration is valuable in that it shows clearly how this power becomes greatest when there is the least gas present in the air-space. The quibble arises over the popular interpretation of the apparently paradoxical statement that the evaporating power of the airis greatest when all the air has been removed. Of course, when the limit is reached and the air-pressure is actually zero, we have to broaden our definition of air so as to let the term mean the space abutting against the evaporating surface into which water vapor may diffuse. That this is necessary at the limit of reduced pressure (which is never really attained) seems to be no reason for changing our term, though if jt seems desirable we are free to admit that the term in question really denotes the evaporating power of the circumambient space in which air usually occurs. Another objection to the term ‘‘evaporating power of the air”’ is parallel to the one always raised against the word suction. The water vaporizes because of conditions resident in its solid or liquid phase, and the energy thus transformed does not come (directly) from the air-space. Just as the term “‘suction,’”’ or sucking power, has to be regarded as referring to the removal or decrease of a resistance, rather than to the application of a driving force, so the evaporating power of the air is to be regarded as proportional to the reciprocal of the measure of the resistance offered by the air to evaporation. The resistance thus 286 ENVIRONMENTAL CONDITIONS. offered may be expressed in terms of the water-condensing power of the air, but, as Livingston has remarked, air without actual tendency to precipitate or deposit liquid or solid water still offers resistance to evaporation: “Air without water-vapor offers resistance to evaporation but has no condensing power; it can not deposit water upon a surface, no matter what its pressure may be. The resistance offered by such dry air can be expressed in terms of an equivalent condensing power, however.” In answering this second objection to the term “‘evaporating power of the air,” a third possible objection is also partly answered. This objection arises from the various senses in which the word power is used. If we are interested only in the statical phase of the matter before us, then the evaporating power of the air is proportional to the reciprocal of the measure of the capacity of the air-space to retard the vaporization of water, from.a liquid or solid water surface exposed to that space. The dynamic phase of the problem of evaporation, how- ever, allows the use of the word “power” in its ordinary physical sense, as denoting the time-rate of doing work. The conditions resi- dent in the air-space are thus thought of as somewhat like a brake on a wheel, and we consider a time-rate of the reciprocal of resistance to evaporation. Thus, our use of the word ‘‘power”’ is not with the meaning of spatial capacity, but we employ the word in its true physical sense, as though the air were a machine acting to retard evaporation. As in other cases of power measurement, it is necessary to measure the power in question in terms of the amount of work capable of being performed in a given time period. Internal conditions, resident in the solid or liquid phase of the water, determine what would be the rate of evaporation if the air-space offered absolutely no resistance, and if these internal conditions remain constant the amount of evaporation occurring per time period is proportional to the reciprocal of the power of the air to behave as though it were condensing water-vapor upon the exposed surface. The reciprocal of the rate at which water would be condensed if all of the tendency of the air-space to retard evapora- tion were effective toward actual condensation is thus proportional to the tendency of the air conditions to allow evaporation to proceed, and this may be relatively measured as a power by determining the amount of water actually vaporized per time period. Of course, the conditions resident within the solid or liquid surface are never even sensibly con- stant for long, and the actual rate of evaporation depends not only upon the evaporating power of the air, as above defined, but also upon the internal conditions. The evaporating power of the air is thus relatively measured as the time rate of the reciprocal of the resistance offered by the air to evaporation, this resistance being measured in terms of equivalent condensation. But condensation is merely negative evaporation, so that when the air conditions aresuch as to make the external resistance just equal to the internal tendency CLIMATIC CONDITIONS OF THE UNITED STATES. 287 (within the solid or liquid phase) to vaporize water, then the apparent rate of evaporation becomes zero. Going further, the external condi- tions frequently become such that the resistance offered by the air to vaporization of water is greater than the tendency of the internal conditions to cause vaporization, this resistance being due toa ten- dency of the air to deposit water on the surface, and actual condensa- tion ensues; 7. e., the evaporating power of the air becomes negative and the evaporating surface gains water instead of losing it. There seems never to have been any attempt to define air according to its chemical content; it would still be air if it were largely carbon dioxide or hydrogen, etc., and it seems unadvisable to attempt a restriction of the terms ‘‘air” and ‘“‘atmosphere” at this late day. For the rest, the expression ‘‘evaporating power of the air” has been in use among students of this power at least since 1906 (when Living- ston used it). Itwill probably appeal to most students of this poweras quite unobjectionable and it need not be dropped. Livingston (1917, 1) has suggested atmometric index as still another term, to avoid the difficulty just mentioned and to avoid the necessity of employing evaporation to mean both the process and one of the conditions controlling its rate. We shall not employ this new expression, how- ever, preferring to allow others to decide the question thus raised. The evaporating power of the air is of the utmost physiological importance to vegetation, and it can be rather readily and directly measured, in relative terms. Nevertheless it has not been seriously studied in the United States, and most of the information so far obtained in regard to it is only indirect. To appreciate what ones of the climatic conditions usually measured may be valuable here, it is necessary to consider the secondary conditions, upon which depends the atmospheric evaporating power. The vapor-tension deficit—Without air-movement, and supposing the air and water temperatures to be the same, the evaporation-rate should be nearly proportional to the vapor-tension deficit; that is, to the difference between the maximum vapor-tension for the given air- temperature and the tension of water-vapor actually present in the air. The actual vapor-tension in the air is a closely approximate measure of the tendency toward condensation and the maximum vapor- tension for the given temperature and pressure is a measure of the whole tendency toward evaporation; the former tendency overcomes a portion of the latter and what remains is very nearly the actual tendency toward evaporation. The maximum vapor-tension of water is, of course, a constant for any temperature and barometric pressure, and its value may be obtained from physical tables. The actual vapor-tension is seldom as great as the maximum; it is so only in the case of water-saturated air. If we allow EH and LH’ to represent evapora- tion-rates from the same surface at different times, P and P’ the 288 ENVIRONMENTAL CONDITIONS. maximum vapor-pressures corresponding to the respective air-tempera- tures, and p and p’ the corresponding vapor-pressures of water actually in the air at those times, then EY Pp Hm Pp Under such conditions P—p and P’—>p’ are measures of the respective forces tending to drive water-vapor off from the surface into the air. To determine the values of p and p’, we may measure the absolute humidity by chemical methods, we may resort to the sling psychrom- eter or any form of wet and dry bulb thermometer with constant and rapid air-movement, or we may employ the Regnault dew-point apparatus. It is clear that the values of p and p’ depend upon the absolute humidity and upon the air-temperature and barometric pressure, while the values P and P’ depend only upon the temperature and barometric pressure. Since the influence of barometric pressure is relatively small under natural conditions, it need not be seriously considered here. The vapor-tension deficit is seen to include the air- temperature influence. Relative humidity—The vapor-pressure deficit is not one of the climatic features usually determined by climatologists, who have rather uniformly followed earlier workers in the employment of the concept of relative humidity in its stead. Relative humidity is the vapor-pressure of the water-vapor actually present in the air expressed as percentage of the maximum vapor pressure for the given temper- ature and pressure; it is simply the percentage of water-saturation of the air. This bears no quantitative relation to atmospheric evaporat- ing power, even with wind and barometric effects left out of considera- tion, for it is obvious that air with a given relative humidity must be more effective in promoting evaporation at a higher temperature than at a lower. It is not the percentage of the maximum vapor-pressure actually present, but the difference between the maximum pressure and the actual, which measures this influence upon evaporation-rate. Since the maximum increases with temperature (though not propor- tionally), a given percentage of deficit must represent a larger actual deficit as the temperature rises. If H and H’ represent the relative humidity of the air at different times, the remaining symbols being the same as above, then 1S ey pd H! p'/P From this it is clear that, if the air-temperature is known in each case, thus furnishing the values of P and P’, the vapor-pressure deficits may be found; from this equation and the one for LF, E’, given above, it follows that E P(i—H) RF’ P'(1—H’) CLIMATIC CONDITIONS OF THE UNITED STATES. 289 In other terms, the rate of evaporation is, under the assumed condi- tions, proportional to the product of the maximum vapor-pressure of water, for the given air-temperature, and the complement of the relative humidity. Relative humidity is commonly measured and discussed in climato- logical studies, and its complement is sometimes employed as a measure of atmospheric dryness. They are both easily seen to have no definite relation to the evaporating power of the air. There is here, however, a general and merely qualitative relation; high relative humidity usually corresponds to low atmospheric evaporating power, and the reverse. We shall have to deal with relative humidity in our discus- sion of the climatic conditions influencing evaporation, but this con- cept is to be clearly appreciated as without logical foundation; it is simply a mathematical abstraction and its value to agriculture or ecology will have to be determined by direct empiricism. It may be here suggested that vapor-tension deficit is the climatic dimension that should be measured by ecological workers, if the analysis needs to be carried so far. Fortunately, the evaporating power of the air can be directly measured, and much more readily and usefully than can this deficit, and it seems not at all necessary at present, for ecologi- cal purposes, to analyze this power into its components. Wind.—Besides the vapor-tension deficit, atmospheric evaporating power is greatly influenced by air-movement; with increasing wind, ceteris paribus, the evaporation-rate is accelerated. Here again, however, the relation between wind velocity and evaporation-rate is not a linear one; with low velocities the effect of alteration in wind velocity is great; with high velocities this effect practically vanishes, and the relation of the two features for any given range of velocity depends upon the kind and upon the exposure of the evaporating sur- face. As has been mentioned, an enormous amount of effort has been expended in attempts to find empirically a formula by which evapora- tion might be calculated from measurements of other climatic condi- tions, and the argument over the wind factor has been greatly pro- longed. Such attempts have failed, as they always must until the problem is first solved by controlled physical methods, which solution has not yet been seriously attempted. When a solution is reached, however, it will obviously hold only for some particular kind, size, etc., of evaporating-pan or other atmometer. As a climatic feature that must surely influence water-loss from plants, but the exact nature of whose influence is still quite beyond our reach, wind velocity will be but briefly touched upon in our study. On theoretical grounds this is not a promising criterion for ecological 1 Livingston, B. E., The vapor tension deficit as an index of the moisture conditions of the air, Johns Hopkins Univ. Cire., Mar. 1917, pp. 170—175.—Johnston, Earl 8., The seasonal march of climatic conditions as related to plant growth, Maryland Agric. Exp. Sta., in press. 290 ENVIRONMENTAL CONDITIONS. climatology, and it is practically unsatisfactory on account of the inadequacy of the information in this regard which is now available. (3) ABSoRBED RApIATION. Reverting again to the conditions controlling water-loss from plants, we have said that there are, generally, two of these—the evaporating power of the air and the intensity of absorbed radiant energy. The first of these has been discussed in sufficient detail for present purposes and the second remains to be considered. By far the greater portion of the radiant energy intercepted by plant surfaces comes directly from the sun; other sources of radiant energy appear to be practically negligible under most natural conditions. It is therefore absorbed sunshine (light and heat) that needs attention at this point. The intensity of absorbed solar radiation is determined by three conditions—the intensity of the impinging rays, the angle at which they meet the exposed surfaces, and the absorbing power of the sur- faces. The last is an internal condition, effective within the plant, with like transpiring power, water-absorbing power, etc. With this, as other internal conditions, practically nothing of a quantitative nature has yet been attempted.! The angle at which the impinging rays meet plant surfaces varies with the time of day, with the season, and with the shape and position of the plant; but since ordinary plants offer absorbing surfaces to solar radiation at all possible angles, it is only in special studies of special species (as of ‘‘compass plants,” for example) that this matter may require attention. We may ignore the angle of incidence in our present discussion.” The intensity of the impinging radiation is obviously the feature dealt with in climatology as sunshine intensity. For the measurement of this, various methods have been devised from time to time (such as the black-bulb thermometer, the bolometer, the pyrheliometer, the Hicks solar radio-integrater and several forms of photographic actinom- eters), but no data are available for a quantitative climatological study of this condition. It appears probable that the radio-atmometer® may furnish adequate information for ecological purposes, when its value in this connection has become appreciated. A very distant approach toward the measurement of sunshine intensity, and the only systematic attempt in this direction thus far 1 See, in this regard, Livingston 1911, a. 2 Briggs and Shantz have argued that only the vertical component of solar radiation is to be considered as effective upon plants. It seems to us that this question requires experimental investigation before its detailed discussion may be attempted. We may add here the remark that the surfaces of most plant leaves occupy almost all conceivable angles with the vertical, so that the exposure of the plant as a whole must approach being equivalent to that of a sphere or of a vertical cylinder with spherical top. For the opposite argument, see L. J. Briggs and H. C. Shantz, Hourly transpiration rate on clear days as determined by cyclic environmental factors, U.S. Dept. Agric., Jour. Agric. Res. 5: 583-649, 1916. 3 Livingston, B. E., A radio-atmometer for measuring light intensity, Plant World 14: 96-99, 1911.—Jdem 1911, b; Idem 1915, a; Idem 1916, b. f | — CLIMATIC CONDITIONS OF THE UNITED STATES. 291 carried out by the United States Weather Bureau, consists in the determination of the number of hours of sunshine occurring each day at the various stations. It is to be emphasized that the sunshine recorders now generally in use give but little information as to the intensity of the sunshine itself; they record the duration aspect of that range of intensities which is called direct sunshine, but the limits of this range have never attracted attention and are not established, so that the whole mass of dataso derived are anything but precise. Never- theless, some of the sunshine data of the United States Weather Bureau will be considered below, since they furnish the only available measure- ments having any bearing at all upon the matter before us. B. ATMOSPHERIC EVAPORATING POWER IN THE UNITED STATES. (1) Very Limitep Nature or AVAILABLE DATA. To obtain data bearing on atmospheric evaporating power it is only necessary to operate a number of atmometers of the same form in the various climatic regions dealt with, being sure that all have similar local exposures. The importance of this condition to plant and animal life and the relative ease with which it may be measured makes it appear surprising that practically no organized study of evaporation throughout the United States has yet been undertaken. Had evapora- tion been recorded as thoroughly as precipitation has been, we should now be able to construct relatively satisfactory atmometric charts, but the almost utter lack of data makes this practically impossible at present. To render our position in this connection still less satisfying, it is to be remembered that observations of any climatic condition, extending through a single year, are of but little value; if evaporation measure- ments were to be systematically begun in the present and were to be systematically continued, it would require many years of records to render these measurements as valuable climatologically as are those of temperature and precipitation at the present time. It seems now, however, that students of climate will hardly be able to persist much longer in their too common attitude of ignoring the evaporating power of the air. As we have emphasized, this climatic feature is probably as important from the standpoint of agriculture and etiological plant geography as either temperature or precipitation, and its investigation seems likely to be carried forward first by agriculturists and ecologists. While it is possible to collect from the literature numerous instances in which evaporation has been measured at a single station for a longer or shorter period of years, such measurements can not usually be correlated with those for other stations, either because the same years are not involved or because different kinds or sizes of atmometers have been employed. Aside from such cases, which are all valuable—at evaporation, Monthly Weather Rev. 36: 181-186, 301-306, 375-381, 1908; 37: 68-72, 103-109 157-160, 193-199, 248-253, 1909. 292 ENVIRONMENTAL CONDITIONS. least in showing the importance of this climatic condition—there are available just two logically planned series of atmometric measure- ments in the United States. One of these series was carried out by Russell for a single year, beginning in the summer of 1887. The other was conducted by the present writers during the summer of 1908. Approximately 20 years elapsed between these two series of observa- tions, and no thorough study of this feature has been completed since the last-named year, although the U.S. Weather Bureau is giving in- creasing (but always secondary) attention to evaporation. It should be noted that the United States Signal Service, the precursor of the United States Weather Bureau, carried out the earlier of these series, the second series being under the auspices of the Department of Botani- cal Research of the Carnegie Institution of Washington. These two series of atmometric observations, and the results derived from them, will now be considered in order. (2) Russevu’s Data oF EVAPORATION IN THE UNITED STATES. Evaporation intensities for period of average frostless season. (Table 11, plate 53 and fig. 14.)—Russell’s! study of evaporation extended from July 1887 to June 1888 inclusive, and this author prepared an evaporation chart of the country, but the data thus used were cal- culated. For the period from June to September 1888, Piche atmom- eters were exposed in louvred instrument shelters at 19 stations. An experiment in a closed room, employing two Piche instruments and two open pans of water, gave data from which Russell calculated that the Piche instrument lost 1.33 times as much water as did his free water surface.” By use of this ratio the readings of the Piche instruments in the louvred shelters were converted into losses from the free water surface of the particular kind of pan used in the labora- tory test, and these, as tri-daily readings, were compared with the corresponding dew-points and wet-bulb temperatures within the shel- ters. From this comparison Russell derived a formula by which he afterwards calculated the evaporational loss from free water surfaces in the shelter, for 140 stations in the United States. In his paper he presents a table of the monthly calculated rates (July 1887 to June 1888), and also the annual totals. In order to make use of these data in connection with the length of the average frostless season, as we have employed the latter, we have proceeded as follows for each station involved. The evaporation data for all whole months included in the average frostless season have been 1 Russell, T., Depth of evaporation in the United States, Mo. Weather Rev. 16: 235-239, 1888. See also Kimball, H. H., Evaporation observations in the United States, Monthly Weather Rev., 32: 556-559, 1905. 2 Of course this relation must vary more or less markedly with temperature and humidity conditions, even where the wind influence is out of account; but Russell seems to have ignored this consideration entirely, along with the other important consideration that the amount of evaporation is dependent on the sort of pan used. ‘ PLATE 53 293 ie ase ae ° ath "| | N bs = 5 CTT Letter cp % Cis | ia ee ir | WO BPN ita Za A 4 AA Lot Pe ER Z WZ iy fii eee ee ay, 1 mee 3 Z; q | i Ae Z : 5 si] | ; table 11, column 7. Full lines o = WS RS WN Ke 3 a A \ f u . SE h O \. As is Lp A = EE AD: BEANS EAS. i 2 ae ARN Se pte Ge OG. | 3 ae ee ots y q <7 ic = , GZ te Z GZ $8 ) Nein : a MORN SZ Zr : Me MIN \ 2 ee : ALA ENS ORR Pe, | : AMEN WCNC ZZ Z : > | Ale WAN \\\\\ <2 Zee g Pa RATT RKC Lu Rei S\N eZ Be QA SSS NN SAQA SLA WSs | t phytic ‘orest poration for period of avera, | | | (East) | | | ie H4 hy Ye , , . ‘= , g ‘ 5 4 evergreen f. rthern meso; No io Sy [| f a ———— Southeastern mesophytic forest em ygrophytic Northwest h evergreen forest = : ~é pee : eae Ss ean : MI De Mean daily rates (thousandths of an inch) of eva YW Grassland- deciduous forest transition Z p into four evaporation provinces. The base is plate 2. divide the ma —e CCC ee —— << s. ‘g oped st oseq oy, ‘sooutAoid wonviodvad moy oyut deur oy} oplArp souly [NY “gE Wamjoo ‘GT eq} Ino Ur UoAls ByEp 8 ,[[oSsNY {(soyoul) sonjva uolyesodvas jenuuy (seq) 318910} UaasTI9A9 SpArdosout wseulON ' 48 tf PLATE 54 47 4/ (/ e an =. i > 7 #4 =: ths C . “ Sheet on 3 ae e., mes Soe Se ieee: SS Nie Wes & jn 4 ee. 8 : g AD; = SS . : ie O = = rer : RO . a = a = | = ) ie ASS sulth i ‘ll iy i Sy 4 IS nw HLS NI b i ‘| \ 294 ie SS , Or CLIMATIC CONDITIONS OF THE UNITED STATES. 29 summed, and to this sum have been added two quantities, representing the evaporation for the two fractional parts of a month at the beginning and at the end of the frostless season, respectively. These added amounts have been derived, in each case, by dividing Russell’s evapora- tion-rate for the month in question by the number of days in that month, and then multiplying the resulting quotient by the number of days of the same month comprised in the period of the average frostless season. The total evaporation for the season thus obtained is next divided by the number of days in the period of the average frostless season. Thus, for example, if the season extends from April 24 to October 3, we sum Russell’s monthly rates for May to September, inclusive, and add to this six-thirtieths of Russell’s rate for April and three thirty- firsts of his rate for October. We then divide this total by 162, the number of days from April 24 to October 3. The seasonal totals and the approximate average daily evaporation-rates (1887-88) for the period of the average frostless season, obtained as just described, for 133 stations, are given in columns 6 and 7 of table 11, and the latter data are spread on the chart of plate 53, where the positions of the stations employed are again shown by small circles. It should be emphasized that Russell’s data for June, and earlier, refer to 1888, while those for July, and later, refer to 1887. We have merely made the best possible use of the available data. The isoatmic lines of this chart are drawn at intervals of 20 thou- sandths of an inch of average daily depth of loss from some hypothe- tical pan of water, Russell’s measurements being in such terms. The total range of values is from 52 (Tatoosh Island, Washington) to 349 (Independence, California). On this chart of the approximate average daily intensities of atmos- pheric evaporating power for the period of the average frostless season, from data of 1887 and 1888, it appears that the isoatmic lines in the vicinity of the oceans have a very pronounced north-south trend. They also have a north-south direction in the plains region. Little relation to latitude, or to temperature, is here discoverable; during the frostless season temperature is not usually a prime condition in the determination of differences between different stations in the evaporat- ing power of the air. The lines for values 120, 160, and 240 are shown on plate 53 as dis- tinct from the others, and these may be taken as dividing the country into four evaporation provinces. Following our usage in the case of the precipitation indices, these provinces will be termed arid, semi- arid, humid, and semihumid. They are shown also in figure 14, which is derived from plate 53. This zonation is different from that shown for precipitation in several important respects. Here the humid province (values above 120) appears again as a western and an 296 ENVIRONMENTAL CONDITIONS. eastern portion. The western portion is much larger in this case, how- ever, occupying the Pacific Slope for practically the full length of the western coastline and widening at the north to include about the western half of Washington. The eastern humid region has an entirely different form from that shown on the precipitation charts. Here it does not occupy the southeastern part of the country, but embraces the northern margin from about the one-hundredth meridian eastward. It also occupies portions of the Atlantic coast as far south as Cape Fear. It appears that the line for value 120 passes into Canada from Wash- ington and reenters the United States in North Dakota, so that these two portions of the humid province are probably to be regarded as a single one. It should be noted, furthermore, that the Atlantic coastal portion from Massachusetts, or New Jersey, southward appears to be separated from the northeastern portion, and that a small area of humid conditions is shown about Corpus Christi and Brownsville, Texas. These features will appear more prominently on the charts of pre- cipitation-evaporation ratios and on those of relative humidity, to be considered below. The arid province (values above 240) occupies much the same region as in the case of the precipitation charts, but it does not here extend west of the Sierra Nevada Mountains. Of course, the western mountains are largely humid, but our charts do not generally present such details. The semiarid province (values between 160 and 240) occupies a belt outside of the area of the arid province, and this belt is extended eastward in the middle of the country nearly to the Appa- lachian Mountains. This eastern lobe of the semiarid province will also be pronounced on the charts of moisture ratios and relative humidities. The line separating the semiarid from the eastern semi- humid area (value 160) does not here bend eastward at its northern end as it does on the precipitation charts; on the contrary, it here bends westward and apparently joins the corresponding line which enters Canada from western Montana. Annual evaporation intensities. (Table 15, plate 54.)—Russell’s table gives the yearly totals for his series of stations, in inches of depth from some hypothetical pan of water, and he also presents a chart to represent these annual data. The data are reproduced in the third column of table 15 and they are shown graphically by the chart of plate 54, which is not exactly the same as Russell’s chart, a number _ of obvious errors in the latter having been corrected here. The total range for the country is from 18.1 (Tatoosh Island, Washington) to 101.2 (Fort Grant, Arizona), and the isoatmic lines are placed at intervals of 10 inches, with full lines for the values 30, 50, and 80. This chart has a pronounced general resemblance to the one repre- senting evaporation for the period of the average frostless season (plate 53), but it differs quantitatively therefrom in several important » yi be < ‘7, oyB[d stoseq eyy, ‘soourAoid uorys10dBAe MOj OJUr deur 9q} OplArp souT] [Ny *g WUINjOo “FT e[qQB} INO UO OATS Byep SJjossNY ‘sy}UOUT JOMIUANS 914} 10} sonjeA UOTyvI0dvAV [v0], G6 oL6 66 LOT ol ° 5 e ° o| ' (seq) uonisuen = S210} Uad13I2A2 S210} use, 1 4sei0} stlonpisep =. onAydosew usayWoN © SBAYdosau ussyION ~puryssessy S371W 009 ‘| =—s fe YH io === EXON NYS =) 48010} onAydosow i iad UJa}SPIYINOS S | Sal) SSN SNS eon IAL CORAL A % GOs ie, WZ 7 & \\' | ‘ ENR NV WA | Y » a : ee { i FN r SNe: Ye AW ge W'S INKS S\\" \ ee SS NS; NV \S & (AS 298 ENVIRONMENTAL CONDITIONS. features. The northwestern portion of the humid province (values below 30) is less extensive, and the Pacific Slope is here depicted as in the semihumid province. The eastern portion of the humid province appears here about as in the case of plate 53, but it does not extend southward farther than Rhode Island, on the Atlantic coast. The arid province (values above 80) is here shown as smaller than in the pre- ceding case. The great eastern lobe of the semiarid province (values between 50 and 80), reaching nearly to the eastern mountains on plate 53, is not present on plate 54, but a large area of values above 80 is shown as extending from St. Louis, Missouri, and Louisville, Ken- tucky, to Key West, Florida. Evaporation intensities for the three summer months. (Table 15, plate 55.)—Because we shall wish to compare the Russell evapora- tion data with those obtained by ourselves (to be considered below), and since it is impossible to employ the length of the mean frostless season as duration factor in connection with the latter, it is expedient here to study Russell’s data for the period of the three summer months, June, July, and August. For a period approximately comparable to this our own data may be studied. TaBLE 15.—Precipitation and evaporation data for the year and for the three summer months June to August. is 6) io. ¢) i 1 8 2B | ee o 2 a E & |sb 8 g (£85 is ee 2 |a2 3 ke ° 8 8 co ae 5 s |5* 3 °e {333 > =8 SA we |RSS 3 & 53 Ba | “9 Bog Station. a = qe ee an eee q q ae 5 = re) oe 2o a 2 ae es Br dee = SS bee ot sag a eo § © a o Se Ro 3 6 |Eo $8 an |She Soci ge 98 | “unl bs are aE Nga gree Nar a ~ S oA Fo | ae |[gcSi8| g8 | a8 lod Sais Z al — Z il & Alabama: inches. inches. inches. | inches. BA TITITS CON tas ahacs a.o uns tie < Semis ISG YL Me arctic, 1 erataete 18.00) |) eo cae Ledtara vbr ted ih: Vc iepeebrns Py ER RW gern Cee AURA hre dst 5a] Dates 18:06) |" sc aaee eee VEN LIG sate ab oc raeis eer ok ee 62.04 42.1 1.47 19.80 bh rf 1.559 DAONtQOMErY ss vies sieve eslenn us 51.16 56.6 0.90 13.13 14.6 0.900 Arizona: Fort Apache (S3)...........| 18.90 65.5 0.29 7.28 22.9 0.318 Fort Grant (83) .........%.- 14.24 | 101.2 0.14 6.12 36.7 0.167 PPG RNIN 2 See iy a oinle eines ba ore EOL Madea erlice eens yo Si a (a 2 Prescott: (G84)... a. die stenesin als 17.40 56.0 0.31 6.29 31.2 0.297 UIT sk ds a ee ee eee bile 3.10 95.7 0.03 0.47 33.8 0.014 Arkansas: Wore SAE vs sves wbiviss eV aie 41.34 49.6 0.83 11.50 14.8 0.777 TIPUTLE RUOGIE. o cine w tae, cadiorea 49.89 61.7 0.97 11.73 15.4 0.762 California: Cedarville‘(616) oi acces os Fort: BiGwWe@llr ca wus lccucaes } 14.58 48.9 0.30 1.22 20.9 0.058 CLIMATIC CONDITIONS OF THE UNITED STATES. 299 TaBieE 15.—Precipitation and evaporation data for the year and for the three summer months June to August.—Continued. Station. California—Continued: 1 ETE) Cs a HLGSPANPEIES: ; 615. 6 5s RCO O RUT aso 35x02 ys atone AMP ICLPO:, 5.4.01 he Sees: San Francisco.......... PST LORE Ne eis o./o aula) cote a\e San Luis Obispo........ Colorado: Colorado Springs (S7)....... SI OR Selon sy ie sicisveieteiers 1 P1512) CES RST, At Connecticut: PRE HLONG a ors 50s stare 3 Cs ate Florida: daeksonville. . 056.0500 ATO eee ee WEG VEWIESb. coc/5 cts 2 we ae New Smyrna (S83)...... SPSGTISVELLE <)3i5%. 65 oe cic k epee Georgia: Idaho: eeatello ss £4 2.0 ose os Illinois: Mises NG Ss ies Aes. cola os bucks New London (H)........ Normal total annual precipitation Total evaporation for year 1887-88 Ratio of normal annual precipitation to annual evaporation for 1887-88 ~~ pre : 3) 38 = 3 cls ot =) A inches. | inches AGZODE eee alike akee 9.73 | 65.8 0.15 “*"1) 9.53 | 100.6 | 0.09 15.64 oleae 0.42 25.03 84.8 0.30 20.09 54.3 0.37 Se PEO 37.5 0.27 DPS Y Oe 0.61 P74 11H be CRS Ps eR 2OSOL Mr ivciatie My aeantene 14.28°*| 59.4 0.24 14.02 69.0 0.20 9.54 68.3 0.14 bt Wao 8H ea Sar een Greet Aer ypc Oell Rate) | SRA es 47.19 31.8 1.48 -..| (43080 31.8 1.38 Sel) kasoe74® 45.7 ila yf ar OU ON eee all ota aye ae af boss 00: 51.6 0.75 elt 50.84 44.2 5 56.25 48.8 P15 PAS POLnoS 49.5 1.04 49 .36 IG) 0.96 47.89 49.3 0.97 a\e {s)/0,e.e)7 |i) 21008 ein © Siviale ale: si, JAhane ee 6 eo ere Sele) OE) s,s) ayers) 6 precipitation for 3 Normal total lila’ one WN RD DSSSSS9 9 OF iw) aa Non summer months (Ps). summer Total evaporation for 3 months, 1887-88 (#5). ee eee ey see eee eee eee tion for summer months, 1887-88 | Ratio of normal precipitation for 3 summer months to total evapora- eee eee ee eeee 300 ENVIRONMENTAL CONDITIONS. TaBLE 15.—Precipitation and evaporation data for the year and for the three summer months June to August.—Continued. = 5 aa5s5 a 8 are bea 42 fees tee 38 eb eee 5 <3 =I my = ° 5B . 3} E Cd & = = OT ae paar geen | em 64 (4° 23e 38 Be |Sa8cig ae ee a A iy ee Station. += .o o | ES Bec al fede My, he eee aw aas Ss + Pirs ose oS Q = AAs g BH bo | ASB o £Sn| 8-1 ooe ee aa | ®= 38° 7 | 2 3] Say |°s 4 oe Fo | a8 lose 8 | £88) a8> loasescle So eh (884 = BSBA | $28 (S385 aN oH Gj Z & & Indiana: inches. | inches. inches. | inches. Evansville......-... ASTOR rece be Nil feck. rotte 11222 NN ae carer ll) © Indianapolis....... 41.48 48.6 0.85 ii Bar ie 20.3 0.580 Towa: Charles City....... BLAS it taser rule tee e.eak 12 VOM cate) | ee Davenport. «0/5... 32.69 39.0 0.84 11.30 Via 0.639 Des Moines........ 32.45 36.0 0.90 12.43 15.5 0.802 Dabwdue:.... see aes 34.01 BRAY 1.02 11.89 fone 0.783 Weolcukes .c 2 ae. e o< 35.07 42.9 0.82 11.62 18.1 0.642 DIOR OLY ces 6 i viele 20.96 |aewancteell eatin. 10.49" 3.822. 0) — Kansas: Concordia... .. . 6 27.47 47 .2 0.58 11.40 18.2 0.626 Woadre City. a. ote 20.84 54.6 0.38 9.29 Doe 0.417 MODEK AL a cin dao Seve 33.76 36.1 0.94 13.95 13.9 1.004 AWachitisiave cpes-wiaererie OGL Yheicerea le we, coterie 12.485 [snc e. | eee Kentucky: Bexington)o ence ai AD OS Ny sverctomtec Uli nil ade erotere 12.00) Seen) sae Louisville.......... 44.33 54.8 0.81 11.58 20.0 0.579 Louisiana: New Orleans....... 57.42 45.4 iy 18,24 12.5 1.459 Shreveport......... 45.68 45.6 1.00 9.54 14.3 0.667 Maine: Bastport....5...66.3 43.27 Duet te 9.92 Chats 1.271 Portland. | ee | ores lee | sa \deese ~~ » ~ Cd — — | as | #8 | febes|Sea|/ 38 |BSaee Station. $5 BL BS a7 | 3 ay 8 a ass re Seu eee. Sra ae tetas a eho 4.2 eet os pee eo doe) bag |g a ees feel Sa he eee | Sie ba a lore Gay cle aa $5 | S88 2 | 38] 828 (Boks oN Z a jeu] Zz a Gj inches. | inches. inches inches Missouri: Gali pias a oa ROOOLS [ils coats Ms shee VAS OZ | sarees arse | webietavtuecs Hannibal)... 5. 2... oe Oy | eee 3 se) NU oe Si POVGSy les ice ee eee Kansas City....... = Ree Ec2s $56 0.90 whee Be UB Lamar (S49).......] 41.24 39.6 1.04 13.51 14.6 0.925 Sie EOUISS oe ek: 37.20 5242 0.71 10.56 20.5 0.515 Springfield.........| 44.57 38.3 1.16 14.29 12.4 1.152 Montana: Crow Agency...... 9 leet Custer)... 14.56 52.0 0.28 5.04 22.5 0.224 15 COTES Eee (Fort Assiniboine) . ee ano a 0.35 6.00 | 16.5 p20? iulenaree es 7h | eh! 0.24 3.87 20.4 0.190 Walispell’: 24... 2 43. NGEOS SIP cae) el tree SPAT Ms stones) || er etetehe uote Fort Maginnis (S30)| 16.52 35.8 0.46 5.20 16.0 0.325 Miles City... 06; POSUCE| ean), aon 1320 dal eee ns onic Poplar (S30)... ... ee in \ 13.59 | 35.4 0.38 5.66 | 16.5 0.343 Nebraska: @rete(S37) 5.6.20. 29.06 35.5 0.82 13.02 14.8 0.880 HRHCOIN.... Sac titles - 7a feta) Uk ee, ee | Sy eee a PLE RGus co aly Ue otereretete North Platte..... 18.86 41.3 0.46 8.39 UPA TL 0.474 (CRS TS ee a ae 30.66 41.7 0.74 13.00 16.6 0.783 Malentine’..4... 2.0/4. 22.46 43.8 0.51 10.03 17.2 0.584 Nevada: BREN Gy sclecicrsc Mater MLOLAS ome ee cee ORGSI vac cen led & aictetoieree Winnemucca....... 8.40 83.9 0.10 0.98 33.6 0.029 New Hampshire: oneord'. -y. hc. fea } 40.11 | 33.3 1.20 10.87 | 12.4 0.877 New Jersey: Atlantic City...... 40.82 Zee 1.62 Myer 9.8 1.133 Wane May .)....2..... BORON isco edly ctleoee NO SOS he oe. s 550 ey eee New Mexico: ] Fort Stanton (S2)..| 16.70 76.0 0.22 8.07 31.9 0.253 ante Nel ts st, sai 3 14.49 79.8 0.18 6.11 53 ie!) 0.192 New York PDN: so5015:5 8 Fhe 36.38 34.8 1.05 11.62 14.2 0.818 Binghamton....... So OAD Cieh ell Nocoe otal NOVAS He reed Ah ov nceaeaee LECT |) Re 37.28 32.9 1)? 9.53 14.0 0.683 (CHT) Re ee SOSUSH wtarcea diy ©. ke (2 oa | [MS ae menue Cgc de ers to LUCE CTS Gey Meee een S57 Wea Ye | are se, | Ris Mage | TORS ZO vas o'ecac5 |), we ieeseleer Be waly OLIGS. o:..6 <'0.<7: 44.63 40.6 1.10 IPE Ss: 14.8 0.834 OBWEPO. oi. 2ackls « 36.18 28.9 1.25 9.35 abe 77 0.799 Rochester......... 34.27 32.4 1.06 9.18 13.6 0.675 302 ENVIRONMENTAL CONDITIONS. TaBLE 15.—Precipitation and evaporation data for the year and for the three summer months» June to August.—Continued. = he 1 fw Lhe 1 ‘ 2 | 2.1882 |48 | oe jee? gq. : 8 a. om er Site ie §3 | as | ass eR i 8 RQ, ok 3 et 2 a aa |2POn% — ae | ah ee ee Ss FI Ss. | £2 ae Saclg o-s.| SA 683 a J 2 al RSoaNY | Ss wo | 8g a“s83 Station. oh ats aaé ou | 28 ag38 rs 5 is On ~“fn!| Ssils Zefa a: ote ii A as|/Sgmi 4 9 2" os | a3 |2t3 8 | £88/a8s loaeescis BA | $5 | S8a 5 | BER | $28 (Boas se Z a a z & ee North Carolina: inches. | inches. inches. | inches. IABHE Ville 2G santos AG 56) ee tcae ll Meese 14500 casi e | - ee Charlotte: 3. wc ce: 49.20 49.0 1.00 15.50 13.8 1.123 Hatteras ey. ckcresor 60.85 31.3 1.94 16.30 10.4 1.567 aleizh: 5t¢aoaso 49.60 37.0 1.34 16.73 12.8 1.307 Wilmington........ 51.05 38.4 1.33 19.10 jh es 4 1.632 North Dakota: SIS ALCIC: oe ee eel 17.64 31.0 0.57 7.66 13.9 0.550 Devils Lake. .2.70.- (Fort Totten)...... 20.16 21.2 0.74 10.07 i eg 0.861 WTIStON von 2 ob tee (Fort Buford)...... 15.07 35.5 0.42 6.91 16.1 0.429 Obio: — Ciena. 4). o2 37.33 52.0 0.72 10.85 19.5 0.556 Gleveland :),.;2aee.: 35.04 3537 0.98 10.38 14.5 0.716 Columbus: '.3...0; 36.92 | 47.8 5 O47 10.36 | 19.1 0.543 Sandusky.......... 34.02 | 36.6 0.93 10.98 | 15.4 0.713 MROLECOM oe shia Peiaies 30.62 38.6 0.79 9.32 17.0 0.548 Oklahoma: Fort Sill (S41)..... 30.85 46.1 0.67 9.83 16.7 0.589 Oklahoma......... SL OO Ne skins. ol stele ees OSB ee ae Oregon: Astoria (S27) .....2.' 75.35 25.3 2.98 5.62 8.6 0.653 Baker! City. 5.22. . T3220) Wars ae. - estes 2/03 | oa... 00) tee Portland). >4).(sea se: 45.13 34.7 1.30 2.97 12.8 0.232 Roseburg:..c.stoc} 34.43 39.2 0.88 2 13.6 0.127 Pennsylvania: STIG secre eee 38.55 33.8 1.14 10.22 14.9 0.686 Harrisburg......... STAD TA ote Velen tess 1 sy Gl ee Philadelphia... .... 41.17 | 45.0 0.91 12.24] 16.6 0.738 Pittsburgh... 20-)-|) seoreo 44.5 0.82 11.49 17.6 0.653 Scranton. ......... SLURS niece il) | pir ees L165) 2.003341) eee Rhode Island: Block Island....... 44.36 24.0 1.85 9.66 8.2 1.178 Providence........ BSE ele ote weer We | geteerese 10:42 }) .ccce< |) ee South Carolina: Gharleston.|..).<2...| 62:07 | 4397 1.19 19.52 13:7 1.425 Columbia..........| 46.08 43.2 1.07 17.02 13.4 1.268 South Dakota: PON ts sie nes 21.10 33.0 0.64 9.36 14.0 0.668 Pierre ster. Petre (Fort Sully) ....... 16.63 41.9 0.40 7.44 17.8 0.418 Rapid Citys... 6... ES SEO Ae ctiaisv Ale! badee weer S.20) | cantaa ls . .ceeeee pr N90) bei ee EY 25.43 31.0 0.82 10.91 race 0.859 Tennessee: Chattanooga.......| 50.68 46.4 1.09 11.93 13.6 0.877 L605 9' 111 49.35 45.9 1.08 12.38 14.1 0.879 IWMGMDBIN: jc chioaels 50.34 50.0 1.01 11.08 15.1 0.734 INashyile. oor .<~ as 48.49 50.1 0.97 12.19 16.9 0.722 CLIMATIC CONDITIONS OF THE UNITED STATES. June to August.—Continued. |__| | z 3 ax ao = a Station. =I 3S B= <5 Be a) Z Texas: inches. PADINENe, 53.26.00 e% 24.74 AMATO is oso sls « (Fort Eliot) ....... \ 22.55 Corpus Christi. .... 27.18 PASO: dese hs enc egese, = 9.84 Brownsville (S1}...| 26.89 Fort Davis (S2)....] 17.46 Fort Ringgold (S?). \ 17.46 (Rio Grande City) . \ HGrbwWVOreh..... 8.0 26.89 Galveston......... 47.06 Palestine! ttacss os: ¢ 43 .02 San Antonio....... 26.83 PR AGLOT ae 5 3) ercicke, aos 35.47 Utah: Modena...) cs..085- 10.15 Salt Lake City..... 16.03 Vermont: IBurhnpione,.))....25| ol.d6 INOrth Ge) Gis: <4... s1 33.84 Virginia: Lynchburg......... 43.42 PNOEFOL KS hae eee sicie 49.54 Richmond s:).%... 0 41.63 Wivitheyille |<<.) 46.71 Washington: 1 North Head........ (Fort Canby) ..... f oe Olympia (S19)..... 55.23 POrtpAneeles: 2 00) 2 sn as psthile tock cite erteve 36.59 SMGkANG 45.2 oicierenssae 18.85 SU RCOMIS 1 24205 che se 45.41 Tatoosh Island.....| 88.78 Walla Walla....... 17.67 West Virginia: PRURSYTASE (2 )che so coteeresiabeodlanee (O242 1122.) 1204: 124 | 0.59] 72 | 0.00} 143 | 0.66; 110 Towa :clowa. City’s: ..nnocs lead ostecleeee cele eetae gol 118 | 0.28] 155 | 0.00} 222 | 1.26) 165 (3) Louisiana: Cameron?....... 3.52] 123 | 3.80 99 | 0.67 275: 1.0.41 lee. <2 4:62). saz 2.66] 167 (3) Maine: Orono?............ 0.70} 121 | 0.00 | 178 | 0.90 117 | 0.86] 91 | 0.39] 122 | 0.57) 126 Manitoba: Winnipeg.......}..... LEO a etectye 720 ee ee es 126) |e ees 180 Ji see 156 Maryland: Easton?........ 0.04] 281 | 0.12 | 301 | 0.34 308 | 4.27] 122 | 0.00] 135 | 0.95) 229 Michigan: Grand Rapids..........| 0.75| 103 | 0.13 | 243 | 0.58 136 | 0.00} 140 | 1.71) 182 | 0.63) 161 IOUGHtOR: jst a dte sheets oe 0.92} 92 | 0.06 | 188 | 1.00 86 | -0.02| 147 | 1.73] 120 | 0.75) 127 Styhelene asc sc atako se Le22i OCs Oso2iiee soe 1 ae 5 a fea a OL00) 7-5. 0.00} 296 | 0.61) 202 (2) Minnesota: Minneapolis....| 0.91) 96 | 0.13 | 119 | 1.77 149 | 47 | 151 | 0.35] 204 | 0.63) 144 Missouri: St. Louis........ 1.70} 146 | 1.31 | 167 | 0.41 125 | 1.36] 126 | 0.00| 206 | 0.96; 154 Montana: Bozeman?....... 0.12} 84] 0.04 | 148 | 0.02 144 | 0.25) 199 | 0.00] 234 | 0.09} 162 Nebraska: HINCOMD ee te se sca eth: Soll 29 Neo G4s New ss OF07* iseraren OP1S) cae: 0.43)- ee 3.81] 29 (1) INOrthePlatte sinc. anna 1.30} 64 | 0.69 | 125 } 0.22 116 | 0.96} 119 | 0.00] 212 | 0.63] 127 VEU ACaS RENO! toc necicias llc careliciece 0.04 | 536 aT 499 | 0.01] 484 | 0.08! 379 | 0.03] 475 (4) New Brunswick: Fredericton| 0.20} 129 | 0.19 | 161 | 1.27 98 | 0.70} 76 | 0.06; 128 | 0.48} 118 New York: INT OL Ks cof senses tei one 0.05} 105 | 47 | 161 | 0.85 i Uy 6 AS «| 0:00);228 0.30) 148 (3) BVIACiuees ty .(. shisha 0).31//340"), 1.20 |... 1.41 270 | 1.08} 225 | ‘7 | 383 | 0.70) 305 (4) North Carolina: Pisgah Forest.......... 4.56} 32 | 0.77 67 | 0.70 104 | 0.78] 68 | 0.27] 81 | 1.42) 70 West italeigh n.:<.ic% ss. 0.70) 140 | 0.43 | 198 | 0.76 216 | 0.74] 190 | 3.52} 74 | 1.23) 164 North Dakota: Dickinson. .} 0.11} 289 | 0.39 | 393 | 0.69 320 | 0.33] 309 | 0.00} 285 | 0.30} 319 IO ROSPORE ois wa, 3 s die de 1.86} 132 | 0.00 | 207 | 0.87 201 | 0.72} 153 | 0.00} 249 | 0.69) 188 Oklahoma: Stillwater...... 1.52) 167 | 1.22 | 189 | 0.04 256 | 0.30} 140 | 0.01] 201 | 0.62) 191 Aaa Bugene,....t.c.. 0.00} 62 | 0.00 | 234 | 0.00 262 | 0.00} 236 | 0.00} 300 | 0.00) 219 uebec: Ste. Anne de Bellevue....| 0.22] 147 | 0.00 | 150 | 0.88 82 | 0.26] 51 | 0.30! 67 | 0.33] 99 Saskatchewan: Regina.....|..... iby gota eS ee oy) ee 109 ae EUSP Mowe 180 ‘hacia 164 Tennessee: Knoxville...... aid eoilacéy aves Os OD Hey sedis O60, tines 0.84] 126 | 0.46] 130 | 0.65) 128 (2) sexses Dalharts cs cish i. 1.64) 390 | 0.00 | 470 | 1.17 329 | 0.03] 427 | 1.05]..... 0.71) 404 (4) Utah: Salt Lake City...... AP Wel 10-00" 19148"10:02 189 | 0.01} 226 | 0.22) 175 | 0.05) 170 Vermont: Burlington...... 0.11; 169 | 0.14 | 349 | 1.58 156 | 0.72} 122 | 0.21} 184 | 0.55) 196 Washington: Seattle....... 0.09} 190 27 | 20010200 128 0.05} 174 | 0.00} 142 | 0.05) 167 2 Precipitation data from monthly weather reports. 4 T means trace, less than 0.01 inch. ® Precipitation data are for Boscommon. 7 Mueh interpolation on account of infrequent readings. , CLIMATIC CONDITIONS OF THE UNITED STATES. 309 TaBLE 16.—Weekly precipitation (P) and weekly rates of evaporation (E), the latter from cylindrical porous-cup atmometers, summer of 1908.—Continued. FourtH 5-wEEK Periop (Aua. 4 To Sept. 7). Aug. 4 Aug. 11 Aug. 18 Aug. 25 Sept. 1 ; to 10 to 17. to 24. to 31. to 7 AVETAEO, Station. BE Bes) Pe E 12 E 12 E P E Te E P E an. | c.c. in. €:C: m. cic: an. | ‘e.c. | am. | e.c. | an. c.c Alabama: Florence?........ 0.85) 34 {0.11 | 132 | 0.59 75 | 0.00} 139 | 1.32] 184 | 0.57] 113 Arizona: Tucson.......... 0.08) 252 | 1.76 | 230 | 1.67 232 | 1.67} 229 | 0.41] 302 | 1.12] 249 California: San Diego...... 0.55] 179 | 0.00 | 204 | 0.00 230 | 0.00) 235 | 0.07} 157 | 0.12} 201 Colorado: Boulder®........ 0.43] 165 | 0.85 94 | 2.02 104 | 0.00) 205 | 0.00} 235 | 0.66} 161 Florida: Miami........... 2.60} 96; 1.81 83 | 0.43 73 | 1.81] 72 | 0.04] 134 | 1.34) 92 Illinois: @harleston*.:.......... OPUS 2S0 a ROL SS a P227. ec. 2 ea line brani eeeIIle, oie ts cil lokercheve terete 0.39] 229 (2) WIGS EEE 5 cine ee AE) L4Sia lel Gow |e sea uid el | Me a Oeioiess ke O48 lea eee 0.00} 148 (1) Iowa: Iowa City”?.......... 0.03} 178 | 5.09 61 | 0.02 153 | 2.33] 133 | 0.28) 170 | 1.55] 139 Louisiana: Cameron?....... WEOS ee ae 2.26) 138@ | 2238 75 | 47 | 123 | 1.30] 130 | 1.49} 116°(4) Renrae: Orono’: .. 6... ss. 2212) (62 e239 83 | 0.18 122 | 0.00}; 97 | 0.04) 73 | 0.95] 87 Manitoba: Winnipeg.......|..... SOs [15 eeoapts | Search [lotsa ates al] fa ave cesta [vere tere tenets ts all suecaretai] ce 2 veel epee 130 (1) Maryland: Easton?........ 2-69)" ‘95 10.62 | 157 } 1:36 113 | 2.93} 59 | 1.60} 112 | 1.84] 107 Michigan: Grand Rapids.......... 0.13] 136 | 3.62 68 | 0.09 120 | 0.14) 154 | 0.00) 160 | 0.80) 128 Ltr 0) 0.16) 164 | 0.12 99 | 0.34 212 | 0.01) 128 | 0.37} 212 | 0.20] 163 ste 13 Cli are 0.88} 249 | 1.13 | 180 | 0.00 314 | 0:00)... .. 0.05} 191 | 0.52] 234 (4) Minnesota: Minneapolis....| 0.53] 127 | 0.24 | 100 | 0.01 159 | 0.14! 177 | 0.25) 231 | 0.23] 159 Missouri: St. Louis........ 1.01} 125 | 0.15 | 176 | 0.36 177 | 0.03} 191 | 0.00) 212 | 0.31] 176 Montana: Bozeman........ 47 | 190 | 0.49 | 105 | 0.24 144 | 0.36] 124 | 47 | 188 | 0.22) 150 Nebraska: Loa OF 2O aoe 1.01 | 108 | 0.52 95 | 0.98] 138 | 0.24] 193 | 0.69] 134 (4) MOGuR PIATtO. 6: 2.65 65s 0.56} 192 | 0.27 | 130 | 0.46 98 | 0.28) 140 | 0.00} 295 | 0.31] 171 Nevada: Reno’........... 0.00} 379 | 0.12 | 367 | 47 367000 -O0le 4. OL TO| eas 0.04] 371 (3) New Brunswick: Fredericton| 1.50} 69 | 1.35 50 | 0.76 66 | 0.00} 73 | 0.40} 37 | 0.80} 59 New York: US Cty 0.63) 62 | 0.35 85 | 2.28 106 | 3.63) 41 | 0.06) 62 | 1.39) 71 UCD eee 0.32) 346 | 0.23 | 299 | 0.11 371 | 0.00) 319 | 0.13) 406 | 0.16) 348 North Carolina: Pisgah Forest.......... 3.11] 42! 3.05 79 | 7.15 14 | 1.24) 38 | 2.20)..... 3.64] 43 (4) West Raleigh.......... L216) LOS" 90. 21 1) 120) | S'09 5009.04) 14 Ws O7i 76> S634) 7 North Dakota: Dickinson?..|} 0.00]..... 0.53 | 181 | 0.54 297 | 0.34) 243 | 0.00) 312 | 0.35) 258 (4) MIRITORMOFG. 2025.55 5...- 0.62) 167 | 0.47 | 190 | 0.00 238 | 0.00) 248 | 47 | 233 | 0.22] 215 Oklahoma: Stillwater...... O24 220 A OT W224 Vt 27. 115 | 0.14} 168 | 3.91} 85 | 1.33] 163 Oregon: Eugene........... 0.00} 305 | 0.00 | 170 | 0.00 174 | 0.00} 110 | 0.00} 134 | 0.00} 179 Quebec: Ste. Anne de Bellevue...| 0.82} 43 | 0.10 60 | 0.77 47 | 0.02} 77 | 0.00} 96 | 0.34) 65 Saskatchewan: Regina.....|..... WSs ae OG) ere Uta ees ke Dis ene O50 trae 76 Tennessee: Knoxville...... E-45 |) (dd) POOL 29 ees Sale ccs. (Oa la | eae 1.254) oes. 0.73) 92 (2) Wexress Oalhart........... 0.00} 504 | 0.28 | 461 | 0.70 353 | 0.10} 485 | 0.15] 447 | 0.25) 440 Utah: Salt Lake City...... 0.72) 178 | 0.62 | 168 | 0.01 181 | 0.04} 195 | 0.00} 170 | 0.28) 178 Vermont: Burlington...... 0.38) 114 | 0.79 | 120 | 0.53 179 | 0.00) 171 | 0.50) 214 | 0.44} 160 Washington: Seattle....... 0.00) 160 | 0.03 | 112 | 0.03 92 | 0.76) 51 | 0.01] 67} 0.17| 96 2 Precipitation data from monthly weather reports. 4 T means trace, less than 0 01 inch. § Precipitation data are for Roscommon. Much interpolation on account of infrequent readings. 310 TABLE 16.—Weekly precipitation (P) and weekly rates of evaporation (E), the latter from ENVIRONMENTAL CONDITIONS. cylindrical porous-cup atmometers, summer of 1908.—Continued. Station. Alabama: Florence?........ Arizona: Tucson.......... California: San Diego...... Colorado: Boulder?........ Florida: Miami........... Tilinoiss Urbana: i... .6 .i3. Iowa: Iowa City?.......... Louisiana: Cameron?...... ; Naine:"Orono*..,:/50% sede Firra 5-wreek Periop (Sept. 8 To Oct. 12). Manitoba: Winnipeg.......|..... Maryland: Easton’?........ Michigan: Grand Rapidss..... 2... Houghton: cies ee ees. Str Heleont’iceeoss asco Minnesota: Minneapolis... . Missouri: St. Louis........ Nebraska: MeINCOMN He: 8 85 he le cede Pes en New Brunswick: Fredericton New York: Syracisens ioe week: se North Carolina: Pisgah Worest: = bce. lies soe West Raleigh: .4,2.3..' North Dakota: Dickinson?. Oregon: Eugene........... Quebec: Ste. Anne de Bellevue... Saskatchewan: Regina.....]..... Tennessee: Knoxville...... esas: Malhart:.. ..0s a S cy Zo RAY \ ee LZ WY) = \\N WAS FANN ar Cy %, uy * ~~, XS \S Ss ‘' a Nw SS 4; 0 LANCIEN PLATE 56 Ff ONIN w= ANS S S AY Sew . \\ Wk SS AR Ry \ ‘ NS D at SS ¥ l Ww aS N x) INN \ Ni SS SS kN rl Al | S, ISS Ul rl A | SX N iW I Ws nus Ul i ff 4 ES , | or == WF ru) ” Vell i i } LS. Bi | Lf BN i, fw S9 9 £9 JL wel 8=6Gh LL . | eee Ce ta SRE Ls os AK om S wal Cas ee Sale PLATE™ 57 % oR St oseq oy, ‘ssoutAoid ainystour oy our deur 94} 9PIAIp Soul] [NY "g WuNyjoo ‘TT apqu 2) 0 I I I TAT I 9B WHOL ByBp ‘(TJassnyy UTO.Ly Q-e 10} Son[va Uorye10dvao Aq poprarp sonyea uoryeyIdroo1d [BUIIOW) WOSBES SS9]}SOIJ ODBIOAB JO porsad 10j Son]vA se I eB a 5 ES 3 COUT meeGOL man RZOL) me OOT EE ELTON MOTT (seq) (say Sa10} uaa1310A0 38910} UdeITIOAd 4S91OJ UsI1B IAI Soins 13K SnAqdosaw ussqyioN = 21}Aydosaw wi1ayzION I Y Mt iil bs, Pe] Ce "4 S371" 009 XY. ool ANS RUN TAN ANN \\ AN SN SOAS = t—} — t—\ ee —S — — —- Se ——J or ysero} 313Aydoseur solo ao purysser5 Vee Za { \ it | a | “Pa 4ZB, Zs z Sh %) if cS ~—=. 45 VN — VC SPER SS ae | NW \\S NINAN + RNY ae Ay A SZ KE NAHAS L ARERR ae LAS LES vas AWA a DER ARS S/ FS ESOS. YG Gb Cane CANS VEEN ooo ae was oe OO t no W2EN SO SK ; Gry Vy Gy T lh Ea G7 eg ee 7 : Ss = pr N S SS we \ N NS =a Ze, ZZ 5G ak SS NV SSN SN \ = K Si eM DA A WN ‘J a SX | We \ | i; K ». ENSS s NS We | ti \ i Mil i | " i i ll, L\ l i! MN Hu R MM ie SY ll i i | l) USS ie WS ,_2—Xt Cc >> \ Ls 1 é ANS oO SS ht A = ' SAA CU PRS = Ly 9 ol9 69 dd BLL old OLB ARB ABMS RG GBB JOT OT GOL LOT COT TIT STI OTT LIN GIT GY etl ont, a — 326 ENVIRONMENTAL CONDITIONS. characterized as follows: The arid province extends northward from Mexico and occupies the Great Basin. The semiarid province is shown as a belt lying outside of the arid area, this belt not being wide enough to reach the Pacific at any point, but extending into Canada at the north and also extending eastward, from Nebraska and Oklahoma, in the form of a large lobe that reaches nearly to the Appalachian Mountains. The semihumid province occupies a narrow belt outside of the semiarid one, but not reaching to the Pacific coast (on plates 53 and 55, at least). This belt broadens at the north, extends into Canada, and lies just outside of the great eastern lobe of the semiarid province. It includes much of the Gulf and southern Atlantic region, a feature that is quite unlike the corresponding state of affairs on the precipitation charts. The humid province occupies a small portion of the northwest (reaches southward along the Pacific coast on plate 54, fig. 14), extends into Canada at the north, reenters the United States west of the Great Lakes, and occupies most of Minnesota, Wisconsin, Michigan, New York, and the three northern States of New England. Another portion of the humid province lies along the Atlantic coast, from Massachusetts (or southern New Jersey, see fig. 14) to North Carolina, and a narrow strip appears on the eastern coast of Texas. C. RATIOS OF PRECIPITATION TO EVAPORATION. (1) PRELIMINARY CONSIDERATIONS. Following the lead of Transeau (1905), we have employed the ratio of precipitation to evaporation as the nearest approach that is as yet possible toward an ideal index of the external moisture-relation of plants. Transeau’s introduction of this ratio marked a very definite and important forward step in climatology, which will, no doubt, be- come more thoroughly appreciated as data of evaporation become available. Of course this ratio, like other intensity factors, may be employed in connection with any duration factor or factors that may seem desirable, and we have thus employed it in six different ways. As has been said, however, it is not yet possible to obtain evaporation normals, and all that can be done is to use Russell’s data for the single year, July 1887 to June 1888. Transeau obtained his ratio by divid- ing the normal annual precipitation, for each station considered, by the corresponding total evaporation for Russell’s year of evaporation data. Our various applications of the Transeau ratio will receive attention below. (2) Ratios or NorMAL ToTaL PRECIPITATION, FOR PERIOD OF AVERAGE FROSTLESS SEASON, TO ToraL EVAPORATION AS OBTAINED BY RUSSELL FOR THE SAME PERIOD, FOR THE YEAR Juuy 1887 ro June 1888. P/E. (Tasue 11, Puare 57, Fie. 16.) The two terms of this ratio, for each of the stations included in our list, are given in the second and sixth columns, respectively, of table 11, and theratio values themselves (P/F) aregiven in the eighth column CLIMATIC CONDITIONS OF THE UNITED STATES. 327 of the same table. These ratios are shown graphically on plate 57, where the numbers on the lines represent hundredths. Inspection of table 11 and plate 57 shows that the moisture-ratio values range from 0.04 (Independence, California, and Winnemucca, Nevada) to 1.76 (Cape Hatteras, North Carolina) and to 3.84 (Tatoosh Island, Washington). The highest values are in western Washington and Oregon and the lowest are in the region of the Great Basin and in the arid Southwest. The isoclimatic lines show that the country may be divided into three main zones or regions: (1) an arid zone (ratio values below 0.20), (2) a humid zone (ratio values above 1.00), and (3) an intermediate zone (ratio values between 0.20 and 1.00). The intermediate zone is conveniently subdivided into a semiarid zone (ratio values between 0.20 and 0.60) and a semihumid zone (ratio values between 0.60 and 1.00). There are thus four climatic zones or provinces to be considered, as in the case of our precipitation and evaporation charts. Their limits are denoted by full lines on plate 57. The arid province extends, roughly, from the Rocky and Big Horn Mountains westward to the Cascades, the Sierra Nevadas, and the Coast ranges. It includes a little of southeastern Washington. The semiarid province forms a belt lying west, east, and north of the arid region, extending westward nearly to the coast of Washington and northern Oregon, and to the coast of southern Oregon and Cali- fornia, northward into Canada, and eastward to about the ninety- ninth meridian of west longitude. The semihumid province is shown northwest and east of the semi- arid area. It includes a narrow strip of western Washington and north- western Oregon and extends eastward from the semiarid region to about the ninety-third meridian at north and south, but is broadened to include most of the country in its middle portion. This zone also includes southern peninsular Florida. The western portion of the humid region occupies western Washington and a little of northwestern Oregon, while the eastern portion includes the Gulf coast east of the Mississippi River (excepting extreme southern Florida) and the Atlantic coast north of southern Florida (excepting the northern part of the New Jersey coast and the coast about Boston). It also occupies the country north of middle New England and extends south- ward from Canada to include northern Michigan, northern Wisconsin, and eastern Minnesota. The most interesting special feature of this chart is the enormous eastward enlargement of the semihumid region, which corresponds to a similar eastern lobe evident on the evaporation chart for the period of the average frostless season (plate 53). It should be remarked that a localized portion of the semiarid region here appears to be located in the middle of this enlargement, occupying southern Illinois, Indiana, and Ohio, and northern Kentucky. A similar localized arid areais shown 328 ENVIRONMENTAL CONDITIONS. on plate 53. There is also apparent here a small localized area with ratio values of 1.00 or 1.03, including Topeka, Kansas, and Lamar and Springfield, Missouri, but this deserves no special attention. It is especially interesting to note, as will need to be done also in connec- tion with a number of the following charts, that the line for value 1.00 apparently leaves the mainland in the middle of the New Jersey coast, returns at the western edge of Connecticut, leaves it again south of Boston, and finally reenters at the southwestern extremity of Maine. This brings it about that the eastern portion of the humid zone is divided into two parts, a northern one extending from Minnesota to northern New England and an eastern and southern one extending from about Boston or New York to the Rio Grande and beyond. This feature appears to be an important one, and it will receive more atten- tion later. (3) Ravios or ToTaAL PRECIPITATION FOR PERIOD OF AVERAGE FROSTLESS SEASON, FOR THE Year Jury 1887 to JuNE 1888, To Toran EvAPorRATION AS OBTAINED BY RUSSELL FOR THE SAME PERIOD AND YEAR (P/E). (Tape 18, Puare 58.) The method of deriving the moisture ratios that has just been des- cribed involves the use of normal precipitation data along with evapora- tion data for the single year of Russell’s study. It was thought that this would give to the derived ratios somewhat more of the character of normals than would be the case if both precipitation and evaporation data had been taken for the single year in question, but the novelty and great promise of this climatic ratio render it worth while to present the values obtained by the latter method. These are given in table 18. To make comparison easier, the ratio values from table 11 are here repeated, in the last column. The second column of table 18 gives the total precipitation for the period of the average frostless season, derived for the actual months of Russell’s observations. His 12 months have been treated as though they all pertained to the same calendar year, and our usual method of approximating average frostless season data from monthly data has been employed, the original monthly data of precipitation for the year July 1887 to June 1888 being taken from the Summary by Sections. We term this precipitation value p, to distinguish it from P. The third column gives the corresponding evaporation values (£) for the period of the average frostless season, being taken from the sixth column of our table 11. The fourth column gives the new ratio values (p/EH). When the name of one station is followed by that of another (the latter in parentheses), the evaporation data are for the latter station and the precipitation data are for the former. CLIMATIC CONDITIONS OF THE UNITED STATES. 329 TaBLe 18.—Data of precipitation and evaporation for the period of the average frostless season, for the year of Russell’s observation (July 1887 to June 1888), and corresponding ratios of precipitation to evaporation, together with similar ratios derived by employing normal data of precipitation instead of those for this single year. hu oO 2 mw oO . S836 | $838. gad -a ples sl Be ooo ee SS peso] 2agon Se |e eo P/E. Station. epee | os 27s p/E (From De joe 3 mos table 11.) By SS | Sud" gq Qer Qers S588 | S8S8E Ss ae a eS Qe a & Alabama: in. in. WY EaY OF URE NC GSI.) Vp Rt ae oe 50.68 35.16 1.49 1.36 IMIONLEOMELY (82) co. cseaie cc sx oe sss 34.20 41.90 0.82 0.75 Arizona: MOEEPADACHE! (S)cccs ac cs css ore eees 10.15 36.74 0.27 0.29 HOT rANta(S) Sas cscs kc Sete os ene 20.71 78.91 0.26 0.14 IPTERCOLUNEL) een Glas « eee eee: 8.54 29.72 0.29 0.27 Arkansas: Port smith (47) 2... ... 500s « 36.24 37.01 0.98 0.85 California: HF ERTON (CIA) ert cs da icnck oS ae 8 a ee Srol 56.87 0.06 0.08 WhosrAnwelesGl3) ... 8500 snk oheises « 8.27 34.81 0.24 0.37 Gade lirie CLS) ees miosis se Sisto won 6.88 70.75 0.10 0.17 DACEATHENUON CLO) seca, cras Gelets oler suave e: 4.10 46.38 0.09 0.21 pan Prancisco) (14)... 6. 6.2. 6s oes = 10.53 32.49 0.32 0.48 Colorado: @oloradoiSprings) (7)... 25.6.2 220+ 6s 12.20 30.48 0.40 0.33 PETIV EE) (S) seers sc Stree te Aivietes 8.76 38.92 0.22 0.19 Connecticut: New Haven (105)........ 22.57 20.05 ibyils? yi Florida: eoksonville|(Sa)!. «2. ve eis ees oe 43.57 39.67 LO 1.16 FE VAWVGSU) (SA) orn <:302 co aise o oic.e sess Seis 384.95 51.6 0.68 0.75 BRACOL A (Ss) hike |e he a iehet oe ae ahet 44.45 41.59 1.07 1.08 PIN AUITIY) AG (OSA) oot toss =o cso) 5, telah cials ele Oe - MC PAT eICOY S| See tets. 5 Gos es bieleels } 55.24 28.35 ag —- Georgia: PA TLAIAL SVD (255)) u's eleyes o1e a ole. slate ites « we 44.21 36.95 1.14 0.74 SHNEgpg Corr SK GS) ie Pees eo reeret aes 21.46 35.00 0.60 jie otic MES OLS 22) ose, we hela eto os! is 4.75 43.80 0.11 0.09 Illinois: Ori (GGO) steer a sate re em aesces 16.71 35.85 0.50 0.64 WhieAvo (GA) Ack ope ssi c.ciskebe tle ee ea 18.26 25.94 0.71 0.74 Srosaberurteiks La) eral PPOs oo en ae 20.08 28.26 (Naval 0.74 Indiana: Indianapolis (68)............. 16.15 34.95 0.46 0.63 Iowa: WAGENPOLE (OD)! ) eee c je, ole sakes oa ae 26.71 OH EFT 0.96 0.74 iDeseMoines (53) 2.4.06 sce wee eae 20.96 24.75 0.85 0.89 LO ASSO UTE Ge: 9 etree A POI a ee 27.30 34331 i lhe) WA 0.95 CCK (D3) se a ae ee 20.36 32.70 0.62 0.75 Kansas: IPG COLOIIA (GS) cts A acess eiscisimte Seen 18.62 28.79 0.65 0.70 Wore Giby. (GO). det. e oe wee ee 13.45 36.69 0.37 0.43 SSID (SO) es tose cc icha c, 0) weet et er ene mie 23.30 25.45 0.92 1.01 Kentucky: Louisville (76)............. 26.67 39.19 0.68 0.57 Louisiana: mow Onleans (45))..i 1... eee eases 67.72 40.61 1.67 eon Bireveporti(4G) .')...c5 0s sates eenis sree 33.60 36.96 0.91 0.82 Maine: DAR UDOLE CLOG) fee occas ‘o, o(us tik ame heaps 14.88 13.97 1.06 Lact Portland 10106) iso... aah. cin. sceut eae 10.02 Lgjols 1.05 1.04 Leet ol eee a WE BN sow tid be te | LR Di a se 1 Numbers in parentheses after station names denote the section number, in the Summary by Sections, under which this station is given. 330 ENVIRONMENTAL CONDITIONS. Tas ie 18.—Data of precipitation and evaporation for the period of the average frostless season, for the year of Russell’s observation (July 1887 to June 1888), and corresponding ratios of precipitation to evaporation, together with similar ratios derived by employing normal data of precipitation instead of those for this single year.—Continued. Station. Maryland: Baltimore (95)e 23. icc heen ctete citys Washington, ID) 'C: (94)... - 0a se. Massachusetts: Boston’ (LOS) Srawetis cts, or etdairrae tim eke Nantucket: (lOS) cap tavtessls eur nee so Michigan: AI eH AMGE) is ai cucterc: opetersiniecabctisye te oyecrte WV SirOG (Ga) erect sss vsiein a ote etake eis ene AGT ANGNELAVENIN(G2) ios ote scrcletsastateioss.s ois Mera (Go) eerie, scl tie init erckhtoe. 6 Ore WE aravietber(GUiyl. tases acm sl cratere bake Bue, e> Port Huron (G3) s).clre vis ot oaeataein ee Minnesota: Plt (SS) keene sacke he Me terercl eats ' IMoorhesdi(S 7a sles cise sielerste ait Sree aul CB) edits, chore 5 caskeue ebausteie sie Blo VANCENtD Me. eteite ose ff aralton eon Mississippi: Vicksburg (80)............ Missouri: Warisas Grtyaal yi screcns srehemeeevey sae TiGA VOL WODEN suo uceic'areis eke usualeaveraiel sie. ae Maran (AG) ar ccleidecice sratetce iis ak Sp OUISt my wa aimrentraice coe enero ne vere cale Springtield'(40))5 5. caw. eset eeete ale Montana: Grower Aeeney (26) a5 ueiatcntea terres oe Hort eAssemiboine ro] now table 11.) Bu SS] og sg 225 | a2 ebe S222 | S8E28 fH HH m. in Utah: Salt Lake City (11). . 22.425... 2k 4.32 Slt 0.08 0.13 Vermont: Northfield (105)............. 19.02 12.28 1.55 1.16 Virginia: Ey nchbure (Os) meek wees nies merece 24.95 28.68 0.87 0.87 Norfolk: (G2) eres, Us Pee Mele ook 39.82 27526 1.46 1.24 Washington: Northibesd (UO) acdc coc. cet ecm Morta@anby. setiat «+ o2.0 .cpieedesi mele 43.68 20.18 2.16 1.73 Olivmpia (CUS) orto serasie ches ane evesvent 11.22 19.85 0.56 0.70 Spokane (20) ie .2kt oaes Pewee on eee 11.28 33.80 0.33 0.23 Walla; Walla (20) k.cc.ck cc eee ee cic. see toot 45.87 0.16 0.18 Wisconsin: Green Bay. (G0) ib. sale cee vce 14.57 19.39 0.75 0.86 Via Crosse (OO) ste caciscs ene ls sates 18.67 21.97 0.85 0.94 Milwaukee (GO)Sexinee one ene eoee el T 263. 19.11 0.92 0.87 Wyoming: Cheyenne (24)............. 7.09 32.48 0.22 0.20 The chart of plate 58 was prepared from these ratio values (p/E), the isoclimatic lines representing increments of 0.20. This chart agrees in its essentials with that of plate 57; the great eastern lobe of the semibumid region (values between 0.60 and 1.00) is here again apparent and the localized area of semiarid conditions, within this lobe, is more pronounced than in the former case. The arid region (values below 0.20) is here indicated much as in plate 57, but the line for the value 0.60, in the middle of the country, here swings farther westward at its northern end than in the other case. In general, it appears to make no serious difference which of these two charts is studied, since all their essential features are so nearly alike. Whenever agricultural climatology begins to receive attention in this country, and when evaporation observations are made for the period without frost, it will be desirable to prepare a chart similar to that of plate 58 for every year. Finally, a normal ratio value for each station may be actually obtained, after which the value for any par- ticular station and season may be stated by comparison with the normal for that station, just as is now done in the case of temperature and rainfall. CLIMATIC CONDITIONS OF THE UNITED STATES. 333 (4) Ratios or NorMAL ToTaL PRECIPITATION FOR PERIOD OF AVERAGE FROSTLESS SEASON PLUS PRECEDING 30 Days, To ToTaAL EVAPORATION FOR THE SAME PERIOD, JuLY 1887 | To JuNE 1888 (7/E). (Tasue 11, Pare 59.) This form of the moisture ratio is based on the idea, already men- tioned, that some of the precipitation occurring before the beginning of the frostless season is effective to supply water for plant activities in the early part of that season. The numerator (P) of the first form of ratio has thus been increased, in each case, by adding to it the normal total precipitation for the 30 days just preceding the beginning of the average frostless season. This increased precipitation value has been termed 7, to distinguish it from P and 7, so that the form of ratio here considered becomes 7/EH. The values of 7 and EH, and those of 7/E, are given, for our list of stations, in table 11, the ratio values occupying the last column. The chart based on these ratios is shown as plate 59. The essentials of this chart are so nearly like those shown by plates 57 and 58 that no special comment is here needed. It may be noted, however, that the full line separating the humid from the semihumid zone is here taken as having the value 1.10 instead of 1.00, as in plates 57 and 58. (5) Ratios or Normat Totat ANNUAL PRECIPITATION TO ToTAL ANNUAL EVAPORATION, JULY 1887 To JuNE 1888 (Pg/Eq). (TABLE 15, Puate 60.) These annual ratios are derived by the method employed by Tran- seau. We have termed the normal total annual precipitation P,, and Wa ine a eae ae ii Ke Ss soa Pile Sy Ch Se ee td NY. 2. 307 orf wr os* or 67 85° Fre. 15.—Moisture zonation of the eastern United States, according to precipitation-evaporation ratio (annual), after Transeau. PLATE 58 334 II Ibi ot lay 18) N ie “A r} yas IN ¢ b q ‘ y Hj SSE Fa = an ace oe A) ers 2 ee SSS Seat As \ (cate o6L NU NY ey \ & e NN Ny ASX oS eYL ‘seouraoid ainystou inoy oyu deur oy} aprarp sour [NT uoryeiodvaa Aq Poptalp g-/SgT OJ sonyea woryeyIdro0Id [eur1ou) y, WAS 18 a 2, IM Rees — o&8 itt: 49 FS . yi SS Sere SS apzs i - =all Mee aN NN \ ¥ S Zk Si NI ~ | s S s Se WN NS ESS Re 8 SN SS ¥ NS Ray 2) . 1% Zo 4 wie s t 2. ‘6 i ti Ae a Hi i Pay yi f b . j s y ‘a ras rs ‘NN o: N K = Sys é a S N SS Me : } , \N: AS ENS : Hy I Si? Za UZ Gy S\ ~~ REIT SS x s IS a Abe ~W a K WF WS % LY SRUAS aa OO ) oP al ety b> 4 - 38910} u9013 1949 onAydosaw wayy0N ee \) G ee ba $ ] ) EO ‘Z 940]d st o8Rq ‘p UUINIOd ‘gy 91qu) WoIy BYP { (JjossNYy UNO ‘IveA OUTTS 1OJ SON|BA WOSBOS S89]}S01j OFBIOAV JO poried IOJ saN[BA uorye1odvao-u0ry By Ider g 18910} usaiBI9A9 on AydoiBAy = eased SS With i wt Ww nie eee | y V ‘iil uit ; if mat SS : ee WA N NN I} \ Lp SS PN eS SS ; i “0x i! Seite di A vannr oAeee INS ie ath a @ RS KS SS Rio ake SRA Ge Q »S EN » any, S RWG , Fo. sexy oe ~. Y. pd Y ve, \ Ns Dare cae % Ae rn ro 4 Y Oy a ft as aon a SAS : = WS SW Ol NGG ‘ i rver;/ G Beam te LY Ea nd | RC ees i AS ERGs Ob 26 SR a & . BC) 4 (ie 0 Te) Ge en yee! | eT 3 336 PLATE 60 A ie ay en | “alt fh (ei Ne isis Pp ia ng i pes NS ii 4 z oT, $ ; (fis Dares: A NRT i 4] ae - =, ie eB Sy eee for 1887-8, from Russell); data from The base is plate 2. A CZ gd meee A eee Chey Ce SN ee \ Tr Za > —t a: | : RN SG A eine: hb bi i NENG SW . x 5 LANDNHIE OR OTT IT RG NNNS = °K Zs CB ESC or so” V4 a by ys" t ih ‘5 i Lee = TRON S AEA RNG ee Dg eh ere “Al TAN divided by evaporation values 97° r, 95 ———— Re NAS Sl Pe | - s Pe 9 NEELYS AS x ~ ha 05° 108 10% 99° 97° 95 «93 (OF cha Be Bei oe S He : & |. AS Nes. & LOSS IE Ee aa: a y ra z a 109” 107° 105” 10% 5 leas Ret alae bi ration values for year (normal precipitation values ~*~ AN SAY Southeastern mesophytic forest Northern mesophytic wre forest West) 119° 117° 115° «118° 111 109° 107? 1 PF) Se - a Py Weeks ee hygrophytic evergreen forest t North western 123° 121° Precipitation-evapo wy «6127? «(125° Grassland deciduous forest transition column 4. Full lines divide the map into four moisture provinces. table 15, PLATE 61 ‘ *z, oye] st oseqoyy, ‘seoutAosd anoj oju1 deur oy) oprArp soul] |My *2 WUINTOO ‘CT 91qvy MOI; BYEp | ([JossNyY UOJ “Aggy ouNL puv JSgT qsn3ny pue Ajng Joy sonyea uoyerodeae Aq poprarp sonyea uoryezidrooid [VULIOU) SYJUOUT ASUIUINS 9914} IOJ SonpBA woneiodvad-uoryeyIdiatg 38910} Ued1B13A9 uolIsSuey 3Sel0} u9018aA2 on Aydoss Ay yse10} snonpisep onAydoseu wreyION T uja}seMyLION ~ pue[sseisy | 001 | 20s 48010} 21yAydosaw 38910} Wra}seayINOS snonpi99q oN AW NS ss . ONS SNR sat : NO RWSNZ 7 se Lo \ . Wt Key “LTE SSeR Os \ i z eas ’ Q as e aay ma Gat R N N: a : FM . S| ‘SN iy NY ; WY as AN ee aor ie age Yyy : | of Ea Z AN a ; yso10} yAqdosour S : ee BOS : wu . Wy he \; - Ze \) cw eo. Vi ae a S SNS y. iy Z t RW Ni \ \\ \ \ _-yeo104 NN RSS hy WY 2 Paw sw ROY % POS Ws ; ydosaw wiaywoN ae ye A, KZ iN 4Seayjnos snonpiseq KRNNNAA AAR ANN Waa AN N a AN NS % 4 1H a, MY PLATE 62 A é is A Weed IN ney i Yay low, } A P yee f aa RY Oe Dy ae fi ; x3 - a h WSN YY f \ . fie oN Say C oe e * Xi ‘ is ' Ty, A NON eee Yt tite. Be ee f RERKVERRSERRS WS? i's PQ Ga we SY \\ ‘ ii ! or we », KO WH NY N S See ‘i rm ra in) ti il Ah i = i l et (0 xt Is ail Si SOT] Ty Saal ) ae-- le! | ~ » 5 ze V : iy CARS f : tes i seca i py ee matse on 4 4 % ts y eee SR eee Be £ CAG, % OE. " a web ett c LIT SOR at Ns — 5 nah A if a Y Y LA aA ONS e od 1: =| 7 if ‘ “oG f Ys 25075 aiater Satie: 5 eee, rs le fs } ¥ = é COL OPALZ ie RTS fest, a £D Z fe 5! ~ OM LLe7. weeteanene ieteg 2 ; - 9¢07 é J d Sooes eee Z RAZZ Z, 7 “ £ , iZ en ae to PAA retains Sat tes 4 a 4 ARALAOES RODS ELS ee - at} = . y ~ A VAAL : seats aor ecetete| oceans a ry . = ee 9 ks Y, i S . x _ et 2 J) | = Cx ea = (CARs NS f Cn ok ohh ol ° ‘ ° se - 26h 6S PLATE 63 O18 SONTA [VOTIOUIN NY ° o> \A \ " eae. aa : Pihale GRYEnN L = Ne SI . KS Ov ye } AY ONE es) é Se kN ‘Z 978 /d SI oseq ayy, ‘Your ue jo syyporpuny "9g WUINOO ‘GT 91GB} UOT BYUp S 2A COR WOSBOS SSI]}SOIJ AFvIDAG OY} JO potrod Oy somnssoid 1odea snoonbe uvour eUION 16910} UsoIBI9Ae (388) on AydossAy 48910} uso1BEA2 dAydosem wieyj0K) 389103 snonpijeq Winns Ks WN . SY Ys SW . rN WY. \ NAS i ARLE > » WROD) \ ~ ) i\} il Hs (it “all tine ds 5 AB 88 = 8 LB AN Oy Sey, { Il IN 68 \S Nes x : S a SLA R 3 Ni AN SAS SS SS tt a J SS cS Mes l ip SS SS i | MUU LS IWS ae SM ih a LS S 8 Ss BSS hey) AC SR * I 1: ey Qi Atlerste hall. onl. , id St > {D3 ° Ge 2 3 % pee \ | DB ™ o 342 ENVIRONMENTAL CONDITIONS. on the other plates. The separation of this eastern humid region into a northern and a southern portion, indicated on all the ratio charts, is here much more pronounced than in any other case. The northern portion is here indicated as occupying eastern Maine, while the south- ern portion extends from eastern Texas to southern Maryland, reach- ing farther inland than in any of the other cases, especially at its southeastern extremity, where its great northern lobe reaches southern Minnesota. This lobe appears here like an exaggeration of the similar but smaller lobe shown on the chart of plate 60. It is thus seen that plate 61 differs from the other ratio charts only in relatively minor details, the main essentials being about the same. (8) CONCLUSIONS FROM THE STUDY OF THE PRECIPITATION-EVAPORATION Ratios. The results of our study of the various forms of Transeau ratios above described lead clearly to the conclusion, which is in full agree- ment with that reached by Transeau from his first use of this ratio, 10” 107° =e ; pase Te rE | Shell | if tI $I LATS mina rT N Keays <{ niall, Bi eee ue NEA CHART A | Jee raise anne Bava! TS VX ig Cee Tl IWEZWA ees AN a ES | ye (1A a LL Ap es | ‘a . q : eS! Wa a 5 4 3 Fic. 16.—Moisture zonation, according to precipitation-evaporation ratio (period of average frostless season). Moisture provinces: Humid, more than 100; semihumid, 60 to 100; semi- arid, 20 to 60; arid, less than 20. (See also Plate 57.) that we have here a climatic index by means of which the zonation of the country with regard to moisture conditions may be clearly shown. As will receive emphasis later, the climatic zonation thus brought out is closely paralleled by certain prominent features of the zonation of vegetation types, and there is little room to doubt that this division of the country into moisture-ratio provinces will be «f very great value in the study of climatology with reference to agric::'’) re and forestry. CLIMATIC CONDITIONS OF THE UNITED STATES. 343 Our charts represent the country as divided into climatic provinces, which may be conveniently considered once more as (1) humid, (2) semihumid, (3) semiarid, and (4) arid. The following general descrip- tion of these areas will serve to summarize the descriptions of the separate charts. Reference may be made to figure 16, which is de- rived from plate 57. Three humid areas are apparent, one in the Pacific Northwest, a second in the region of the Great Lakes and northern New England, and a third in the Southeast along the Gulf of Mexico and the Atlantic as far north as northern New Jersey or southern New England. The last two humid provinces are nearly continuous, and may be repre- sented as such by charts drawn from certain forms of moisture ratio or from data of certain years. Our plate 60 shows them as continuous by a narrow belt that embraces about the southern half of Delaware and the eastern half of New Jersey. The two semihumid areas are irregular in shape. The eastern one occupies all of the country not included in the humid area and east of about the one-hundredth meridian of west longitude. The western semihumid area occupies a rather narrow strip of country east of the western humid province and extending southward along the Pacific, south of that province, to about the middle of the California coast. These two areas are almost surely joined at the north, in Canada, though this point is not actually demonstrated by any of our data. The semiarid area appears to be made up of an eastern and a western portion, joined together at the north. The eastern portion extends, approximately, from the one-hundredth meridian westward to the Rocky Mountains and to the San Francisco Mountains of the Arizona Plateau. Its eastern boundary appears to cross the Rio Grande some- what southeast of El Paso. In its northern part this eastern semiarid region apparently broadens westward and extends through the lower passes of the mountains to Washington, where it joins the western portion of the same region. This western portion lies east of the western semihumid area as above described and extends southward to the Mexican boundary, along the coast of southern California. From Washington to Lower California it is a rather narrow belt. The arid area occupies the intermontane region of the West, extend- ing from the Rocky Mountains to the Sierras and from southern Cali- fornia and Arizona to Idaho and southwestern Washington. It is thus surrounded, on three sides within the United States, by the belt of the semiarid area. Of course, it is understood that all the higher mountain ranges are to be considered as belonging to the humid area, at least in their upper portions, and that such high mountain masses are surely each bordered by a zone of semihumid conditions when they lie in an arid area. No attempt is here made to consider the innumerable small areas of 344 ENVIRONMENTAL CONDITIONS. humid, semihumid, and semiarid conditions included in the arid, semiarid, etc., areas as above described. A detailed moisture-ratio chart of a single county in Arizona or Nevada would doubtless be vastly more complicated than is any of our general charts for the entire country. D. AQUEOUS-VAPOR PRESSURE. (1) PRELIMINARY CONSIDERATIONS. The pressure of aqueous vapor in the air should be an index of the relative influence of the air (aside from its rate of movement) toward the retardation of evaporation from wet surfaces. Roughly, this index should be somewhat nearly inversely proportional to the evapora- ting power of the air, disregarding the wind factor. This is, therefore, a climatic dimension the measurement of which should be valuable in studies of the relations between plant activities and environmental conditions. The vapor-pressure data for the United States, for the years 1873 to 1905, have been reduced to a system of homogeneous monthly and annual means by Bigelow (Bull. 8., 1909), and these means have fur- nished the basis for our work in this connection. We have employed (1) the normal mean vapor-pressure for the period of the average frost- less season, and (2) the normal mean annual vapor pressure. (2) Norma, Mean AQueEous-VAPOR PRESSURES FOR PERIOD OF AVERAGE FROSTLESS SEASON (TABLE 19, PLATE 63.) The indices here employed were derived from the monthly means of Bigelow, in the way already described for such cases. Our results are given in the last column of table 19, and plate 63 represents them graphically. TABLE 19.—Normal mean relative humidities, for the year and for the period of the average frostless season, mean relative humidities for the three summer months, 1908, and normal mean vapor-pressures for the year and for the period of the average frostless season. Mean relative humidity. Mean vapor-pressure of water. Station. For period | For June, For period eee of average | July, and Aeeuaseh of average , te frostless August - ’ frostless season. 1908. season. Alabama: p. cl. p. ct. p. ct. inch. inch. Bittainwharm soe cave theta std] sik whe Lethe eae a 76 ..) lint.cee 4. bles eee La] Nye ORG O ROeee PE CRE,» 80.9 80.6 78 0.530 0.599 DIONUZOMEry «tas soe es ccs 72.4 71.3 71 .457 .553 Arizona: LV VTd- 11:41 (ane he SPN ee TEMP CPC NG [OT ESE eR yy RISERS POA 64... |. Skawiee ss saree IPHOROIE ve dies A wate ore bine cle 38.7 36.3 35 278 308 Wa lieel (Ay COE Moe BRAT Reh» ADT SIE Sida She ths 45 334. | .\. ese Arkansas: Hort SMI, hac ee lees ten 70.6 70.8 75 399 522 Littl Hoes ts tase bt seen se 72.8 72.4 74 424 539 CLIMATIC CONDITIONS OF THE UNITED STATES. 345 TaBLE 19.—Normal mean relative humidities, for the year and for the period of the average frostless season, mean relative humidities for the three summer months, 1918, and normal mean vapor-pressures for the year and for the period of the average frostless season.— Continued. Mean vapor-pressure Mean relative humidity. of water. Station. For period | For June, For period of average | July, and of average Annual. frostless August Annual. frostless season. 1908. season. California: p. ct. p. ct. p. ct inch. inch MIRE Keeeee sd biol Es sacs 86.8 87.5 89 a2 348 HREERTIO Me yspcdse ee apie a sicnasck hee 48.9 30 271 279 Imdependence: ....%. 2... 42%:.. 29.7 22.6 BOAO! is cctv cthe eaten ce AOBTAN PELE 73/6 bbs adi ace C7. (222 74 367 378 ROCIBES INTE oe aveere oie oneuneehel siete 48.1 49.9 36 281 296 MACTAMENtOMW: «2 =< -- acts. 66.7 62.8 55 .324 347 SET] OS GT.20 ae en CdS ey Met See Ge 3 ae 73 Pate Ten apenas, aes SIAM MEAT CIACO «cis yo5 icici eier< es 79.9 80.0 83 340 348 San Duis! ObISpo:.. 5 ......eeic.o 69.0 69.9 Zirh 314 335 Colorado: Dieter aie Se SrA eae 48.7 46.7 54 185 285 TD Tae RO 5 15 RR RAT cn eM 7 ah, 5, Rea ne 5S SA een comer (wa De SE TARGEIIMCHON 6) o/s 0552) Se ees ote As Le chs BE SOK a sa illexepers ayes ecard Rega ep cas LETGiol ee See Seen aoe 48.1 45.8 47 Mais, 274 Connecticut: LE EATH hia eo [a ee eed Cote] | aerate enter (Fa 0 ae Ce eee ee ay 2 he LIGAT Aa 8 02) ee a 75.9 77.6 72 298 447 Florida: Paeksonivallers. jo/275) 72.6 71.9 75 343 495 Montana: Ea VTG ae ae es dias we nie es 66.4 56.7 62 186 308 PT OLeN a ae Mere isiacrcths conte merareie 56.1 48.0 56 165 249 LASCAT yofc| MU aes Paice Pam ric oc ke RCC On Reecechc Grito (;) Re SIE (eco cte e Nines Cxtyie tess ssw icae ararteene.s 69.7 57.8 59 223 367 Nebraska: UTIGO Mestre oo ck Uap usa donts tise acess 70.0 66.9 73 ilivweteealOcle as ae North Platters: sees. cisco 66.3 65.3 69 250 421 mana see ssc ae eles se 69.2 66.4 71 289 463 RY RIGHT Weer. iccacta elie nials:a! 66.5 63.8 67 222 386 Nevada: MOBO SolUU es y oak ne e's Stam sk 50.2 39.6 LAO) 1a eine ere . PISTON Er tates isha Scie ire. Calliskuse S cuers ee oLdtctcreia stomras 28. [wwaDoahyls la oe eee | DV EMTIGIA UI OUR cc ie'.cts cues sa > ko 46.5 30.4 45 .148 .183 | New Jersey: AMlemitiG: CHG << 2s-o.0 ar itp a 81.4 82.3 78 . 356 491 New Mexico: RRR aey GL ne ance el ck ata e all ob ivinselets brs, | eitane Sl aterars GB ea vk vee ee ic ORR AUCH Odea: ci occikes Sepia 45.4 41.5 50 .169 . 233 1 Reno instead of Carson. CLIMATIC CONDITIONS OF THE UNITED STATES. 347 TasieE 19.—Normal mean relative humidities, for the year and for the period of the average frostless season, mean relative humidities for the three summer months, 1908, and normal mean vapor-pressures for the year and for the period of the average frostless season.— Continued. Mean relative humidity. Mean vapor-pressure of water. Station. For period | For June, For period of average |} July, and of average Annual. frostless August Annual. frostless season. 1908. season. New York: p. ct. p. ct. p. ct. inch. inch. JUS Ro OSCE Es 3 Se 76.2 74.0 68 284 405 Bitaloemitas oe oekc weds ee 73.4 Ae 72 271 414 New. ¥ Oke nits sSxcjec)ccstte ses 73.2 fous 69 314 433 OSWELOM MEE. sin stools cc oheth ss « 75.8 73.4 72 270 408 RROCHOSLET Nes. ete sis. oie! cdetoore 73.4 70.4 67 267 410 North Carolina: PSHE VAN ry eater ea ci eat cs shcte are, cals | a RA AS Wiel Raats aiate munee SB clits caterer pe cetera Meth AU aArlOsteeeiecins. casio s cco oe (fker Wes 78 382 500 taht erasers. aoc vers bien 82.9 82.7 86 484 573 Batty naw: so. 8 vo.<-oRjeise ¢ LUE CCIM I Rtrens Hin 2.0 Ro | eee Iota [OO crac onal SS istoiee oie oe Teed he Ee 73.8 74.6 78 394 527 Wilmington 36.352 ccs acc 80.1 80.9 82 471 588 North Dakota: Bersrnicinclk tole coves as oa eae osc 69.9 65.8 64 206 377 WM errr See a iacsiae sesve c cuaeele Soil Grtatoiertt aay cleirals ea ee GBs UAiksrecees, Cx, shawsl tenons PRU SAIS UOT etek svc enw evera's'epeta ons 68.8 59.7 60 184 334 Ohio: MOTI CITITIAGL ho ayo vavs 4 6 ausietes « 69.0 65.8 60 .325 459 Wleveland sae =: ~ oso0 cele ss 72.9 70.9 68 . 290 414 SP OLUMBUA S26 cise Siete ee eiieds, 6 72.6 68.9 66 305 442 PETIA KCY?: = Er t ¢ YO , Py . . KO QED eS “ 20 \ x . WS x oi 4) x A e + Yh \ PES Z NS: \ SY nO N NZ N \ AS SI Ri i ty fe VSR | ae ZG: SS N . f--} _} RY ms 2) | Mell, Wie ssi pe © ox Va i yy Ha Ife MT a2! 4 i. to J : | y 2 | > DS e 5 a \ — ony el : e Hi \3 Wetans A NS & ° ob L I 302 "71 of ° ‘Z ayeid stoseq oy, ‘soouraosd Ayrpromny anoj oyar deur oy} oprArp sour 12 *g WuuNoo “GT o[qe} WO VYVp SMOsvoS sso]}sO1y OBv.AOAG Jo ported 10y AyIpruMy We savor WROUT [eur10u Surjuosoida. so8vyu00I10g ° ° ° ove ° ° : i ° ° o o' of o! ) am) _ -- en) ' | (seq) (S989) 1S910} udeI310A0 contsuey an : a E 04 48910} uaais1en0 S910] usaIZ 1000 ay AydosBAy 4S0J0} snonpldep j : 9 i ayAydosow wseywoN = AY w19}sa. -purjsseiy $371 009 EP Ses asia ‘, an Fi Wi a \ “ >) enone. pugjssess \\ ty) ee f N\ . NY YY BA KX 7 V4 Wace ate EEN pV] Bs. 4 iy 47/47% \ WHO % \Y \\\ Y ( \ NY ‘ 2 YS \) S Dr \' ~ yy h \ \\ \\ . We S\ AY N . “yf Y g 277770 es : . . x ? >i \ SS \ f ea y YW, 7 ZA/ 2 \ SA SK Sed) 5 \y te é RCS 4 Ry Oy VSS 7 1 i 4 Pe Ww i fe $ ‘ ¥ Uy 77 a < \ d : AX) i) csi a SY REN le £8 TS YSN PLATE 65 LAR Ss ad me 2 A 4, DEE, Le SEE rad 2 h “a nS ‘4 Vip Aes 77, 3 : AA, f. , a a >, Ce \ WME De ,, Ae Ld 4 S VC st tY d z ify WW y ds H [7 > y ye yy Zp Za) S 4 A 4 ‘ wi RQ BE GUM OO ps SSA eee te WY Nearer try “Ss So | s Ty ©) peer sve genpal RENN vA yea e, i SS ~ A PA ele xX} MNS il in : j i Pu mI ZA a — ym is # | te Y Lal —== ( ee } = : < Sy akc IA . o gh, 5 | I] | i il ="\iall | (37 i: te; —{ ee J6 686 354 ENVIRONMENTAL CONDITIONS. plates 57 to 62. This relative-humidity chart also agrees, in general, with the moisture-ratio charts just mentioned, by showing an east- ward projection of the semihumid area from Nebraska to New England. The eastern humid area occupies the Gulf coast from the Rio Grande eastward, practically all of the Atlantic coast (excepting northern New Jersey and southeastern New York), northern New England, northern New York, and northern Michigan. In its northeastern portion this chart resembles that of plate 61 more than any of the other moisture-ratio charts. (3) PERCENTAGES REPRESENTING NoRMAL MEAN RELATIVE AIR HuMIDITY FOR THE YEAR. (TABLE 19, PLATE 66.) Our data for this index are taken directly from Stockman’s table. They are reproduced in the second column of table 19, and are repre- sented by the chart of plate 66. This chart also shows a pronounced general agreement with our charts for the Transeau moisture ratios. The general zonation of the country is again shown by broad lines, but the lines for the value 65 on plate 65 are here represented by those for the value 70. The semi- arid region is here thus characterized by values between 50 and 70, and the values for the semihumid region lie between 70 and 75. The line for the value 72 has been added in the central part of the country, to emphasize the eastern lobe of the semihumid area (shown also on plates 57 to 61 and on plate 65). An area of values a little below 70 is Shown about St. Louis, Louisville, and Indianapolis, thus differing from plate 65, but agreeing, in general, with plates 57 to 61. The eastern humid region is here shown as much like that of plate 65, but the line for 75 here lies considerably farther south in the regions of the Great Lakes and of New England. In this respect plate 66 resembles plates 57 to 60 more than plates 61 and 65. (4) PercENTAGES REPRESENTING MEAN RELATIVE AIR HuMIDITY FOR JUNE, JULY, AND AUGUST 1908. (Tasie 19, Pate 67.) The data for these indices were obtained from the Monthly Weather Review for 1908, the means for the three separate months being simply averaged in each case. The results are given in the fourth column of table 19 and are charted in the usual way in plate 67. This chart has the lines for the index values 50, 70, and 75 repre- sented as full lines, to exhibit the general zonation of the country, just as in the case of plate 66. The northwestern humid area (values above 75) here extends southward, along the Pacific coast, nearly to Los Angeles, California, and the corresponding semihumid area reaches to the Mexican boundary. The arid area is depicted in much the same way as on the other two charts of relative humidity, but is not quite as extensive here as in the other cases, this being due, no doubt, to the effect of the characteristic summer rains in western Texas and New CLIMATIC CONDITIONS OF THE UNITED STATES. oa Mexico. Instead of completely surrounding the Rocky mountains, as in the other two charts, this zone extends eastward only to these mountains, but there is here a small local area of values of 50 or below, indicated by the data of Santa Fe, New Mexico, and Pueblo, Colorado. The eastern limit of the semiarid region (70) is here placed about the 99th meridian and this line has nearly a north-south direction. The region here called semihumid, in the east, is somewhat restricted in this case, but shows a very large eastern lobe, reaching from Nebraska to Maine. The localized area of semiarid conditions within this lobe is here very large, extending from Minnesota and Illinois to Maine. It will be remembered that the area in question is not shown at all on the humidity chart for the period of the average frostless season (plate 65) and is represented as much smaller on that for the year (plate 66). The eastern humid region is here exceptionally wide in the southeast and is narrowed at the north, somewhat as on the chart for the moisture ratio for the year (plate 60). This chart of relative-humidity values for the three summer months of 1908 was prepared for comparison with the chart of evaporation for a similar period of the same year (plate 56). The stations for the 1908 - series were not numerous enough to give a satisfactory chart, but these two charts agree, in a general way, in showing the great eastern exten- sion of the region of semihumid and semiarid summer conditions, from Oklahoma to northern Michigan and New England. For details, the two charts can not be compared. (5) GENERALIZATIONS FROM THE THREE CHARTS OF RELATIVE Humipity VALUES. (PuateEs 65 To 67, AND Fic. 17.) The main points brought out by our study of relative humidity values may be briefly summarized as follows, reference being made to figure 17, which is derived from plate 65. The country may be divided, on the basis of relative humidity—as on that of other moisture features—into four humidity provinces, which may be termed arid, semiarid, semi- humid, and humid. The humid relative humidity province occupies: (1) western Wash- ington and Oregon and a variable portion of the California coast region (depending on the form of index employed); (2) the whole of the Gulf coast region and that of the Atlantic coast as far north as Long Island or Massachusetts; (3) northern New England and New York and a variable portion of Michigan, Wisconsin, and Minnesota. It appears probable that portions (1) and (3), as just defined, are con- tinuous through Canada, so as to form a single northern humid region. The southeastern portion (2) appears to be quite cut off from the northern one, but only by a narrow neck. The semihumid province lies just interior to the humid one, being very narrow in the Northwest and West. The eastern and western portions of this zone, as shown on PLATE 66 306 *z oynid stoseq oy, ‘soouraoid Ayrpromy MO} FUT deur oy} OPLAIP Soul] [[NJ *Z WuANjoo “GT o[qB} Woy BYP ‘rvoX soy AJIPAMMY OANVepoI Weou [eUIOU ZuTuoserdos sodezU90I0g ol8 68 16 386 6 lb 66 = JON &Ol 901 LO 601 ol ll fll ofl, lll bu — ; GS59M) 48010} us91B. 1949 uorsuesy on AydosB Ay 78010) snonpisep Wa SaMyION -purjsses5) Ps f Baa a tid § jp: Yj ra WE wis == EN og sag - a ssastelclin RA ; > deol be Nt Wey WW SGZ7 WN =—. WRAY TS WWOQy — QA . ih ~S NN SN \ Ser aN : eC weal mit Dy er rs N S iS aw 39 S y = x aS RS AGE LeSN PRES NARAGN- See e Oe A GL / SR ORR aH Pee Se Va fd a oe eS 1 io. SON SY ; alin \ 4 L KEY OOS NSOBIEZ SAN SY YU: GY, eh YEA 1 5 7 0 ime i D A oH ( af 4 oy ‘ 4 7A y J i 3 pags se 43.724 “ a 4 . ose = dg| 3 |2°3 43| 4 |e" Station. 5 q 2B Sthyon. 2 q Sy BG] Fy [ses 3G] 93 [Be © oo ) AS © 2° rs) 33 > ir) to S38 +> ns os ao by Si, 2 ao BS ty 3 MO) SS ISSE wo) 86 |B 3 5) > > Ho Oo 8 5m fae = < < ss} < <= Alabama: feet. | miles. | miles. || lowa: feet. | miles. | miles. Birmingham............ 48 7.4 6.7. Charles City s..02 005% oe 8 7.4 6.2 VEO DLC sig cereie re ets a 106 7.4 7.2 Davenport teers sides ice 79 8.2 7.3 IMONTCOMECNY sor gee per 112 6.1 5.8 Des Moines............- . 98 8.3 he ¢ Arizona: Mubuque.. s0i.en bere data 115 6.6 6.0 Ml agatati ae sissy oe testes 57 7.4 6.7 ieo kik. te eta ene ars 79 7.5 6.8 | PROEDIK Se 6 sis,cte.2 ie tose Be 56 4.3 4.5 SloUssCley = eert scat 164 12°32 11.4 VW ume hfe sage es eee. 58 Go. ALES Kansas: Arkansas: COnCOrdiae® histo ride nese 50 7.6 re MOLG UNM a tie ace saclay terecas 94 ats 7.0 DOdRG dank se cee Boor ee 51 i ie 4 11.5 Little Rok: ke sect: 147 733 627 Wichita: ck Sah ate sae 121 9.3 8.8 California: Kentucky: ; NYG See ecobic ce beeen oir 88 6.6 6.6 bexington'... . +... - .<2see 102 10.8 9.0 Bremen ta). chan kitonies 70 5.6 6.1 Towisvalle.c a4 od 2 oe tae 132 7.9 6.9 Hos Angeles’) ...2 53sec s | LOM 4.5 4.5 || Louisiana: Point Reyes Light....... 18 LOW Flin Access New Orleans... ..0..-.~- 121 8.5 8.3 Red Blut Nas. eee ere 56 5.8 5.5 Shreveport. oi /..0sth eee 74 6.8 6.4 Sacramento. siren ae. ala lire 8.2 8.4 || Maine: San, DiegOs snitch cenc 102 ye | meted 6 MaSGDOLts.o ¢ sees « ecto, ee 85 11.0 BT San Francisco........... 204 9.7 10.1 Pontland st. hil, sek cee 117 8.5 7.8 San Luis Obispo......... 54 522 5.2 || Maryland: 8S: By Parrallom..i1s.0% «i 48 T5eOulew a os ce Baltimore's ..2.o.5. series 11S 6.8 6.5 | Colorado: Washington, D. C........ 85 6.7 6.0 IB Tasks) ee ee, Se io emerge 172 7.8 7.4 || Massachusetts: PWTADEO A272 Soc ti soln re 56 ars 5.8 (Bastons. ox sas wists «alee 183 10.8 9.8 Grand Junction......... 51 5.0 5.6 Nantucket ii > a < WAN! conan SS WESSS = Gyan Gai TSS — On MOP wee, Oe i = y aS oR Sn) SS: Ne GRY —~., NS 5 rt IAS Rs Le +. Le . SS > J s D ae SS Ce . 4 Be = | ie y a [| Sy) “Ree SPS e t | ome J) ss A Nama NS i 99 of 69 AL CL G2 ptt BE JS 88 98 ob Cy ——l = 366 ENVIRONMENTAL CONDITIONS. The bolometer and the pyrheliometer seem not yet to have been em- ployed in climatological comparisons and the initial cost of these instruments, as well as the labor required in obtaining readings there- from, make it probable that they will not, in their present forms at least, be of much climatological value. It must be remembered that, for such studies as we are considering, the operation of the instruments must be simple, the results must be satisfactory, and the cost must be low. Livingston' has discussed and compared various other simple forms of instruments intended for measuring light intensity, including the Hicks solar radio-integrator, two forms of actinometer employing photographic paper, and his own radio-atmometer. Reference should be made to his paper for what little has been done by way of comparing the readings obtained from these instruments with the corresponding degrees of light influence upon plant transpiration. This extremely important subject is deserving of much further study. The Hicks instrument is faulty in its theoretical conception, in several ways; its readings are as much determined by air-temperature as by the intensity of the light which it aims to measure, and they are also greatly influenced by the changing amounts of liquid in the exposed bulb, in the shaded reservoir, ete. The various forms of photographic-paper actinometers, such as the Wynne photographic exposure meter, the Clements ‘‘ photometer,’ and the instrument employed by Wiesner’ are all valuable in measur- ing and comparing sunlight intensities with reference to their possible photochemical effect upon the particular paper or film employed, but they show nothing in regard to the corresponding possible photo- synthetic or evaporational influence upon plants, since the photosyn- thetic process in plants has nothing essentially in common with the photochemical alteration of silver salts, excepting that both are photochemical, and transpiration has nothing at all in common with the photographic process. . Livingston’s radio-atmometer has been greatly improved since its first description,* and it seems probable that this instrument may prove to be of very great value in climatology as well as in plant physiology, ecology, agriculture, etc., but its general employment in such lines of study is yet to be accomplished. The most improved form of the radio-atmometer consists of two spherical porous-cup atmometers, one of the spheres being black and the other white, but the two being otherwisealike. Theseareseparately mounted and are operated side by side in the location where the light 1 Livingston, (191la). See also, for the best sunshine records yet available, Briggs and Shantz, 1916. 2 Clements, F. E., Research Methods in Ecology, Lincoln, Nebraska, 1905. 3 Wiesner, J., Der Lichtgenuss der Pflanzen, Leipzig, 1907. 4 Livingston, 1915, b. CLIMATIC CONDITIONS OF THE UNITED STATES. 367 intensity is to be measured. The white surface absorbs but little radiant energy, while the black one absorbs a large proportion of the sunlight that reaches it. Both instruments are similarly affected by alterations in the evaporating power of the air, due to whatever cause, and the difference between their losses for any given time period is the amount of water vaporized on account of the energy absorbed by the black surface, over and above what is absorbed by the white one. This difference is thus an approximate measure of the radiation inten- sity for the given period, as this might accelerate evaporation from moist exposed surfaces of the kind here employed. The instrument is calculated to integrate the effects of sunshine throughout the time period that occurs between the readings, and it is exceedingly sensitive to relatively weak light intensities, so that it can give a wide range of rates. The period of exposure may of course be made of any conven- ient length. Attention should be called to the fact that the readings are to be interpreted as the time-rates of work done in vaporizing water from the standard surface, so that it thus becomes possible to consider sunshine, from the climatological point of view, as to its power to do the work of accelerating evaporation from the instrument. For a simple term to denote this power we may use ‘‘the evaporating power of the sunshine.’”’ No doubt this expression can be shown to be faulty in certain respects, if interest seems to lie in such a direction, but until the very important climatological factor of light intensity begins to attract serious attention it makes little difference in what terms we emphasize its present neglect and its great importance. The sunshine records of the United States Weather Bureau consist of observations on the number of hours of sunshine occurring each day at each of the stations provided with the Marvin sunshine recorder. This instrument is virtually a differential thermometer, having two bulbs, the surface of one being a very good reflector and that of the other being blackened. The automatic recording device records the time periods when the blackened bulb has a temperature higher than that of the other bulb by more than a certain small amount. During periods of sunshine these two temperatures differ in this way. It is thus seen that the instrument is not calculated to give any information regarding comparative intensities of the impinging or absorbed radiant energy. It simply records for each day the amount of time when the sunshine was intense enough to produce the stated difference between the temperatures of the two bulbs. While this recorder leaves much to be desired, its records are probably more valuable than are periodic ocular observations of the amount of cloudiness during daylight hours.’ 1Tn this connection see F. T. McLean, A preliminary study of climatic conditions in Maryland as related to the growth of soy-bean seedlings, Physiol. Res. 2: 129-208, 1917. See also: F. Merrill Hildebrandt, A method for approximating sunshine intensity from ocular observa- tions of cloudiness, Johns Hopkins Univ. Cire., March, 1917, pp. 205-208.—Idem, 1921. 368 ENVIRONMENTAL CONDITIONS. From the records of the Marvin sunshine recorder, the United States - Weather Bureau has derived data of the normal number of hours of sunshine, for each day of the year, for each of the stations included in the sunshine study. These numbers are expressed as percentages of the possible daily hours of sunshine in each case, the possible number of hours being, for each day and station, the number of hours between sunrise and sunset. Through the kindness of Professor P. C. Day, of the United States Weather Bureau, we have been able to obtain these data of the percentage of possible hours of sunshine for each month and for 57 stations in the United States, and these have been employed as basis for our sunshine computations. From the present point of view the percentage of possible hours of sunshine is quite without interest; what affects plant activities is, of course, simply the amount of sunshine, and, if we consider this in terms of hours of light intensity above the threshold of the sunshine recorder, it is the actual number of hours of sunshine which should attract our attention. Our first step was, then, to calculate the normal number of hours of sunshine for each month, in each case. This was done by finding the number of possible hours of sunshine for the latitude of each station and for each month included in the period of the average frostless season, from Marvin’s Sunshine Tables,’ and then multiplying this number by the corresponding percentage of the possible, con- sidered as hundredths. The next step was to sum the numbers thus obtained for all whole months occurring in the average frostless season for the station in question, and to add to this sum quantities calculated to represent the fractions of a month with which the average frostless season generally begins and ends. The final sum represents the normal number of hours of sunshine occurring in the period of the average frostless season, for the particular station in question. The sums thus obtained are given in table 21, the summations being plotted on a chart, with isoclimatic lines drawn in the usual way. The chart is given as plate 69. It is obvious, from the small number of stations for which data are available, that this chart is very crude and super- ficial. Nevertheless, a rational and self-consistent arrangement of cli- matic zones is here brought out, and this zonation is very similar to that based on temperature conditions. The stations receiving the most sun- shine (measured in terms of hours by the Marvin recorder) are in the extreme Southwest, while those receiving the least lie near the northern boundary of the country or in the mountain regions. The lines of the western portion of this chart are shown as distinct from the rest, to suggest the greater uncertainty with which they have been placed. 1 Marvin, C. F., Sunshine tables, Edition of 1905, giving the times of sunrise and sunset in mean solar time and the total duration of sunshine for every day in the year, latitudes 20° to 50° North, U. 8. Dept. Agric., Weather Bur., 1905 (numbered ‘‘ W. B. No. 320°’). CLIMATIC CONDITIONS OF THE UNITED STATES. 369 TaBLE 21.—Normal total number of hours of sunshine within the period of the average frostless season. Normal Normal total total Station. duration Station. duration of of sunshine. sunshine. hours. hours. Arizona: Fiagstaff.........'...-. 1,134 New Mexico: Santa Fe......... 1,892 Arkansas: Little Rock.......... 2,166 New York: California: Alibamiyetcefe clase) ahale a setelese)< 1,504 Neos AMIPCLER cruise tise saecies 2 2,995 IES UIST el Olen a taeey areas = eS tal cate, ere 1,479 San) HPANCIsSGOl. . f2)5\2 « elelsiaa/eie 2,615 New Viork ‘hfe a53,2ha2 fy Ase 1,626 Colorado: oehester 4-70.95). cee aye 1,418 MON VEL ci) se2 Helenai. ... ....)5505 s.- 1,286 @heyenne<. Mesaecs aheeue ee LESS ebrasica: Omaha: foc ce an oe 1,548 MG aM eLere oie aurece a. ctate soe eee 1,167 New Jersey: Atlantic City...... 1,827 Sheridansthi +t ws 5.5 Ate 1,307 370 ENVIRONMENTAL CONDITIONS. IV. MOISTURE-TEMPERATURE INDICES. A. INTRODUCTORY. An attempt on the part of Livingston’ to obtain a single climatic index for moisture and temperature efficiencies combined resulted in a climatic chart of the United States that has certain interesting charac- teristics. These indices are based on the tentative suppositions: (1) that the temperature efficiency of a climate, to produce plant growth, is proportional to the temperature summation-index of that climate (obtained by whatever method may prove most satisfactory), and (2) that the moisture efficiency is proportional to the Transeau ratio of precipitation to evaporation. This last supposition considers that if two stations differ only in rainfall and in the intensity of the evaporating power of the air, then plant growth at these two stations should be directly proportional to the rainfall and inversely propor- tional to the atmospheric evaporating power, as far as climatic condi- tions are concerned. In short, the moisture-temperature index of a climate (for any given duration factor) is taken as the product of the temperature summation-index and the moisture ratio. To employ Livingston’s terminology, if J, represents the moisture-temperature index, if J, represents the temperature summation-index, if J, repre- sents the summed precipitation for the period considered, and if I, represents the total evaporation from some standard a{mometer for the same period, then Oe PB ae Es Inspection of this formula shows that the value of this moisture- temperature index is increased by higher temperature (supposing that the optimum temperature for plant growth is not surpassed) and also by lengthening of the time period taken into account. The higher are the daily temperature-index values and the more of them are summed, the greater must be the resulting sum (J,), and the product index is of course increased by increasing its first factor. Also, this product index is increased by higher values of the Transeau moisture-ratio (I,/I,.). This ratio value, in turn, is increased by more rainfall and by lower atmospheric evaporating power. The efficiency, for plant growth, of the moisture-temperature complex is thus greatest with a long growing-season, with high temperatures (not surpassing the optimum), with great rainfall, and with low evaporating power. Livingston’s product indices were based upon the duration factor of the length of the period of the average frostless season and upon the physiological summation-index of temperature efficiency. We have calculated these values also for temperature indices derived by the remainder method and for those derived by the exponential method. The results obtained for these two forms of moisture- JLivingston (1916, 2). CLIMATIC CONDITIONS OF THE UNITED STATES. 371 temperature index are presented below, and these are followed by those obtained by Livingston. It is perhaps not out of place here to remark that these moisture- temperature indices represent no more than a first rough approxima- tion toward an environmental index, which might state the efficiency of the environment as a whole to produce plant growth. It is quite obvious that such an environmental index will not really be attainable for a very long time; it must embrace many other terms besides those representing climatic conditions, and also terms for all of the influential climatic ones, and, as has been emphasized, methods for the measure- ment and weighting of most of the environmental conditions are yet to be devised. Nevertheless, progress can best be favored by employ- ing the two climatic indices that seem most promising, with the hope that the shortcomings of the resulting interpretations may suggest closer approximations to the form of index required. It may also be remarked that but little real progress can be hoped for in this direction until laboratory facilities become available, by which the relations between plant growth and environmental condi- tions may be experimentally studied. As has been emphasized, this sort of experimentation will require well-planned physical equipment for the control of environmental conditions. It will also require a group of workers who can bend their energies toward gaining a com- mon end, for a single individual, no matter how well equipped with apparatus, can not hope to find it in his power to enter very deeply into these complex relations. Nevertheless, expensive and difficult as the project may seem at present, there can be no doubt that it will be eventually undertaken, nor can it bedoubted that the benefits to be derived from properly. planned and conducted experimental studies on plant environmental relations will prove fully as great and as valuable to the human race as have been those derived from experi- mental physics and astronomy. It is in the laboratories and observa- tories of these sciences that the nearest approach to the sort of work here contemplated is now being carried on. On the practical, bread- winning side, it needs only to be suggested that the greatest and most important of all human industries, agriculture, rests entirely upon what little knowledge we already happen to possess in regard to the relations between plant growth and environmental conditions. When- ever a workable environmental index for plant growth may be approached, it is certain that the arts of agriculture and forestry will be markedly improved. B. MOISTURE-TEMPERATURE INDICES BASED ON TEMPERATURE SUMMATION- INDICES OBTAINED BY THE REMAINDER METHOD (ABOVE 39° F.), FOR THE PERIOD OF THE AVERAGE FROSTLESS SEASON. (TABLE 22, PLATE 70.) These values were derived by multiplying the summation index for the period of the average frostless season, for each station (table *z, oye] st oseq oy, *Z WUIN]OO ‘zz a[qQB} UOJ BBP {(poyjJoUT JepureUIE! Aq poureyqo senyvA UOT}eUIUINS AoUBIOYJo o1nyesedule4 Aq poydynu pue g-/ggy 10} sonywa uo1ye1odvas Aq poprlArp senyea UoryeyzIdIdeId [eUIOU) UOSBeS Sse[}SOIj OABIOAG JO potsod OJ SeoTpul oAnyesoduI04-01n4S10 J (seq) OSAMA 38010) ussB19A9 uonisuesy 38910} Uve1Ts9A0 3Sa10} U9a1819A9 78910} snonpiloep a Aydosaw wiayyi0N on Aydosaw wWiayioN -puelssel5 yso10j d1Aydosauw Us19}SEaYINOS AAA yy il S32 UW 009 a | ‘Nia AN Boe —_~ SSS SS N eo Gs i> A 4 , ve % % y, Wh 4 G7. f jj > ‘7 i AS rm y ce, F Oo ™ wW = qt = a V6) +Y we ( 4 \2 YUp,Cs A © Op a EB ZG #3 PRR S (A Save Ik |S ad == = r = [=e 4 y [A JY ‘ ae eo \ : N Z) ee cv \ t a > ~ te 7 ee SS oe 4477. i ote Xo. Bet ol. eet : (ate a! Zita of ofl. ohh 6L ALS) eecuern a TA fe et ea } ° 6 o o T GOL QUE SIE SIE Zit GIL JOE 62% Set Let 621 CLIMATIC CONDITIONS OF THE UNITED STATES. 373 6, column 5), by the corresponding Transeau ratio (table 11, column 8). For simplicity, we may represent the first of these factors by the letter 7’ and retain the expression already used (P/E) for the second, whence we may term the resultant index P/EXT. The results obtained are set forth in the second column of table 22. The chart for this series of values is shown as plate 70. Its discus- sion may best be combined with those of the two following charts. TABLE 22.—Moisture-temperature indices (P/E XT) jor the period of the average frostless season, by remainder (above 39° F.), exponential, and physiological methods. The moisture ratios (P/E) from table 12, and the summation indices (7’) from tables 9 and 10. Temperature summation obtained by— Station Remain- : der Expo- | Physio- method | nential | logical (above | method. | method.? 39° F.). Alabama: Gg: re 12,106 | 1,314 | 23,652 Montgomery........ 6,114 665 | 12,400 Arkansas: Hort smiths... .!s-: 5,543 601 | 10,782 Little Rock......... 5,795 627 | 11,246 California: SHITE aiey—. 5, «559s a 8\ ote’ « 625 68 1,186 Independence....... 254 27 449 Los Angeles......... Zee pail 283 3,127 zl iE LCT telecast ae 1,285 ae hate AGCEAMENLO:. 2.0/2 sisi. 1,399 , 04 San Francisco....... 2,598 283 1,991 Colorado: WI EHVEM Es 55:5 els c 2s = 786 81 1,204 Connecticut: Wew Haven... ... 5,330 556 7,869 Florida: Jacksonville......... 10,813 1,196 | 21,760 Key West.......... 10,877 | 1,155 | 23,266 Pensacola.... 10,175 1,113 | 20,465 De ct... \ nh & (Cedar Keys)!....... i a2 1,271 | 23,155 Georgia: PENANG: conc se 5,175 549 9,686 PAUIPMISGH cic coc ks cee 6,420 695 | 12,879 Savannah.......... 9,385 1,014 | 18,294 Idaho: (US) he 405 42 598 Illinois: Od ae 4,174 446 7,807 Se a 3,384 355 5,100 Springfield.......... 3,971 418 7,032 Station. Indiana: Indianapolis......... Iowa: Davenport... ae ne Des Moines......... Kentucky: Louisville Louisiana: New Orleans........ Shreveport... .400 Maine: HASEDOEL= Soclysuiak a. Rortland see ote Maryland: Baltimores* eens . Washington, D.C... Massachusetts: Michigan: IDEN Ss fess ih is certie. Marquette.......... Port, Ekuron essen. oe Minnesota: LOTT (Hl Cg See et uchetor Temperature summation obtained by— Remain- der Expo- | Physio- method | nential | logical (above | method. | method.? 39° F.). 3,376 359 5,967 3,672 384 6,255 4,364 456 | 7,457 4,583 478 7,472 4,138 440 | 7,241 3,781 401 7,114 2,410 256 4,474 5,744 611 | 10,599 3,498 371 6,590 11,956 | 1,304 | 23,381 6, 846 751 | 13,874 3,406 391 2,747 3,712 386 4,528 4,540 483 7,947 4,704 497 8,322 4,097 429 5,714 4,396 474 5,193 3,008 312 3,300 2,996 309 4,569 3,147 327 4,189 2,997 314] 3,113 2,918 301 3,819 3,751 401 | 4,064 1 Where a second station is named in parentheses, the evaporation value is for this station. 2? The values in this column have appeared in Livingston’s paper (1916, 2). 374 Taste 22.—Moisture-temperature indices (P/E XT) for the period of the average frostless season, by remainder (above 39° F.), exponential, and physiological methods.—Continued. The moisture ratios (P/E) from table 12, and the summation indices (T) from tables 9 and 10. i) Station, Nee Expo- method | nential (above | method. 39 F.). Minnesota—Continued: Moorhead.......... 3,163 315 Biebaul pees ae eee 4,269 442 Mississippi: VICKBDUNE \7.-).ps ns eile 7,663 $34 Missouri: Kansas City........ (Leavenworth, Kans.)! 4,895 525 Stp Wouise.. caer ren 3,635 390 Springfield.......... 5,631 591 Montana: 15 hig eens Premera (Fort Assiniboine)! .. } 1,139 116 Welenain.ic.scanetin ts 691 (P Nebraska: North Platte........}| 2,205 229 Om ahareisesaciawe oto 4,151 436 Valentine®:... «ss. 2,309 238 Nevada: Winnemucca........ 127 13 New Hampshire: @ontords:.)¢ sites (Manchester)!....... } 3,356 344 New Jersey: Atlantic City....... 6,858 707 New Mexico: fof ieh tl cy; See POR ene 12 81 New York AND ANy ee fis3% sists 3,780 394 Shit aie e Apismeneroe 3,257 340 New) Work, 24 chai. 4,541 481 OBWEZ02 soe eee a yale 373 Rochester. ..cic¢ss'..« 2,962 307 North Carolina: Charlotte: .c< << % wes 5,914 630 HMatteraay <.c).nieeees 135512 1,418 PU BLOMEN ES siete ac ccarers 7,992 851 Wilmington......... 9,781 1,034 North Dakota: IBISMarOk, «ets onapas 1,590 188 Devils Lake......... (Fort Totten)!...... 2,210 227 Walliston) css 5. ches (Fort Buford)!...... 1,327 135 Ohio: Cincpanatyy: sec sens =e 3,007 319 Cleveland's i.:Joc teats 3,782 399 COMBOS +. sees 3,019 316 Sandusky... o » ~ ~ ‘\ RA SANS SAN INS aN k S IN PLATE 71 EOS TSN aes ane A. ey fe, ¢ \s oD — Se \ va 4qo sanyea uoKworUNs Aouatogye aanyesoduiay Aq SS0]}S01J OFBIVAT JO pOttod Joy sooTpUT aINyRsedui44-91N4SIO] (seq) 38910} u9aIB19A2 amnAydosow wiayyioN = Ay -puejsses) 2 : Y . i = — — bod Sos N SS Z| 4 V 7 477) VRS Ss SACFE , = = ps A ee Ne i 7 SS il Rea Seat 376 IS ‘ ] Lf n | \S ees \ es ca. \S ~ NSS y SS SEN Q i RS , ANS BS SN = Nee > SAN SAN A ; SSS ¢ 6 SSAND - N B) \ SS -} . x Ny OdSNN r ‘\ ‘ hk = ores rN Eyei -eeh) 2 PLATE 72 '% o78]d sl aseq ayy, ‘seoUTAOId 91NyB1edUIE}-91N4SIOUI BAY OFUI duu ay} OpTAIp souyy [INT “p UMN [09 “Zz 9[qB} MOL pourE}qo vy ep : (poyyour [BordojorsAyd Aq pourezqo sonyea uoT}eUIUINS AouaTOWyo a1nyesodurey Aq pordy[nu pus S-L88T 10} sonyva uoryviodvas Aq paplAlp sonyBva UOr}e4zId199Id [BULIOU) UOSvAS SSo]}SOIJ OSBIOAB JO POrsod IO} SoOIput o1NyeIOdUI9}-91N4SI0 JL (seq) (3S9M) ysa10j u9a1819A9 uonisuety 38010} u901319A9 3Sa10} U9a1T19A9 a1 Aydo13 Ay ysai0j Snonpioap dAydosaw way WON dn Aydosaw usdyIION uiojsamyqIoN -pur[sse1y h ate RK, a 4 W1a}seay Nos snonpoaq puryssei5 SOY WS CLAAAOA AEP Si : f Z ng 4 ih SS c fi voi "| y \. Qe ;. S Sat 39, 7 ae NA VY lasaq{-1ulas P LIVE oe a : AV ELE? : ) — IN ee ; SS SS .\\ %, ? & a at =A yieseq aS EASY eS Pe Bie rr 7, RY “\ WS RAN ASA TANS ee" x *) SY ». ea NG Se. SS 2 WS ECE \ ab 8 lh Ute: ib " \ NR wed § o of 26 ( 6 66 JOL gOt .SOl 9 ,ccshanietelaret austere axe SSeS raisers Gulehae gs .66 1.63 60 rrTIL EGS YORT fo) oon cic ois nie aie weds ities SIS 51 1.85 Vapor pressure: 63 Normal mean, fs (hundredths inch)................. 411 600 + Humidity: 65 Normal! mean, fa (per CONt) o5..cni6. dicscjaleis elo cteidin tavaiale wisps 65.6 83.9 66 Normal mean, year (per cent). ............-ee2ecees 67.5 82.1 Wina: 68 Normal mean hourly velocity, fs (miles)............. 3.6 12.6 Sunshine: 69 Normal total duration, fs (hours)..................- 1,468 2,300+ Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method.............. 301 1,100 71 Normal P/E XT, fs, exponential method............. 2,914 10,0002 72 Normal P/E XT, fs, physiological method............ 3,819 20,0004 TABLE 28.—Climatic extremes for the northwestern hygrophytic evergreen forest. Plate | Temperature: Low. High. 34 Days in normal frostless season (fs)..............0: 103 316 35 EEO UIA AV S50 JS orcie/aro oie si es os velop akana,/ayotel Mie ster atatey sere 6: ata: taneranters 0 0 36 KE LAA YS EIR oral oieiw wap ore a 1 bisheraieiwie toheliabonaye daebaebarrteonme 0 0 37 Remainder summation above 32°, year (thousands).... iil Se Seve 38 Remainder summation above 39°, fs (thousands)...... 3.8 4.6 39 Exponential summation, fs (hundreds)...... ate Sayers eeant 4.1 5.0 40 Physiological summation, fs (thousands)............. 1.9 4.8 41 FA solu G6: EN ETN UT 555. cj ois oye 0) ue wietel ae tole lov avoceheney tilepalle vei Gee —6 +30 43 Normal daily mean, coldest 14 days of year (°F.)..... 354 458 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 64.4 th 45 Normaltdaily/mean} year (CR) ac. o1staacdinraisreiotciereysiavdicte. aisle .091 .131 47 Normal No. rainy days (over 0.10 inch), fs........... 35 124 48 Normal No. dry days (0.10 inch or less), fs........... 28 136 49 Dry days, percentage of total, fs (per cent)........... 18 80 50 Days in longest normal rainy period, fs.............. 22 106 61 Days in longest normal dry period, fs................ 9 58 52 Mviean! totalsyear (INCHES)!. <1 \s6/a\cre\e ore o'< sysicvete she yeuz eum) aie 30- 50+ Evaporation: 53 Daily mean; 1887—88, fs (inch)... «6%. ciyalseleeiele ane .084 .149 54 Total annual, 1887-88 (inches).............e0ecee005 21.3 30+ Moisture ratios: 58 INormal {P/E} $85.50. h Sve Oe Neil NSS Mas 71 1323 eM NOTMaAl 4:/Ei, FB isc a « o wreravareeruerttel da ei avere mr emai e aneval 81 1.52 60 armal (P/E, \VOak:s « o~os ewinicesemis Sree we ae 82 1.72 Vapor pressure: 63 Normal mean, fs (hundredths inch).................. 345 450 —4 Aumidity: 65 Normal‘mean, fs (per cont)!o sees Sosietectae aes eis teletsholeis 70.4 81.8 66 Wormal'mean; ‘year’ (percent) ic acre «ola ncieea ce seietaiov 73.4 80.2 Wind: 68 Normal mean hourly velocity, fs (miles)............. 4.7 12.9 Sunshine: 69 Normal total duration, fs (hours)..........e...e000% 1,225 1,500 +2 Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method.............. 301 401 71 Normal P/E XT, fs, exponential method............. 2,997 3,780 72 Normal P/E XT, fs, physiological method............ 2,747 7,000 +4 406 CORRELATION OF DISTRIBUTIONAL FEATURES. TaBLE 32.—Climatic extremes for the Boreal Region. Plate | Temperature: 34 35 36 37 38 39 40 41 43 Days in normal frostless season (fs) Hot days, fs Cold days, fs Remainder summation above 32°, year (thousands).... Remainder summation above 39°, fs (thousands) Exponential summation, fs (hundreds) Physiological summation, fs (thousands) Absolute minimum Normal daily mean, coldest 14 days of year (°F.)..... Normal daily mean, hottest 6 weeks of year (°F.)..... Normaldaily mean, year '(CH:))41.5..06 oi vin tsuate ioe alors Precipitation: Normal daily mean, fs (inch) Normal No. rainy days (over 0.10 inch), fs Normal No. dry days (0.10 inch or less), fs Dry days, percentage of total, fs (per cent) Days in longest normal rainy period, fs Days in longest normal dry period, fs Mean total, year (inches) Evaporation: Daily mean, 1887-88, fs (inch) Total annual, 1887-88 (inches) Moisture ratios: Normal P/E, fs Normal r/E, fs Normal P/E, year Vapor pressure: Normal mean, fs (hundredths inch) Humidity: Normal mean, fs (per cent) Normal mean, year (per cent) Wind: Normal mean hourly velocity, fs (miles) Sunshine: Normal total duration, fs (hours) Moisture-temperature indices: Normal P/E XT, fs, remainder method Normal P/E XT, fs, exponential method Normal P/E XT, fs, physiological method Plate | Temperature: Days in normal frostless season (fs) Hot days, fs Cold days, fs Remainder summation above 32°, year (thousands).... Remainder summation above 39°, fs (thousands) Exponential summation, fs (hundreds) Physiological summation, fs (thousands) Absolute minimum Normal daily mean, coldest 14 days of year (°F.)..... Normal daily mean, hottest 6 weeks of year (°F.)..... Normal daily mean, year (CF.)........c.ccccccsccccs Precipitation: Normal daily mean, fs (inch) Normal No. rainy days (over 0.10 inch), fs Normal No. dry days (0.10 inch or less), fs Dry days, percentage of total, fs (per cent) Days in longest normal rainy period, fs Days in longest normal dry period, fs Mean total, year (inches) SEES — Plate 53 54 58 59 60 70 71 72 CORRELATION OF DISTRIBUTIONAL FEATURES. TABLE 33.—Climatic extremes for the transition zone—Continued. Evaporation: Low. Daily mean 1887-885 far CGneh))< see adsosses oh cinceee 50— 60+ Evaporation: 53 Daily mean 1887-88, 7s Gneh) cits. sce ec meee ee .130 . 146 54 Total annual, 1887-88 (inches).................-.0+- 38.4 49.6 Moisture ratios: 58 NOrMalER EU fans: cere ee tetas ee 1.08 1.36 59 INOrMaal as feic ak 535 oe ee ee eee met oe nee 12 1.52 60 NOFA Ee VOaR irs kee ke Se ee ate ee eee 1.09 1.47 Vapor pressure: 63 Normal mean, fs (hundredths inch).................. 585 610 Humidity: 65 Normal:mean, fe'(per'cent)itctec- ce ee ce cnet eae be Tone 80.6 66 Normal mean, year (per cent).................0.08- Uvhes: 80.9 Wind: 68 Normal mean hourly velocity, fs (miles)............. 5.1 9.9 Sunshine: 69 Normal total duration, fs (hours)................+5- 1,942 2,297 Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method.............. 1,014 1,314 71 Normal P/E X17, fs, exponential method............. 9,385 12,106 72 Normal P/E XT, fs, physiological method............ 18,294 23,652 416 CORRELATION OF DISTRIBUTIONAL FEATURES. TABLE 47.—Climatic extremes for region ee od to 14 species of evergreen broad-leaved trees (West). Plate | Temperature: Low. 34 Days in normal frostless season (fs)..........-2+++0: 237 35 IGE AAV QUIS s accvcteueleis aleers'o:5.b ais, o'o 0.0 ss SIMO aI 0 36 COAG RV AAIS s chcieicelevorrecrw wreeyrts © oon bcpe pinto areas mie iete 0 37 Remainder summation above 32°, year (thousands).... 18.0+ 38 Remainder summation above 39°, fs (thousands) . patie ake Dee 39 Exponential summation, fs (hundreds) tee SO Rene. 5.01 40 Physiological summation, fs (thousands)............. 4.1 41 Abbsoliite! minima 2s o.cysve.cis:2 ans aroret olaaeat ae lehe ayeeleteiene +13 43 Normal daily mean, coldest 14 days of year (°F.)..... 49 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 64.4— 45 Normal’ daily mean; year (CE) 00 cic «ia cn sislaiurs sfateve ters 60 — Precipitation: 46 Normal'daily mean, fe’(Gineh)). .. aceon et ote ceteris bores .033 47 Normal No. rainy days (over 0.10 inch), fs........... 25 48 Normal No. dry days (0.10 inch or less), fg........... 235 49 Dry days, percentage of total, fs (per cent)........... 81 50 Days in longest normal rainy period, fs.............. 22 51 Days in longest normal dry period, fs................ 232 52 Mean‘ total; year (anches), ««/s:s/\svsjetelsh. oteie tetas ate elsisie 20- Evaporation: 53 Daily mean, 1887-88; 78 (ANCH))....6 ce ors © cist ec, c eerie ers .102 54 Total annual, 1887-88 (inches).................2-+8- 32.5 Moisture ratios: 58 NOME Lefer oa oe eee eee ee eae eee BY 59 MEA ey OTE Sc aes a0 bas sede e han igadde many an eeen 45 60 SRBEIAA ECA VGAE 5 gas ib o.3s 3:3 we so tpeeeee hes an a sere 20 Vapor pressure: 63 Normal mean, fs (hundredths inch)...........-....-.- | 335 Humidity: 65 INormalimean;, fa" (per CONt) oom. csiee exe tus es aio revels 7 69.9 66 Normalimean, year (per Cent)... ccc: «ste es oe ae owe cle 69.0 Wind: 68 Normal mean hourly velocity, fs (miles)............. 4.5 Sunshine: : 69 Normal total duration, fs (hours)................-.. 2,615 Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method.............. 283 71 Normal P/E XT, fs, exponential method............. 2,598 “2 Normal P/E XT, fs, physiological method............ 1,191 High. 334 54 0 18.0+ 8.0+2 8.0+ 8.4 +32 54 64.4 60+ .048 62 294 90 50 299 30+ -104 36.7 -48 -61 .60 378 80.1 79.9 if :% 2,995 283 2,721 3,127 TABLE 48.—Climatic extremes for region with 15 to 63 species of evergreen broad-leaved trees (East). Plate | Temperature: Low. 34 Days in normal frostless season (fs)........-...+-+5+ 311 35 LOMUAV Aa Bie ccusctacaiece cise so 2 0.0 oben &6.8-0s famines Giana aa 210-2 - 36 WOM BRV Biles bak tess Soko tiwisyoace erdseraneceasiwiet ave Ae ein elete 0 37 Remainder summation above 32°, year (thousands).... 18.0+ 38 Remainder summation above 39°, fs (thousands)...... 9.0+4 39 Exponential summation, fs (hundreds)............... 10.0+ 40 Physiological summation, fs (thousands)............. 17.5+ 41 AS BOLT CAPE EIN UIENY » oats; s 1s 016: ose c.0 wi ors (a issete MIRE henna +16 43 Normal daily mean, coldest 14 days of year (°F.)..... 552 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 78.8+ 45 Normalidalyimean, year (OR .). 060.0... sosaatmeniens 70+ Precipitation: 46 INormalidally mean: 78) (NON). «icc. <:scem areietars Sierelavermiala 147 47 Normal No. rainy days (over 0.10 inch), fs........... 165 48 Normal No. dry days (0.10 inch or less), fs........... 100 +4 49 Dry days, percentage of total, fs (per cent)........... 30 —4 50 Days in longest normal rainy period, fs.............. 137 51 Days in longest normal dry period, fs................ 252 52 DIGAT LOVEE VERE CADLODOS) . vic sins Wa ksdic dard scs steiner Ares 50+ CORRELATION OF DISTRIBUTIONAL FEATURES. 417 TABLE 48.—Climatic extremes for region with 15 to 63 species of evergreen broad-leaved trees (East)—Continued. Plate | Evaporation: Low. High. 53 Darnly mean, 1887-86, fs GNCH) 2y<,<.< < fais ois 9 «sue sicteknsiove .124 .138 54 Total annual, 1887-88 (inches).................ccee0:- 44.2 50+4 Moisture ratios: 58 ee LINAM pe Phra o« Sock ace eae tn iste g aia aie ety Stes toe 1.08 1.19 59 OoerH MU eis ta cs Retr aot te ec ot) Ate 1.15 1.28 60 IN Onn allele (Es OAR jorava) score cjaiss< 4 © iebcasnctove ee Pe oo 1.07 1.36 Vapor pressure: 63 Normal mean, fs (hundredths inch).................. 612 6504 Humidity: 65 INormalemiean 7s) (OCTSCENE) siepcsss hye Sickoisirs Bex insoles 80.0—24 80.4. 66 Normal mean, year (per cent) . 2... 606562 cs ness ce cws 80.0—2 80.5 Wind: 68 Normal mean hourly velocity, fs (miles)............. 6.7 8.044 Sunshine: 69 Normal total duration, fs (hours)..................- 2,300 +4 2,300 +4 Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method.............. 1.200 —4 1,271 71 Normal P/E XT, fs, exponential method............. 11,000 —¢4 11,722 7P Normal P/E XT, fs, physiological method............ 22,0002 23,155 TaBLE 49.—Climatic extremes for region with a to 87 species of evergreen broad-leaved tree s (East). Plate | Temperature: Low. High. 34 Days in normal frostless season (fs)................. 312 348 35 1S EET R GRAYS hu fst SIGNI PAC R ER SCPE POCNRE Core RBC Ne UN dl 2404 330 +4 36 EON GEA SY Senfs hey As NINO gna eid WRN Se ee Aa MR 0 0 37 Remainder summation above 32°, year (thousands).... 26.0+ 26.0+ 38 Remainder summation above 39°, fs (thousands)...... 10.8 12.0+4 39 Exponential summation, fs (hundreds)............... 11.0+ — 12.0+ 40 Physiological summation, fs (thousands)............. 20.0 27.5+ 41 PNOSOLUGE TURIN UTI oye) ier oa er shots aioe Sores eee Oe es +19 +30 43 Normal daily mean, coldest 14 days of year (°F.)..... 60 — 64 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 78.8+ 78.8+ 45 Normalidaily mean, year (CM) acess ence cee eee. 75 — 75+ Precipitation: 46 Normal daily/means fsiGneb)ia... se ee eek als . 140-4 .173 47 Normal No. rainy days (over 0.10 inch), fs........... 175 —4@ 234 48 Normal No. dry days (0.10 inch or less), fs........... 84 1754 49 Dry days, percentage of total, fs (per cent)........... 26 50+4 50 Days in longest normal rainy period, fs.............. 1254 174 51 Days in longest normal dry period, fs................ 19 1004 52 Mean totaly year Gncehes)ia ce mes ek Sa els aioe 50—- 60+ Evaporation: 53 Mailysmean, 1S887—SSy7saneh))..sacccomecectke cance: . 140-4 Bcc 54 Total annual, 1887-88 (inches)...............2.c.000- 50-4 50+4 Moisture ratios: 58 1. (ere Za OAR Re Re i tle ase Ms LRM As ete ah .804¢ 1.00+4| ° 59 Normalan Ms fein strc ce eo raceeae t ee 1.00—4 1.20 60 Normale) H yearss. cicero ee mien ee 1.00 —2 1.36 Vapor pressure: 63 Normal mean, fs (hundredths inch).................. 650 —4 692 Humidity: 65 Normal'mean, fs (percenl)saseaveena tyes ooe eto. 80.0—4 80.5 66 Wormal mean, year (per cent). 305.3... 025.05 0-00 - 80.0—4 80.5 Wind: 68 Normal mean hourly velocity, fs (miles)............. 8.0-4 8.0+4 Sunshine: 69 Normal total’ duration, js (hOurs)).<.- seers oe se oe 2,300 +4 2,300 +4 Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method.............. 1,200 —2 1,200 +4 71 Normal P/E XT, fs, exponential method............. 11,000 —4 11,000 +4 72 Normal P/E XT, fs, physiological method............ 23,000 +4 23,000 +4 418 CORRELATION OF DISTRIBUTIONAL FEATURES. TaBLe 50.—Climatic extremes for region with 88 or more species of evergreen broad-leaved trees (East). Plate | Temperature: Low. High. 34 Days in normal frostless season (fs)................- 3404 365 35 a Coy ris b: 6 fret ie nua ca pokes aS Retr bas are pal 300 + 365 36 Cold: GBy8) Jac Sees cones co One a ee om Os CREB acer 0 0 37 Remainder summation above 32°, year (thousands).... 26.0+ 26.0+ 38 Remainder summation above 39°, fs (thousands)...... 12.04 14.5 39 Exponential summation, fs (hundreds)............... 12.0+ 15.4 40 Physiological summation, fs (thousands)............. 25.0+ 31.1 41 Absolute: minimums. 6: <5 so se seas chsionens eee +29 +41 43 Normal daily mean, coldest 14 days of year (°F.)..... 60+ 69 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 78.8 78.8+ 45 Normal daily mean, year (CE). 7..62..2 sess cee ese re 75+ 75+ Precipitation: 46 Normal“‘daily mean,ifs Gnch): 2:22. ..-<)-- 22.0040 .106 . 1604 47 Normal No. rainy days (over 0.10 inch), fs........... 161 2004 48 Normal No. dry days (0.10 inch or less), fs........... 1504 204 49 Dry days, percentage of total, fs (per cent)........... 30 —4 56 50 Days in longest normal rainy period, fs.............. 112 150+4 51 Days in longest normal dry period, fs................ 75 —4 182 52 Mean totaly year (inChes))-./40. ene cee eee eee. 50+ 60+ Evaporation: 53 Daily-amean* 1887-88) fa) (nich) 22 10sec ic oe ac wleusheloveon . 140-4 .141 54 Total annual, 1887-88 (inches)...................+-. 50—- 51.6 Moisture ratios: 58 INOMmInLE MEN BAL cast Goxxloe caine Rice ee ROIS Eimeine 7 .80+2 59 Normals (Hes) 2) Mok. 0 cris Soa. ae eee eee eee 75 .80+4 60 IB AERIS hss \y jatacetusce'e oS siete eee 75 1.00+4 Vapor pressure: 63 Normal mean, fs (hundredths inch)...............-.. 700 —2 707 Humidity: 65 Normalémean) fs! (per cent) penis seem eee eee Cree neh. 80.0+4 66 Normalimean, year (per cent); <\./).2iuee ae cc .119 .159 Normal No. rainy days (over 0.10 inch), fs........... 166 284 Normal No. dry days (0.10 inch or less), fs........... 26 88 Dry days, percentage of total, fs (per cent)........... 8 34 Days in longest normal rainy period, fs.............. 95 196 Days in longest normal dry period, fs................ 17 34 Mean total; year Gnches) ecu sa so hone celac sls saneers 50— 60+ Evaporation: Daly meanylSS/—SS,, 18) (INCH) |.) sc: eyosstater s cia cretan bet wre a ltaly/ .165 Total annual, 1887-88 (inches)...................e2. 38.4 50.0 Moisture ratios: Normal / BS 16:5 25...) ses I IA Oe eee oe 75 1.35 Normalia/ To fsi. iy. /2 So 5 aR RA Oe OI rhe iE .92 1.47 Normals 23 /E sy Years ro cae tts CERRO oes 1.00 1.33 Vapor pressure: Normal mean, fs (hundredths inch).................. 545 588 Humidity: Wormalymean, fs (per, Cent) iissas's tsaacisie ie Poe eigeiess rp Wes 79.0 Wormal mean, year (per cent))s <0. o6se5 os sis cece - 71.8 80.1 Wind: Normal mean hourly velocity, fs (miles)............. 6.3 8.0 Sunshine: Normal totali duration, fs (hours). pci. cis cisusilet dere ue 1,895 2,301 Moisture-temperature indices: Normal P/E XT, fs, remainder method.............. 593 1,304 Normal P/E XT, fs, exponential method............. 5,253 11,956 Normal P/E XT, fs, physiological method............ 10,837 23,381 424 CORRELATION OF DISTRIBUTIONAL FEATURES. TasBLe 59.—Climatic extremes for region with all species of a selected group of 15 deciduous trees of the southeastern states. Plate | Temperature: 34 Days in normal frostless season (fs) 35 Hot days, fs 36 Cold days, fs 37 Remainder summation above 32°, year (thousands).... 38 Remainder summation above 39°, fs (thousands) 39 Exponential summation, fs (hundreds) 40 Physiological summation, fs (thousands) 41 Absolute minimum 43 Normal daily mean, coldest 14 days of year (°F.)..... 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 45 Normal: dailyamean, year: (Cis) icc.c:clce sre oe a ee ere Precipitation: 46 Normal daily mean, fs (inch) 47 Normal No. rainy days (over 0.10 inch), fs 48 Normal No. dry days (0.10 inch or less), fs 49 Dry days, percentage of total, fs (per cent) 50 Days in longest normal rainy period, fs 51 Days in longest normal dry period, fs 52 Mean total, year (inches) Evaporation: Daily mean, 1887-88, fs (inch) Total annual, 1887-88 (inches) Moisture ratios: Normal P/E, fs Normal x/E, fs Normal P/E, year Vapor pressure: Normal mean, fs (hundredths inch) Humidity: Normal mean, fs (per cent) Normal mean, year (per cent) Wind: Normal mean hourly velocity, fs (miles) : 9.7 Sunshine: Normal total duration, fs (hours) 2,300 +2 Moisture-temperature indices: Normal P/E XT, fs, remainder method 1,314 Normal P/E XT, fs, exponential method 12,106 Normal P/E XT, fs, physiological method 23,652 TABLE 60.—Climatic extremes for region with no species of a selected group of 13 deciduous trees of the eastern states (North and West). Plate | Temperature: Low. High. 34 Days in normal frostless season (fs)............-.+5+ 25 318 35 ERO EBV rattan w sich siete cic stariediors visio tere ao ee eon 4 211 36 CSOMATI RUA, Ecotec et losin oe eays ore 8 pins Ci Ae eR 0 158 37 Remainder summation above 32°, year (thousands).... 10.0— 26.0 38 Remainder summation above 39°, fs (thousands)..... . 2.8 10.1 39 Exponential summation, fs (hundreds)............... 291 1,184 40 Physiological summation, fs (thousands)............. 1,947 20,640 41 PA DAOLULGO TOTUETEPULTID eateries wher 01 cealaierstiigie vib eee ROG Cerone TREE —49 +22 43 Normal daily mean, coldest 14 days of year (°F.)..... 0 54 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 64.4— 78.8+ 45 Normal daily mean, year CB.) vce. seer seauawe teenies 35 70+ Precipitation: 46 Normal daily. mean, fs Gnoh)....... \co00ss $2. 0ekea eee .009 .199 47 Normal No. rainy days (over 0.10 inch), fs........... 0 199 48 Normal No. dry days (0.10 inch or less), fs........... 48 294 49 Dry days, percentage of total, fs (per cent)........... 27 100 50 Days in longest normal rainy period, fs.............. 0 99 51 Days in longest normal dry period, fs................ 54 299 52 Mean total; Year Gnohes) sc sckt loses eet deeeuenh ls 10— 30+ CORRELATION OF DISTRIBUTIONAL FEATURES. 425 TABLE 60.—Climatic extremes for region with no species of a selected group of 13 deciduous trees of the eastern states (North and West)—Continued. Evaporation: Low. High. Davlyamean el Ssi—Soa 78 ANC) skis eis solace veuiever ete! reve toe .052 .349 otal annual) 1887-88 /Gnehes)); ia 60% 3s cece c sos eve 18.1 101.2 Moisture ratios: NOIR AVE! MEANS R atric ME Ihe he ete center he tree tots .04 3.84 Norn alla HH, fie 2 rosacea ate aie Mitea ch She uote Mavclert igh c ie aeons .06 4.48 INORDAAL EE HE VCAP ss eval he ik ees ee SIRI tHe .03 4.90 Vapor pressure: Normal mean, fs (hundredths inch).................. 183 6004 Humidity: INermalsmean, fs-(per Cent <7. ioe c Po:c bg aie os Dictal recto: 22.6 704 Normalimean, year (percent)... 65.6 « ws oo le nace Matinee 29.7 86.8 Wind: Normal mean hourly velocity, fs (miles)............. 4.5 16.4 Sunshine: Normal total duration, fs (hours)................... 1,134 2,995 Moisture-temperature indices: Normal P/E XT, fs, remainder method.............. 13 6002 Normal P/E XT, fs, exponential method............. 127 11,724 Normal P/E XT, fs, physiological method............ 197 13,0004 TaBLE 61.—Climatic extremes for region with no species of a selected group of 13 deciduous trees of the eastern states (South). Plate 34 35 36 37 38 39 40 41 43 44 45 46 47 48 49 50 51 52 53 54 58 59 60 63 65 66 68 69 70 71 72 Temperature: Low. High. Days in normal frostless season (fs)................. 312 365 ERGa Ly AVS eS erteh org oes coped ey ckeese a Psuayen cle iey'e Sehake ose) oS eo ok 285 365 WOLAVABYS Skiers Ra OAS TOLER EOE 0 0 Remainder summation above 32°, year (thousands)... . 26.0 26.0+ Remainder summation above 39°, fs (thousands)...... 11.3 14.5 Exponential summation, fs (hundreds)............... 1,260 1,542 Physiological summation, fs (thousands)............. 24,872 31,063 PADSOMIte aTingMA UT as aire ara aren ee Sart ee a eee +19 +41 Normal daily mean, coldest 14 days of year (°F.)..... 60 — 69 Normal daily mean, hottest 6 weeks of year (°F.)..... 78.8-+- 78.8+ Normal. daily.mean; year (SH .)).5 ois ciara oc os b'sie oe oc 7O+ 75+ Precipitation: Normal daily mean fs) (nen) 2.3). «a2, 465k aoe es.c.ceuc .106 palr6ss Normal No. rainy days (over 0.10 inch), fs........... 161 234 Normal No. dry days (0.10 inch or less), fs........... 84 204 Dry days, percentage of total, fs (per cent)........... 26 56 Days in longest normal rainy period, fs.............. 112 174 Days in longest normal dry period, fs................ 19 182 Mean total, svear,Gnches)) a ..:.\2\1c o:. os oe Soreness 06— 60+ Evaporation: Daily mean: 1887—88. 7s (ineh)); © o.6 o.ccs oe eae ec . 140-4 .141 Total annual, 1887-88 (Gmches)).. 0:06. sees oes ee cc pec 50-4 51.6 Moisture ratios: : Normal P/E. 0 ss Ho eee oe patna: .75 1.00+4 Normal’ g/ Fe fscc cls > sje 5 eae RT etter te nO cla Ass 1.204 Normal Pye, Vea 5 svc.c sold Ae ce Le ints .75 1.36 Vapor pressure: Normal mean, fs (hundredths inch).................. 650 —4 707 Humidity: Normal mean, fs! (perscent)eic cos fy ente we foe elonh « Hzfenl 80.5 Normal mean, year (per CENb)iac <2 js seaiswmilee ouleen Tio 80.5 Wind: Normal mean hourly velocity, fs (miles)............. 8.0-—4 9.7 Sunshine: Normal total duration, fs (hours)................... 2,300 + Moisture-temperature indices: Normal P/E XT, fs, remainder method.............. 1,155 1,200 +2 Normal P/E XT, fs, exponential method............. 10,877 11,000 + Normal P/E XT, fs, physiological method............ 23,000 + mene 426 CORRELATION OF DISTRIBUTIONAL FEATURES TABLE 62.—Climatic extremes for region with 1 to 7 species of a selected group of 13 deciduous trees of the eastern states. Plate | Temperature: Low. High. 34 Days in normal frostless season (fs)..............+-- 85 335 35 LOGOS AT Aiths oo clots ta ase ietele ove iohav av clic sho ole a eee eee 0 226 36 Coldidavay so ieee oe SAIS Ie th acts leteders lave iorere oie 0 150 37 Remainder summation ab »\e 32°, year (thousands).... 10.0— 26.0+ 38 Remainder summation above 39°, fs (thousands)...... 2.6 10.6 39 Exponential summation, fs (hundreds)............... 3.0 Li 40 Physiological summation, fs (thousands)............. 2.1 21.4 41 SA DSOMLGG) RIMLI TALLINN ora ay aveies s etic \snsys a. aie ee ee ei —59 +22 43 Normal daily mean, coldest 14 days of year (°F.)..... 0 57 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 64.4— 78.8+ 45 Normal datlyimean, year (OW) ic 00.061 oo « 6 etka aon 35 70+ Precipitation: 46 Nornial dallymean:ifs/(neh)i.. +-.cssiieee severe ae .077 TZ 47 Normal No. rainy days (over 0.10 inch), fs........... 13 284 48 Normal No. dry days (0.10 inch or less), fs........... 28 259 49 Dry days, percentage of total, fs (per cent)........... 8 92 50 Days in longest normal rainy period, fs.............. 14 235 51 Days in longest normal dry period, fs................ ll 88 52 Biean total wear. (inches) )cva.. tii. teleieiels elcves wits aloes : 20 — 60+ Evaporation: 53 DalysmeansS87—88, se neh) ks. os sae cio etac aerate were .084 .188 54 Total annual, 188-788) Gnches)): one ssc «Sines sche 22.1 54.4 Moisture ratios: 58 Normal) Eps alee at ieee Se ee eT Re ene .39 1.36 59 iINormialva Ae Acteite ctor cies caren ee ee ER eee .43 : 1.52 60 INOTMAl PE SVEALE. 8: irs an crate sn ice oe ae ae ne :38 1.72 Vapor pressure: 63 Normal mean, fs (hundredths inch).................. 322 675 Humidity: 65 Wormal’mean;)J/s'(per Cent) acne ss ciae cree cats ee cie eee 53.2 81.9 66 Normal mean: year (percent) as. cent oe oe ce ce eee 59.8 85.2 Wind: 68 Normal mean hourly velocity, fs (miles)............. oul 12.4 Sunshine: 69 Normal total duration, fs (hours)................... 1,225 2,650 Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method.............. 179 1,314 ial Normal P/E XT, fs, exponential method............. veg lil 12,106 72 Normal P/E XT, fs, physiological method............ 2,747 23,652 TABLE 63.—Climatic extremes for region with 8 to 12 species of a selected group of 13 deciduous trees of the eastern states. Plate | Temperature: Days in normal frostless season (fs) Hot days, fs Cold days, fs Remainder summation above 32°, year (thousands).... Remainder summation above 39°, fs (thousands) Exponential summation, fs (hundreds) Physiological summation, fs (thousands) Absolute minimum Normal daily mean, coldest 14 days of year (°F.)..... Normal daily mean, hottest 6 weeks of year (°F.)..... Normalidailp moan, year ((C.). 0.006. «cs s.« Gane wills Precipitation: Normal daily mean, fs (inch) Normal No. rainy days (over 0.10 inch), fs Normal No. dry days (0.10 inch or less), fs Dry days, percentage of total, fs (per cent) Days in longest normal rainy period, fs Days in longest normal dry period, fs Mean total, year (inches) \ CORRELATION OF DISTRIBUTIONAL FEATURES. 427 TABLE 63.—Climatic extremes for region with 8 to 12 species of a selected group of 13 deciduous trees of the eastern states—Continued. Evaporation: ! igh. 53 Daly meant SS—So. 78) GUCM) I ca yseeicl oleyie sore evctsrale, -)e y= .088 .195 54 Total annual, 1887-88 (inches)..............----00-- 20.3 56.6 Moisture ratios: 58 MAREE Ph GB ola oo «keene ee .vear) (inCHES)) ao) vaya e atsieveiereh = ya eletere te aepens 40 — 60+ Evaporation: 53 Daly! mean; S87—SSi 7s NCD) lee ser s1e sielsis e eleie ec ee as .108 . 200 54 Total annual, 1887-88 (inches).......... aS SPP Sta re Bint nia Zoe 54.8 Moisture ratios: 58 NOrMalVe EIS enemas ceRine eC rise ect hee .51 1.30 59 Normal:2)/E vised tore cote coe ee een .60 1.50 60 Normal (E/E SVenrs cc. eee cere oe et mc seine ate 1.85 Vapor pressure: 63 Normal mean, fs (hundredths inch).................. 405 520 Humidity: 65 Wormal mean. js (per Cont) cenceiiaccrse se caeac «lee 65.6 84.0 66 Normal mean, year (PeNiCONG) er. is rei crsraiolere.c ols e cus aes 69.0 81.4 Wind: 68 Normal mean hourly velocity, fs (miles)............. 4.2 12.9 Sunshine: 69 Normal total duration, js (DOUTS). occ a fciec. eces 1,403 1,836 Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method.............. 303 707 71 Normal P/E XT, fs, exponential method............. 2,914 6,858 72 Normal P/E XT, fs, physiological method............] 4,511 10,241 428 CORRELATION OF DISTRIBUTIONAL FEATURES. TasBLe 65.—Climatic extremes for Pinus taeda, area 1. Plate | Temperature: 34 Days in normal frostless season (fs) 35 Hot days, fs 36 Cold days, fs Remainder summation above 32°, year (thousands).... Remainder summation above 39°, fs (thousands) Exponential summation, fs (hundreds) Physiological summation, fs (thousands) Absolute minimum Normal daily mean, coldest 14 days of year (°F.)..... Normal daily mean, hottest 6 weeks of year (°F.)..... Normalidaily mean; yveari(Gh;)\....% iiss eae anes Precipitation: Normal daily mean, fs (inch) Normal No. rainy days (over 0.10 inch), fs Normal No. dry days (0.10 inch or less), fs Dry days, percentage of total, fs (per cent) 51 Days in longest normal rainy period, fs 233 Days in longest normal dry period, fs 78 Mean total, year (inches) 60+ Evaporation: Daily mean, 1887-88, fs (inch) .138 Total annual, 1887-88 (inches) 49.5 Moisture ratios: Normal P/E, fs ; 1.36 Normal 7/E, fs ; 1.52 Normal P/E, year : 1.62 Vapor pressure: Normal mean, fs (hundredths inch) 612 Humidity: Normal mean, fs (per cent) ‘ 80.6 Normal mean, year (per cent) t 81.4 Wind: Normal mean hourly velocity, fs (miles) , 9.6 Sunshine: Normal total duration, fs (hours) 2,300 +4 Moisture-temperature indices: Normal P/E XT, fs, remainder method 11,722 Normal P/E XT, fs, exponential method 1,271 Normal P/E XT, fs, physiological method 23,155 Plate | Temperature: 34 Days in normal frostless season (fs) 35 Hot days, fs 36 Cold days, fs 37 Remainder summation above 32°, year (thousands).... 38 Remainder summation above 39°, fs (thousands) 39 Exponential summation, fs (hundreds) 40 Physiological summation, fs (thousands)............. 41 Absolute minimum 43 Normal daily mean, coldest 14 days of Year (°F.)..... 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 45 Normal daileamean, *yeari(°E.) 29.7 82.9 Wind: 68 Normal mean hourly velocity, fs (miles)............. 4.5 14.9 Sunshine: 69 Normal total duration, fs (hours)................... 1,134 2,995 Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method.............. 13 1,418 71 Normal P/E XT, fs, exponential method............. 127 13,511 72 Normal P/E XT, fs, physiological method............ 197 24,265 TaBLE 76.—Climatic extremes for total range of all species of Cylindropuntias. Plate | Temperature: 34 Days in normal frostless season (fs) 35 Hot days, fs 36 Cold days, fs Sf Remainder summation above 32°, year (thousands).... 38 Remainder summation above 39°, fs (thousands) 39 Exponential summation, fs (hundreds) 40 Physiological summation, fs (thousands) 41 Absolute minimum 43 Normal daily mean, coldest 14 days of year (°F.)..... 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 45 Normal daily mean, vear (CE :) c.j026 2.5 «<= «elevate & Precipitation: 46 Normal daily mean, fs (inch) 47 Normal No. rainy days (over 0.10 inch), fs 48 Normal No. dry days (0.10 inch or less), fs 49 Dry days, percentage of total, fs (per cent) 50 Days in longest normal rainy period, fs 51 Days in longest normal dry period, fs 52 Mean total, year (inches) Evaporation: 53 Daily mean, 1887-88, fs (inch) 54 Total annual, 1887-88 (inches) Moisture ratios: 58 Normal P/E, fs 59 Normal z/E, fs 60 Normal P/E, year Vapor pressure: 63 Normal mean, fs (hundredths inch) Humidity: 65 Normal mean, fs (per cent) 66 Normal mean, year (per cent) Wind: 68 Normal mean hourly velocity, fs (miles) Sunshine: 69 Normal total duration, fs (hours) Moisture-temperature indices: 70 Normal P/E XT’, fs, remainder method 71 Normal P/E XT, fs, exponential method 72 Normal P/E XT, fs, physiological method 436 CORRELATION OF DISTRIBUTIONAL FEATURES. TaBLE 77.—Climatic extremes for Tsuga heterophylla. Plate | Temperature: Low. High. 34 Days in normal frostless season (fs)..............+-. 25 316 35 NGL AUER ss sis lelcis\ets cs or0.5i0(4.0rescsoisis level a « oe ate ee 0 30+4 36 MOOS Hite cok acces ois ecole cele srl alas oops ns a tae abe a 0 120 37 Remainder summation above 32°, year (thousands).... 10.0— 18.0+ 38 Remainder summation above 39°, fs (thousands)...... 3.5 4.6 39 Exponential summation, fs (hundreds)............... 3.0—4 5.0 40 Physiological summation, fs (thousands)............. 1.9 4.8 41 ABsoluite stim WM 5.0 j6:055 <2 00.0.2 ces usivin so SR BE eC —46 +30 43 Normal daily mean, coldest 14 days of year (°F.)..... 204 46 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 64.4— 71.6+ 45 Norma! idaily mean, year (CE) <2... 0. -<.solletento%te le Bu 45 — 55+ Precipitation: 46 Normaldaily. mean, fa’ (inch). ... sw uns +) a/ besles- ats 0404 .199 47 Normal No. rainy days (over 0.10 inch), fs........... 28 199 48 Normal No. dry days (0.10 inch or less), fs........... 72 257 49 Dry days, percentage of total, fs (per cent)........... 27 90 50 Days in longest normal rainy period, fs.............. 21 99 51 Days in longest normal dry period, fs................ 56 187 52 Mean totaly year Gnches).-., .\. Ficjeteachs so eielerse bees << a.ceee eee aiieee -—15 Normal daily mean, coldest 14 days of year (°F.)..... 40 Normal daily mean, hottest 6 weeks of year (°F.)..... 71.6— Normal!daily: mean, Vear (CF). <<<) 54.5 sisbererait eidackebete 60 Precipitation: ; Normal!daily, mean, fs (Gnch),...)5-t).4% J) seep eek .119 Normal No. rainy days (over 0.10 inch), fs......... S 3 144 Normal No. dry days (0.10 inch or less), fs........... 0 Dry days, percentage of total, fs (per cent)........... 0 Days in longest normal rainy period, fs.............. 92 Days in longest normal dry period, fs................ 0 Mean total> year) Gnehes)::, «<< Sewage ters tetcte ates ot 50— Evaporation: Daily mean, 1Ssr—o6, Jai QUCH) no... cle eee eter .096 Total annual, 1887-88 (inches)...............-...0-- 31.3 Moisture ratios: ST ORSISILE) By JOR, ou's sac a0 sk vadeee ub see en See aD Narrvitibray ie, Fob cc.) is 05.8 Shidc Seek ees ome .90 Normal P/E, Year... 21.../010'< o:dratrg 0 ty) SERRE aA 1.00 Vapor pressure: 527 Normal mean, fs (hundredths inch).................. Humidity: Normal mean, fs (per cent) ire e ties. aan ee 71.0 Normal mean, year (per cent)........-.-------+-+0-- 72.4 Wind: Normal mean hourly velocity, fs (miles)............. 5.0 Sunshine: Normal total duration, fs (hours)................-.. 1,900 —2 Moisture-temperature indices: Normal P/E XT, fs, remainder method.............. 600 —¢ Normal P/E XT, fs, exponential method............. 6,114 Normal P/E XT, fs, physiological method............ 10,0002 1,314 13,511 24,265 Plale 34 35 36 37 38 39 40 41 43 44 45 46 47 48 49 50 51 52 Temperature: Low. Days in normal frostless season (f3).............+-+.- 143 EMO f CAVE css stesso os2 oinyed less slain ae RRS ne 63 ON ONS YE J Bic cesta co elele rac isc oss Bion peee MaRS eines halal ices 0 Remainder summation above 32°, year (thousands).... 11.5— Remainder summation above 39°, fs (thousands)...... 3.9 Exponential summation, fs (hundreds)............... 3.9 Physiological summation, fs (thousands)............. 5.7- AUTISGUN GG SXRUALAUIIDY cai csv rose je 1510s <5) 0 0:8 ie tae ease —25 Normal daily mean, coldest 14 days of year (°F.)..... 25 Normal daily mean, hottest 6 weeks of year (°F.)..... 64.4— Normoalbaiy mean, veari(Cl.) «0.50.5 esis ene oe ae 50 —- Precipitation: Normal daily mean, fs (inch)... ........00..c0000ecee: .112 Normal No. rainy days (over 0.10 inch), fs........... 102 Normal No. dry days (0.10 inch or less), fs........... 0 Dry days, percentage of total, fs (per cent)........... 0 Days in longest normal rainy period, fs.............. 48 Days in longest normal dry period, fs..............+: 0 MGayi TOC) Y CRT CDONGS) 5's)... Saniora sa Senin teats ae 40 lt i a CORRELATION OF DISTRIBUTIONAL FEATURES. 441 TaBLE 84.—Climatic extremes for Pinus echinata—Continued. Plate | Evaporation: Low. High. 53 Daily-mean, 1887-88; ‘fs; Gneh) 0.5/0.0, epee ale gieveita. os .O81 .172 54 Total annual, 1887-88 (inches)...............-..-055 251.2 56.6 Moisture ratios: 58 Normale E278) <5 SEG AIS Mpoces JAR aicrsie Inve ty .64 1.76 59 IN ONT aay PE IS hac asi <5 LC TAGHGER ae INS Re GG cl ibis me 1.96 60 arma Bivear 35.05.00 !a5s Mee a) eon tees .85 1.94 Vapor pressure: 63 Normal mean, fs (hundredths inch).................. 428 600 Humidity: 65 INormalimean,) fs. (pericentss dela 8A Geer Biskagebsid des 68.6 82.7 66 Normal mean; year (per cent)... 6... 56 <5 obijeie os ore 69.5 82.9 Wind: 68 Normal mean hourly velocity, fs (miles)............. aj! 14.9 Sunshine: 69 Normal total duration, fs: (hours)... s. 22S F ete sees 20.3 Moisture ratios: ‘ 58 NOT sli Efe ees fois att ctetitensiele lnstene ats oot ohare .66 59 IN ormnalliar Epi 6 aye ett oie lors is! hs ed Siete Tele artes aerate Hf: 60 INGIMALSE ENV ORT ei Me wctotiasee ee nek ae ate eee .82 Vapor pressure: 63 Normal mean, fs (hundredths inch).................. 345 Humidity: 65 Normalymean ifs) Mer cents: seaise cio rcs @ a cee cere 68.6 66 Normaljmeanyy esr (per Cent) oa eaters Othe Bip 0 36 Cala Gavas facie, Boose, brea eieye io our eis asnlicinle’e elelatee veiethine ¢ 146+ 37 Remainder summation above 32°, year (thousands).... 10.0— 38 Remainder summation above 39°, fs (thousands)...... 2.6 39 Exponential summation, fs (hundreds)............... 3.0 40 Physiological summation, fs (thousands)............. 2 41 PA GOLULE IPAMINNSVAUUTEN sg. asi’ wiynsiassas 0a ces sere of eben aap Rar oleae 5 —59 43 Normal daily mean, coldest 14 days of year (°F.)..... 0 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 64.4 — 45 Normakidaily, mean, year (CR) o:ij% 5, «210s, aidinisyn taal zat 35 Precipitation: 46 Normal: daily mean, fs (inch) > 2)-j-;thes » oye eee 4s .099 47 Normal No. rainy days (over 0.10 inch), fs........... 43 48 Normal No. dry days (0.10 inch or less), fs........... 28 49 Dry days, percentage of total, fs (per cent)........... 17 50 Days in longest normal rainy period, fs.............. 22 51 Days in longest normal dry period, fs................ 9 52 Meani total.syear (inehes)) «1st, cicyeieds nisirlnmeaciasetcas 3 - 30-— Evaporation: 53 Daily*meany 1887-83, feiGnch)) 22.2.2 ee ee eee .084 54 Total annual, 1887-88 (inches)..............--.+200- 24.3 Moisture ratios: 58 GMB ALE Te, PR's otis) toa nS ockjarkamichore hike Oe je eee ee 74 59 Der AMPs PO Gt hea fo S12 n-ne Meds a eigen a ea .92 60 Normal PYMES SCAT; ab ies, « year (per Gent) 5.22.60. eset cee ee Zid bash 80.2 Wind: 68 Normal mean hourly velocity, fs (miles)............. Silt 12.4 Sunshine: 69 Normal total duration; fs (hours)".......-....-2..2.. 1,225 1,500+4 Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method.............. 312 500 +4 71 Normal P/E XT, fs, exponential method............. 2,997 4,097 ie Normal P/E XT, fs, physiological method............ 2,747 7,0004 , TaBLE 91.—Climatic extremes for Quercus alba. Plate | Temperature: Low. High. 34 Days in normal frostless season (fs). .............2-- 101 331 35 TE OLN CTR RS coat Bente ee ar ecee ioeode Iie SOP SI Means IRN eeine E 0 215 36 Gold Gaysnfsr ce} occa ck eRe ne echo ee, 0 137 37 Remainder summation above 32°, year (thousands).... 10.0— 18.0+ 38 Remainder summation above 39°, fs (thousands)...... 2.6 10.6 39 Exponential summation, fs (hundreds)............... 3.0 TE 40 Physiological summation, fs (thousands)............. 221 2132 41 Absolute minimums. cricciac. os.6-3 ee Oe ee —48 +19 43 Normal daily mean, coldest 14 days of year (°F.)..... 11 53 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 64.4— 78.8+ 45 Normal daily,mean: year (CE!) io 0... asso. 0s ee 40 65+ Precipitation: 46 Normal daly mean,.fs.(ineh); <\ese es k .091 .170 47 Normal No. rainy days (over 0.10 inch), fs........... 26 284 48 Normal No. dry days (0.10 inch or less), fs........... 0 136 49 Dry days, percentage of total, fs (per cent)........... 8 83 50 Days in longest normal rainy period, fs.............. 17 256 51 Days in longest normal dry period, fs....... a 0 91 52 Megan totale-yeari(Inches)\riisictoc coe diane were teen 30—- 60+ Evaporation: 53 Baily mean; 1887-8) feGnch).esscee eek eee. .O81 . 200 54 otal annual; 1887-887 Gnches)s.. 2s ook Be tees 24.3 54.8 Moisture ratios: 58 Normal PB fart tere eae te eae eee ye ee 51 1.76 59 INOLMAL A). fae eee ee LE Seen ee .60 1.96 Fie ee NOrmal P/E years). t he teen ss pn ae eet ote RN eee 74 1.94 Vapor pressure: 63 Normal mean, fs (hundredths inch).................. 345 622 Humidity: 65 Worms mean, jsi(percent)e acest. oo eee 64.5 83.9 66 Normal mean; ‘year (per'cent)e vias t. ee eee 69.0 85.2 Wind: 68 Normal mean hourly velocity, fs (miles)............. 3.1 14.9 Sunshine: 69 Normal total duration; fs (hours)'> .).2--). se s. kee ee 1,225 2,650 Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method.............. 301 1,418 71 Normal P/E XT, fs, exponential method............. 2,914 13,511 72 Normal P/E XT, fs, physiological method............| 2,747 24,265 446 CORRELATION OF DISTRIBUTIONAL FEATURES. TABLE 92.—Climatic extremes for Fagus atropunicea. Plate | Temperature: Low. High. 34 Days in normal frostless season (fs)..........-++.0.- 95 281 35 PRON GAN BIBI. in) Are ciclatwie crwie eia.5 Sineesninin Ac. eee Id 0 215 36 OLOK CAVE I Bite5 oS. 5 cote one ere nw cio vata ere CRASS 0 149 37 Remainder summation above 32°, year (thousands).... 10.0— 18.0+ 38 Remainder summation above 39°, fs (thousands)...... 2.6 10.6 39 Exponential summation, fs (hundreds)............... 3.0 1P.7 40 Physiological summation, fs (thousands)............. Zt 204 41 (A DB OlUbO Sa Ma G1IM F720, 5 oss eo intern cisielata er ee —44 +12 43 Normal daily mean, coldest 14 days of year (°F.)..... 12 52 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 64.4— 73.8+ 45 Normalidaily mean’ vear (CE) e.o5)0.1c octein Seine Senet cre 40 — 65+ Precipitation: 46 Nornialidaily mean, /s\Gnch));..... sites ois epee oe .O91 .172 47 Normal No. rainy days (over 0.10 inch), fs........... 26 256 48 Normal No. dry days (0.10 inch or less), fs........... 0 154 49 Dry days, percentage of total, fs (per cent)........... 0 83 50 Days in longest normal rainy period, fs.............. 17 256 51 Days in longest normal dry period, fs................ 4 91 52 Mean totaliivear | Gmches)\ 3% se). is LAO E Okiccae SITE & Normal?) He year. sh.) ssc fal Sa en owe Vapor pressure: Normal mean, fs (hundredths inch)................ Humidity: iNormal mean,/fs' (per cent)!s s saws ee 6 sae a ook ky. Normal mean, year (per cent).............ecee00- Wind: Normal mean hourly velocity, fs (miles)........... Sunshine: Normal total duration, fs (hours) ................. Moisture-temperature indices: Normal P/E XT, fs, remainder method............ Normal P/E XT, fs, exponential method Normal P/E XT, fs, physiological method.......... ee) High. 240 +4 150+ 0 26.0 10.0+2% L0°0--4 15.0+4 714 8 50 +2 78.8+ 70+ .020 +2 0 275 +4 100 0 250 +2 10+ =180--e 90 +2 .20+4 .20+4% -10+4 300 +4 504 40 +4 2,700% 100 +4 1,000 +2 1,000 +2 454 CORRELATION OF DISTRIBUTIONAL FEATURES. TaBLE 104.—Climatic extremes for Cephalanthus occidentalis. Plate | Temperature: Low. High. 34 Days in normal frostless season (fs)..............- 111 365 35 FIOUIGBYS fo dissc cao uice b acvervins Fh 5/5, 4b Se ore patos 0 365 36 CGE Va Sis CC eI Coo OR PTS CPS Ch Pe ETC Peer eee 0 137 of Remainder summation above 32°, year (thousands). . 10.0— 26.0+ 38 Remainder summation above 39°, fs (thousands).... 2.6 14.5 39 Exponential summation, fs (hundreds)............. 3.0 15.4 40 Physiological summation, fs (thousands)........... PAs k 31.1 41 SA DSOLULE DAMEN UTE. 5 sic 52 a's) a tesa omc atevateoeae eoperehae a —43 +41 43 Normal daily mean, coldest 14 days of year (°F.)... 11 69 44 Normal daily mean, hottest 6 weeks of year (°F.)... 64.4— 78.8+ 45 Normal daily mean, year (°F.)............-.-.006- 40— 75+ Precipitation: 46 Normal daily mean, fs (inch)|)>..).- 262+ os .2 see. .017 .172 47 Normal No. rainy days (over 0.10 inch), fs......... 0 284 48 Normal No. dry days (0.10 inch or less), fs......... 0 283 49 Dry days, percentage of total, fs (per cent)......... 0 100 50 Days in longest normal rainy period, fs............ 0 256 51 Days in longest normal dry period, fs.............. 4 283 52 Mean total. year 'Gnehes).;«:..:/3. ise eee hele sete 10— 60+ Evaporation: 53 Daily mean, 1887—S88, 76: Gneb) ..<'<.5 6, <.16 oieroyoievs o's 'ai oat .081 . 240 +4 54 Total annual, 1887-88 (inches).................-2- 24.3 101.2 Moisture ratios: 58 IN prate Eyhlg J Os arse hos oe, Sole ae 28 tess emit Ie ee .08 1.76 59 DUR CRI Lae PeM o oars shes 3 wn sn. in 0 '0cr0.48 ol late eee aaa .10 1.96 60 MRONIBL £16. VEO Sees Wind: 68 Normal mean hourly velocity, fs (miles)............. Sunshine: 69 Normal total duration, fs (hours)................... Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method.............. 71 Normal P/E XT. fs, exponential method............. 72 Normal P/E XT, fs, physiological method............ TABLE 111.—Climatic extremes for Carnegiea gigantea. Plate | Temperature: Low. High. 34 Days in normal frostless season (fs) ..............-. 262 305 35 IOC AAV Hy iSie ec sti cone Ru ote hala can spss 186 211 36 MONON MOR Faia dic Pocw sete cic cre aieele wlohe sare, soem 0 0 37 Remainder summation above 32°, year (thousands). . 11.5+ 26.0+ 38 Remainder summation above 39°, fs (thousands).... 6.0= 10.1 39 Exponential summation, fs (hundreds)............. 6.0= 11.8 40 Physiological summation, fs (thousands)........... 12.5 20.6 41 FADSGII LO MHITISEIRUNERRG csc a'ete os wa «ab oo aioe ne eee hae +10 +22 43 Normal daily mean, coldest 14 days of year (°F.).. . 45 50+ 44 Normal daily mean, hottest 6 weeks of year (°F.)... 71.6 78.8+ 45 Normal daily mean; ‘year (CF.)..). 05.6. wx, «devia eine 65 — 70+ Precipitation: 46 Normal daily mean, fs (inch)..............eceeees .020—2 .040 +4 47 Normal No. rainy days (over 0.10 inch), fs......... 0 0 48 Normal No. dry days (0.10 inch or less), fs......... 2254 283 49 Dry days, percentage of total, fs (per cent)......... 100 aint 50 Days in longest normal rainy period, fs............ 0 a 51 Days in longest normal dry period, fs.............. 1752 283 52 Mean total, Rent (OHOA) so. 66. cuicivieaeh cis icrwitiaieceretels 10-— 10+ CORRELATION OF DISTRIBUTIONAL FEATURES. 459 TABLE 111.—Climatic extremes for Carnegiea gigantea—Continued. Plate | Evaporation: 53 Daily mean, 1887-88, fs (inch) 54 Total annual, 1887-88 (inches) Moisture ratios: 58 Normal P/E, fs 59 Normal x/E, fs 60 Normal P/E, year Vapor pressure: 63 Normal mean, fs (hundredths inch)................ Humidity: 65 Normal mean, fs (per cent) 66 Normal mean, year (per cent) Wind: 68 Normal mean hourly velocity, fs (miles) Sunshine: 69 Normal total duration, fs (hours) Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method vil Normal P/E XT, fs, exponential method Normal P/E XT, fs, physiological method Plate | Temperature: Low. High. 34 Days in normal frostless season (fs)................. 119 328 35 EROUIG AYE 9 Syke a5 ois @ cka.cko oie sare sere iaret rn clanstore oe oiare sisal 55 215 36 CE GEG EGE 13. Senne eee oe we ORIN A NMEA Ce Srey eee dee 0 140 37 Remainder summation above 32°, year (thousands).... 15 26.0— 38 Remainder summation above 39°, fs (thousands)...... 4.3 10.6 39 Exponential summation, fs (hundreds)............... BEY tb Ley 40 Physiological summation, fs (thousands)............. 5.6 21,2 41 Absolute sms nam 67/503) vars. sehelatbevontels SRT SE —43 +20 43 Normal daily mean, coldest 14 days of year (°F.)..... 9 53 44 Normal daily mean, hottest 6 wéeks of year (°F.)..... 71.6 78.8+ 45 Normal daily;smean: year (CE). .cc..8-ae sie. dc% asters 45 — 70+ Precipitation: 46 Normal: daiky: mean; fs) Gneh) =. /. ee tSc cca clseinc beets < .080 YP? 47 Normal No. rainy days (over 0.10 inch), fs........... 29 284 48 Normal No. dry days (0.10 inch or less), fs........... 26 211 49 Dry days, percentage of total, fs (per cent)........... 8 81 50 Days in longest normal rainy period, fs.............. 18 172 51 Days in longest normal dry period, fs................ 11 83 52 Mean total, year (inches)..... NE HELA Ret OS ca 20 60+ Evaporation: 53 Daly mean, 1887—Ss,je(Gneh) AG... Saks ee See LE . 200 54 Total annual, 1887-88 (inches)...................... 31.0 56.6 Moisture ratios: 58 Morinal- F/R fae: osc eRe Ft Sn wats 43 1.36 59 Normale) JNias 3.35.22 so ee ee ee oe 47 1232 60 Normals P/He yearet: os ean atk ae eee .38 1.47 Vapor pressure: 63 Normal mean, fs (hundredths inch)........:......... 415 622 Humidity: 65 Normal (mean, fs! (percent). 8 ee ae eee ae boo iek ue 59.4 80.6 66 Normal mean’ year (per Cent) oni. o'c)s s/s celeste 59.3 85.2 Wind: 68 Normal mean hourly velocity, fs (miles)............. 4.7 14.2 Sunshine: 69 Normal total duration, fs (hours) ................... 1,267 2,650 Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method.............. 229 1,314 71 Normal P/E XT, fs, exponential method............. 2,205 11,956 72 Normal P/E XT, fs, physiological method............ 3,682 23,652 460 CORRELATION OF DISTRIBUTIONAL FEATURES. TABLE 113.—Climatic extremes for Solidago missouriensis. Plate | Temperature: Low. High. 34 Days in normal frostless season (fs).............++.. 53 323 35 Hotidsys. farses ccdews waids on as vaesvas ee ceerere 0 215 36 CDIGIARVE IBS tes SRO ta et oni LO aE eee 0 158 37 Remainder summation above 32°, year (thousands).... 10.0— 18.0+ 38 Remainder summation above 39°, fs (thousands)...... 7 10.6 39 Exponential summation, fs (hundreds)............... 3.0 117 40 Physiological summation, fs (thousands)............. 4.0 AZ 41 FA DSOMUCO IMIDIMUIIN 25) 2)2.4,5.4 © o.4-a:c\5, 21 retake renteea tetas —59 +15 43 Normal daily mean, coldest 14 days of year (°F.)..... 0 53 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 64.4— 78.8+ 45 Normal daily mean; yeati CH.) :.sv1coe0s once hee ees 45 70 Precipitation: 46 Normal daily mean, fs:(neh)-*....\ 20 + ac.c see cere .029 .147 47 Normal No. rainy days (over 0.10 inch), fs........... 0 257 48 Normal No. dry days (0.10 inch or less), fs........... 26 211 49 Dry days, percentage of total, fs (per cent)........... 14 100 50 Days in longest normal rainy period, fs.............. 0 172 51 Days in longest normal dry period, fs...............- 11 202 52 Mean.total,' year’ Cinchés)).«.: «2... <2 9R28Gk Ds eee 10— 40+ Evaporation: 53 Daily mean, 1887-88; Js Geb). <-50.6 6 occ cc oo 6 cies oon .118 275 54 Total annual, 1887-88 (inches)...............e.e000- 27:2 76.5 Moisture ratios: : 58 INGIIMAWR LE Ope ecaous Saeiiamied aoe Sate Se Eee ou 1.01 59 © WOrmallne ie site asteried satan nme eee ace peas soe 26 1.05 60 Normal P/E year 22 2yo foe oee ek hee re Tene 18 1.16 Vapor pressure: 63 Normal mean, fs (hundredths inch).................. 253 622 Humidity: 65 Normal! mean: fe (percent)... .:- -esenc tack eseoe eee 47.9 80.2 66 Normal’ mean, year'(per cent)-o2.2>-+-.)ss tee ents ee Sart 85.2 Wind: 68 Normal mean hourly velocity, fs (miles)............. 6.0 14.2 Sunshine: 69 Normal total ‘duration, fs (hours))... 7.0.22. 2 ee eee: L127 2,650 Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method.............. 58 114.2 (fl Normal P/E XT, fs, exponential method............. 563 10,331 72 Normal P/E XT, fs, physiological method............ 710 20,570 Plate 34 35 36 37 38 39 40 41 43 At 45 46 47 48 49 50 51 52 TABLE 114.—Climatic extremes for Gutierrezia sarothre. Temperature: Days in normal frostless season (fs) Hot days, fs Cold days, fs Remainder summation above 32°, year (thousands)... . Remainder summation above 39°, fs (thousands) Exponential summation, fs (hundreds) Physiological summation, fs (thousands)............. Absolute minimum Normal daily mean, coldest 14 days of year (°F.)..... Normal daily mean, hottest 6 weeks of year (°F.)..... Wonmal) daily mean, verti (OB): icccay ww machine ieee Precipitation: Normal daily mean, fs (inch) Normal No. rainy days (over 0.10 inch), fs Normal No. dry days (0.10 inch or less), fs Dry days, percentage of total, fs (per cent) Days in longest normal rainy period, fs Days in longest normal dry period, fs Mean total, year (inches) CORRELATION OF DISTRIBUTIONAL FEATURES. 461 TaBLE 114.—Climatic extremes for Gutierrezia sarothre—Continued. Evaporation: Waily;méan;) 1887-88, fe (neh). «...s,<0%)s dcteniseaeekioas Total/annual pIS8%—SS8. GNCHES)/s... 0.00.0 sslaloccccwsoe 27. Moisture ratios: NormaltP ie ais... hata srsetrh Setar. ca gos Ord Siders Normale HP if sit eo 715s SERA TWAT Phe SORE ots screen: INormalee i Svear ioc... s 2.2 ceptors Akl lusteoers Vapor pressure: Normal mean, fs (hundredths inch).................. Humidity: iNormalimean 7s (percent) sje) scorer ate Seok aie lete och 2 INormalimean,, year) (per cent))... .-. <. -.c% od avmnen ees 2 Wind: Normal mean hourly velocity, fs (miles)............. Sunshine: Normal total duration, fs (hours) ................... 1,127 Moisture-temperature indices: Normal P/E XT, fs, remainder method.............. 27 Normal P/E XT, fs, exponential method............. Normal P/E XT, fs, physiological method............ 710 4,080 Plate 70 71 72 Temperature: Days in normal frostless season (fs)................. 53 ID UGA EI78 csc h tos Hee ae ER ere ee Peo Cte) Gl CERES CORRS PM epee eee OE ee Mea Neh eno sm OE We Remainder summation above 32°, year (thousands) .... Remainder summation above 39°, fs (thousands)...... Exponential summation, fs (hundreds)............... Physiological summation, fs (thousands)............. FAT DSOMELOAIIINITNUNR siya ee oks arc oe iy eanieie nisi Be ee Normal daily mean, coldest 14 days of year (°F.)..... Normal daily mean, hottest 6 weeks of year (°F.)..... Normalidaily,mean year (Cl.)icecs cc. snes ee ee ane Precipitation: Wormalidailvimean? fsiGneh) 45,525 acorns ke Normal No. rainy days (over 0.10 inch), fs........... 0 Normal No. dry days (0.10 inch or less), fs........... 41 Dry days, percentage of total, fs (per cent)........... 23 Days in longest normal rainy period, fs.............. 0 Days in longest normal dry period, fs................ 13 — | on AkRODNWNOOO On | a2Oooo i | Mean; total; -vear iGnches))..)5.:0)s 23 sc sists Sloe Scleeean. 10— Evaporation: Daily, mean; 1887—S8. fa) (inch) 55 3. oiene ssc) eisieesusiarswsa tienes Total annuals 1887-88 Gnehes))..< (5... 6: 5.6 cede cs caugaoe 22" Moisture ratios: IN oy gst) 27 Ot fee os a, Ae fale a eae Normalan fe)5. ic sceeh tee pee Precipitation: Normal daily mean, fs (inch) Normal No. rainy days (over 0.10 inch), fs Normal No. dry days (0.10 inch or less), fs Dry days, percentage of total, fs (per cent) Days in longest normal rainy period, fs Days in longest normal dry period, fs Mean total, year (inches) Evaporation: Daily mean, 1887-88, fs (inch) Total annual, 1887-88 (inches) Moisture ratios: Normal P/E, fs Normal x /E, fs Normal P/E, year Vapor pressure: Normal mean, fs (hundredths inch) Humidity: Normal mean, fs (per cent) Normal mean, year (per cent) Wind: Normal mean hourly velocity, fs (miles) Sunshine: Normal total duration, fs (hours) Moisture-temperature indices: Normal P/E XT, fs, remainder method Normal P/E XT, fs, exponential method Normal P/E XT, fs, physiological method Plate | Temperature: 34 Days in normal frostless season (fs) 35 Hot days, fs 36 Cold days, fs 37 Remainder summation above 32°, year (thousands).... 38 Remainder summation above 39°, fs (thousands) 39 Exponential summation, fs (hundreds) 40 Physiological summation, fs (thousands)............. 41 Absolute minimum 43 Normal daily mean, coldest 14 days of year (°F.) 44 Normal daily mean, hottest 6 weeks of year (°F.) 45 Normal daily mean, year (CF). occ sccvc Liu We eee Precipitation: 46 Normal daily mean, fs (inch) 47 Normal No. rainy days (over 0.10 inch), fs 48 Normal No. dry days (0.10 inch or less), fs 49 Dry days, percentage of total, fs (per cent) 50 Days in longest normal rainy period, fs 51 Days in longest normal dry period, fs 52 Mean total, year (inches) — MORO | o AkROSHwWNHOOOn as | oD _ to om —] ~I he to to oc Plate 53 54 58 59 60 63 65 66 68 69 71 CORRELATION OF DISTRIBUTIONAL FEATURES. 469 TABLE 126.—Climatic extremes for Dulichium arundinaceum—Continued. Evaporation: Daily mean, 1887-88, fs (inch) Total annual, 1887-88 (inches) Moisture ratios: Normal P/E, fs Normal x/E, fs Normal P/E, year Vapor pressure: Normal mean, fs (hundredths inch) Humidity: Normal mean, fs (per cent) Normal mean, year (per cent) Wind: Normal mean hourly velocity, fs (miles) Sunshine: Normal total duration, fs (hours) Moisture-temperature indices: Normal P/E XT, fs, remainder method Normal P/E XT, fs, exponential method Normal P/E XT, fs, physiological method TaBLE 127.—Climatic extremes for Spartina michauziana. Plate 34 35 36 37 38 39 40 41 43 44 45 46 47 48 49 50 51 52 53 54 58 59 60 63 65 66 68 69 70 71 72 Temperature: Low High Days in normal frostless season (fs)................. 53 231 ROTARY Bf Bie cass tes oto ows erento rnc a's Rima aitersia ee eee 0 153 CATH i FAA 2) OR A a i Pe A ae ORE ed Mp Ue 0 158 Remainder summation above 32°, year (thousands).... 10.0— 18.0+ Remainder summation above 39°, fs (thousands)...... 2.6 5 Exponential summation, fs (hundreds)............... 2.8 8.1 Physiological summation, fs (thousands)............. 2.1 14.6 PSH OLULS TOINEDIUIN. <6: 5) os; anh) oireyane lose leharn) or aye exer srovalin ta) cle —65 Ses Normal daily mean, coldest 14 days of year (°F.)..... 0 42 Normal daily mean, hottest 6 weeks of year (°F.)..... 64.4 — 78.8+ Normal daily mean; year (CH) 5.6. << «.<.cis's oe ote a sieloine 35 60+ Precipitation: Normal dailvamesn/s Gnch)).... ee ec ae peice cien 045 136 Normal No. rainy days (over 0.10 inch), fs........... 0 182 Normal No. dry days (0.10 inch or less), fs........... 19 140 Dry days, percentage of total, fs (per cent)........... 11 100 Days in longest normal rainy period, fs.............. 0 144 Days in longest normal dry period, fs................ 4 144 Mean total year (INCHES) ices score cles Chie a slo's Uista sieloeks 20- 60+ Evaporation: Daly mean; 1887—88, Ja) Ginch))z\.5).20) ««e'c\s 4 ule we nusiel tere .084 234 Motabanniual, W887—SS:GNnobes) na. eaciciercietclorce ee cscoe. 20.3 54.8 Moisture ratios: INGIMNALE MEALS A. acid nate RoR Ga ee ee 21 1.39 Normalan Hs Jeo. As sence oe Ont ete tees oe aioe 26 1.63 Normale i. year s,s score ereace ae eee eee 24 172 Vapor pressure: Normal mean, fs (hundredths inch).................. 249 545 Humidity: INormalimean, fs\(pericent) i cioe soos ey eee 48.0 84.0 Normal mean; year’ (per Cent): cece. 6 cctee ste e 56.1 82.1 Wind: Normal mean hourly velocity, fs (miles)............. ou 14.9 Sunshine: Normal total duration, fs (hours) ................... 1,225 2,166 Moisture-temperature indices: Normal P/E XT, fs, remainder method.............. 72 707 Normal P/E XT, fs, exponential method............. 691 6,858 Normal P/E XT, fs, physiological method............ 809 11,246 470 CORRELATION OF DISTRIBUTIONAL FEATURES. TaBLE 128.—Climatic extremes for Arceuthobium cryptopodum. Plate | Temperature: Low High 34 Days in normal frostless season (fs).......-+++++++5- 83 237 35 MT Ota BASSE Rio skcce telat © nied onjove inte oie oieuss 4, « Sagat ate 0 156 36 GOLAN EVA asics neers cores lae ela ets inlelaisrawit atte etaievete @ 0 88 37 Remainder summation above 32°, year (thousands).... 10.0— 18.0+ 38 Remainder summation above 39°, fs (thousands)...... 2.4 7:6 39 Exponential summation, fs (hundreds)............... 2.4 8.3 40 Physiological summation, fs (thousands)............. 2.6 15.0 41 IA SOLU CE sIMUMNETEAUINLL ooo hc ciclo a veloieic oan 5 RiNele Bete tala oer —45 +2 43 Normal daily mean, coldest 14 days of year (°F.)..... 24 454 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 64.4 78.8 45 INormalidarly meat year i(k s)\4 nic iojore)... svete stn.) -tole torent ots .020—4 .073 47 Normal No. rainy days (over 0.10 inch), fs........... 0 25 +4 48 Normal No. dry days (0.10 inch or less), fs........... 153 2752 49 Dry days, percentage of total, fs (per cent)........... 90 —2 100 50 Days in longest normal rainy period, fs.............. 0 10 51 Days in longest normal dry period, fs................ 91 2504 52 Mean total, year: (Gnches)\.\.(..-1.- 5 tstoal ole etare mielala eet siete 10+ 30+ Evaporation: 53 Daily mean, 1887- 88, fs (inch).............+eeeeeee- .199 .330 54 Total annual, 1887-88 (inches).............-2eeeee0- 56.0 101.2 Moisture ratios: : 58 DUCHY PATE, cing cca sats orp etn eee ale ane maee wile .34 59 UN CRUE To G8 a evo cie-c ois ce sla oa anion ae whe setae wee 2 .39 60 (Normal /ae Year ss iaia ass Ss -iis'e iain ke ene x12 .28 Vapor pressure: 63 Normal mean, fs (hundredths inch).................. 233 300 Humidity: 65 Normal\mean: fs (per Cent)iccess oer sielicis oie otettee cele 37.0 46.7 66 Normal mean, year (per cent)........-.......-...-.; 38.8 63.9 Wind: ° 68 Normal mean hourly velocity, fs (miles)............. 6.7 10.2 Sunshine: 69 Normal total duration, fs (hours)................26- 1,134 1,892 Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method.............. 81 98 cA Normal P/E XT, fs, exponential method............. qe 906 72 Normal P/E XT, fs, physiological method............ 979 1,790 TaBLE 129.—Climatic extremes for Arceuthobium americanum. Plate | Temperature: Low. High. 34 Days in normal frostless season (fs) ..........-.0005- 25 eae a 35 LOT GAVE Sette wi cie bie force cre tetc sas oe. eee bel vee atten te 0 105 36 Wolaidavepiyaricmstie bee ct else nie etauieme Saunas ob okies 26 121 37 Remainder summation above 32°, year (thousands).... 10.0 18.0+ 38 Remainder summation above 39°, fs (thousands)...... 2.4 5.4 39 Exponential summation, fs (hundreds)............... 2.4 5.7 40 Physiological summation, fs (thousands)............. 2.6 8.4 41 PA TSMCLERCG VAXINIDRENI EDT oy oh, /kicic vena fnyace! vce w © & 9 ae OR are ranean —49 +11 43 Normal daily mean, coldest 14 days of year (°F.)..... 17 34 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 64.4— 78.8+ 45 Normal daily mean, Year (CRs) ...2 5.00.5 0x. cawsoielonls pie bie 40— 60 Precipitation: 46 Normal daily mean, fe (inch). ...........0..ceeecceee .009 .067 47 Normal No. rainy days (over 0.10 inch), fs........... 0 25+4 48 Normal No. dry days (0.10 inch or less), fs........... 104 216 49 Dry days, percentage of total, fs (per cent)........... 90-4 100 50 Days in longest normal rainy period, fs.............. 0 rf 51 Days in longest normal dry period, fs................ 100 —4 216 52 Riear total, wear (NOHOd) « silirc cet cbc beitonicc mee 10+ 50+ CORRELATION OF DISTRIBUTIONAL FEATURES. 471 TABLE 129.—Climatic extremes for Arceuthobium americanum—Continued. Plate | Evaporation: Low. High. 53 Daily means L887—Ss, fs GNCh) s..c.. yr ek ewes ee ecla ss .100 .3849 54 Total annuah 1887-88: Gnches) .2 2 cccae ces awe se ess 42.8 100.6 Moisture ratios: 58 IN Or alle HME I8) bears ae tector ais eco Aantal. GRE .04 .34 59 Normal aa 76's. cls ote 2 AO Reese ene aot elas tae .06 .39 60 INOrmalUeEe Veal: ctors/ acters tow cetera aioe Me oe alate .09 1.302 Vapor pressure: 63 Normal mean, fs (hundredths inch).................. 183 285 Humidity: 65 Wormalmean? fs (per Gent)s.. sO ass aes ocenm ae. 22.6 54.6 66 Wormial mean. year (per eCent)).) = <<: 2c. oreo = 2 oe oe eels 29.7 64.8 Wind: 68 Normal mean hourly velocity, fs (miles)............. 4.9 11.4 Sunshine: 69 Normal total duration, fs (hours) ................... 16127, 1,927 Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method.............. 13 3002 71 Normal P/E XT, fs, exponential method............. 127 950 TP Normal P/E XT, fs, physiological method............ 197 1,475 TaBLE 130.—Climatic extremes for Phoradendron flavescens and varieties. Plate | Temperature: Low. High. 34 Days in normal frostless season (fs)..............4-. 148 365 35 GHIA VS sf Soo atars chase | Plate | Temperature: 34 Days in normal frostless season (fs) 35 Hot days, fs 36 Cold days, fs 37 Remainder summation above 32°, year (thousands).... 38 Remainder summation above 39°, fs (thousands) 39 Exponential summation, fs (hundreds) 40 Physiological summation, fs (thousands) 41 Absolute minimum 43 Normal daily mean, coldest 14 days of year (°F.)..... 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 45 NOMmMalidesly mean, Year (Es) <6. i axiscen nee wee Precipitation: 46 Normal daily mean, fs (inch) 47 Normal No. rainy days (over 0.10 inch), fs 48 Normal No. dry days (0.10 inch or less), fs 49 Dry days, percentage of total, fs (per cent)........... 50 Days in longest normal rainy period, fs 51 Days in longest normal dry period, fs 52 Mean total, year (inches) CORRELATION OF DISTRIBUTIONAL FEATURES. 475 TaBLE 135.—Climatic extremes for Spermolepis echinata—Contin ued. Plate | Evaporation: Low. High. 53 Daly meanwlS8(—sS.7s) Neh) ino. e6 6 des «eae see .102 .330 54 Total annual, 1887-88 (inches)...................06- 36.7 101.2 Moisture ratios: 58 (MormaliF les fay: |. ¢ .(pleigets te. 2 Seas ewe reak .12 1.36 59 Digna ep 8 ao 3, oso, < SNS Ela SO wrTae: HAO 3 1.52 60 israel EA A VOAN 5, 215 x o'eise ata 4 a < o OSS AR SY, ees .03 1.47 Vapor pressure: 63 Normal mean, fs (hundredths inch).................. 300 675 Humidity: 65 Normal mean, (fs (per; cent))..)2c)h4 oe sia ae He hac 36.3 81.9 Goer Normal:mean, year. (per. cent)).)...624 <<. is sats Woes = 38.8 85.2 Wind: 68 Normal mean hourly velocity, fs (miles)............. 4.5 1223 Sunshine: 69 Normal total duration, fs (hours) ................... 1,895 2,995 Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method.............. 98 1,314 71 Normal P/E XT, fs, exponential method............. 625 12,106 72 Normal P/E XT, fs, physiological method............ 1,186 23,652 TaBLE 136.—Climatic extremes for Daucus pusillus. Plate | Temperature: Low. High. 34 Days in normal frostless season (fs)................. 166 365 35 PROTO RY Ss ofS ore ie tete avs srercuebays oh oie oa ayer aus evs eas ae ee 0 365 36 SONG 87S ator oy odes avclinse tates eich lel oVronS cate hepa ae oes 0 43 37 Remainder summation above 32°, year (thousands).... 10.0 26.0+ 38 Remainder summation above 39°, fs (thousands)...... 3.5 14.5 39 Exponential summation, fs (hundreds)............... re 15.4 40 Physiological summation, fs (thousands)............. 23.9 ail 41 PAs Ol Ube MMI GAUITIA re, Sie he iors leretopeeancnsl cals soedolete cane —30 +41 43 Normal daily mean, coldest 14 days of year (°F.)..... 31 69 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 64.4 78.8 45 INOnMAlGally, mean, Vearn(CH) ip ies cess a :cic cies oun enue 50- 75+ Precipitation: 46 Normalidaslyamesn: f6 GmMCh)).\./- tus .cpchaus cacirystensucisievdveveie tc .020 .199 47 Normal No. rainy days (over 0.10 inch), fs........... 0 284 48 1 Normal No. dry days (0.10 inch or less), fs........... 0 294 49 Dry days, percentage of total, fs (per cent)........... 0 100 50 Days in longest normal rainy period, fs.............. 0 256 51 Days in longest normal dry period, fs................ 0 299 52 Meanstotal, yeariGnches) jects cr aisles cielo oie hs eee 10-— 90 Evaporation: 53 Daily mean; 1887-88, fs (neh): «.- .<. << Sei Semele aielease .052 .330 54 Total annual, 1887-88 Gnches)).......2..n0.ccecccees 18.1 101.2 Moisture ratios: 58 Normal Py dn IS 58 «5 oS AEG Aas oe BES, Toke a2 3.84 59 Normal’ ay Efe. osc MELO. Ee eee ae eee als 4.48 60 IN GTM S PE NG CAN atascciers seco oe a oie .03 4.90 Vapor pressure: 63 Normal mean, fs (hundredths inch).................. 300 707 Humidity: 65 INormalimean, fs (per cent) ences eee eee Soe eioek 36.3 87.5 66 Normal mean, -year\(pericent)): ws aesneie. cs cee dete. 38.7 86.8 Wind: 68 Normal mean hourly velocity, fs (miles)............. 3.5 16.4 Sunshine: 69 Normal total duration, fs (hours) ..................- 1,578 2,995 Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method.............. 68 1,418 71 Normal P/E XT, fs, exponential method............. 625 13,511 72 Normal P/E XT, fs, physiological method............ 1,186 24,265 476 Plate 70 CORRELATION OF DISTRIBUTIONAL FEATURES. TABLE 137.—Climatic extremes for Parietaria debilis. Temperature: Days in normal frostless season (fs) ROU OB YS Oks ore ofc Api coacioasino cle Meioln d sienna meee CLONE NBs PBS vies oe ouskelicniel ae ed eke ovoid cian eee oe Remainder summation above 32°, year (thousands)... . Remainder summation above 39°, fs (thousands)...... Exponential summation, fs (hundreds)............... Physiological summation, fs (thousands)............. Absolute minimum.......... ey Normal daily mean, coldest 14 days of year (°F.)..... Normal daily mean, hottest 6 weeks of year (°F.)..... Normal daily mean, year (°F .)........5.0.0 oases sleidslleere Precipitation: Normal daily mean, fs (inch) . oa 4 told wales oc O)6. © wt 0 inte O's @ Normal No. rainy days (over 0.10 inch), fs........... Normal No. dry days (0.10 inch or less), fs........... Dry days, percentage of total, fs (percent) .......aseeee Days in longest normal rainy period, fs.............. Days in longest normal dry period, fs................ Mean total, year (inches)... . Evaporation: ole 6 '9 6) 0)e 8 ste we» vy wis = Ss we te Daily mean, 1887-88, fs (inch)...........-...e.20005 Total annual;\1887—88\ Gnches) hostels Pen Moisture ratios: Normale //E rience eines Normalan (P58. sts cts aenee Normal P/E, year.......... Vapor pressure: Normal mean, fs (hundredths inch).................. Humidity: Normal mean, fs (per cent)... Normal mean, year (per cent) Wind: Normal mean hourly velocity, fs (miles)............. Sunshine: Normal total duration, fs (hours)................... Moisture-temperature indices: Normal P/E XT, fs, remainder method.............. Normal P/E XT, fs, exponential method............. Normal P/E XT, fs, physiological method............ C6) 0-0 6 0 w owe! wig 6a nie 6 81.9 85.2 12.3 2,995 1,314 12,106 23,652 Plat e| Temperature: 34 35 36 37 38 39 40 41 43 44 45 46 47 48 49 50 51 52 Days in normal frostless season (fs) ..........-...+.. TR OUTLRVBIIIG «6.5.0 doesn civ-ners ae COlddave: JO. oe ilecccn oe « Remainder summation above 32°, year (thousands).... Remainder summation above 39°, fs (thousands)...... Exponential summation, fs (hundreds)............... Physiological summation, fs (thousands)............. Absolute minimum.......... Normal daily mean, coldest 14 days of year (°F.)..... Normal daily mean, hottest 6 weeks of year (°F.)..... Normal daily mean, year (PR .)\. ssi(inoh) jcc. See cece eee eee .O89 .131 47 Normal No. rainy days (over 0.10 inch), fs........... 26 161 48 Normal No. dry days (0.10 inch or less) fs........... 19 136 49 Dry days, percentage of total, fs (per cent)........... ll 83 50 Days in longest normal rainy period, fs.............. 21 117 51 Days in longest normal dry period, fs................ 4 91 52 Niean totaly vear (inches). <0) 7 ese cocces cee 30- 60+ CORRELATION OF DISTRIBUTIONAL FEATURES. 483 TABLE 147.—Climatic extremes for Flerka proserpinacoides—Continued. Plate | Evaporation: Low. High. 53 Daily mean, 1887-88, fs (inch) jo... 5.0. See. ee eee .084 . 200 54 Total annual, 1887-88 (inches)...............-.00025 20.3 54.8 Moisture ratios: 58 Pnrial Ppl ae.) LAs BUYS, OR AIA A 51 1.39 59 IN OETN Salton EMIS is ois ce NR eae, re eee .60 1.63 60 Normale TA VOCAL ss (sor), coy asta ee vee Cee See —5l1 +30 43 Normal daily mean, coldest 14 days of year (°F.)..... 17 42 44 Normal daily mean, hottest 6 weeks of year (°F.)..... 64.4 — 71.6+ 45 Normal daily meany, year (CE)! ones ccs s 2/<.s are Secletate bi 40 — 60 Precipitation: 46 iNormal dailyanean® fs\(inch))...'. Sutsiiaus de eiereniiavein. oie .025 .199 47 Normal No. rainy days (over 0.10 inch), fs........... 0 199 48 Normal No. dry days (0.10 inch or less), fs........... 72 179 49 Dry days, percentage of total, fs (per cent)........... 27 100 50 Days in longest normal rainy period, fs.............. 0 99 51 Days in longest normal dry period, fs................ 56 216 52 Meanttotal sy eari(Gnehes)is tacts oct is oe cise bie ee 20+ 90 Evaporation: 53 Daly mean Ss7—-sen7s neh). a. see See ee ae eee .052 .293 54 Total annual, 1887-88 (inches)...............-....055 1821 79.8 Moisture ratios: 58 IOrmal 2/5; fab sicrcvnic Se ee tere etait: Tee ee oe eee .18 3.84 59 Normal asi {sins kee systoc, SO ee ee eee .20 4.48 60 Normal PE ACVeaTy s\cicizpcrhafastiatis Meee ee ee cate es 18 4.90 Vapor pressure: 63 Normal mean, fs (hundredths inch).................- 233 329 Humidity: 65 Normalimean, fs (pericent)).see ae Sees oe eee 41.5 73.6 66 Normal mean, year (per cent).................e000: 45.4 76.2 Wind: 68 Normal mean hourly velocity, fs (miles)............- 3.5 16.4 Sunshine: 69 Normal total duration, fs (hours)................+5- 1,167 1,578 Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method............-. 81 1,566 71 Normal P/E XT, fs, exponential method............. 691 11,724 72 Normal P/E XT, fs, physiological method............ 809 7,475 484 CORRELATION OF DISTRIBUTIONAL FEATURES. TABLE 149.—Climatic extremes for Trautvetteria carolinensis. Plate | Temperature: Low. High. 34 Days in normal frostless season (fs)............6-- 145 231 35 EL OUOSVES Gs 5: 1s c.ale Glare} 'eieleievere suche. piece oe ee 63 160 36 Oolidavanysay sd cit oa oc bien iene bitoaias meenes ee 0 66 au Remainder summation above 32°, year (thousands).. 11.5— 18.0+ 38 Remainder summation above 39°, fs (thousands)... . 3.9 7.0 39 Exponential summation, fs (hundreds)............. 3.9 8.2 40 Physiological summation, fs (thousands)........... OE 15.0 41 CADBOLUTO HERETO UINI ) o.'s- 6, csoceyi.s. 0/00 yarns sa ee ae —35 +4 43 Normal daily mean, coldest 14 days of year (°F.)... 16 45 44 Normal daily mean, hottest 6 weeks of year (°F.)... 64.4— 78.8+ 45 Normal daly: mean, year (CE.)is <5 <0: «sa eer Gee 50 — 60+ Precipitation: 46 Normal daily mean, fs (inch)...............eee02. .099 .147 47 Norma! No. ra ny days (over 0.10 inch), fs......... 91 161 48 Normal No. dry days (0.10 inch or less), fs......... 19 93 49 Dry days, percentage of total, fs (per cent)......... 11 49 50 Days in longest normal rainy period, fs............ 52 172 51 Days in longest normal dry period, fs.............. 4 56 52 Mean ‘total;iyear (inches)... .... 204: siaeee eee ae 40— 60+ Evaporation: 53 Daily mean, 1887-88, fs (inch).................22: .1490 —4 . 200 54 Total annual, 1887-88 (inches)...............-.20. 38.3 54.8 Moisture ratios: 58 Normal HE Sia es cm orc araiie asian ee Alsi 1.00 59 VERSIE Ge hon 02% Bosasas ah wettest ah rated .60 1.25 60 Normal: PVA, year fF. ocskccsscc cae SO ee 172 1.16 Vapor pressure: 63 Normal mean, fs (hundredths inch)................ 345 545 Humidity: 65 Normalimean, fe (per:cent)s. 4-5 ens cee 65.6 73.7 66 Normal mean, year (per cent)...............c.0e- 67.5 75.7 Wind: 68 Normal mean hourly velocity, fs (miles)........... sul 9.4 Sunshine: 69 Normal total duration, fs (hours)................. 1,403 1,878 Moisture-temperature indices: 70 Normal P/E XT, fs, remainder method............ 316 593 71 Normal P/E XT, fs, exponential method........... 3,007 5,631 72 Normal P/E XT, fs, physiological method.......... 5,112 10,061 Plate | Temperature: 34 Days in normal frostless season (fs) 35 Hot days, fs 36 37 Remainder summation above 32°, year (thousands).... 38 Remainder summation above 39°, fs (thousands) 39 Exponential summation, fs (hundreds) 40 Physiological summation, fs (thousands) 41 Absolute minimum 43 Normal daily mean, coldest 14 days of year (°F.) 44 Normal daily mean, hottest 6 weeks of year (°F.) 45 Normal daily mean, year (°F.) Precipitation: 46 Normal daily mean, fs (inch) 47 Normal No. rainy days (over 0.10 inch), fs 48 Normal No. dry days (0.10 inch or less), fs 49 Dry days, percentage of total, fs (per cent) 50 Days in longest normal rainy period, fs 51 Days in longest normal dry period, fs 52 Mean total, year (inches) Plate CORRELATION OF DISTRIBUTIONAL FEATURES. 485 TABLE 150.—Climatic extremes for Cebatha diversifolia—Continued. Evaporation: Low. Daily mean, 1887-88, fs (inch).................2.00% .188 Totalcannual; 1887—SS (Gnches))o). 5:25 << c's. 0s ccc wees 60 —4 Moisture ratios: ING a TANG 2 OBS TSS ot Bie ied ele rit cA CR ee Bee ay AZ INGYNL Al alg ES cl ne aes: coe Rio epee ee, a apes a venches ee Ail INGE Ase Ary CARR hepsi ceiente tare\« eteithe thst ae ek one mene 2 Vapor pressure: Normal mean, fs (hundredths inch).................. 300 — Humidity: Normsalemeansy/S) (Der CENt) co kaaie vio etac a aisistseetorerod eer 30.0—4 Normal mean; year (per Cent). siodic.c.01. oe cake se oa 40.0—4 Wind: Normal mean hourly velocity, fs (miles)............. 6.0—4 Sunshine: Normal total duration, fs (hours)................... 2,3004 Moisture-temperature indices: Normal P/E XT, fs, remainder method.............. 100 —4 Normal P/E XT, fs, exponential method............. 1,000 —4 Normal P/E XT, fs, physiological method............ 1,000 —4 High. 101.2 NI TABLE 151.—Climatic extremes for Cebatha carolina. Plate 34 35 36 37 38 39 40 41 43 45 69 70 71 72 Temperature: Low. Days in normal frostless season (fs)................. 172 HAOUOAVS JE a ci re tite so oa are eats dis Diane as et ha wees 129 Orc EMR SS create ae cree Ce arena Mica ELAS Concise ons ee 0 Remainder summation above 32°, year (thousands).... 11.5 Remainder summation above 39°, fs (thousands)...... 6.3 Exponential summation, fs (hundreds)............... 6.7 Physiological summation, fs (thousands)............. 11.9 PN DSOLMILE TINIE =, | =.= Sys! 6 she Seb ees Sd a aS Gea eo aes —32 Normal daily mean, coldest 14 days of year (°F.)..... 32 Normal daily mean, hottest 6 weeks of year (°F.)..... 71.6 Normal daily mean;-year (CH.) ao06 cst oerest s eee on 55+ Precipitation: Normal daily mean: fs (nGh)).,. s s-s csi 5 cis-ace «ase ess .052 Normal No. rainy days (over 0.10 inch), fs........... 39 Normal No. dry days (0.10 inch or less), fs........... 26 Dry days, percentage of total, fs (per cent)........... 8 Days in longest normal rainy period, fs.............. 25—- Days in longest normal dry period, fs................ 14 Mean: total,.veat GQnehes)\: ec Sock siacieeiate sa evecare 20 Evaporation: DWarly«meana LSS 7—Se. 7s) (neh): sate seats) whe tein cvs ote e eas .102 Totaliannual,, [887—88)(Gnches))./. seers ceyo nce cs «oars. eh: « 38.4 Moisture ratios: Nonmasl 22/1 ?}s seu Se eas eae a iss CRN ree at ok Keo .34 ANorma allan / EG S8\- coi vevers eee aS ot are SS Open aie SE Res 47 Normal Ps year’. s/fc see cvs cee Se ie ae ec he = 24 Vapor pressure: Normal mean, fs (hundredths inch).................. 4004 Humidity: iINormial mean; fsi(perjscent)). Be - cee saya alos «ots d6 dees 504 Normal mean year (per Cent) ic a. steeds cigael abe eis roe 63.9 Wind: Normal mean hourly velocity, fs (miles)............. Sal Sunshine: Normal total duration, fs (hours)................... 1,878 Moisture-temperature indices: Normal P/E XT, fs, remainder method.............. 300 —2 Normal P/E XT fs, exponential method............ 3,000 —2 Normal P/E XT, fs, physiological method............ 4,0004 2,650 1,314 12,106 23,652 CORRELATION OF DISTRIBUTIONAL FEATURES. 486 ‘O 'N ‘88103987 edey ‘OCN ‘88109987, ede) “yse Ay ‘Ar0z9B]Y edvy 99¢'T ‘[eQ ‘sopeduy soy 266% “ysv Ay ‘yuourjuroddesiq ede FOL [BO “eyoing 8°98 ‘yeO ‘eyoing g° 28 "BL ‘980 AO LOL ‘ysem ‘A109e,T dv | 06'F ysty ‘A10998 [7 edey StF “yse\y ‘10998 [ adey 8 'E “ZITy ‘YUBINH) WOT 101 *[BQ ‘esouepuedepuy 6FS" “YsB 06 ‘BO ‘sepesuy so'T 66% ‘O(N ‘svioqyqeH odey 99% “UOIBeL Y1OSa(T 001 "BQ ‘sepesuy soy $62 “eT ‘SUBOTIO AONT FRST ‘O(N ‘sBs0qgey odes) OLT” “BLA +¢L ‘BA RL “XAT, “ZNy 8°82 "BLA ‘989. 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NORTHERN MESOPHYTIC EVERGREEN Forest (East) (__ —yCSCC“(SNSNCNC(#NNNNNCCC“*SYS VEGETATION 009 DESERT ———=—E___—S—t—CS Semi-DESERT = LULhLULULCC( Ue GRASSLAND es a GRASSLAND-DECIDUOUS-FORESTTRANSITION Decibuous FOREST ________________ ann) NORTHWESTERN HYGROPHYTIC EVERGREEN FOREST ae SOUTHEASTERN MESOPHYTIC EVERGREEN FOREST CC T__VNu¥TNVTyNy rrr NORTHERN MESOPHYTIC EVERGREEN Forest (West) (Ee SSeS a NORTHERN MESOFHYTIC EVERGREEN Forest (East) (C__——s———————— 494 CORRELATION OF DISTRIBUTIONAL FEATURES. marked but more surprising exist between the Semidesert and the western section of the Northern Mesophytic Evergreen Forest, and between the Grassland and the eastern section of the Northern Meso- phytic Evergreen Forest. These incongruities probably find their explanation in the differences in the seasonal distribution of rainfall and in the compensating influences of other conditions. The western section of the Northern Mesophytic Evergreen Forest (which is much drier than the eastern section) does not include markedly higher values than does the Desert, a fact which must be considered in con- nection with the temperature and evaporation differences between these two vegetations. Mean total annual precipitation (plate 52, fig. 24).—The graph repre- senting these data is given a conventionalized appearance from the fact that we have used the map prepared by Gannett and have not drawn the extremes from the readings of individual stations, as in the case of most of the other climatic conditions. This graph bears a generic resemblance to the one showing the normal daily mean pre- cipitation for the frostless season. Owing to the smoothed nature of the data on which it is based, it shows an even more pronounced gra- dation between the several vegetational areas. No new features are brought out in this figure, as compared with the one just discussed, and indeed some of the indications of the latter are partially con- tradicted by this one. Owing to the fact that the data from plate 52 are not the readings of individual stations, it is impossible to tell in how far the differences between this figure and the preceding are due to this circumstance and in how far they are due to the fact that the PRECIPITATION. Mean ToTat, YEAR VEGETATION 0 90 DESERT SSS SS SEMI-DESERT SS EE See aaa GRASSLAND an | a a ERE GRASSLAND-DECIDUOUS-FORESTTRANSITION CME MET e mame DecinuoUS FOREST Err) NORTHWESTERN HYGROPHYTIC EVERGREEN FOREST C rrr SOUTHEASTERN MESOPHYTIC EVERGREEN FOREST = NORTHERN MESOPHYTIC EVERGREEN Forest (West) (_________ — a > NORTHERN MESOPHYTIC EVERGREEN FOREST (East VEGETATION 052 = DESERT ( ae Semi-DESERT OO are GRASSLAND C{N“™“"w“v"”w” oor GRASSLAND-DECIDUOUS-FORESTTRANSITION a ae ad DecipuoUs FOREST FEE ————E——= NORTHWESTERN HYGROPHYTIC EVERGREEN FOREST Pe PE SEEN SY STS i ae SOUTHEASTERN MESOPHYTIC EVERGREEN FOREST [Se ————_—_E_— ee - =e _ a NORTHERN MESOPHYTIC EVERGREEN Forest (West) (a es a) NORTHERN MESOPHYTIC EVERGREEN Forest (East) (Daan SSS ee Fig. 24. CORRELATION OF DISTRIBUTIONAL FEATURES. 495 former is based on the precipitation data of the frostless season and the latter on those of the entire year. Evaporation (plate 58, fig. 24)—The daily mean evaporation for the frostless season reaches its highest values in the Desert region, although some of the stations situated in this vegetation have readings so low as to give the Desert an amplitude of evaporation conditions which is half that for the entire country. The Semidesert and Grass- land have limits and amplitudes which are closely similar. The Grassland Deciduous-Forest Transition exhibits a minimum which is the same as that of the Grassland, but it has a much smaller amplitude of evaporation conditions, nowhere reaching values as high as the lowest ones for the Desert. The Deciduous Forest has a much greater amplitude than the transition region between it and the Grassland, and its highest values are as great as the minimum values for the Desert, corresponding to the southward prolongation of the Deciduous Forest into Texas. The lowest evaporation values are found in the North- western Hygrophytic Evergreen Forest, and relatively low minimum values are found in the Southwestern Mesophytic Evergreen Forest and in the eastern section of the Northern Mesophytic Evergreen Forest. The maximum values of the three areas just mentioned are remarkably similar. The limits and amplitude of evaporation condi- tions for the western section of the Northern Mesophytic Forest are higher than for any of the other evergreen forest areas, and closely similar to the Semidesert and Grassland values. Moisture ratio (plate 59, fig. 25.) —The data here considered are those derived from the precipitation index for the frostless season and the 30 Moisture Ratio. Normat r/E, F.S. VEGETATION 06 4.48 Desert |___REPORT We | GEES Ser eae an RET | SEMi-DESERT TI oc ne SE ES] GRASSLAND ea) SERRE EE ee PO RT ET ETE GRASSLAND-DECIDUOUS-FORESTTRANSITION ss = —S || } DECIDUOUS FOREST a et NORTHWESTERN HYGROPHYTIC EVERGREEN Forest — (_ ESE UNE aT Eee SOUTHEASTERN MESOPHYTIC EVERGREEN FOREST baa SS ee NORTHERN MESOPHYTIC EVERGREEN FOREST (West) [x ——) NORTHERN MESOPHYTIC EVERGREEN Forest (East) 9 (______ VEGETATION 22.6 Ss” DESERT Res ) Semi-DESERT (ee GRASSLANO Eee) GRASSLAND-DECiDUOUS. FORESTTRANSITION aT Sa =—«F$ er Deciouous Forest (Se ee en |] NORTHWESTERN HYGROPHYTIC EVERGREEN Forest 9 (OE) SOUTHEASTERN MESOPHYTIC EVERGREEN FOREST [ Pa aS ee = =—S NORTHERN MESOPHYTIC EVERGREEN Forest (West! [aa NORTHERN MESOPHYTIC EVERGREEN Forest (East) [___ eC COCOCOCONCOCNCNCNC#”NONON”C(#NNNNNNN¥N’S. Ls) 496 CORRELATION OF DISTRIBUTIONAL FEATURES. days preceding its commencement, and from the evaporation data for the frostless season. The differentiation of the values of this ratio for the vegetational areas is restricted by reason of the extremely high ratios for the Northwestern Hygrophytic Evergreen Forest. The col- lective amplitudes of the other vegetations cover less than half the total range for the United States. The narrow amplitudes of this climatic feature for all the vegetations except the Hygrophytic Forest are an indication of the importance of the moisture ratio as an expression of the conditions which are critical in determining the distribution of the vegetation of the United States as we have charted it. The five vegetations in group A exhibit progressively higher limits for their values of the moisture ratio. The evergreen forest areas of group B are very dissimilar. The western and eastern sections of the Northern Mesophytic Evergreen Forest stand well apart, the former overlapping with the Northwestern Hygrophytic Evergreen Forest, just as these vegetations merge into one another in their actual occur- rence. The Southeastern Mesophytic Evergreen Forest possesses a greater amplitude than the eastern section of the Northern Mesophytic Evergreen Forest, but embraces almost the whole scale of values found in the latter vegetation. The amplitude of the western section of the Northern Evergreen Mesophytic Forest is covered by that of the Semidesert, again emphasizing the relative aridity of this forest region. The amplitude of the eastern section of the Northern Meso- phytic Evergreen Forest is also covered by that of the Deciduous Forest, which is accordant with the overlapping and intermixture of these two forests. Normal mean relative humidity (plate 65, fig. 25).—The relative humidity values for the Desert are so low that they have the effect of restricting the differentiation of the values for the remainder of the country, just as the Hygrophytic Forest does with respect to the moisture ratios. The lowest humidity is to be expected in the Desert, but the highest readings would be looked for in the Northwestern Hygrophytic Evergreen Forest rather than in the western section of the Northern Mesophytic Forest, as is the actual case for our data. This is doubtless due to the unfortunate circumstance that there are no humidity records available for localities in the most humid portions of the coast or mountains of Washington and Oregon. The maximum value is recorded for Eureka, California, which is situated in the red- wood type of mesophytic forest. The Semidesert greatly exceeds the Desert in the range of its humidities, as would be expected in the con- trasting of arid coastal regions with an arid interior region. The blocks showing the range and amplitude of humidity for the Grassland, Grass- land Deciduous-Forest, and Deciduous Forest overlap in a manner which is quite analogous to the occurrence of these vegetations. The transition region possesses an amplitude of humidities which is nearly CORRELATION OF DISTRIBUTIONAL FEATURES. 497 the same as the overlapping of the Grassland and Deciduous Forest. The total amplitude of humidity conditions in the western section of the Northern Mesophytic Evergreen Forest is half that of the entire United States. The other three evergreen-forest areas have ampli- tudes and limits which are extremely similar, and fall within the range for the western section of the northern forest. The range of humidity conditions which appears to favor the evergreen type of forest also falls within the range for the Deciduous Forest. This indicates that the forest regions of the United States as a whole are to be found under very similar humidity conditions (about 70 to 80 per cent) for the frostless season, and that the diversified types of forest comprised in the western section of the Northern Mesophytic Ever- green Forest extend into regions of both lower and higher humidity, overlapping slightly with the highest values of the Desert Region. Normal daily duration of sunshine (plate 69, fig. 26).—The sunshine conditions of the frostless season are imperfectly known for the Desert region, where they might be expected to possess their highest values. Much lower minimum values are found in the Desert than in the Semidesert, and in the latter region are the highest known values. The sunshine conditions are very similar throughout the Grassland, Grassland-Deciduous Forest, and Deciduous Forest regions, reaching the lowest value for the United States in the first-named of these regions. The sunshine conditions are very dissimilar in the four ever- green forest areas, reaching a wide amplitude in the Southeastern Mesophytic and western section of the Mlarehers Mesophytic Ever- green Forests. Sunsnine. Normac Daicy Dunmation, F. S. VEGETATION 1127 2995 OgSERT Semt-DESERT GRASSLAND ee l CEE GRASSLANO-DECIOUOUS-FORESTTRANSITION EES ay ee se ( DECIDUOUS FOREST NORTHWESTERN HYGROPHYTIC EVERGREEN FOREST 2 SOUTHEASTERN MESOPHYTIC EVERGREEN FOREST (___O0____ nes = — NORTHERN MESOPHYTIC EVERGREEN FOREST (WES!) [i $}\| ||) NORTHERN MESOPHYTIC EVERGREEN Forest (East) €¢ ——————————_> °° — zs] VEGETATION 197 24265 DESERT ee SG | SEMI-DESERT $e RSE - Seeeu} GRASSLAND Dis al GRASSLAND-DECIDUOUS-FORESTTRANSITION — Bea DECIDUOUS FOREST a NORTHWESTERN HYGROPHYTIC EverGREEN Forest =e SOUTHEASTERN MESOPHYTIC EVERGREEN FOREST a NORTHERN MESOPHYTIC EVERGREEN Forest (West, [ 3 #) NORTHERN MESOPHYTIC EVERGREEN Forest (East) [srry GTS Soa ne ik a ey 498 CORRELATION OF DISTRIBUTIONAL FEATURES. Normal moisture-temperature index for the growing-season (plate 72, fig. 26).—This composite climatic feature is remarkable in the fact that it possesses a steep gradient of change on passing from the central to the southeastern part of the United States, and possesses a relative uniformity over the western third of the country. The highest values are consequently to be found in the Southeastern Mesophytic Ever- green Forest and the second highest in the Deciduous Forest, both of which areas show wide amplitudes of this condition. The lowest value is found in the Desert, which is closely approached by the Grassland and the western section of the Northern Mesophytic Evergreen Forest. The last-named vegetation, by reason of its western position, possesses a very narrow amplitude of the conditions expressed by this index. The distribution of the various values of the moisture-temperature index is such as to give closely similar limits and amplitudes to such dissimilar vegetations as Desert, Grassland, Northwestern Hygro- phytic Evergreen Forest, and the eastern section of the Northern Mesophytic Evergreen Forest, with the Deciduous Forest overlapping into this range of conditions. III. CONDITIONS THAT PROBABLY DETERMINE THE GENERAL VEGETATIONAL AREAS. 1. OBSERVATIONS FROM THE CHARTS. The foregoing review of the comparative ranges and intensities of 12 of the leading climatic conditions for the 9 vegetational areas of the United States has served to throw some light on the question as to which of these conditions are most important in controlling the limits of the vegetations. The wide amplitude of all of the temperature conditions has indicated that in the case of many of the vegetations, and particularly those with a wide north-and-south extent, there may be found parallel series of temperature conditions in two cr more vegetations. The moisture-temperature index also fails to exhibit differences between the several vegetational areas such as to give it importance as representing a controlling factor. The precipitation and evaporation data fall much more nearly into groups of intensities and amplitudes which show dissimilarity throughout the series of vegetational areas. The moisture ratio and the relative humidity also show striking differences between the various vegetations. Figures 27 to 35 give, in diagrammatic form, the limits and ampli- tudes of 17 selected climatic factors for each of the 9 generalized vege- tational areas. These diagrams make it possible to view the correla- tion of climate and vegetation from a different angle to that employed in the immediately preceding pages. Here it is possible to see a diagrammatic picture of the climate of each of the vegetations, to note whether each of the conditions ranges through a series of values which CORRELATION OF DISTRIBUTIONAL FEATURES. 499 are high or low as contrasted with conditions in other parts of the United States, and to observe whether the amplitude of each condition is wide or narrow. The last has already been mentioned as a valuable means of discovering the climatic conditions which are most critica] in controlling the distribution of a given vegetation or plant. It will be fruitful to discuss these diagrams in connection with a comparison between the vegetational boundaries and the isoclimatic lines of the corresponding plates. In this manner it will be possible to test out the indications given by narrow amplitudes of the condi- tions (short blocks in the diagrams), and not only to discover which of the various conditions appear to be the most potent in controlling distribution, but also to find the particular intensity of the condition which seems to be critical in each case. Desert (fig. 27).—Wide amplitudes are exhibited by the Desert with respect to all of the temperature conditions, the number of days in the longest normal dry period of the frostless season, and also the daily mean evaporation for the frostless season. Narrow amplitudes are exhibited by the number of days in the longest normal rainy period of the frostless season and by the moisture ratios and sunshine dura- tion (plate 69). An examination of the 6 plates showing the isoclimatic lines for the temperature conditions used on this diagram (plates 34, 35, 36, 40, 43, 45) will discover that numerous lines cross the Desert region in a northwest-southeast direction, indicating, as we have already been prepared to find, from the wide amplitudes in figures 21 and 22, that the Desert possesses a wide range of temperature conditions, which may also be found to the eastward in three or four other vegetations. Temperature Days in Norma Frostiese Season (F. S.) ee Hor Days, F. S. PO Coup Days, F.S. ay > PrysioLocicat Summation, F. S. OE ee Norma Daity Mean, covoest 14 oars or Yeam (_—C—~—“‘t*~*~*~*~*C*C«i ee Normat Daicy Mean, Year =] % PRECIPITATION Normmat Dairy Mean, F. S. fo Sei NE Se a erat ee = Days In LONGEST Normal Rainy Perioo, F.S. HE } Days In LONGEST Normat Dry Peniop, F. S. fe PPE vache oT arts RY sect ee ae Mean Torat, Year BRETT See | EvaPoraTion Dany Mean, 1887-9, F. S. t 25 Moisture Ratios Norma P/E, F. S. ea aaa | Norma 7/E, F.S. a ne wey Normat P/E, Year a ee SET Se | Humioiry Normat Mean, RS. a | SunsHine Normat Dairy Duration, F. S. MoisTURE-TEMPERATURE INDICES Norma. P/E xT, F.S., Puysio.ocicat Mctxoo MER) Fia. 27. Climatic extremes for the Desert. 500 CORRELATION OF DISTRIBUTIONAL FEATURES. We may not, therefore, look for any of the factors controlling the dis- tribution of the Desert region in these features of the temperature. The amplitude of the number of days in the longest normal dry period of the frostless season is also great for the Desert (from 127 to 283). This is a case, however, in which even the minimum value for the Desert indicates dry conditions, and the maximum value signifies conditions of extreme aridity. The lower values of this condition may be of importance elsewhere in differentiating. vegetation, and the various values within the Desert area may be of importance in connec- tion with the minor vegetational differences of that diversified region. The position of the isoclimatic lines of plate 51 does not indicate that any particular intensity of this condition is critical in limiting the Desert region, which is in accord with the evidence of figure 27. The case of the daily mean evaporation is a very similar one. The number of days in the longest normal rainy period in the frost- less season ranges, in the Desert region, from its minimum value of no days at several stations to a maximum of 10 days. The isoclimatic line of 25 days embraces the entire Desert region and portions of the Semidesert, the Grassland, and the western section of the Northern Mesophytic Evergreen Forest. This factor appears to be an impor- tant one and it is possible that numerous and adequately distributed stations would show that the isoclimatic line of 10 days lies near the boundary of the Desert area. The three derivations of the moisture ratio (plates 58, 59, 60) are very similar here, as they are in all of the vegetations and other botani- cal areas. The narrow amplitude of this condition for the Desert area suggests an importance which is borne out by an examination of the isoclimatic lines. The line representing values of 0.20 for the moisture ratio is roughly similar to the outline of the Desert, and suggests that the isoclimatic line of the ratio 0.25 might be a still closer approxima- tion to the limits of the Desert. The area with values below 0.25 would then include more of western Texas, would separate the Mogollon Plateau of Arizona from the Rocky Mountains, and would extend somewhat further north, thereby coming into still closer agreement with the outlines of the map of generalized vegetation. The bend of the isoclimatic line for 0.20 which extends through the Tehachapi Pass into the San Joaquin Valley of California is in accord with the pro- nouncedly desert character of this valley, which has been included in the composite Semidesert Area. All the evidence which we have been able to bring out in this case points to the moisture ratio as being the climatic condition of most importance in determining the boundaries of the Desert region. Semidesert (fig. 28).—The Semidesert embraces two areas—one in California and one in Texas. These areas are not only diversified within themselves, but are somewhat dissimilar in their vegetation and CORRELATION OF DISTRIBUTIONAL FEATURES. 501 still more unlike in the general character of their climates, particularly with respect to the seasonal distribution of precipitation and other moisture conditions. In amore detailed study of climatic correlations these two areas would repay separate investigation. The evidence of the amplitudes and also of the position of the iso- climatic lines indicates that temperature conditions are of more impor- tance in limiting the Semidesert than is the case with the Desert. The normal daily mean of the coldest 14 days of the year appears particularly to be of importance, ranging only from values slightly below 45° to 54° for different sections of the region. The amplitudes of the moisture ratios are greater than in the case of the Desert, and the position of the isoclimatic lines corroborates this indication that the conditions expressed by the ratios are not so important in the limitation of the Semidesert. In passing from the coastal to the interior portion of the Texas section of the Semidesert there is a rapid fall in the values of the moisture ratio (plate 59) from 0.81 (Brownsville) to 0.35 (Fort Ringgold). A similar diversity in the California section of the area is indicated by the value 0.10 for Fresno, as compared with the value 0.45 for Los Angeles. The value for Fresno is well above the minimum value for the Desert and the maxi- mum values in each section of the area are well above the maximum (0.27) for the Desert. Grassland (fig. 29).—The longest axis of the Grassland region runs in a north-and-south direction nearly across the United States, with the result that all of the leading temperature conditions exhibit wide amplitudes within its boundaries. The number of cold days in the frostless season runs through the entire gamut for the United States, TEMPERATURE Days in Nornmat Frostiess Season (F. S.) es || Hort Days, F. S. Es) Coro Days, F.S. | CR PrysioLocicat Summation, F. S. Ces Normat Daity Mean, covoesr 14 cays of Yean (TS Norma Daity Mean, Year PRECIPITATION Normat Daicy Mean, F. S. LLL ““@Z ©) Days IN LONGEST Norma: Rainy Perico, F.S., Days IN LONGesT Norma Dry Perion, FS. (C—O Mean Torat, Year (=a se ee) EVAPORATION Dany Mean, 1887-8, F. S. ar Moisture Ratios Normat P/E, F. S. (NT a Norma. 7/E, F.S. (NE are ee Norma P/E, Year 9 ES eee Humioiry Normat Mean, F. S. T__!MVy§— eee) SUNSHINE Normat Dairy Duration, F.S SSS a a MoistTure- TEMPERATURE INDICES Norma. P/E x T, F.S., Pxysiovocical MeTHoo |_ ae ) Fig. 28. Climatic extremes for Semidesert. 502 CORRELATION OF DISTRIBUTIONAL FEATURES. and the other five conditions shown in figure 29 run through about half the gamut for the country. Although the mean total precipita- tion is both low in amount and narrow in its amplitude for this region, the normal daily mean precipitation ranges through a greater ampli- tude of values and reaches maximum values which are slightly more than half as great as the maximum values for the Northwestern Hygro- phitic Evergreen Forest. This difference points to the importance of the seasonal distribution of rainfall in the Grassland region and to the heavier precipitation of the frostless season. The evaporation index for the frostless season ranges from a daily mean of 0.117 inch, at Moorhead, Minnesota, to 0.275 inch, at Cheyenne, Wyoming, which is nearly half the‘amplitude for the United States. Numerous isoclimatic lines cross the central portion of the Grassland, as this would indicate. A comparison of the position of the Grassland (plate 2) with the location of the isoclimatic lines showing the values of the moisture ratio 7/E for the frostless season (plate 59) indicates a correspondence as close as that exhibited for the Desert. The isoclimatic line for the value 0.40 lies slightly to the east of the western edge of the Grassland in Montana, South Dakota, and Nebraska, although very close to it in Texas. The line for the value 0.30 would lie much nearer this boundary along the northwestern edge of the area. The eastern edge follows closely the line for the value 0.60 in Texas and Oklahoma, but crosses rapidly in Kansas and Nebraska to the lines for higher values, and reaches its maximum of 0.117 in western Minnesota. A corre- spondence almost as close is shown between the Grassland and the area comprising normal mean relative humidities between values of 50 per cent and 65 per cent for the frostless season (plate 65). Along TemPcraturc Dave in Normal Frosticss Season (F. S) rT Hor Days, F. S. | Coto Days, F. S. EIA ED, TEE FT ae SE IE EE PE Prysiococicat Summation, F. S. Ces? Normat Daity Mean, covoest 14 cave or Venn [=a > Norma Daicy Mean, Year PRECIPITATION Nommat Daicy Mean, F. S. OT Ss Days IN LONGEST NORMAL RAINY Penion, F. S. Sg $$$ $$ $$$} Days im LONGEST Nonmac Day Penioo, F.S. (SSN $$ Mican TOTAL, Yean —ESS— See Evaronartion Dany Mean, 1887-8, F. S. ————E]E[EE[EEEE EEE Moisture Ratios Norma P/E, F. S. a eee Nonmac 1/E, F.S. at ie 2 ee Nonmac P/E, Ye I ee Humioir Pz Nonmat Mean, F.S C EEE——————= ST) Sunsnin c Nonmat Daity Duration, F. S a | MoistTune- TemMPcmaTuR C Inorces ae Normar P/E x T, F. S., Prvsiovocicat MetHoo [ ———————————— Fig. 29. Climatic extremes for Grassland. CORRELATION OF DISTRIBUTIONAL FEATURES. 503 both edges of a region with such great north-and-south extent as the Grassland there is abundant opportunity for the interaction of condi- tions of critical importance, or for the influencing of the effects of a critical condition by one that is nowhere of primary critical importance itself. This is well exemplified in the changing values of the moisture ratio which are found to coincide with the western edge of the region. If this composite condition is as critical as its general distributional relations seem to indicate, we have here a case in which the vegetation of the Grassland is able to endure lower and more exacting values of this condition at the north than it is at the south, owing to the differ- ence between the temperature conditions at north and south. On the eastern edge of the Grassland there is a similar change in the apparent controlling condition, but in this case we have the Grassland extending into the region possessing the same values for the moisture ratio that are found in the forested regions. It is possible that this is due to the inter-operation of temperature conditions and those expressed by the moisture ratio. Such a possibility is indicated by the fact that the values of the moisture-temperature index which are found in the Grassland also extend eastward in the Northern States. Grassland Deciduous-Forest Transition (fig. 30).—A comparison of the temperature blocks in figures 29 and 30 will show that the Transi- tion region is very similar to the Grassland in the limits and ampli- tudes of its temperature conditions. The principal divergence lies in the fact that the frostless season is slightly narrower in its amplitude in the former area, and does not reach such low values. A comparison of the precipitation conditions shows a difference in each case in the direction of more moist conditions for the Transition region, although Temperature Days in NonmAL Frostices Season (F. S200 Ce Hor Days, F.S. ————Z*~Z=Z£Z£~&E«___—ri=C<“‘i‘;272 7S: Coto Days, F. S. Prysiotocica, Summation, F. S. PO Normat Daicy Mean, covocsr 14 cays oF Yean (__ - i — Normat Daicy Mean, Year Se EO ee PRECIPITATION Norma. Daicy Mean, F. S. | Days in LONGEST Normat Rainy Perioo, F.S. (_ i =} Days in LONGEST Nonmat Dry Penioo, F. S. PS ee ee | Mean Torat, Year SS OR =} Evaroration Daity Mean, 1867-8, F. S. SSS ee | Moisture Ratios Norma P/E, F. S. ——— #4 J Norma 7/E, F. S. (SSS ee ee ee Normat P/E, Year a ee | Humioity Norma Mean, F. S. Se ee ee SunsnHine Norma Daicy Duration, F. S. Os MoisTurc-TemPcratune Invoices Nonmat P/E x T, F.S., Paysio.ocicac Mctxoo (> Fig. 30. Climatic extremes for the Grassland Deciduous-Forest-Transition. 504 CORRELATION OF DISTRIBUTIONAL FEATURES. the amplitude of its normal daily mean is less, and also that of its num- ber of days in the longest normal dry period. The lowest values for the daily mean evaporation are very similar in the Grassland and the Transition, but the much narrower amplitude in the Transition region gives it much lower maximum values (0.166 inch as ages with 0.275 inch). The moisture ratios show a narrower amplitude in the Transition region than they do in either the Grassland or the Deciduous Forest, falling, in general, between the values for these regions, as has been shown in the earlier discussion. The narrow amplitude of the moisture ratios and of relative humidity would point to these cortditions as having a strong controlling importance for the Transition region. Its western edge coincides with the eastern edge of the Grassland and the conditions with regard to the moisture ratio along that line have just been mentioned. Along the eastern edge of the Transition region there is not a close correspondence with any of the isoclimatic lines of the moisture ratio, although there is a good agreement with the line for 0.110 in the north and an approximation to the interpolated line for 0.90 in the south. The Transition region lies almost precisely over the area that is comprised between the lines for 65 per cent and 70 per cent normal mean relative humidity for the frostless season, pointing to a strong controlling importance in this condition. Deciduous Forest (fig. 31).—The leading temperature conditions of the Deciduous Forest are of an intermediate character as compared with those of the entire country, reaching an extreme value only in the ease of the minimum number of cold days. The amplitude of these conditions is, in general, similar to that found in the Grassland and the Grassland Deciduous-Forest Transition, although the north-south Tempcratunc Days in Nonmar Faostiess Scason (F. S.) [OR Hor Days, F. S. es) Coto Days, F. S. a Prysiovocicat Summation, F. S. Tr —E——~~LE Norma Daity Mean, covocst 14 oavs of Year (Es) Nommat Daicy Mean, Year * LO PRECIPITATION Nonmar Dairy Mean, F. S. _ ES 8&8 °° }°»88 ss Days in LONGEST Nonmat Rainy Penioo, FS. (es Days in Loncest Nonmmac Day Penioo, F.S. (i Mean Tora, Year I Evarorartion Oaicy Mean, 1887-8, FS OC ) Moisture Ratios Norma P/E, F.S a _ EE SSS Norma r/E, F.S a = Norma P/E, Year a _——__———_=_— Humioiry Nonmar Mean, F.S | eT ee Sunswine Nonmar Dairy Duration, FS —— a ——— EE: ST Moisrunc-TemPcaatune inoices Noamay P/E x T, F. S., Poysiovocica, Metnoo a — rrr Kia. 31. Climatic extremes for Deciduous Forest. CORRELATION OF DISTRIBUTIONAL FEATURES. 505 extent of the Deciduous Forest is somewhat less than that of the two vegetations just mentioned. The most significant changes of ampli- tude that may be observed in comparing these three vegetations are found in the normal daily mean for the 14 coldest days and in the normal daily mean for the year. Both of these conditions are much narrower in their amplitude for the Deciduous Forest than they are for the Grassland and the Transition region, indicating that these con- ditions are of increasing importance as we pass from pure grass to pure forest. It has already been shown that a comparison of Grassland, Transi- tion, and Deciduous Forest exhibits respective increase in the values for all of the moisture conditions, except, of course, that a reciprocal relation exists with regard to the number of dry days. While this fact might well be anticipated, it is somewhat surprising to find that the amplitude of several of the moisture conditions is greater for the Deciduous Forest than for either of the other two vegetations men- tioned. The amplitude of the normal daily mean precipitation for the frostless season is slightly greater for the Grassland than for either of the other areas, and the number of days in the longest normal dry period for the frostless season is greater for the Grassland than it is for the Deciduous Forest, although it is less for the Transition area than for either of these. The mean total precipitation for the year and the number of days in the longest normal rainy period of the frostless season are both conditions that show the widest amplitude in the Deciduous Forest region. The evaporation conditions, which show such wide amplitude in the Grassland and such narrow amplitude in the Transition region, again show a relatively wide amplitude in the Deciduous Forest. This fact determines the great amplitude of the moisture ratios in the Deciduous Forest as compared with the Transition region. While the moisture ratio appears to be of great importance in controlling the limits of the Desert and Grassland, and also those of the Deciduous Forest, the evidence shows it to be of even more critical importance in controlling the boundaries of the Grassland Deciduous-Forest Transi- tion. To continue our comparison of the three vegetations which have already been contrasted, we find that the value for relative humidity becomes progressively greater from Grassland to Deciduous Forest, and that its amplitude is very narrow for the Transition region, while it is relatively broad for the Grassland and Forest, ranging through a third to a fourth of the total amplitude for the United States. The evaporation conditions bear a reciprocal relation to relative humidity, but show the same narrow amplitude for the Transition region and wider amplitudes for the adjacent vegetations. It appears from this circumstance that relative humidity is the strongest determinant of 506 CORRELATION OF DISTRIBUTIONAL FEATURES. the rate of evaporation for these areas. The influence which it exerts in this respect is therefore embraced in the moisture ratios, and addi- tional evidence is supplied to the view that the moisture ratios are of particular importance in controlling the bounds of the Transition region. While the moisture-temperature index shows little difference when the Grassland and Transition areas are contrasted, it exhibits a very wide amplitude for the Deciduous Forest, indicating that the interplay of moisture and temperature differences causes a wide diversity of conditions in the Deciduous Forest region. Northwestern Hygrophytic Evergreen Forest (fig. 32).—There are many respects in which this region possesses the most marked set of climatic conditions of any of the vegetational areas of the United States. Although the Hygrophytic Forest merges into the western section of the Northern Mesophytic Forest, both southward along the California coast and in isolated areas on the western slopes of the Rocky Mountains, nevertheless the climatic characteristics of the most pronouncedly hygrophytic region, as indicated on plate 2, cause it to stand out in sharp contrast with the Mesophytic Forest. The length of the frostless season in the Hygrophytic Forest varies from 103 days at McKenzie Bridge, Oregon, to 316 days at Cape Disappointment, the greatest amplitude to be found in any vegetation in the country. There are no hot days and no cold days, in our sense of these terms, in any part of the region. The physiological summation of temperature for the frostless season is both low in its values and small in amplitude, resembling closely the eastern section of the Northern Mesophytic Evergreen Forest. The normal daily mean temperature of the coldest 14 days of the year ranges from about 35° Tempcmatunc Dave in Nonmat Faostices Season (F.S) Ce) Hor Days, F. S. |e ee Coto Daves, F. S. I ee ae a ES See ers TE ES Prysiovocicat Summation, F. S. ae ee Sa Se Nonmac Daity Mean, covocst 14 pave oF Year (__ aS ies Nonmat Dairy Mean, Year DAP Sr Se PRECIPITATION Nonmar Daicy Mean, F. S. ES TE Te Dave iw LoncesT Nonmat Rainy Prsioo, FS es) Dave in LonacsT Nonmat Dry Penioo, F.S. 0 (i Man Torta, Year Ee Evaronation Dany Mean, 1887-6, F. B. —_—_— ST MotsTunc RaTice Nonmar P/E, F. S. ES a a Nonwac r/E, F. S. ESS SLL Nonmac P/E, Year ES A TT ET Humiorry Nonmar Mean, F. &. CTE SSS! Sunsnine Nonmar Daicy Dunation, F. S. SS ee ee Morerurnc-Tempcaarune Invices Nommar P/E x T, F. S., Prreoooca: Meroe (_ i Fig. 32. Climatic extremes for Hygrophytic Forest. CORRELATION OF DISTRIBUTIONAL FEATURES. 507 to about 45°. Inasmuch as the coldest portion of the year is coincident with the moist portion, this circumstance is of great importance, as indicating that the highest temperatures of the coldest days of the year are still well above frost, while the winter days outside the coldest period present conditions which are favorable for photosynthesis and growth. The normal daily mean temperature of the year ranges from below 50° for the coldest stations to above 55° for the warmest. The whole series of moisture conditions, including evaporation and the moisture ratios, is remarkable in this region for the extremely wide amplitudes exhibited. The highest values for all of these conditions are secured from Cape Disappointment, and the lowest values from Roseburg, Oregon, a town located in the valley of the Umpqua River, in the driest conditions that are to be found in the region. The normal daily mean precipitation exhibits an amplitude which is about five- sixths of that for the entire United States. The values and amplitude of the number of days in the longest normal rainy period of the frostless season are here very similar to these conditions for the Grassland, while the greatest number of days in the longest normal dry period of the frostless season (198) is greatly in excess of the smallest number of days in this period (127) for the Desert region. The mean total pre- cipitation for the year is not only higher for this vegetation than for any other, but it also shows a greater amplitude than elsewhere. The daily mean evaporation for the frostless season ranges from the lowest value in the United States, 0.052 inch at Cape Flattery, Washington, to a value of 0.143 inch at Roseburg. The values and amplitudes of evaporation conditions in the Hygrophytic Forest are closely similar to those in the Southeastern Mesophytic Evergreen Forest and in the eastern section of the Northern Mesophytic Evergreen Forest. The maximum values for the three vegetations are nearly the same, but the very low values for the Hygrophytic Forest are not found in the two latter regions (see fig. 24). There is a slight overlapping of the evapora- tion conditions with those of the western section of the Northern Mesophytic Evergreen Forest, into which the Hygrophytic Forest merges to the east and south, and with those of the Pacific Semi- desert region, the conditions of which are approached in all of the broad valleys of coastal Washington and Oregon which lie in the lee of the mountains. The values for all three forms of the moisture ratio are remarkable, in the Hygrophytic Forest, for their great amplitude. It has already been seen (fig. 25) that the highest values of the ratios for this region far exceed the maximum values recorded for any of the other vegeta- tions of the United States. The relatively low values of the South- eastern Evergreen Forest and of the eastern section of the Northern Evergreen Forest are completely overlapped in the Northwestern Forest, and the moisture conditions of such localities as Astoria and 508 CORRELATION OF DISTRIBUTIONAL FEATURES. Cape Disappointment are far exceeded by the still more moist condi- tions of Cape Flattery. In short, the difference between the values of the moisture ratios in moderately moist localities in the Northwestern Forest and in the most moist localities is as great as the differences which have already been seen to play such an important rdéle in deter- mining the distribution of the vegetation in the remainder of the United States. It is unfortunate that the inadequacy of the climatological data gives this region the appearance of being exceeded in the normal mean relative humidity of the frostless season by the western section of the Northern Mesophytic Evergreen Forest, an appearance that is prob- ably misleading. The sunshine conditions are also imperfectly known for this area. . The moisture-temperature index for the frostless season has already been shown to be closely similar in values and in amplitude for this most hygrophytic of the vegetations of the United States and for the Desert. In forming the product by which this compound condition has been secured, the high temperature summation of the desert, together with the low moisture indices, and the low temperature sum- mations of the Hygrophytic Forest, together with its high moisture indices, have given results which are closely identical. Southeastern Mesophytic Evergreen Forest (fig. 33).—In viewing the entire series of blocks drawn to represent the chief climatic conditions of this region, it will be noted that there is, in general, a wide ampli- tude of nearly all conditions, including those of temperature and precipitation, and excepting only the imperfectly exhibited mean annual precipitation and the relative humidity. The amplitude of the moisture ratios is no greater than in the case of the Deciduous Forest Temperature Days in Nonmat Fxostiess Season (F. S.) {= x A RRAGREAE ORE Hor Days, F.S a Co.o Days, F. S. Po a ate ssi PrHysioLocicat Summation, F. S. (a Noamac Daicy Mean, covogsr 14 vays of Year (_ z TG ein ES TGR Normat Daicy Mean, Year = os | RS PRECIPITATION 2 ae ie 1 Normac Daicy Mean, F.S fe eee . EE) Days IN LONGEST Nonmat Rainy Penioo, F. S. (Sg FSR RE SS Days in Loncest Noamai Dry Penioo, F. S Ms Mean TorTat, Year a ar Se —— Se EVAPORATION Daiy Mean, 1887-8, F. S. —-~ |. & ar = > _ e Moisture Ratios Normat P/E, F.S a ———_—————=Ez_— se ae Nonamac n/E, F.S a 2|8|8LDLDLDLDLhLLhLlLU Passes = an Nonmat P/E, Yean t a $e Humioiry Nommac Mean, F.S. Co S EEE) Sunsnin c P in Norma Dairy Duration, F. S OOO a) MoisTUAc~-TEMPERATURE INDICES Nenmar P/E x T, F.S., Pxvsiovocicat Metwoo [_ OO arr Fic. 33. Climatic extremes for Southeastern Evergreen Forest. CORRELATION OF DISTRIBUTIONAL FEATURES. 509 and but little in excess of the corresponding amplitude for the Grass- land. The wide amplitude of the temperature and precipitation con- ditions, taken in themselves, might be held to indicate that there is not a close correlation between the distribution of this vegetation and that of any of the important controlling physical conditions. Such a view would have in its support the fact that this vegetation is one that is well known to be closely correlated in its distribution with the extent .of the Atlantic Coastal Plain and with the series of soils typical of that physiographic province. However, the types of vegetation which seem to be strictly controlled by topographic features and by soils in the northern part of the Coastal Plain are not so controlled in the southern part and in the lower Mississippi Valley, where the Coastal Plain is not so sharply defined. In spite of the wide amplitude of many other conditions, the moisture ratios are of a significance with respect to this region which must not be omitted from consideration. In the Southeastern Mesophytic Evergreen Forest all of the leading temperature conditions of the country reach their maximum values, except the number of cold days, which, reciprocally, reaches its mini- mum value. Five of the six temperature conditions represented in figure 33 range in this vegetation through more than half of the ampli- tude found in the United States as a whole, due to the far northward extension of the region along the Atlantic coast. All of these condi- tions overlap with those of the Deciduous Forest region, which is quite to be expected in view of the overlapping and admixture of the vege- tations themselves. A transition region between the Deciduous Forest and the Southeastern Evergreen Forest has been outlined in the detailed map of vegetation (plate 1), and there is considerable evidence (some of which will be discussed) that portions of the Southeastern Evergreen area are of such a character climatically as to support a deciduous forest. . The amplitude of the normal daily mean precipitation is nearly equal to that of the Deciduous Forest, but the minimum and maxi- mum values are somewhat higher. The number of days in the longest normal rainy period of the frostless season exceeds even the great amplitude exhibited by this condition in the Deciduous Forest, and reaches, at Cape Hatteras, the highest value for the United States. The number of days in the longest normal dry period exceeds the amplitude of this condition for any of the other forest regions, and is even greater than that for the Grassland and the Desert, although the actual range of this condition is from the lowest value for the country, no days at Cape Hatteras, to 182 days at Key West. The amplitude of evaporation conditions for the Southeastern Ever- green Forest is narrow, and the values are relatively low, being almost equal, as already shown, to the evaporation values for the eastern section of the Northern Evergreen Forest. ‘The amplitudes of the 510 CORRELATION OF DISTRIBUTIONAL FEATURES. moisture ratios have also been seen to be closely similar for these two forests, and their minimum values are almost identical (see fig. 25). There is also little difference between the amplitudes and extremes of the humidity conditions for these two forests and for the Northwestern Hygrophytic Forest as well. The narrowest amplitudes for the South- eastern Evergreen Forest are found in the evaporation, relative humidity, and moisture ratios, and these must be regarded, therefore, as the most important of the various conditions in controlling the dis- . tribution of this vegetation, in so far as its control is a matter of climate. A comparison of the position of the isoclimatic lines for the values 0.100 and 0.110 of the moisture ratio based on the conditions of the frostless season and the preceding 30 days (plate 59), with the position of the boundary of the Southeastern Mesophytic Evergreen Forest, shows a close correspondence. This is least satisfactory in the vicinity of Arkansas, where the Southeastern Forest extends into a region with lower values for the ratio. This region, however, is the one in which there are the largest areas of mixed forest forming a tran- sition to the Deciduous Forest region (see plate 1). The moisture-temperature index reaches its highest values in this vegetation, and has an amplitude almost exactly equal to that for the Deciduous Forest. The high values of the physiological summation of temperature are responsible for the high values of this index, when the relatively low values of the moisture ratios are taken into account. The very high values which are rapidly attained by this form of the moisture-temperature index on approaching the southeastern corner of the United States may be taken to signify that this region presents the optimum conditions for plant activity in the entire area studied, as far as climate is concerned. Northern Mesophytic Evergreen Forest, western section (fig. 34).—The Northern Mesophytic Evergreen Forest, when considered as a whole, is so widely distributed and so varied in its character and specific com- position that it assumes a unity only when contrasted with the other evergreen-forest areas of the country. The only natural subdivision of this region is that which is made possible in the United States by the geographical separation of the eastern and western portions. In the study of the correlation of this vegetation with the climatic condi- tions it has seemed desirable to determine the climatic extremes separately for the eastern and western sections, which are sharply separated by the northern arms of the-Grassland and the Grassland Deciduous-Forest Transition. The most striking feature of the diagram which shows the leading climatic features of the western section of the Northern Mesophytie Evergreen Forest (fig. 34) is the relatively narrow amplitude of the majority of the conditions which accompany this widely and irregularly distributed forest area. There is a particularly strong contrast in this CORRELATION OF DISTRIBUTIONAL FEATURES. aL | respect with the conditions for the Southeastern Evergreen Forest and for the Deciduous Forest, indicating that the western section of the Northern Evergreen Forest is confined in its distribution to a region in which there is a relatively narrow range of physical conditions, in spite of its wide geographical extent. Owing to the mountainous and thinly settled character of most of this region, there is an inadequate series of climatological stations from which data may be obtained. This is particularly true of the southernmost portions in New Mexico, Arizona, and California, and of the portions which face the Desert region on all sides of the Great Basin. The climatic conditions of the western Xerophytic Evergreen Forest (see plate 1) are even more poorly known, and this has been a strong consideration in omitting that vegetation from the generalized map. Its conditions would doubtless be found to be generally intermediate between those of the Desert and those of the western section of the Northern Evergreen Forest. The length of average frostless season ranges from the minimum for the United States to 307 days, a remarkably great amplitude, which is matched only for the Desert region. The lowest values of this condition are recorded for several stations in the Klamath Lake region, where frost has been observed on every day in the year. The shortest normal frostless season outside these stations is 25 days in length, and this has been taken as the minimum for the United States in view of the fact that there is actually, in any given year, a growing-season for plants even in localities where frost is of average daily occurrence, and that the length of this season is at least 25 days even for Klamath Lake stations. The maximum length of frostless season for this vege- Temperature Days in Nommat Frostiess Season (F. S) Hor Days, F. S. Coro Days, F. S. Prysiotocicat Summation, F. S. Normar Dairy Mean, CoLoest 14 Oaye oF Year Normat Daicy Mean, Year PRECIPITATION Norma Dairy Mean, F. S. Days in LONGEST Normat Rainy Penioo, F. S. Days in LoNGest Norma. Dry Penioo, F. S. Mean Tora, Year EVAPORATION Daiy Mean, 1887-8, F. S. Moisture Ratios Norma P/E, F. S. Norma 7/E, F.S. Norma P/E, Year Humpty Norma Mean, F. S. SUNSHINE i" Normac Daicy Duration, F. S. MoisTurRe-TEMPERATURE INDICES CERRO XT AE 2 Sh.5) PHYSIOLOGICAL BAT HDD | a= MMO eee aaa ee ures Fic. 34. Climatic extremes for western section of Northern Mesophytic Evergreen Forest. Oe CORRELATION OF DISTRIBUTIONAL FEATURES. tation has been recorded at Fort Bragg, California, where the condi- tions and vegetation of the ocean front are more nearly those of the Hygrophytic Forest than of the western section of the Mesophytic Forest. At stations just inland from Fort Bragg the frestless season is about 100 days shorter. The number of hot days ranges from 0 to 105, which is about one-third of the amplitude for the entire country, and the number of cold days ranges from 78 to 149+, closely approach- ing the maximum values, which occur in the Grassland. The physio- logical summation of temperature has a relatively narrow amplitude, from values approaching the minimum to 9,921 at Grand Junction, Colorado, a station which must be regarded as approximating the conditions of the forests in that vicinity. Much higher values of the physiological summation might be expected for stations located in the more southern extensions of the Mesophytic Evergreen Forest, but our data fail to bear out such an expectation. Santa Fe, New Mexico, is located close to bodies of Mesophytic Evergreen Forest, and is so situated as to enjoy higher temperatures than the forest, yet the sum- mation for Santa Fe is only 5,350, a figure considerably below the maximum at Grand Junction. Flagstaff, Arizona, which is situated in the midst of forest, has the very low summation of 2,652. The cold nights of the mountain altitudes at which the Mesophytic Forest of New Mexico and Arizona occurs are responsible for low daily mean temperatures, and consequently for low values of the summations for the frostless season. The normal daily mean for the coldest 14 consecutive days of the year is a condition ranging through a moderate amplitude and reaching extreme values which are neither very high nor very low, overlapping with the conditions in the Hygrophytic Forest. The normal daily mean temperature for the year is of wide amplitude, but also fails to attain the extremes for the country. When the climatic extremes for the precipitation conditions are compared, as a whole, with those for the temperature conditions, the former are seen to be of narrower amplitude, which points to the limitation of the western section of the Mesophytic Evergreen Forest as being due more largely to moisture conditions than to those of temperature. This is confirmed by the narrow amplitude of the moisture ratios, at least of those which relate to the frostless season only. Although the actual extremes of the moisture ratio 7/E for the frostless season are 0.12 and 0.60, it will be seen by an examination of plate 59 that the major portion of the western Mesophytic Forest is comprised between the lines for values of 0.20 and 0.40. The lowest evaporation values and the highest relative humidities for this region are recorded for the extreme coastal stations of northern California, and they are not typical of the great bulk of the forest, although they permit its occurrence in a form approaching the character of the North- western Hygrophytic Forest. CORRELATION OF DISTRIBUTIONAL FEATURES. 5IS No vegetational region exhibits a narrower amplitude of the mois- ture-temperature index than does the western section of the Mesophytic Forest. This derived and complex expression of climatic conditions has been shown to differentiate the evergreen-forest regions of the United States, in spite of its failure to bring out consistent differences between the other regions. Its extremely narrow amplitude for the western section of the Mesophytic Evergreen Forest points to its impor- tance as an expression of the conditions controlling this vegetation. Northern Mesophytic Evergreen Forest, eastern section (fig. 35).—A comparison of the climatic extremes of the eastern and western sec- tions of this forest shows a general similarity of temperature condi- tions, and a dissimilarity of moisture conditions and the expressions derived from them. The length of frostless season is much narrower in its amplitude in the eastern section, ranging from a length of 85 days at Thomaston, Michigan, to 167 days at Fitchburg, Massa- chusetts, values which lie well within those of the western section. The normal daily mean temperature for the coldest 14 days of the year ranges through lower values in the eastern section, as does also the normal daily mean temperature for the year. The only precipitation condition in regard to which the eastern and western sections of the Northern Mesophytic Evergreen Forest are at all alike is the normal annual total. The normal daily mean for the frostless season ranges in the eastern section through values which are well above those of the western section, the maximum of the latter being 0.070 inch and the minimum of the former being 0.091 inch. In accordance with this fact the number of days in the longest normal rainy period of the frostless season is greater in the eastern section than in the western, and the number of days in the longest normal dry period is pronouncedly less in the eastern section. TEMPERATURE Days in Normac Frostiess Season (F. S.) [ate a Hort Days, F. S. a ee Coup Days, F. S. Prysiovocicat Summation, F. S. De Wig seks aa) Norma Daicy Mean, coLoest 14 vaya oF Yean (__ —- ee Normat Daicy Mean, Year MEE —LCCCCCCC‘“‘“C;CSCSC;COC™C*C*#*‘“‘“‘CSNNNCNNC*‘AR; PRECIPITATION Norma Darty Mean, F. S. [SSS See) ee Days IN LONGEST NornMAL Rainy Period, F.S. | mr Days IN LONGEST Nonmat Dry Penioo, F.S. (MMM Mean Tota, Year Sa 5 een aeaaa | EVAPORATION Daicy Mean, 1887-8, F. S. fase EE soos eee) Moisture Ratios Norma P/E, F. S. (SSS eee ee Norma. 7/E, F. S. CCE ee Norma. P/E, Year [ae es ne ee) Humipity Normat Mean, F. S. Sip Se ee ES SuNsHING Normat Daity Duration, F. S. eS MoisTURE-TEMPERATURE INDICES Normat P/E x T, F.S., Prysiovocicat MetHoo C___ aT Fic. 35. Climatic extremes for eastern section of Northern Mesophytic Evergreen Forest. 514 CORRELATION OF DISTRIBUTIONAL FEATURES. The amplitude of evaporation conditions is much narrower in the eastern section than in the western, agreeing closely, as already stated, with the amplitude and values for the Southeastern Mesophytic Ever- green Forest. The humidity conditions also exhibit a much narrower amplitude, although they nowhere reach in the Hast the maximum values that are to be found at the Pacific coast stations of the western section. The moisture ratios all exhibit higher values for the eastern section, and the ratio /E for the frostless season fails to overlap with the range of this condition in the western section (see fig. 25). Here again there is a close correspondence between the Evergreen Meso- phytic Forests of the Northeastern and the Southeastern States. The sunshine values are low and of narrow amplitude for the Northeastern Evergreen Forest, the maximum value for this region being lower than the maximum in any of the other vegetational areas. The moisture-temperature index for the eastern section ranges from a minimum value which is slightly above the maximum for the western section to a maximum which nearly coincides with that for the North- western Hygrophytic Evergreen Forest. The close correspondence between the range of this condition in the last-named region and in the eastern section of the mesophytic forest doubtless affords a means of explaining the features of resemblance between these two vegeta- tions, which are so widely separated in the United States but have a narrow strip of connecting forest in Canada. The Climatic Conditions for Evergreen and Deciduous Forests.—In connection with the inquiry into the climatic conditions characterizing the vegetational areas, a comparison has been made of the climatic extremes for the Deciduous Forest and for the four evergreen forest areas considered collectively. In figure 36 are shown graphs for the TCMPERATURE Days in Normar Frostcess Season (F. S)) * Hor Days, F.S Co.o Days, F.S. PrysioLocicat Summation, F. S. Normat Daicy Mean, Year PRECIPITATION Nommar Dany Mean, F. S. Days in LONGEST Nommat Rainy Penioo, F. S. Days in LonaesT Norma Day Penriono, F. S. Mean ToTar, Year EVAPORATION Dany Mean, 1887-8, F. S. Moisture Ratios Nonmar P/E, F.S Normac n/E, F. S. Nonmmat P/E, Year Humiorry Nonmar Mean, F.S Sunsnine Nonmar Dairy Dunation, F.S MoistTurc-TemMPecnaTune tnoices Nommar P/E x T, F.S., Puysiovocica, MetHoo £ Fig. 36. Contrasted climatic extremes for Evergreen (black) and Deciduous (shaded) Forests. EOE CORRELATION OF DISTRIBUTIONAL FEATURES. 515 principal features of the climate for these two regions, brought together for ready comparison. The widespread occurrence of evergreen forest, extending into the northwestern, northeastern, and southeastern corners of the United States, gives a very wide amplitude of conditions for this type of forest. In 5 cases the amplitudes of conditions are as great for the collective evergreen regions as for the entire United States, and in 6 cases they are nearly as great. These wide amplitudes are found among temperature and moisture conditions alike. The narrowest amplitude is that of relative humidity, which ranges through the upper half of its scale for the entire country. The importance which humidity is here indicated to have for evergreen forest is‘borne out by the detailed humidity extremes for each of the evergreen areas (see fig. 25). The amplitude of the conditions found in the Deciduous Forest region is narrower in every case than that of the collective evergreen regions, and for none of the conditions does it approach the entire amplitude for the country. It is also to be noted that the extremes for the Deciduous Forest are, in nearly every case, well within the extremes for the evergreen regions. In the number of cold days in the frostless season the two have the same minimum, and in the number of days in the longest normal dry period of the frostless season the two have nearly the same minimum. The only case in which the two maxima approach each other is that of humidity. If we disregard the diversities in the evergreen forests which have led us to their separate treatment, it is possible to say that this type of forest in general is capable of withstanding a much wider range of climatic conditions than is the deciduous type. 2. DISCUSSION OF THE OBSERVATIONS. We have now reviewed the observed correlations between the general vegetation areas and some of the leading climatic conditions, both from the point of view of the vegetation and from that of the conditions. In the comparison of the amplitudes and extremes of each single condition, as shown for each of the vegetational areas, it has been possible to see to what extent that condition is unlike in the several vegetations. It is obvious that a condition which ranges through nearly the same values in two vegetations can not be looked upon as one that is important in determining the optimum conditions for each of these vegetations, nor as one that plays an important réle in controlling the limit between the two. It is evident, for example, that the physiological summation of temperatures can not be held to play a primary part in establishing the optimum conditions for Grass- land, Grassland Deciduous-Forest Transition, and Deciduous Forest (see fig. 22). If, on the other hand, a given condition exhibits a sliding scale of values for several adjacent vegetations, it is evident that such 516 CORRELATION OF DISTRIBUTIONAL FEATURES. a correlation points to this condition as playing an important part in the existence and distributional limits of these vegetations. This should be true even if there is some overlapping of the values of the condition between the adjacent vegetations, for there is always such overlapping of the vegetations themselves. A comparison of the humidity conditions for Grassland, Grassland Deciduous-Forest Transition, and Deciduous Forest, or a comparison of the moisture ratios for these vegetations and for Desert and Semidesert, shows a progressive change of position of the climatic extremes with respect to the extremes for the entire country (see fig. 25). A comparison of the amplitudes and extremes of all conditions for a single vegetational area makes it possible to discover which conditions tend to exhibit great differences in the various parts of the area and which ones tend to show a relative uniformity throughout the area. We have already seen that this method of evaluating the conditions makes it possible to use their relative amplitude as a measure of their comparative importance in establishing the limiting conditions for the vegetation in question. A cartographic representation of the distribution of vegetation and of the distribution of the various intensities of the climatic conditions makes it possible to compare distributional limits and isoclimatic lines, and to search for correspondence between the two. But such a search is best carried out by the aid of suggestions from the graphs showing the climatic extremes. The use of the three methods of correlation has shown them to be consistent in their indications. If a condition shows a sliding scale of values for a given series of vegetations, it is also found to show ampli- tudes, in each of the vegetations, which are narrow as compared with those of other conditions, and the isoclimatic lines showing the dis- tribution of the intensities of this condition are found to approximate the distributional lines between the series of vegetations. The first two methods serve for the discovery of the relative importance of various conditions in determining distribution, and the third method serves to show the critical intensities of the condition which appear to be important. In a general review of our examination into the correlations between climatic conditions and the general vegetational areas we find the most salient fact to be the great controlling importance of moisture conditions, embracing precipitation, evaporation, relative humidity, and the moisture ratio, as compared with the small controlling impor- tance of temperature conditions, embracing length of frostless season, number of hot and of cold days, and the temperature summations. The moisture-temperature index partakes strongly of the character of a temperature condition when it is brought into this comparison. In the vegetations of group A (including our series from Desert to Deciduous Forest; see figs. 21 to 26) the temperature conditions show CORRELATION OF DISTRIBUTIONAL FEATURES. 517 particularly wide amplitudes and a pronounced tendency to range through about the same values, except in the case of the length of frostless season and the normal daily mean of the coldest 14 days of the year, in both of which conditions the Semidesert departs from the conditions of the other vegetations of the group. A comparison of the extremes of the temperature conditions for the vegetations of group B (the evergreen forests) shows a much greater dissimilarity, indicating that temperature conditions play a more important rdle in differen- tiating our evergreen-forest areas than they do in differentiating the other vegetations of the country. This applies to the moisture-tem- perature index, as well as to the purely temperature conditions. It is in the vegetations of group A that the moisture conditions show their greatest differentiation and appear to exert their strongest con- trol. This is shown with diagrammatical clearness by the smoothed data for the mean total precipitation of the year (fig. 24), and it is also shown by the moisture ratio and by relative humidity (fig. 25). There is a much less marked differentiation of moisture conditions among the evergreen-forest areas, as will be seen in the close agreement of the extremes of daily mean evaporation for the Southeastern Mesophytic Evergreen Forest and for the eastern section of the Northern Meso- phytic Evergreen Forest (fig. 24), and also in the similarity of humidity conditions for the above forests and for the Northwestern Hygrophytie Evergreen Forest (fig. 25). The preceding pages have brought out the fact that the moisture ratio t/E is the most important single expression of climatic conditions with respect to the vegetation as a whole. The nar- row amplitude of this condition in all of the vegetations except the Northwestern Hygrophytic Evergreen Forest, and the distinctness of its extremes for all of the vegetations give an indication of its impor- tance which is well borne out by a comparison of the climatic and vegetational lines of plate 2 and plate 59. The isoclimatic line for the ratio value 0.110 closely follows the limit of the Southeastern Meso- phytic Evergreen Forest from Alabama to New Jersey, and then swings westward in such a manner as to approximate closely the southern limit of the eastern section of the Northern Mesophytic Evergreen Forest, failing to dip with the vegetation along the Alle- ghenies (where there are no data for this climatic map), but closely following the vegetational line to Minnesota. The similarity of the conditions expressed by the moisture ratio for the evergreen forests of the Southeastern and Northeastern States is indicated in figure 25, but no indication is there given of the closeness with which the inner limits of these vegetations follow asingle isoclimaticline. The line for the value 0.80 is an equally close approximation of the inner limit of the Northwestern Hygrophytic Evergreen Forest. The line separating the Grassland and the Grassland Deciduous-Forest Transition is closely followed by the isoclimatic line of 0.60 in the South and by that 518 CORRELATION OF DISTRIBUTIONAL FEATURES. of 0.80 in the North. The area lying inside the line for values of 0.20 is entirely occupied by Desert, which exceeds the line only to short distances in Texas and Washington. The importance of the moisture ratio in controlling the leading vegetations was shown by Transeau for the eastern United States,’ and our investigation has served to confirm his deductions, as well as to extend their application to the entire country. The comparisons which we have made between the vegetational areas and the various other climatic conditions have served to emphasize the importance of the moisture ratio even more than was done by Transeau, since no other single datum has been found in our work to approach it as an expression of the controlling conditions for forest, grassland, and desert. The importance of the moisture ratio is due partly to the fact that it is a combined expression of several other conditions, and still more to the fact that these conditions are ones which have a combined effect upon the physiological processes of the plant. The moisture ratio is, in brief, an expression of the relation existing between the water available for plants and the amount of water lost as a result of atmospheric conditions. The moisture ratio gives us a single numerical expression for the group of conditions which control a single important physiological condition of the plant, namely, its maintenance of a balance between intake and outgo of water. When we secure the product of the moisture ratio and the temperature summation we have a still more comprehensive expression of conditions, the moisture- temperature index. In this index, however, we have no such succinct expression of a set of conditions that are closely coordinated in their relation to the physiology of the plant, in spite of the individual impor- tance of each factor in the product. The moisture-temperature index is correspondingly of less value in interpreting distributional etiology than is the moisture ratio itself. A much more ideal derivation of the moisture ratio is one employing the soil-moisture rather than the precipitation, since it is the former rather than the latter condition that is of immediate relation to the activities of the plant. Shreve has used the ratio of soil-moisture to evaporation in a discussion of the annual march of moisture conditions at Tucson, Arizona,’ and also in describing the gradient of conditions from the base to the summit of the Santa Catalina Mountains, in southern Arizona, a range surrounded by desert and capped by heavy forest. For detailed work, and particularly in arid regions or regions with pronounced periodicity of rainfall, the ratio of soil-moisture to evaporation will be found to express the prevailing conditions for 1Transeau, E. N., Forest centres of eastern North America. Am. Nat., 39: 875-889, 1905. *Shreve, Forrest, Rainfall as a determinant of soil moisture, The Plant World, 17: 9-26, 1914.— Idem, 1915. CORRELATION OF DISTRIBUTIONAL FEATURES. 519 plants more precisely than the ratio of precipitation to evaporation. In dealing with large areas, however, the soil-moisture is so closely a function of the precipitation that the two expressions frequently approach identity, or at least proportionality. Of course, it is under- stood that in all cases where the soil-moisture content (percentage of soil- moisture on a volume or weight basis) is employed as the index of the soil-moisture condition, great differences in the water-retaining power of the soil in different areas must upset this clear relation. If the soils compared are all clay or all sands, etc., the soil-moisture content itself is probably a relatively good index of the water-supplying power of the soil, but this is not true when sands are to be compared with clays, for a 10-per cent water-content in a sand may be physiologically equivalent to a 50-per cent content in a clay, other conditions being considered as alike. IV. CONDITIONS THAT PROBABLY DETERMINE THE LIFE-ZONES OF MERRIAM. 1. OBSERVATIONS FROM THE CHARTS. In a discussion of the climatic extremes of the life-zones of the United States, as outlined by Merriam, it is necessary for us to dis- regard the Boreal and Tropical Regions, and also the Gulf Strip of the Lower Austral Zone, on account of the very small number of climato- logical stations comprised in those areas. As originally drawn by Merriam, the life-zone map of the United States was in actuality a climatological map, based on a summation of temperatures and slightly modified, particularly on the Pacific coast, by data on the temperature of the hottest 6 weeks of the summer. It is not necessary, therefore, to make correlations of the range of the life-zones with the distribution of the different intensities of the remainder summation of temperatures above 32° for the year (plate 37, which is Merriam’s chart), nor with the normal daily mean of the hottest 6 weeks of the year (plate 44, which is also Merriam’s). In figures 37 to 42 are shown the graphs for the leading climatic dimensions given in tables 33 to 38 for the Transition, Alleghanian, Upper Sonoran, Carolinian, Lower Sonoran, and Austroriparian zones. These 6 subdivisions comprise the western or arid and the eastern or humid subdivisions of the transcontinental zones based on temperature conditions. They represent, in other words, an exact but special sub- division of the country on a basis ofcertain temperature conditions, together with a rough subdivision on the basis of moisture conditions. The line separating the arid and humid divisions of these zones is drawn by Merriam along the one-hundredth meridian from Oklahoma to South Dakota, departing a little to the eastward at the north and south. This line corresponds rather closely with the isoclimatic line of a value of 0.60 for the moisture ratio7/E. It does not take account, 520 CORRELATION OF DISTRIBUTIONAL FEATURES. however, of the fact that humid conditions exist in the extreme North- west, as shown by the return of the isoclimatic line of 0.60 for the moisture ratio, running from the Upper Columbia River to San Francisco Bay (see plate 59). Transition Zone (fig. 37).—The dimensions of nearly all of the leading climatic conditions exhibit a very wide amplitude in this zone, which embraces the northern Great Plains, portions of the humid Pacific Northwest, and mountain areas throughout the Western States. The length of frostless season and the number of cold days in the frostless season both range through nearly their entire amplitude for the United States. The normal daily mean precipitation and the mean total precipitation of the year likewise range through very wide amplitudes. The amplitude in the number of hot days is not great, and the extremes for this condition are low, ranging from 0 to 105. The physiological temperature summation exhibits a narrow range, as might be expected from the similarity of its derivation to that of the remainder summa- tion above 32°, the use of which in outlining these zones has been mentioned. Evaporation and humidity both show a much wider amplitude in this zone than in any of the vegetational areas that have been discussed, exceeding greatly the amplitude for the western section of the Northern Mesophytic Evergreen Forest, with which this zone has some distributional features in common. The moisture ratios all show wide amplitude for the Transition Zone, P/E and 7/E ranging through about half the total amplitude and P/E for the year through a still greater amplitude of conditions. The Northwestern Hygrophytic Evergreen Forest is the only one of our vegetational areas that exceeds the Transition Zone in the amplitude of the second of these ratios. The entire gamut of sunshine conditions for the United States is to be Temperature Days im Nonmar Frostiess Season (F. S) ee eee es — Hor Days, F.S. / ee Covp Days, F. S. CRE a cies Ee PHysioLocicat SummaTiON, F. S. aa ee eee Norma Daity Mean, cocogst 14 cays of Yaar (i Normat Daicy Mean, Year PRECIPITATION Normat Daicy Mean, F. S. ee | Days In LONGEST Normat Rainy Perioo, F.S. (_ — xy: = ee Days IN LONGEST NORMAL Dry Penioo, F.S.00 (0 Mean Torat, Year SE EG A a SS RPL a ESSARY Pe Be WSL ea at a. Gal EVAPORATION Dairy Mean, 1887-8, F.S. es Moisture Ratios Nonmat P/E, F.S = Nonmar 7/E, F.S. PO Nonmat P/E, Year OO Humoioiry Normat Mean, F. S. Sunsnine Norma Dairy Duration, F. S ETE A, PD Moisturc-Tempcaarune Invices Norma P/E x T, F. S., Puysiovocica: Mecrxoo (—_ i a3 ) Fic. 37. Climatic extremes for the Transition Zone. : CORRELATION OF DISTRIBUTIONAL FEATURES. 52t encountered in the Transition Zone. The moisture-temperature index possesses almost as narrow an amplitude as that characteristic of the western section of the Northern Forest region. Alleghanian Zone (fig. 38).—The temperature conditions of this zone, excepting the length of the frostless season, are very similar to those of the Transition Zone, while the precipitation and other moisture conditions are quite unlike. The frostless season ranges from a length of 106 days to one of 211 days, extremes which lie well within those of the Transition Zone. The amplitude and extremes in the number of hot days and the number of cold days are very similar, while the normal daily mean of the coldest 14 days of the year is somewhat higher in its extremes, although very similar in its amplitude. In all of the precipitation and other moisture data the Alleghanian Zone exhibits much narrower amplitudes than those of the Transition Zone, because of the high values characteristic of the portions of the latter zone which lie in the extreme Northwest. In nearly all cases extremes for the Alleghanian Zone lie within those of the Transition Zone, the exceptions being the number of days in the longest normal rainy and dry periods in the frostless season. Owing to the winter occurrence of rainfall in the Northwest, these features indicate more moist conditions for the Alleghanian than for the Transition Zone. The amplitude of sunshine duration for the Alleghanian Zone is much less than that for the Transition, ranging from low values upward through about one-third of the entire amplitude for the country. The moisture-temperature index has a much wider amplitude in the former than in the latter zone, slightly exceeding the amplitude for the eastern section of the Northern Mesophytic Evergreen Forest. TEMPERATURE Days in Norma Frostiess Season (F. S.) es == Hot Days, F.S. Dasa NS Fe | Coto Days, F. S. ED REDE EER EE PHYSIOLOGICAL Summation, F. S. | RES TT =eay Norma Dairy Mean, CoLoesT 14 pays oF Yean MESS a CSCS Normat Daicy Mean, Year OS | PRECIPITATION Normat Daity Mean, F.S. SSS Days In LONGEST Normal Rainy Penioo. F.S. (rms Davs IN LONGEST Nonmat Dry Penioo,F.S. [__ = Mean TorAt, Year are EvaPoraTion Daity Mean, 1887-8, F. S. aS a eS | Moisture Ratios Nonmat P/E, F. S. SSS Ee Norma 7/E, F.S. | = Normat P/E, Year SS Humioity Normat Mean, F. S. Oo892—"——Nuo0.T-"-™7N"°"VN™—_ arr Sunsnine ; Norma Daicy Duration, F. S , ee eT | MoisTuRE-TEMPERATURE INDICES Normat P/E x T, F.S., Puysiotocicat MetHoo (_____ ar > Fic. 38. Climatic extremes for the Alleghanian Zone. 522 CORRELATION OF DISTRIBUTIONAL FEATURES. Upper Sonoran Zone (fig. 39).—In this zone are comprised portions of the Desert and more than half of the Grassland region. The ampli- tude of the conditions is not, in general, as great as it is for either of these two vegetations, since the zone does not include the more moist half of the Grassland nor the extremely arid portions of the Desert. This zone is largely one of arid grassland and comprises the Desert- Grassland region of our detailed vegetation map. There is a wide amplitude in length of frostless season and in number of cold days, . both seen to be characteristic of the Grassland, and there is a wide amplitude in the number of days in the longest normal dry period. Both the evaporation and humidity conditions range through wide amplitudes, the former from 0.166 to 0.330 inch and the latter from 40 per cent to 80 per cent. The most moist conditions in this zone are to be found in the narrow strip which follows the coast of southern California. Both the physiological temperature summation for the frostless season and the moisture-temperature index show relatively narrow amplitudes for this zone. The narrowest ones, however, are those exhibited by the number of days in the longest normal rainy period of the frostless season and by the moisture ratios. The latter criterion appears to express the conditions which are critical in the limitations of this zone just as it does in the case of the Desert and Grassland. Carolinian Zone (fig. 40).—This zone bears about the same relation to the Upper Sonoran that the Alleghanian does to the Transition. That is to say, the temperature conditions are generally similar in the two, while the moisture conditions are dissimilar. The temperature similarity does not hold with respect to the length of frostless season, as it did not in the case of the Transition and Alleghanian Zones. The TemPcrature Days in Normat Frostiess Season (F. S) a Eee Hor Days, F. S. Coto Daves, F. S. EE Prysiotocicat Summation, F. S. yy — Ez Norma Daicy Mean, covoest 14 vays oF Year | i Norma Daicy Mean, Year PRECIPITATION Normat Daicy Mean, F. S. TEES) Days in LONGEST Normal Rainy Penion, F.S. . ——_ °° — ~_ = ee Days in Lonaest Nonmat Day Penioo, F. S. OOS aes) Mean TOTAL, Year — a meses EVvAPonaTion Daity Mean, 1887-8, F. S. = SES SN Moisture Ratios Nonmac P/E, F. S. — = ae = eee Nonmac 7/E, F. S. = va = aes ee Normac P/E, Year —= = eT SS PERT oll Humiorry Nonmmar Mean, F. S. ao. —— as Sunswine Nonmac Dairy Duration, F, S. = a Morstunc-TemPcrature Inoices Norma P/E « T, F.S., Pxysiovocicac Metxoo | - aaa ee 3 Fig. 39. Climatic extremes for the Upper Sonoran Zone. CORRELATION OF DISTRIBUTIONAL FEATURES. 523 maximum length is very similar in the two, being 237 days for the Upper Sonoran and 241 days for the Carolinian, but the minimum value for the former is 25 days and for the latter 127 days. Whereas the moisture conditions of the Alleghanian Zone were found to lie well within the extremes for the Transition Zone we have in the case of the Upper Sonoran and Carolinian Zones a more sharp separation of the ranges of these conditions. In every case there is an overlapping of moisture conditions, by which the minimum values of the Carolinian are seen to be lower than the maximum values for the Upper Sonoran. This circumstance is due to the fact that the highest moisture values of the Upper Sonoran are registered on the Pacific coast, while the lowest values of the Carolinian are experienced along the one-hundredth meridian. The minimum sunshine values for the Carolinian and Upper Sonoran Zones are very similar, but the amplitude of the former is only half that of the latter. The moisture-temperature index is higher in its extreme values and wider in its amplitude in the Carolinian Zone, reaching a maximum which is about midway of the amplitude for the United States. Among the relatively narrow amplitudes for this zone should be noted the normal daily mean precipitation for the average frostless season and the number of days in the longest normal dry period within that season. It is of interest to note that the length of the dry period shows a narrow amplitude in the Carolinian Zone, indicating its critical limiting importance, whereas the longest rainy periods are demonstrated to have a critical value for the Upper Sonoran. Con- versely, the length of the rainy period shows a wide, but imperfectly TEMPERATURE Days in Nornmat Frosticss Season (F.S) 009 CEE) Hor Days, F. S. SaaS aaa Coto Days, F. S. . Te OMEN Te Prysiococica, Summation, F. S. CC oT—K———=—=—=ZSCSC“‘“CSCSCSCSCNCN(N(NNCQ Norma Daity Mean, cocoest 14 vas of Year (_— s — i ——SCSC—“‘=~SCSD Normat Daity Mean, YEAR CO ee PRECIPITATION Nonmmac Daity Mean, F. S. ee Days in LONGEST Normat Rainy Penioo, F.S. (Ca — —COCOCCOC—C—C—C—CS;3;3}XKEhC Mean Torta, Year << «4+« Te EvaProraTion Daity Mean, 1887-8, F. S. EE) Moisture Ratios Nonmat P/E, F. S. FEE Normat 7/E, F. S. OT ae) Norma. P/E, Year SE EE aS Humipity Normac Mean, F. S. ——————— eS Sunsnine Norma Daicy Duration, F. S. TEE ————E——_ MoistTunc-TemPeRature Invoices Norwat P/E x T, F.S., Paysiovocicat MetHoo (a Fia. 50. Climatic extremes for Pinus echinata. Pinus strobus (fig. 51).—The distribution of Pinus strobus is coex- tensive with that of the eastern section of the Northern Mesophytic Evergreen Forest, and exceeds it to some extent toward the south and west, carrying the tree into the Deciduous Forest region and into the Grassland-Deciduous Forest Transition region. The conditions in the area of Pinus strobus are therefore similar to those of the vegetational area in which it reaches its greatest abundance. The southernmost extension of the tree carries it into a region with longer frostless season and with no cold days, in our sense of this term. A slightly greater value for the physiological temperature summation and a greater normal daily mean temperature are also encountered by Pinus strobus in its extension toward the Atlantic Coastal Plain and southward to Georgia. The number of days in the longest normal rainy period and the number in the longest normal dry period both reach maximum values which are greater for this tree than for the Evergreen Forest, although the minimum values are the same for the two. There are no very narrow amplitudes for this pine. The narrowest, however, are those for the moisture ratios and for relative humidity, Eo CORRELATION OF DISTRIBUTIONAL FEATURES. 543 and those for the number of hot days, the normal daily mean pre- cipitation, the physiological temperature summation, and the number of days in the longest normal dry period. From a comparison of the distributional limit with the positions of various isoclimatic lines, the southward range of Pinus strobus appears to be determined by temperature conditions, of which the physiological summation and the number of hot days in the frostless season are the most important, while its westward range appears to be determined by moisture conditions, of which the normal daily mean precipitation and the number of days in the longest normal dry period are the most important. The position of the isoclimatic lines for the moisture ratio, /E, would indicate that this compound factor is one of strong importance in determining both the southern and the western limits of this tree. Temprratune Days in Nonmat Frostices Stason (F.8)00 COO Hort Days, F. S. _—— LCCC CO? Coup Days, F. S. ee Ee PHYSIOLOGICAL Summation, F. S. MT ||. —~)j Normat Daicy Mean, covogst 14 cave o7 Yean as Nonmat Daicy Mean, Year Be PRECIPITATION Normat Daity Mean, F. S. [SSS Es | Days In LONGEST Normat Rainy Peaion, F.S. (_ res S—sS?>Sh Days in aoncest Norma Dry Penioo, F.S. (_ Mean Torta, Year aay eae Ts EvaronaTion Dany Mean, 1887-8, F. 8. em | Moisture Ratios Monua. P/E, F.S. SS ee Es | Norma 7/E, F.S. SS EE ee Nonma. P/E, Yean SSS ee Humioity Nonmac Mean, F. 5. See) Sunsning Normat Daicy Duration, F. S. ———L__ MoisTure-TemPrrature Inoices Norma: P/E x T, F.S., Puvsiovocica, Metnoo (i >? Fic. 51. Climatic extremes for Pinus strobus. Quercus alba (fig. 52).—This oak is found throughout the eastern United States, with the exception of northern Minnesota and Michigan and peninsular Florida. In keeping with its wide distribution it encounters a very wide range of practically all of the climatic condi- tions, exceeding in a number of cases the extremes for the Deciduous Forest region, in which it finds its greatest development. The nar- rowest amplitudes for Quercus alba are those of the number of days in the longest normal dry period, the normal mean relative humidity of the frostless season, and the moisture ratios. The first and last of these conditions appear to be responsible for the western limit of distribu- tion. This edge is roughly paralleled by the line for 25 days in the longest dry period and by the line for a value of 0.60 for the moisture 544 CORRELATION OF DISTRIBUTIONAL FEATURES. ratio. Owing to the extension of the northern ‘edge into Canada it is impossible to speak regarding its probable controls in that direction. Temperature Dave In Nommac Faostices Season (F. S.) C—O arr Hor Days, F, S. LS E—————————————— Coip Days, F. S. CEE LEO, OE EM Prysio.ocicat Summation, F. S. OB Nonmac Daicy Mean, cococst 14 oavs of Year (rs Nonmat Daity Maan, Year a PRECIPITATION Nommat Dairy Mean, F. S. OOOO eee Days IN LONGEST NORMAL Rainy Perio, F. S. (axa ag a Dave in Lonazet Nommac Day Penioo,.F. S. gy Mean Tora. Yean SE Evaronation Daity Mean, 1887-8, F. S. SS Moisture Ratios FEE) Nonmar P/E, F. 5. Normat 7/E, F, S. — i #= ae Nonmmat P/E, Year eS SS Humioiry Nonmac Mean, F. S. Es Sunsnine Nommat Daicy Duration, F. S. Cerra =) MolsTURC-TeEmPrRmaTuRE Invices Nonmat P/E x T, F.S., PHysiovocicat MeTHoo (a Fig. 52. Climatic extremes for Quercus alba. Quercus falcata (fig. 53).—This tree is found throughout the Atlantic Coastal Plain and the southern Mississippi Valley, reaching its greatest abundance in the Southeastern Mesophytic Evergreen Forest, but ex- tending well into the Deciduous Forestregion. Itencountersextremely TEMPERATURE Days in Normac Frostiess Season (F. S.) Hor Days, F.S Coro Days, F.S. Prysio.ocicat Summation, F. S. Norma Dairy Mean, co.oesT 14 vays oF Year (Eee a | CC § eee (eS a eee Pe [ Ea Se Norma Daicy Mean, Year i rrr) i ee ——— Se ee PRECIPITATION Nowmmar Dairy Mean, F. S. Days IN LONGEST Nonmat Rainy Perion, F. S. Days in LoncesT Normar Day Perioo, F. S. Mean Torar, Year EVAPORATION Dany Mean, 1887-8, F. S. Moisture Ratios Normat P/E, F.S ee Normac 7/E, F.S el ae Normar P/E, Year | _ Humioiry Normmat Mean, F.S if = = _ EEE SuNsHING Normat Daicy Duration, F. S 3 MoisTurc-TcmPcratune Inoices Nonma P/E x T, F.S., PaysioLocical MeTHoo (aa Fic. 53. Climatie extremes for Quercus falcata. wide amplitudes of all but one of the temperature conditions here dealt with, and its diminishing occurrence in southern Florida is coincident with the maximum values for these conditions. This is one Ce a CORRELATION OF DISTRIBUTIONAL FEATURES. 545 of the cases already mentioned in which the southernmost limit of plants in Florida is not well known, so that it is impossible to state just how far Quercus falcata may fall short of extending into the region of the temperature conditions represented by Key West. The one temperature condition for which Quercus falcata shows a narrow ampli- tude is the number of cold days, due to the fact: that its range barely extends into the region in which any cold days are experienced. The moisture conditions in the area of this oak are very similar to those of the Southeastern Evergreen Forest, except in the case of the number of days in the longest normal dry period. While the Evergreen Forest withstands a maximum of 182 days (the reading for Key West), Quercus falcata has its maximum at 63 days, at the southwestern edge of its range. The normal dry period, the moisture ratios, and the relative humidity of the frostless season appear to be important con- ditions for the limitation of Quercus falcata. Its distributional area may be defined as the region in which the frostless season is more than 180 days in length, the moisture ratio greater than 0.80, the humidity greater than 70 per cent, the normal number of cold days in the year less than 30, and the number of days in the longest normal dry period not more than 63. Quercus macrocar pa (fig. 54).—The area occupied by this oak covers the Northeastern States, omitting the Coastal Plain, and extends as far west as eastern Montana and central Oklahoma. Its southern limit is roughly coincident with the northern limit of Quercus falcata, and its northern limit is in Canada. Quercus macrocarpa is like Q. alba in exhibiting broad amplitudes for nearly all of the climatic condi- tions. It encounters approximately the lower half of the scale of TEMPERATURE Days in Noamat Frostices Season (F. S.) (es =| Hor Days, F. S. 2 a a) Coto Days, F. S. EES Ra I ERENT RED ESR RAIS LEE OE LEERY Prysiovocical Summation, F. S. Bec Cb IAP Ee SS eee ge Esl ay Norma Dairy Mean, coLvest 14 pays or Year ERT SP RAEN =) Nommat Daicy Mean, Year rs ——+; PACCIPITATION Nommat Daicy Mean, F. S. (es - : Days IN LONGEST Norma: Rainy Perioo, F. S. (rere — Days Im LoNGest Nonmmat Dry Periop, F. S. A Mcan oral, Year —— ee EVAPORATION Daiy Mean, 1667-6, F. S. == a = z Moisture Ratios Normac P/E, F. S. — €=€6=ESEererti(‘C:sSC Sees Se == Nonmar 7/E, F. S. — — 2 zi Nonmar P/E, Year EE Humioiry Normac Mean, F. S. (48@M§V@"@"VWnNruououot."—7} oo Sunsnine Nonmmac Daicy Duration, F. S. es MorsTurt-TemPeratune Invices Nonmat P/E x T, F. S., Puysiovocicat MetHon L ————_—L_SdLULULmCOCCC—S—SSSSSSS— Fic. 54. Climatie extremes for Quercus macrocarpa. 546 CORRELATION OF DISTRIBUTIONAL FEATURES. amplitudes of temperature conditions for the United States, but reaches areas with the maximum number of cold days. One of the narrowest amplitudes among the moisture conditions is found in the case of the number of days in the longest normal dry period, for which the extreme values are 9 and 88 days. A previous allusion was made to the western limit of this tree as showing the manner in which wholly distinct conditions cooperate in controlling the ranges of plants. It will be seen by a comparison of plate 52 (dry days) with plate 18 (dis- tribution of Quercus macrovarpa) that the longest dry period encoun- tered at the western edge of this tree in Oklahoma is less than 50 days, whereas the maximum number encountered near the Canadian boundary is 88 days. The potence of this moisture condition is evi- dently modified by the differences in temperature conditions which are encountered along the western limit of the tree. The values of the moisture ratio (1/E) encountered in Oklahoma are about 0.60 and those encountered in Montana are about 0.40. An ability on the part of Quercus macrocarpa to withstand the same values for these two con- ditions in the latitude of Oklahoma that it does in Montana would carry the tree to the eastern borders of New Mexico with respect to the moisture ratio, and well into the borders of that State with respect to the longest dry period. These two conditions, as modified in their influence by temperature conditions, may be regarded as setting the limit of the westernmost occurrences of this oak, which (like the western limits of so many deciduous trees) are to be found in alluvial bottoms characterized by moisture conditions which are higher than those of the adjacent upland. The southern limit of Quercus macrocarpa corresponds roughly with the isotherm of 120 hot days (see plate 36), and this condition, possibly in conjunction with closely related conditions, may be regarded as probably controlling the southern edge of the distributional area. The mean temperature of the hottest 6 weeks is apparently one of the most important of these related conditions, as the isotherm of 78.8° lies near the southern limit (see fig. 45). Ilex opaca (fig. 55).—The occurrence of this Jlex is rather closely confined to the Atlantic Coastal Plain throughout all but a small part of its range, where it extends into the Piedmont and mountain sections of Georgia, Alabama, and Tennessee. This range is characterized by wide amplitudes of the temperature conditions, reaching the maximum values in all cases except the number of cold days. With respect to the latter condition, the amplitude is relatively narrow and the maximum is 55 days, encountered only at the northernmost attenuated limit of occurrence, in Massachusetts. The moisture conditions for the area of Jlex are nearly those of the Southeastern Mesophytic Evergreen Forest, the amplitudes being CORRELATION OF DISTRIBUTIONAL FEATURES. 547 wide in all cases except the imperfectly determined mean annual pre- cipitation. Although this plant encounters a wide range of the condi- tions expressed in the moisture-temperature index, there is still a close correspondence between its limit and the isoclimatic line of 11,000 for this index. At the localities where it occurs in southern Illinois and Indiana it encounters index values of only 7,000, and at its northern limit in Massachusetts it encounters its minimum value of 5,193. Numerous isoclimatic lines have such a direction as to indicate that there are belts of relatively similar climatic conditions extending parallel to the Atlantic coast for long distances (see plates 46, 50, 53, 59, 65, and 72). The conditions of the southern part of the Missis- sippi Valley, which is technically a part of the Coastal Plain, are almost always different from those of the coastal strip of this physiographic province. Numerous plants occur throughout the Coastal Plain from ' New Jersey or Virginia to Georgia or Mississippi, but fail to range coextensively with it in the southern Mississippi Valley, and terminate their distribution before reaching the mouth of the Rio Grande. Ilex opaca, Pinus teda, Itea virginica, and Quercus falcata are all examples of this type of distribution. In all of these cases we undoubtedly have to do with three sets of limiting conditions; those operating in the Atlantic coast region, those in the Mississippi Valley, and those deter- mining the extreme southern limit in Texas. In at least the first of these regions we have to reckon with the modifications of climatic conditions which are due to the soil. Tempcearure Daye im isonmar Frostiess Scason (Ff. S.) foesesiro ener MRE Se OE eS BLS Hor Days, F.S. I 28S oe RNS ST a LE ht abe] Corp Dars, F.S. eae ee zy PHYSIOLOGICAL SUMMATION, F. S. (RRS eT ES Noamat Daity Mean, co.ogcst 14 oars or Yean (_ i(‘“‘(CMCO!COC*«*URS Nonmat Dairy Mean, Year Cs PRECIPITATION Nonmat Daity Mean, F. S. | ONS Days Im LONGEST Normat Rainy Pesiop, F.S. (__ xe Days IN LONGEST Nonmat Day Peaico, F.S0 SE Mean Torta, Year (ear Ee oe erie eres EVAPORATION Dany Mean, 1867-8, F. S. PO Moisture Ratios Normat P/E, F.S. aa ee Normat 7/E, F. S. SSS aa Se Nonmat P/E, Year SSS ee) Humioiry Normat Mean, F. S. Se a SUNSHINE Nonmat Daity Duration, F. S. | NS MolsTuRe-TEMPERATURE INDICES Normat P/E x T, F. S., Pursiotocical Merwoo (Ee Fie. 55. Climatic extremes for Ilex opaca. 548 CORRELATION OF DISTRIBUTIONAL FEATURES. Magnolia grandiflora (fig. 56)—The distribution of this magnolia covers only the portion of the Coastal Plain between Cape Fear and the Trinity River, and south of the northern boundary of Louisiana. This is a region with no cold days, in our sense, and with high ranges for all of the temperature conditions studied. It is also a region with a high normal daily mean precipitation, although the amplitude of this condition is not great. The amplitude in number of days of both the wet and the dry periods is very wide, and those of evaporation, humidity, and the moisture ratios are narrow. The range of Magnolia is remarkable from the fact that its limit corresponds with a large number of isoclimatic lines, giving the follow- ing indications regarding the conditions which it encounters: a frostless season of 240 days or more, a normal daily mean temperature of 45° or more for the coldest 14 days of the year, an annual mean temperature of 65° or more, a mean relative humidity in the frostless season of 65 per cent or more, and a value for the moisture-temperature index of 15,000 or more. None of the isoclimatic lines for moisture conditions coincide, even roughly, with the limit of Magnolia, although the narrow amplitudes of evaporation, moisture ratios, and humidity indicate that this tree encounters only a small part of the total range of these conditions for the United States. The limit of Magnolia corresponds rather closely with the line to the south of which are found 5 or more species of evergreen broad-leaved trees (see plate 3), and the climatic characteristics just mentioned may be taken as defining the region which is favorable for the abundant occurrence of trees of this type. TEMPERATURE Dave in Nonmat FaosTiees Season (F. S.) (eee Hor Days, F. S. Ee Coto Days, F. S. CSS eee Prysiovoaicat SumMATION, F. S. a Norma Daity Mean, Coronet 14 pays of Yeam (0 Norma Daicy Mean, Year SSS eee PRECIPITATION Norma. Daity Mean, F. S. LE Days im LONGEST Normal Rainy Pemion, F.S. 0 (0 ) Dave in Lomarst Nommat Dry Penioo, F. S00 (ee MEAN Torat, Year Evaronartion Day Mean, 1887-8, F. S. — awa Morsrune Ratios Nonmat P/E, F. S. a eee Nonmac r/E, F. 8. a Sanna Nonmat P/E, Year SSS eee Humioiry Nonmar Mean, F. 8. SS a PS SARE SSP Sunsnine ® Nonmacr Daity Duration, F. S. OOWvwcV“V“llvl”V"VQ_.(MD9D EEE MoieTunc-TemPcaatunc inoicee Nommar P/E xT, F. &., Prverorocicaa Metwon CF _—ETe_.f}N}}}N}TYjN}N}NYYYYYN’nN’N"runNN oe Fig. 56. Climatic extremes for Magnolia grandiflora. SS CORRELATION OF DISTRIBUTIONAL FEATURES. 549 Serenoa serrulata (fig. 57).—Only the extreme edge of the southern Atlantic Coastal Plain is occupied by this palm, from southern South Carolina to the eastern border of Texas. It occupies the warmest and moistest portion of the area which has just been stated to be favorable for the development of evergreen broad-leaved trees, with which it may be classed. Considering the small area occupied by Serenoa, it encounters a wide range of conditions in both temperature and pre- cipitation, together with narrow ranges of evaporation, humidity, and the moisture ratios. It encounters a frostless season of 231 days or more and no cold days, in our sense. Its limit coincides closely with the line of 50° for the normal mean temperature of the coldest 14 days of the year, although the palm does not follow the region of these tem- perature conditions into southern Texas. The encountering of the conditions expressed by a moisture ratio of 1.00 appears to be respon- sible for the westward limitation of a plant which is elsewhere con- trolled by temperature conditions. Temperature Days in Norma Frostiges Season (F. 8) 9 (es Hor Days, F. 8. (ree Cotn Daves, F. S. | Cee aE Ss CRT ee ay BASE SS ESL RE eT CR A | Pursio.oaicat Summation, F. S$. es lll Nonmat Daity Mean, covogst 14 pave of Year (rr Normat Daicy Mean, Year [a a SECS | PRECIPITATION Norma: Daicy Mean, F. S. | CCC | Dave im Lonaest Nonmat Rainy Perioo, F. S$. (rs) Dave in tonarst Normat Dry Penod, F.So 0 (ee Mean Torat, Yeaa SE f ee ee ee Te a Evaporation Dany Mean, 1687-8, F. 3. GSS LETS) Eakis esl esey eos Fi baal Cree eee Moistuac Ratios > Norma P/E, F. S. SS ee ee Normat 7/E, F. S. ae a ee Norma P/E, Year SIs | Humipity Normar Mean, F. S. | SS SES = ESTES Sunsnine s Nonmat Daicy Duration, F. S. 0 MoisTure-TemPcaature invoices Nonmat P/E x T, F. S., Payaiovocica, MetHop Co Fig. 57. Climatic extremes for Serenoa serrulata. Cephalanthus occidentalis (fig. 58)—The distribution of Cepha- lanthus is remarkable from the fact that it is one of the very few woody perennials of the southern United States which has a nearly trans- continental distribution. It is very infrequent west of the one-hun- dredth meridian, apparently being absent from New Mexico, but appearing again in southern Arizona and in the San Joaquin Valley of California. A‘distribution which is so extensive both latitudinally and longitudinally naturally encounters a wide amplitude of conditions, both with respect to temperature and moisture. None of the ampli- tudes of conditions for Cephalanthus are sufficiently narrow to give any 550 CORRELATION OF DISTRIBUTIONAL FEATURES. suggestion of their importance as limiting the plant. Its distributional edge, extending from central Michigan to the mouth of the Colorado River, is 2,300 miles in length, which in itself suggests that very dis- similar constellations of conditions are involved in its limitation in different sections of this line. In a plant of palustrine habitat it is not surprising to find that the normal moisture conditions of upland habitats have no apparent importance. We find Cephalanthus oceur- ring in localities where the moisture ratios approach their minimum values for the United States, and extending from there halfway through the gamut of values for this compound condition. It also encounters extremely low values for the normal daily precipitation and high values for the number of days in the longest dry periods. These con- ditions, however, have no apparent influence on the plant in the habitats where it occurs, although they are probably responsible for the fact that there are very few favorable habitats for it in the localities where these extremes are registered. In the San Joaquin Valley the atmospheric conditions are extremely arid, but there are numerous areas of moist soil, and Cephalanthus is there abundant. It is by no means true that all palustrine or swamp plants are able to withstand extremely arid atmospheric conditions if they are supplied with an abundance of soil-moisture, and only a relatively small number of the plants associated with Cephalanthus in the southeastern United States are found growing with it in southern Arizona and the San Joaquin Valley. . It is in the eastern half of its range that Cephalanthus encounters the greatest amplitude of temperature conditions. It is there found in localities with no hot days, as many as 137 cold days, where the Spe mamarorsoricasaaste Hort Days, F. S. Coro Days, F. S. Prysiovocicat Summation, F. S. Days in Loncest Nonmmat Rainy Peniop, F. S. Days im LonGcest Norma: Dry Peniop, F. S. Mean Torat, Year | | aH Ul Evaronation Dany Mean, 1887-8, F. S. Moisture Ratios Nonmat P/E, F. S. Yormac m/E, F. S. Nonmat P/E, Year | ae Humioirry ‘a Norma Mean, F.S Sunsnine | Nonmac Daicy Duration, F. S. MoisTune-TEeMPcRATURE INDICES 4 J | | Nonmac P/E x T, F. S., Pxysiovocicat MetHon C____ Fia. 58. Climatic extremes for Cephalanthus occidentalis. ————— — - SZ ———— I i CORRELATION OF DISTRIBUTIONAL FEATURES. 551 physiological summation is as low as 2,100, and where the normal daily mean of the coldest 14 days of the year reaches 11°. Some of these conditions may, indeed, be exceeded at the extreme limit of this plant in Canada. Much more complete information regarding Cephalanthus will be needed before it is possible even so much as to suggest some of the conditions that may be keeping it from spreading into other parts of the United States, if indeed it is not now making secular movements to the west and north. A knowledge of its relative abundance in different parts of its area and of the character of the habitats which it occupies throughout the edge of its distribution might aid in solving the problem which it presents. It seems to be a plant that would be well worthy of a thorough ecological study. Decodon verticillatus (fig. 59).—This is an aquatic or palustrine shrub found throughout the States east of Wisconsin, Missouri, and Louisi- ana, with the exception of southern Florida. The temperature condi- tions which it encounters are almost as wide in amplitude as those encountered by Cephalanthus. The amplitudes of the moisture condi- tions are somewhat narrower than in the case of that plant, but the only condition that can be regarded as having a significantly narrow amplitude is the number of days in the longest dry period, which reaches a maximum value of 78 days. The isoclimatic lines for the latter condition indicate that the area of Decodon is roughly limited by the line for 25 days in the longest dry period. Another line approxi- mating the limit of Decodon is that for a mean annual precipitation of 30 inches, the distribution of the plant extending over the region in which the rainfall is greater than that amount. TEmPeRaTuRE Days in Norma. Frostcess Season (F. S.) a Hor Days, F. S. EE Pe Rs SS + } Co.p Days, F. S. EE Se ee PHYSIOLOGICAL SUMMATION, F. S. | AE TP LE TR 3 Normat Daity MEAN, coLoest 14 bays or Year [__ Eee reeere ee Norma Daity Mean, Year SEE SS I PRECIPITATION Normat Dairy Mean, F. S. ee DAYS IN LONGEST NORMAL Rainy Perion, F. S. (— SSSSSIIISISST sy apres gree pr ereree arr cere ere Days in LONGEST Norma Dry Peniop, F. S. Ea eee a Mean Torat, Year { - Pea RS } EVAPORATION Daity Mean, 1887-6, F. S. es zea Moisture Ratios Normat P/E, F. S. res Norma 7/E, F. S. a J Norma P/E, Year : ae | Humipity Norma Mean, F. S. = 77s SunNsnine Normac Daicy Duration, F. S. Oe Ee Moisture-TemPcRATURE INDICES Nonmat P/E x T, F.S., Pxysiovocical McTHoo (SISSIES era Fic. 59. Climatic extremes for Decodon verticillatus. 552 CORRELATION OF DISTRIBUTIONAL FEATURES. The palustrine and semipalustrine shrub [tea virginica (see plate 23) has a more restricted range than Decodon, very similar in its general outlines to that of Ilex opaca, and apparently limited, like the latter, by high values for temperature and moisture conditions. Artemisia tridentata (fig. 60).—This plant is the dominant element of the vegetation of the Great Basin, and it extends in diminished abun- dance eastward to the edge of the Great Plains, upward into the moun- tains, southward to northern New Mexico and Arizona, and still more sparingly into southern California. The map of its distribution (plate 22) is not drawn to indicate the mountain areas from which it is absent. When these breaks in the distribution are taken into account the shrub is found to occupy an area whichis much more homogenous in its climatic conditions than its wide extent would seem to indicate. The amplitude for Artemisia is wide with respect to the number of days in the average frostless season, the number of cold days in the year, the normal daily mean for the coldest 14 days, and the normal daily mean for the year. With respect to the number of hot days and the physiological summation of temperature the amplitudes are nar- row, however. The precipitation conditions also exhibit narrow ampli- tudes, with the exception of the number of days in the longest dry period. The amplitude of evaporation and humidity conditions is made to appear wide because of the extension of its area to the Pacific coast, where the plant is extremely rare, its most westward abundant occurrence being in the Cuyamaca Mountains, 40 miles from the coast. The values for the moisture ratios exhibit narrow amplitudes, reaching, in two cases, the lowest values for the country. It is manifest that the southern limitation of Artemisia is not solely a matter of its inability to withstand extremely arid conditions, since Temperature Daya in Nonmac Frostices Szason (FS. (__ es Hor Days, F. S. _———E__ Coin Days, F. S. SL a SR Prvsio.ocicat Summation, F. S. 0 EO ee Nonmat Daity Mean, coLocsr 14 cava or Yean (a Nonmat Dairy Mean, Year +) PRECIPITATION Nonmmat Daity Mean, F. S. Days in LonacsT Nonmat Rainy Penioo, F.S. a Ser | Days In LONGEST Nonmat Dry Pemion, F.S. 0° (0 Man ToTAt, Year — [EEF EvaPoration Dany Mean, 1887-8, F. S. EG EE ES 5 TTS Moisture Ratios Nonmac P/E, F. S. EEE SST Nonmat 7/E, F. S. a Nonmac P/E, Year | SEINE REMEG METS Humipiry Nonmac Mean, F. S. ye —————————— SunsnHine Norma Daicy Duration, F. S Sk PR eek Sd eS MoisTunc-TemPcrarure inoices Nonmar P/E x T, F.S., Puysiovoacicat MeTHoo ——= = SAEED SIRE MT Fira. 60. Climatic extremes for Artemisia tridentata. a pS as ee CORRELATION OF DISTRIBUTIONAL FEATURES. 553 it encounters the lowest normal daily mean precipitation (Reno, Nevada), the highest evaporation (Winnemucca, Nevada), and the lowest moisture ratio (Winnemucca, Nevada). The most trying con- ditions with respect to these three important moisture conditions are, therefore, not found near the southern edge of the distribution of Artemisia, but well within the region of its greatest abundance. The narrow amplitude of the number of hot days suggests that this may be a condition of importance in limiting this plant at the south, and the maximum of 118 hot days for its area is close to the value of the isoclimatic line of 120 days, which is seen to approximate the dis- tributional limit in Arizona and Nevada. The amplitude of the physiological summation of temperature is also narrow, and the maxi- mum value encountered by Artemisia is 8,400. The evidence would indicate that these and associated temperature conditions are respon- sible for the southern limit, or else that they cooperate with the low moisture conditions in rendering the deserts along the lower Colorado and Gila Rivers untenable for this plant. The eastern limit of Artemisia appears to be set by some of the several moisture conditions which present isoclimatic lines closely paralleling its course. The indications of these correlations are that the plant nowhere encounters a mean annual rainfall of more than 20 inches, reaches no areas in which the longest normal rainy period is more than 25 days, nor the moisture ratio more than 0.40, and that it is accustomed to normal longest dry periods of at least 75 days in length. It is more than probable that the northern limit of Artemisia is set by conditions similar to those that appear to be responsible for its eastern boundary, with the possible cooperation of low temperature conditions. Low temperatures accompanied by arid atmospheric conditions appear to permit the northward extension of the plant into Canada, but low temperatures accompanied by more humid conditions appear to keep it from the northern Rockies and the Coast Range, as well as from the higher slopes of the Sierra Nevada. Covillea tridentata (fig. 61).—The range of Covillea extends from southern Nevada and interior California through southern Arizona and New Mexico to the lower part of the valley of the Rio Grande. The area which it occupies in the United States is less than half of its total range in North America, which extends southward through the deserts of central Mexico. No cold days, in our sense, are encountered by Covillea, and the amplitude of the normal daily mean temperature of its area is rela- tively wide. In other respects the temperature conditions present nar- row amplitudes, especially when compared with those for Artemisza. The data for moisture conditions reveal the extremely arid conditions under which Covillea exists, showing that it also extends into regions 554 CORRELATION OF DISTRIBUTIONAL FEATURES. with much more favorable conditions, particularly with respect to the longest dry periods, daily mean evaporation, and normal humidity. The distributional area of this plant contains few climatological sta- tions, and it is certain that the conditions actually met by it include the lowest values of normal daily mean precipitation, the longest normal dry periods, and the lowest mean total precipitation o the year. In the vicinity of Death Valley and in the arm of the Mojave Desert which stretches southeast toward the Colorado River are to be encountered the most arid areas in the United States, and in them Covillea is one of the most ubiquitous plants. The narrow amplitude of the moisture ratios indicates here, as in the case of Artemisia, that conditions of greater general favorableness with respect to this condition are either directly inimical to Covillea, or else that they are accompanied by associated conditions which are of importance in limiting its range. The extremely narrow amplitude in number of days in the longest rainy period signifies that there is a great importance in this factor, probably having to do with the effect of prolonged wet periods in making the conditions of soil aeration injurious to Covillea. The distributional boundary lies close to the northern limit of the area in which there are no cold days, and this condition, with its associated conditions of low winter temperatures, is undoubtedly of great importance in controlling the northward limitation of the plant. Covillea appears, in brief, to be confined to a region in which there are no prolonged periods of rain and no severe periods of cold. The narrow amplitude of the physiological summation of temperatures for the frostless season indicates importance for this condition, but the fact that Covillea is an evergreen points to the low temperatures of winter having a greater significance in its limitation than do the summations for the growing season. Tempcrarure Days in Norma Frostiess Season (F. S00 (CT Hor Days, F. S. Ly = #8 }° }»=i aS Coup Days, F. S. Ce ee PHYSIOLOGICAL SumMATION, F. S. Ls i i | i Tat ar Normat Dany Mean, cotocst 14 pave or Yean (CC OC CCT Normat Daicy Mean, Year Faas ae LA PE PReciPiTaTion Norma Daicy Mean, F. S. LLL |) DAvs in Conagers Nonmat Ramey Panioo, .&. Ue Days in LonarsT Nommat Dry Penioo, F.S. 00 (a ——) MEAN TOTAL, Year a) ~NgRP TEET EvaPoration Daity Mean, 1887-6, F. S. ( Moisture Ratios Nonmat P/E, F. S. [EN aE SESS SRS SSS SSS Normac 17/E, F. S. cs ae Nonmac P/E, Year | _—_ aaenRS ee Humioiry Nonmmat Mean, F. S. A Sunswine Normar Daicy Dunation, F. S. LE = RELL MoisrTure-Tempcrarure invoices Norma P/E x T, F.S., Prysiovocical Mcrxo0 (ss Fic. 61. Climatic extremes for Covillea tridentata. a a A be, CORRELATION OF DISTRIBUTIONAL FEATURES. 5595 Silphium laciniatum (fig. 62).—The range of this plant extends from Texas to South Dakota and from Alabama to Pennsylvania, but itis mostabundant in the Grassland Deciduous-Forest Transition and in the Grassland. The distributional area is such that the plant encounters only 140 cold days in the frostless season. The amplitudes are relatively wide in all of the temperature conditions and are also wide in all of the moisture conditions except the number of days in the longest normal dry period. The moisture ratios are also relatively narrow in amplitude, the extremes for z/E in the frostless season being, minimum 0.47, maximum 1.32. The eastern limit of Silphium is very roughly approximated by the isoclimatic line of 0.80 for the moisture ratio 7/E, isotherms of 45° and 50° for the annual daily mean temperature, and is closely followed by the isotherm for a physiological summation of 7,500°. The western limit is closely approximated, at least in part, by the isoclimatic lines of 20 inches annual mean total precipitation, 75 days in the longest normal dry period, 25 days in the longest normal rainy period, and a moisture ratio of 0.40. Although the extreme values of the moisture ratio for Silphium are 0.47 and 1.32, nevertheless the location of its entire range with respect to the isoclimatic lines for this condition indicates that the plant is found mainly where the moisture ratio is between 0.40 and 0.80. The northward extension in the area pre- senting this range of conditions is apparently controlled by tem- perature conditions, among which the physiological summation is most important. The central location of the range of Silphium laciniatum gives it a distributional edge about 3,300 miles long, with only its southern limitation formed by the ocean. These circumstances make it a par- TEMPERATURE Days in Nonmat Frostiess Season (F.S,) (es Hot Days, F. S. (eee 5 Cop Days, F.S. Se eS ieee | PHYSIOLOGICAL SUMMATION, F. S. (a ars | Normat Daicy Mean, covoest 14 pays of Yean (__—#§§ «COA es Normat Daicy Mean, Year | ee PRECIPITATION Norma Daicy Mean, F. S. (eee Days IN LONGEST Normal Rainy Period, F.S. (ee Days In LonGest Normat Drv Pernioo, F.S. (MM CC“‘“O™;SOSOSOSOSOSOOOOOVVVVVVVVTCOY Mean Torat, Year [ORM | EVAPORATION Daity Mean, 1887-8, F. S. LL) Moisture Ratios Norma P/E, F. S. Cs =a Norma 7/E, F. S. ae J Norma P/E, Year ee, EAS | Humioity Norma Mean, F. S. ees) SuNSHINE Normat Daicy Duration, F. S. a MoisTURE-TEMPERATURE INDICES Normat P/E x T, F. S., PHysiotocicat MctHop (________ aaa Ta | Fia. 62. Climatic extremes for Silphium laciniatum. 506 CORRELATION OF DISTRIBUTIONAL FEATURES. ticularly favorable subject for fuller investigation. Its ability to with- stand a considerable range of temperature conditions between Missis- sippi and South Dakota and its ability to range through dissimilar moisture conditions from Ohio to Texas should be investigated in terms of the habitat requirements of the plant in the various portions of its range. From such information as is available, this plant seems to be confined to the most arid situations on the eastern edge of its area and to relatively moist situations or seasons on the western edge, so that it would be particularly valuable to have parallel series of data for the local conditions met by the most widely separated colonies. Bouteloua oligostachya (fig. 63).—This abundant and characteristic grass of the Great Plains is found throughout the Grassland, in the western part of the Grassland Deciduous-Forest Transition, and in the southeastern part of the Desert. It is most abundant in the Grassland and the Desert-Grassland Transition, becoming infrequent at the southeastern and southwestern corners of its area. The temperature conditions encountered by Bouteloua are those of the Grassland, but with slightly higher maxima in each case (except number of cold days). The amplitude of the precipitation conditions is much greater for Bouteloua than for the Grassland, the maxima being higher and the minima lower. With respect to the length of the longest normal dry period particularly, Boutelowa exhibits its ability to range from the conditions of the Grassland far into those of the Desert, enduring 283 days in the vicinity of Phoenix, Arizona. Like many other perennial grasses, it is able to withstand prolonged and severe conditions of drought in its resting condition and to take advan- tage of moist periods of relative infrequency. Tempcnatunc Dave in Nonmat Frosticss Season ‘F. S.) Hor Days, F. S. Coto Days, F. S. PHYSIOLOGICAL SUMMATION, F. Ss. Norma Daicy Mean, covocst 14 pays oF Year Norma Daicy Mean, Year PRECIPITATION Normat Daicy Mean, F. S. Days In LONGEST Norma Rainy Penion, F. S. Days im LONGEST Norma Day Peniop, F. S. Mean TorTat, Year EVAPORATION Baicy Mean, 1887-6, F. S. Moisture Ratios Norma P/e, F. 8. Normac 7/E, F. S. Norma P/E, Year Humiorry Normat Mean, F. S. SUNSHINE Nonmat Daicy Duration, F. S. MoisTuRc-TeMPeRature Inoices Normmar P/E x T, F. S., Pxysiovocica, MeTHoo LE eS ee Fia, 63. Climatic extremes for Bouteloua oligostachya. “ OE CORRELATION OF DISTRIBUTIONAL FEATURES. 557 The minimum conditions of evaporation and the maximum condi- tions of humidity are very similar for Boutelowa and the Grassland region, indicating that conditions which determine the eastern limit of the Grassland also limit one of its most characteristic plants, and this is also true of Bulbilis dactyloides and Bouteloua hirsuta. The maximum conditions of evaporation and the minimum conditions of humidity, however, are respectively higher and lower for Bouteloua than for the Grassland. The moisture ratios for the frostless season are very similar for this plant and for the Grassland as a whole, although the minimum values for Bouteloua are lower. The eastern limit of Bouteloua, like that of many of the grasses associated with it, is apparently set by some one of the moisture con- ditions, or by a combined operation of several of them. Neither the area of Grassland nor that of Boutelowa extends very far into the region with more than a daily mean precipitation of 0.100 inch with more than 75 days in the longest rainy period, or with a mean annual precipitation of more than 25 inches. The position of the western boundary of Bouteloua indicates that it is there again limited by mois- ture conditions. Although we have presented no data bearing directly on the seasonal distribution of precipitation, it is apparent that this grass is unable to penetrate far into the portion of the Desert, in which the summer rainfall is hght. The ability to withstand dry periods of as much as 283 days has enabled it to range as far as the area of uncer- tain summer rains in the lower Colorado Valley. It is not able, how- ever, to extend its area into the region in which there is frequently no summer rain for many successive years. Agropyron spicatum (fig. 64).—This grass is found throughout the Grassland north of Oklahoma and New Mexico and in the outlying portions of that vegetation which fringe the northern edge of the Great Basin Microphyll Desert. Although withstanding the entire amplitude of cold days this grass encounters lower and narrower amplitudes of the other temperature conditions. It has a relatively low maximum (125) for the number of hot days and a low maximum (11,600) for the physiological temperature summation. With respect to precipitation it shows wider amplitudes than those of the Grassland, and this is true of evaporation and the moisture ratios for the frostless season. Agropyron appears to have its eastern limit set by the same con- stellation of moisture conditions that controls the Grassland and other grasses, but it does not follow the conditions favoring Grassland as far as the southern limit of that vegetation. Nowhere does it encounter more than 100 to 120 hot days, nor does it range into regions with a physiological summation of temperature greater than 12,500. These and associated temperature conditions appear to be responsible for its southern limitation. 558 CORRELATION OF DISTRIBUTIONAL FEATURES. The failure of Agropyron to extend farther into the northern part of the Great Basin Desert is apparently due to the moisture conditions of that region. A more precise correlation of its distribution with that of the moisture ratio (7r/E) would probably show that it does not occur where the values of this ratio are lower than 0.20, the same value that limits it in Arizona and New Mexico. The actual moisture-ratio conditions of the areas occupied by Agropyron in Utah and northern Nevada are poorly exhibited by our data, which are from stations located in the valleys. TemPematune Days in Nonmac Frostices Season (F. S) Hor Dars, F. S. Coro Days, F. S. Pwysiovoaica, Summation, F. S. Normac Daicy Mean, CoLoesr 14 Days oF Year “Wormac Daicy Mean, Year PRECIPITATION Nonmmat Daicy Mean, F. S. Days iW LONGEST Normat Rainy Peaioo, F. S. Days IW Lonorst Nonmat Day Penioo, F. S. Maan Tora, Year Evaroration Dany Mean, 1687-8, F. S. Morstunc Ratios Nonmac P/E, F. S. Nonmac 7/E, F. S. Nonmac P/E, Year Humioity Nonmmar Mean, F. S. Sunsnine Nonmmac Dairy Duration, F. S. MoisTure-TemPERATuRE INDices Nonmar P/E x T, F.S., Puysiovocica, Metxoo (i Fic. 64. Climatic extremes for Agropyron spicatum. Hilaria jamesi¢ (fig. 65).—This is a characteristic grass of the Desert- Grassland Transition region, ranging from western Texas to southern Nevada and northward to extreme southwestern Wyoming. All of the climatic conditions exhibit narrower amplitudes for Hilaria than for the Grassland, with the exception of the number of days in the longest normal dry period. The narrowest amplitude among the temperature conditions is that of the normal daily mean for the coldest 14 days of the year, which ranges from 27° to 45°. At few points does the limit of Hilaria extend north of the isoclimatiec line for 25° as the aily mean of the coldest fortnight, and the correspondence of these lines would indicate that this condition is an important one in limiting the northward distribution of the grass. The eastward extension of Hilaria in Texas is such that it nowhere encounters moisture ratios (7/H) higher than 0.40, nor rainy periods of more than 25 days. These are the same conditions that appear to limit the eastward range of other grasses, but different intensities are involved in the case of Hilaria from those mentioned in connection with Bouteloua. CORRELATION OF DISTRIBUTIONAL FEATURES. 559 With respect to the westward range of Hilaria, it appears that the same limiting conditions are operative that have been mentioned in connection with Boutelowa oligostachya. Extremely long normal dry periods, in excess of the 250-day maximum for Hilaria, are inimical to it as to all other perennial grasses. Temperature Days im Nonmat Frostiess Season (F. S.) Hor Days, F. S. Coro Dars, F. S. Prysiotocican Summation, F. S. Norma Darcy Mean, covoest 14 pays oF YEAR Normat Dairy Mean, Year PRECIPITATION Normat Dany Mean, F. S. Days In LONGEST Normat Rainy Peanion, F. S. Days In Loncest Noamat Dry Penion, F. S. Mean Toray, Year EVAPORATION Dairy Mean, 1687-8, F. S. Moisture Ratios Norma P/E, F. S. Nonmat 7/E, F. S. Nonmac P/E, Year Humiony Norma. Mean, F. S. Sunsnine iM Nonmac Daicy Duration, F. S. Moisturc-TemPerature Inoices Norma: P/E x T, F. S., Prvsiovocicat Menon (_ RN > Fic. 65. Climatic extremes for Hilaria jamesii. Sparganium americanum (fig. 66).—Sparganium is widely distributed throughout the eastern United States, except in peninsular Florida, and from its westward extension in Canada it reappears within our limits in Washington. A considerable number of northern plants exhibit distributions of this type, Dulichiuwm arundinaceum being another example (see plate 27), with a range closely like that of Spar- ganium. The range of another palustrine plant, Siwm cicutefoliwm, is of an analogous character (see plate 28), its limits being far enough south, however, for it to present a continuous area within the United States. The distribution of each of these palustrine plants is such that they are absent from the arid and semiarid regions, where they might find localities with suitable soil-moisture conditions, although much more widely separated than in the moist regions. The western edges of the ranges of Sparganium and Dulichium in the Eastern States run parallel to the isoclimatic lines for moisture conditions. Although these are plants of wet habitats, there is here a suggestion of their inability to extend into the regions with very high evaporation. In other words, the conditions expressed by the moisture ratio are of importance to them even when the numerator of the ratio is constant and of high value. Their ability to withstand lower moisture ratios in eastern Washington than they endure along their western edge in the Central States is doubtless due to the interaction of temperature con- ditions. 560 CORRELATION OF DISTRIBUTIONAL FEATURES. Wide ranges are exhibited by Sparganium for both temperature and moisture conditions, due to its extended north-and-south range in the Eastern States, and to its occurrence from the Atlantic coast to the arid interior of Washington. Its climatic extremes are of interest in comparison with those of Sium, which shows the broadest amplitudes of any of the plants that we have selected for investigation. The dis- tribution of Stum indicates that it is able to withstand the entire gamut of temperature conditions for the United States, excepting those encountered in peninsular Florida, and that it is excluded only by the lowest conditions expressed by the moisture ratio (0.40 or lower). Sium, Dulichium, and Sparganium are apparently alike in being unable to withstand the highest intensities of evaporation, in spite of the saturated substrata in which they are invariably found, apparently belonging to that already well-known group of plants in which the transfer of water from absorbing to transpiring organs is internally limited. TemPemarure Dave in Monmat Faosriaces Season (F. S.) Hor Davs, F. S. Coro Dares, F. S. Prvsiovocicat Gummarion, F. S. Nonmmat Dairy Mean, co.coger 14 oars of Year Norma. Daicy Mean, Year PacciPivaTion Nommar Dairy Mean, F. S. Daye in concesT Nonmat Rainy Penion, F. S. Days in Lomacst Nonmat Dry Penioo, F. S. Mean Tota, Year | Evaporation Dany Man, 1687-6, F. S. Moisture Ratios Norma P/E, F. S. Norma. m/E, F. S. Nonmac P/E, Year Humioiry Norma Mean, F. S. Sunsnine Nommac Davy Dunation, F. S. Moistunc-TempPcratune INDICES Normac P/E x T,F.S., Pavsiovocicac MetHoo (a Fic. 66. Climatic extremes for Sparganium americanum. Arceuthobium americanum (fig. 67).—This mistletoe is found throughout the Rocky Mountains and the mountains of the Great Basin and its western edge. Its actual occurrence is limited to the forested portions of the area indicated for it in plate 29 (see plate 1). A number of the climatological stations located within the area credited to it are not in situations actually occupied by Arceuthobium, and consequently some of the moisture conditions, in particular, are higher than indicated in figure 67. Arceuthobium americanum is chiefly confined to Pinus contorta (including P. murrayana) as a host, but appears to be absent from it in Washington, Oregon, and California east of the Cascade Mountains. ee CORRELATION OF DISTRIBUTIONAL FEATURES. 561 The area of Arceuthobium is therefore closely similar to that of Pinus contorta, with the exception of its absence from the northern Pacific coast. The area of the mistletoe (plate 29) has been drawn in a more generalized manner than has that of Pinus contorta (plate 16), because less information is available regarding the occurrence of the former. The climatic extremes for Arceuthobium are very similar to those for Pinus contorta, except with respect to the high conditions of precipita- tion, moisture ratios, and humidity encountered by the host in the Pacific-coast part of its range. This is a case in which a sap parasite is not able to accompany its host throughout the entire range of the latter, apparently as a result of the operation of limiting climatic con- ditions upon the parasite. The well-known xerophytic character of the mistletoes apparently brings their distributional behavior into accordance with that of other xerophytic plants in their inability to invade regions of high moisture conditions. The only mistletoe that is found in Washington west of the Cascade Mountains is a locally abun- dant form of Arceuthobium douglasw growing on T'suga heterophylla. Temperature Dave in Nonwat Frostisse Sxaeon (F.S) EE > Hor Dava, F. S., a Cod Dars, F. 8. rs Puysiovocicat Summanom, F. S. ETS | Nona Dai.y Maw, co.pest 14 wave or Year ON SSS? Monwat Day Mean, Year rs > Precirration Nomeat Dany Mean, F. S. ————E= sss? Days im LonogsT Norma, Rainy Penicd, F.S. Mi Ss | Dars im Lonaest Nqnwat Day Peron; F. S. > ‘Mean Torat, Year rs Evaroration Day Mean, 1867-6, F. S. EEE TAT Molsturt Ratios Normar P/E, F. S. a ee Normat r/E, F.S. ___— ee Re gE a aT | Normat P/E, Year > Humioiry Nonmac Man, F. S. | Sunswine Normar Daicy Duration, F. S. ————EE eee Moisture-TemPeaature Invices Normat P/E x T, F. S., PHysiovocicat METHOD a Fic. 67. Climatic extremes for Arceuthobium americanum Phoradendron flavescens (fig. 68)—A remarkably wide range is exhibited by this plant and its varieties, extending from coastal Oregon, through California and the extreme southwest, to Texas, Florida, Indiana, and New Jersey. The species itself is found from New Jersey to Louisiana, variety orbiculatum in Arkansas, Oklahoma, and Texas, variety pubescens in Texas, variety macrophyllum in Arizona, and variety villosum in California and Oregon. Both the species and the varieties of this mistletoe are found on a number of different host trees, so there is no such restriction of its range as that shown for Arceuthobium americanum. Little is known 562 CORRELATION OF DISTRIBUTIONAL FEATURES. regarding the water-supply of sap parasites and the relation between the seasonal conditions of the host and the maintenance of the tran- spiration-stream in the parasite. It is safe to assume, however, that the water-supply for mistletoe is not subject to as sharp nor as pro- nounced fluctuations as that of most autonomous plants rooted in the soil. The influence of precipitation and soil-moisture conditions is exerted very indirectly on the mistletoes, and we may regard them, for the purposes of our investigation, as somewhat analogous to palus- trine plants. Phoradendron encounters wide ranges of all temperature conditions, except in regard to the number of cold days. The wide range of mois- ture conditions which it meets is to be anticipated from its independ- ence of these conditions as they affect autonomous plants. The extremely wide amplitudes of evaporation conditions through which it ranges apparently indicate that it is able to secure supplies of water sufficient for the maintenance of high rates of transpiration. In the most arid parts of its range, however, Phoradendron flavescens var. macrophyllum is found only on trees that occur in relatively moist situations, and not on the small microphyllous trees, in which the maintenance of the transpiration-stream is precarious. The temperature condition which appears to be most potent in limiting the range of Phoradendron is the number of cold days. A very small part of its area lies inside the region in which cold days are encountered, and in this part (the Ohio Valley) it reaches the maximum of 44 days for this condition. Tempenatunt Dave in Nonmat Faosticss Stason (F.S.) (‘___ es Hort Days, F. S. Sa ET Ca os We TS Coin Days, F.S. —— St Prysio.ocicat SUMMATION, F. S. a IS Te Nonmat DaiLy Mean, covoest 14 cays or Yean [_______—————C—C—CSCSC Nonmat Daicy Mean, Year rr PRECIPITATION Nonmat Daicy Mean, F. S. a Days in LONGEST Norma Rainy Perioo, F. S, ee ee ees) Days im LONGEST Nonmat Day Pcrion, F.S. (SSS ag a ay ea a Mican ToTAt, Year PO EVAPORATION Day Mean, 1687-8, F. S. aS ee LE ee Moisture Ratios Norma P/E, F. S. SE — Norma 7/E, F. S. SE SO ia Nonmat P/E, Year Bee Humiorry Nonmac Mean, F. S. se ee! SuNsHiIne Nonmmac Daicy Duration, F. S mmm Moistunc-Tempcrarure inoices Nonmac P/E x T, F. S., Pxysiotocicat MetHop PEA a | Fig. 68. Climatic extremes for Phoradendron flavescens. Daucus pusillus (fig. 69).—This annual herbaceous plant ranges from the coastal region of Washington and Oregon through California a ee loop) CORRELATION OF DISTRIBUTIONAL FEATURES. 563 and the extreme Southwest to Oklahoma, Mississippi, Florida, and North Carolina (see plate 31). The relation of Daucus to the climatic conditions of its wide range requires interpretation in terms of its seasonal behavior in different sections of the range. In the States east of Texas it is an early summer plant, reaching maturity in July or later; in the Desert region it is an early spring plant, reaching maturity in March or April; on the Pacific coast it is a late spring or early summer plant, reaching maturity from May (in southern California) to July (in Washington). In order to evaluate properly the conditions under which it actually lives in these sections of its range, we should take into account only those climato- logical values in each section that refer to the period of its activity. A separate investigation of Daucus and other plants of the same facul- tative seasonal habits would yield results of great value. The wide amplitudes of moisture conditions shown in figure 69 would thus doubtless be greatly narrowed and the temperature amplitudes would be made somewhat narrower also. Whereas this plant appears at first to encounter a remarkable gamut of conditions through its range, nearly 4,000 miles in length, a study of the conditions in its particular seasons and in the habitats which it occupies would undoubtedly show that it grows only under a relatively limited set of conditions. The northern (and eastern) limit of Daucus follows certain of the isothermal lines so closely as to indicate that its controlling conditions are to be looked for in the temperature series. The danger of attempt- ing a final explanation of distributional limits by correlational methods alone is shown very clearly in the case of this plant. The distributional limit is closely parallel to the line for a length of growing-season of 240 days, but the length of this season is obviously of no direct importance oT Days, F. S. Corp Days, F. S. Puysiovocicat Summation, F. S. Normat Daicy Mean, covoest 14 oars of Year Normat Dai_y Mean, Year PRECIPITATION Normat Daity Mean, F. S. Days in LONGEST Normat Rainy Penion, F. S. Days In LonGcest Norma Drv Peniop, F.S. Mean Torar, Year EVAPORATION Daicy Mean, 1887-8, F. S. Moisture Ratios Normat P/E, F. S. Normat 7/E, F. S. Norma P/E, Year Humipity Normac Mean, F. S. SUNSHINE Normat Daicy Duration, F. S. MoisTuRE-TEMPERATURE INDICES Normat P/E x T, F. S., PuysioLocicat MetHoo [_ Fig. 69. Climatic extremes for Daucus pusillus. 564 CORRELATION OF DISTRIBUTIONAL FEATURES. to an annual plant which uses only a portion of the season in any part of its range. There is also a close correspondence between the dis- tributional limit of Daucus and the line for a daily mean temperature of 40° for the coldest 14 days of the year, but here again there is obviously no significance for this plant in the temperatures that super- vene while the entire race is represented only by seeds. ‘There is nevertheless a parallelism between the different phases of any climatic condition, as has been remarked before, and this may well mean that there are other indices of temperature conditions critical for Daucus which have not been elaborated in our work, and the isotherms for these may be parallel to the ones mentioned above. It may also be true that there is an unsuspected importance for this plant in the length of the total frostless season and in the low temperatures of winter. Spermolepis echinatus and Parietaria debilis have ranges which are similar to that of Daucus, although not so extended either to the northwest or the east. These plants are also annuals which have different seasonal habits in the different sections of their areas, and the remarks made about Daucus will apply also to their relation to climatic conditions. Ozxybaphus floribundus (fig. 70).—This herbaceous root-perennial ranges throughout the central United States between the Deciduous Forest region and the Rocky Mountains and north of central Texas. It encounters the entire amplitude for the country, of cold days, and relatively wide amplitudes of the other conditions, both of temperature and moisture. The narrowest amplitude is that of the moisture ratio (r/E), which ranges from 0.25 to 0.89. Tempcnatunce Dave in Norma: Frostuces Season (F.S) (___ ae} Hor Daves, F. S. ——~LL__ Cote Dave, F. S. NTS PT PO ee ee EE PE ETL Puysro.oeica: Summation, F. S. yk —~—~E—E~—~—~—————— SSCS Normac Daity Mean, coLocs7 14 ovvs oF ¥cxn es Nonmat Dairy Mean, Year Paccierrarion Nommat Daicy Mean, F. S. EE __———&& Days In Loncest Nonmat Rainy Period, F.S, es = ) Dave in concest Nenmat Dry Peniod, F. So (is) Mean Tora, Year (RETR a EVAPORATION Day Mean, 1887-8, F. S. i ——————~eE_ Moisture Ratios Nonmac P/E, F. S. ee Nonmai’rr/E, F. S. a eee Nowmat P/E, Year a Humiorry Normac Mean, F. S. EEE Sunsrine Nonmmar Dairy Duration, F.S ——<*~~L_;&<~«aE_ Mortstuac-TemPcrarunc inoices Nommac P/E x T.F.S., Prrgiovoaicat Metxoo [ R——E———E——~E—————— Fic. 70. Climatic extremes for Oxybaphus floribundus. en CORRELATION OF DISTRIBUTIONAL FEATURES. 565 The area of Oxybaphus is somewhat similar to that of Silphium laciniatum with respect to eastern and western limits, but the latter plant is definitely limited at the north, failing to reach the Canadian boundary, while Oxybaphus is limited at the south, failing to reach the Gulf coast. The eastern and western limits of Oxybaphus, like those of Silphium, appear to be set by definite constellations of moisture conditions, while the southern limit must apparently be sought in the temperature conditions. The western edge of the distribution of Orybaphus is set by the lines for dry periods of 75 to 125 days, according to the latitude, and nowhere does this plant enter regions with a moisture ratio of less than 0.20. Its eastern limit is like that of Solidago missouriensts (see plate 25), in being located along the western boundary of the Deciduous Forest more nearly than along any of the climatic lines. The actual ecological conditions for these plants of the open prairies are more radically changed on passing from the Grassland Deciduous-Forest region into the Deciduous Forest than the data of ordinary climatological stations are capable of showing. A normal mean relative humidity above 70 per cent and rainy periods of more than 100 to 125 days doubtless serve as limiting intensities of important conditions for these plants in a much more precise manner than is indicated by the positions of the isoclimatic lines as shown on our charts. At its southern edge Oxybaphus does not enter the regions with more than 200 to 240 days in the frostless season, does not encounter more than 180 hot days, and grows at no place with a physiological summa- tion of more than 17,500. Trautvetteria grandis (fig. 71) and Trautvetteria carolinensis (fig. 72).— These closely related species are the only North American representa- TemPcratuac Days in Norma: Faostiges Season (F.S) EE Hor Days, F. S. NN 23 ne Cowp Days, F. S. I A CE es CEE Pursiovocicat Summarion, F. S. DE eee Normat Daicy Mean, covvest 14 cave of Year (es > Norma Daity Mean, Year PS PRECIPITATION | Nonmat Daity Mean, F. S. ET EPS PEO ET PRT ee Pe PSE LEE BET Days IN LONGEST NoRmat RAINY Period, F.S. [ey sss? Days In LoNGesT Nonmat Dry Perion, F.S. 9 [nn Man Torat, Yean rv] EVAPORATION Daity Mean, 1887-8, F. S. a Moisture Ratios Nonmac P/E, F. S. (DRT EL PO OT PRR EO Normat 7/E, F.S. (PIES ETE a BOTT EOL Ee RE TE Norma P/E, Year TS TENET EID BD Humiorry Normat Mean, F. S. WwYtuvNROOoO._ es SUNSHINE Norma Daicy Duration, F. S. [I rsesrcececs ey Moisture-TemPeRATuRE INDICES Normat P/E x T, F.S, Puysiotocicat MetHoo ( Fig. 71. Climatic extremes for Trautvetteria grandis. 566 CORRELATION OF DISTRIBUTIONAL FEATURES. tives of a genus of Ranunculacee which is found elsewhere only in eastern Asia. They are herbaceous root-perennials of moist situa- tions, and neither of them is common nor characteristic of very exten- sive plant communities. The ranges of both the eastern and western species are based on a very small number of known localities of occur- rence, and are so broadly drawn that they comprise climatological stations near which the plants are doubtless absent. Here, as in many other cases, a much more detailed study should be given to the dis- tribution of the plants involved and to the nature of the climatic condi- tions before attempting to apply the methods of a general study of this character to an investigation of the climatic controls involved. It is, nevertheless, of interest to make even a broad comparison between the conditions under which these related but geographically segregated species exist. The amplitude of the climatic conditions for the area of Traut- vetteria grandis is wider than for that of 7’. carolinensis in all cases except the number of hot days, physiological summation, and the number of days in the longest normal rainy period. The former species exhibits a very narrow range in number of hot days, from 0 on the coast of Washington to 57 at Sante Fe, New Mexico (near one of the southernmost stations for the plant in the Sante Fe Mountains). This condition appears to be an important one in determining the limits of the western species. Trautvetteria carolinensis encounters from 63 to 160 hot days, and also has a relatively narrow amplitude of conditions with respect to the length of the frostless season, 145 to 231 days. The amplitudes of the daily mean precipitation and of the mean annual precipitation are much narrower for Trautvetteria carolinensis TremPcratuRe Days in Norma, Faosriges Season (F.S.)00 (a Hort Dave, F. &. / OE E—E—=I Coto Dare, F. S. ——_—_——— SD Puysiotocicat Summation, F. S, lr ————*=*~*~ Normat Daicy Mean, covogst 14 cave of Yean (es Nonmat Daicy Mean, Year Se ee PRECIPITATION Norma Daicy Mean, F. S. ———————S—SS ee Days In LONGEST NonmaL Rainy Penioo, F.S.0 (eT) Days in LoNGesT Nonmat Day Penioo, F.S. (i MEAN Tora, Year | EVAPORATION DaiLy Mean, 1887-8, F. S. se) nasa Sas i Moisture Ratios Nonmac P/E, F. S. SN Nonmat 7/E, F. S. a a eee Nonmat P/E, Year ee Humipity \Nonmac Mean, F. S. SSS Sunsuine 4 Nonmat Daicy Duration, F. S. CD TA Moilstunc-Tempcnatune Inoices Nonmay P/E x T, F, S,, Prysiolocicat MeTHoo Cr) Fig. 72. Climatic extremes for Trautvetteria carolinensis. CORRELATION OF DISTRIBUTIONAL FEATURES. 567 than for the western species, but are comprised in the wide amplitudes of the latter. The amplitudes of the rainy periods and the dry periods, on the other hand, are such as to indicate that the two species have almost nothing in common with respect to these conditions. The occurrence of Trautvetteria grandis over both interior and coastal regions in the Northwest is responsible for wide amplitudes of evapora- tion, humidity, and moisture ratios, for all of which the amplitudes are narrow for the eastern species. Climatological data from such locali- ties as Helena, Boise, Walla Walla, and Spokane are not suited, however, to giving an accurate conception of the conditions for this plant. The moisture conditions of those stations are doubtless much more severe than those of the southernmost mountain localities for Trautvetteria grandis in New Mexico. Even though our graphs may indicate conditions of evaporation that are too high for Trautvetteria grandis and conditions of humidity and moisture ratio that are too low, there remains, nevertheless, a marked difference between the amplitudes of these conditions for the eastern and western species, since the low range of evaporation and the high range of humidity and moisture ratios are the conditions in which the western species is most abundant. In this case our climatological stations are located in the midst of the conditions in which it actually grows. The differences in the extremes and amplitudes of the principal climatic conditions for the two species of T'rautvetteria are sufficiently great to indicate that it would be difficult to grow either of them throughout the range of the other. The distinctive conditions under . which the two species now grow, together with their complete geo- graphical segregation, must be taken to mean that they are neither recent nor immediate derivatives from a common ancestral stock. Populus balsamifera and Sapindus marginatus (fig. 73).—A ready comparison of climatic extremes for these two trees has been made possible by placing the blocks for the two on the same diagram. Sapin- dus is a tree of southern range, extending from central Texas and southern Kansas to Florida. Populus is a tree of northern range, extending from Connecticut to North Dakota, and through Canada to the northern Rocky Mountains, where it occurs in a form which has been recently regarded as a distinct species (see plate 19). Since the distributional areas of these two trees are quite separate and yet are nowhere less than 400 miles apart, it is not surprising that the ranges of temperature conditions are so unlike as to overlap only in the case of the length of the frostless season, where the maximum for Populus is slightly in excess of the minimum for Sapindus. The pre- cipitation conditions for these two trees are such that there is a con- siderable range of precipitation values common to both of them, although the extremes are by no means the same. The amplitudes of evaporation and humidity are much greater for Populus than for 568 CORRELATION OF DISTRIBUTIONAL FEATURES. Sapindus, but the values of all three of the moisture ratios are nearly the same. The amplitudes of sunshine duration do not overlap at all, and those of the moisture-temperature index are very dissimilar. The southern limit of Populus balsamifera and the northern limit of Sapindus marginatus have such a direction as to indicate that both trees are controlled by temperature. The edge of the area of the former corresponds closely to the line for 30 hot days in the normal year, although the tree extends far enough south to encounter 88 days at the edge of its range, at Toledo, Ohio. The area of Sapindus is limited in central Kansas by an average frostless season of slightly less than 180 days, by slightly more than 60 cold days, and by a physio- logical summation of 10,000. The position of the northern limit of Sapindus is so placed, however, as to indicate very clearly that its range is determined by the interaction of temperature and moisture conditions in such a manner as to require a detailed investigation based on a more accurate knowledge of the distribution of the tree than is yet available. The arid conditions of the Grassland and Desert regions apparently limit the western extension of both Populus and Sapindus. Tempenatunc * Daye in NORMAL Frostiess Season (F, SQ 2 TITTITTITITITTT E ——————} Hor Days, F. S. EL Covo Days, F. S. (cpr 202020 TEIN} Puysiotocicat Summation, F. S. (LTT J} Normat Daiy Mean, covoest 14 vavs oF Year 2 2zLII TITTLE TELE TITEL TT Nonmmat Dairy Mean, Year PRECIPITATION Norma Dairy Mean, F. S. Days IN LONGEST Normal Rainy Perioo, F. S. 9 (222272 Days 1m LONGEST Normat Dry Penioo, F. S. Mean Torat, Year EVvaPorATION Daicy Mean, 1887-8, F. S. Moisture Ratios Norma P/E, F. S. Norma 17/E, F. S. Norma P/E, Year Humipity Nonmacr Mean, F. S. Sunsnine Nonmac Dairy Dunation, F. S. Moistune-TemPerature Invoices Normat P/E x T, F. S., PHYSIOLOGICAL METNOD 2 Fic. 73. Climatic extremes for Populus balsamifera (shaded) and Sapindus marginatus (black) Cornus canadensis and Spermolepis echinatus (fig. 74).—These plants are examples of species with northern and southern transcontinental distributions, respectively, and their climatic extremes have been placed together in the:same diagram for comparison. Cornus ranges from northern California and the Sierra Nevada through the Rocky Mountains and the Black Hills to the extreme Northeastern States, being nearly coextensive with the Northern Mesophytic Evergreen Forest (plate 30). Spermolepis ranges from central California through southern Arizona and western Texas to Arkansas, western Tennessee, and western Florida (plate 31). CORRELATION OF DISTRIBUTIONAL FEATURES. 569 Wide amplitudes are exhibited by both of these plants for a number of the temperature conditions. Cornus shows a narrow amplitude for the number of hot days and the physiological temperature summation, and Spermolepis shows narrow ones for the number of cold days and for the annual daily mean temperature. The region with less than 60 hot days coincides roughly with the area of Cornus, except in the Great Basin region, where the plant is absent. The isotherm for a physio- logical summation of 5,000 corresponds in a striking way with the limit of Cornus, from the Pacific to the Atlantic. Spermolepis nowhere encounters a daily mean temperature of less than 55°, and barely enters the region with cold days. These temperature conditions are manifestly the strongest determinants operating to limit the ranges of these two plants. Both of the plants under comparison exhibit wide ranges for all of the moisture conditions, and in most cases their amplitudes overlap te a considerable extent, or even show closely similar extremes. The moisture ratios for Spermolepis show much narrower amplitudes, and much lower maxima, than those for Cornus. The sunshine conditions are more nearly wholly dissimilar than any other condition, even than those of the temperature series. These plants are of particular interest as exhibiting the influence of temperature conditions in controlling the distribution of individual species. Such plants as these and Arenaria lateriflora, Parietaria pennsylvanica, and others of transcontinental distribution are able to range through widely diversified conditions of precipitation, evapora- tion, humidity, and moisture ratio at the same time that they are strongly controlled by temperature conditions. Tempcaaturc Days in Nonmar Frostiess Season (F. S.) Hor Days, F. S. Coun Days, F. S. Puysiovocicat Summation, F. S. Nonmat Daicy Mean, covoest 14 pays of Year Norma Daicy Mean, Year PRECIPITATION ‘NormaL Daicy Mean, F. S. | Days in Loncest Normac Rainy Perioo, F. S. | Days in Loncest Nermat Dry Penion, F. S. ' Mean Torta, Year EvaProraTion Dai_Ly Mean, 1887-8, F. S. ‘Moisture Ratios Normat P/E, F. S. Normac 7/E, F. S. Norma P/E, Year Humioity Norma Mean, F. S. SUNSHINE !Normat Daicy Duration, F. S. MolsTuRE- TEMPERATURE INDICES ‘Normat P/E x T, F.S., PHYSIOLOGICAL MeTHOo ©” etait tte a Fig. 74. Climatic extremes for Cornus canadensis (shaded) and Spermolepis echinatus (black). 570 CORRELATION OF DISTRIBUTIONAL FEATURES. Among such species of northern transcontinental range we find chiefly herbaceous and shrubby plants of the evergreen forests, while among those of southern range we find herbaceous plants of facultative seasonal habits, or else palustrine and aquatic forms. We see, there- fore, in each group a set of circumstances which apparently tend to equalize the moisture conditions for these plants—the northern species are subordinate associates of the evergreen forests; the southern species are active in different portions of the frostless season, according to the seasonal distribution of rainfall, or else they occupy perpetually moist situations. VI. CORRELATION OF VEGETATIONAL AREAS WITH GENERALIZED CLIMATIC PROVINCES. 1. INTRODUCTORY. The generalized climatic provinces roughly defined from the results of our climatic studies should be of value in comparing the geographic distribution of vegetation with the distribution of different degrees of intensity and duration of climatic conditions throughout the country, and we have therefore carried out such comparisons between the vegeta- tion charts (plates 1 to 33) and the generalized climatic graphs of figures 18 to 28. Of course, it is not to be expected that any vegeta- tional area will be found te correspond perfectly with any climatic area. Probably the only method by which close areal correlations may be attained lies in the employment of several climatic conditions, as in our two-dimensional systems of climatic provinces. A number of cases have been discovered, however, in which the correspondence between areas of plant distribution and simple climatic provinces is very good. The mention of these will be valuable in the formation of a conception of what sort of plants may be expected to occur in the various climatic provinces. Owing to the complexity of the conditions to be compared and to the varying degrees of precision with which the climatic zones can now be defined—as well as to our own limitations, no doubt—these correlations are but preliminary and very tentative. The first observation to be made in beginning these comparisons is one that might have been expected on general grounds, namely, that relatively few of the vegetational areas show any pronounced corre- spondence with any single climatic province, defined by whatever method. The second observation is perhaps a little surprising, after all, considering how crude are our climatic charts, namely, that a num- ber of good agreements have been found. As has been previously mentioned, it appears that correlations between moisture provinces and vegetational areas are more frequent than those between tempera- ture provinces and the same areas. This may perhaps be due to the fact that the range of moisture conditions in the United States is very great (from very arid to very humid), while the range of temperature CORRELATION OF DISTRIBUTIONAL FEATURES. 571 conditions is relatively not nearly as great. If the humidity of our most humid areas were to be increased (within the limits set by world climate), little or no alteration in the vegetation would be expected. Nor would any great alteration in the vegetation of our most arid areas be expected from increased aridity, excepting that vegetation would finally be prohibited altogether. On the other hand, if the intensity or duration factor of the temperature conditions of our warmest provinces were increased, or if that of our coldest provinces were decreased, great changes in vegetation would be expected. Both north and south of the area of the United States the same humidity conditions are concomitant with very different vegetation characters, and this difference is to be related mainly or entirely to temperature differences. We present below some of the most definite cases of concomitancy, considering first the temperature provinces, then the moisture proy- inces, then the provinces based on the temperature-moisture product, and finally the two-dimensional provinces based on temperature and moisture. 2. TEMPERATURE PROVINCES. Two charts of temperature provinces have been employed for these comparisons, the one based on the average frostless season (plate 34) and the one based on physiological summation indices (plate 40). It will be convenient to consider the comparisons in the order of the vegetation features as these have been presented in plates 1 to 33. As has been mentioned several times, there is no primary correlation between temperature conditions and the general types of vegetation as shown by plates 1 and 2, and a comparison of these plates with those of the temperature provinces emphasizes this statement once more. In the case of plate 3, if the evergreen broad-leaved trees and the microphyllous trees are taken together as a single group (characterized by relatively low transpiring power), it is found that the geographic area occupied by this group very nearly corresponds with the area of the very warm, warm, and medium temperature provinces as brought out by the chart of the average frostless season. Near the Pacific and Atlantic coasts the correspondence is not good when the chart of physiological summations is employed. The two eastern palms, Sabal palmetto and Serenoa serrulata, occupy nearly the same area as does the very warm temperature province on either of the two temperature charts here employed. On plate 29, Phorodendron flavescens shows an area of distribution that closely agrees with the form and extent of the combined very warm, warm, and medium temperature provinces, as shown on the frostless-season chart. As in the case of the broad-leaved and micro- phyllous group of trees, the correlation is not good with the physiologi- cal summation chart. 572 CORRELATION OF DISTRIBUTIONAL FEATURES. Parietaria pennsylvanica, and, to a less extent, Arenaria lateriflora (plate 30), generally agree in their distributional area with the area of the very cool and cool temperature provinces, based on the length of the average frostless-season. On plate 31, Daucus pusillus is shown to occupy an area that corre- sponds, in a very satisfactory manner, with an area composed of the very warm and warm climatic provinces, taken with the warmer half of the medium province, as shown on the frostless-season chart. Other correspondences are suggested by our charts, but these are the most satisfactory. The generalization is at once suggested that the length of the period of the average frostless season (plate 34) exhibits much more striking correlations to vegetational areas than does the chart based on physiological summations (plate 40). The results of these comparisons are shown graphically by the following scheme: Temperature provinces based on frostless season. Warm. Medium. Cool. Sabal palmetto Serenoa serrulata Daucus pusillus Broad-leaved and microphyllous Parietaria POUNSYIVANICA. of. 5 sess «wa o 5 Sele bois lor OR OED | celta et areis coolest 3. MOISTURE PROVINCES. For these comparisons with the vegetation charts we have employed four charts showing moisture provinces: (1) mean daily normal pre- cipitation (P, plate 46); (2) mean daily evaporation, 1887-88 (£, plate 53); (3) precipitation-evaporation ratio (P/E, plate 57); and (4) mean normal relative humidity (H, plate 65). The main cases of agreement brought out by these comparisons are given below. The generalized vegetation chart of plate 2 shows many coordina- tions with the charts of moisture provinces. The best correlation occurs with the moisture-ratio chart (P/E), which alone will be con- sidered here, although a study of the other moisture charts is well worth while in this connection. Desert occupies approximately the arid province. Northwestern Evergreen Forest occupies about the humid and semihumid provinces, western subdivision. Western Evergreen Forest occupies the western and northern subdivisions of the semiarid and semihumid provinces. Grassland occupies most of the eastern subdivision of the semiarid province, extending eastward approximately to the line joining western Hudson Bay with western Gulf of Mexico. Deciduous-Forest Grassland Transition (the so-called prairie type of vegetation) occupies the more arid portion of the eastern subdivision of the semihumid province, merging imperceptibly into the next type. CORRELATION OF DISTRIBUTIONAL FEATURES. o73 Deciduous Forest occupies the remainder of the eastern subdivision of the semthumid province, so that if the two last-named vegetation types are grouped together they occupy the whole eastern subdivision of the semihumid province. Northeastern Evergreen Forest occupies very nearly the northern portion of the eastern subdivision of the humid province, but this vegetation type extends southward in the Appa- lachians, which is not shown for the corresponding climatic area by the precipitation-evaporation ratio; but this southward extension of the northeastern humid conditions is shown by the line for 140 on the evaporation chart (plate 53, figs. 3 and 22). Southeastern Evergreen Forest occupies nearly the same area as the southern portion of the eastern subdivision of the humid province. This agreement is about as close as could be hoped for in work of this kind, and the correlations are very nearly what might have been expected. Two apparently important features require brief mention: (a) the relation of the Deciduous Forest area to that of the Deciduous- Forest Grassland Transition, and (b) the relation between the climatic conditions concomitant with the Northeastern and Southeastern Ever- green Forests. (a) It will be noticed that no one of the climatic maps shows any line that may be considered as approximating the position of the boundary between the deciduous forest and the prairie. While this boundary, like the other lines of plate 2, does not represent a sharp line of demarcation, nevertheless it is one of the most pronounced and clearly recognizable vegetational boundaries presented by the United States. It is actually a simple matter—for example, in Minnesota, Indiana, or Illinois—for an observer to step, within a very few meters, from what is clearly and unequivocally decid- uous forest into what is just as unquestionably prairie. This is not nearly so easy in the case of the other vegetation boundaries as actually encountered in the field; deciduous and evergreen forest usually mingled near their common margins, and désert, grassland, and prairie usually intergrade quite imperceptibly, so that their bound- aries frequently have to be regarded as bands or zones many kilo- meters wide, even by an observer in the field. Furthermore, various species of trees have recently been introduced upon the upland of the prairie region, which originally was forested only on the flood-plains of the streams, while the deciduous forest of Pennsylvania, Ohio, etc., has been largely removed, so that the general aspect of the country is now much the same as in Iowa or eastern Kansas and Nebraska. ‘This fact has led many students to regard the prairie region as potentially a deciduous-forest region, as far as climatic conditions go, and various non-climatic conditions have been suggested as explaining the original absence of trees from the prairie uplands. Looked at from the dynamic standpoint, it seems clear to us that the difference in environmental conditions that has to be postulated as 574 CORRELATION OF DISTRIBUTIONAL FEATURES. related to the difference in vegetation here considered must be rather recondite and subtile in its nature. We are, however, strongly inclined to maintain that this environmental difference will prove to be a cli- matic one, though probably not measurable in terms of any of the simpler climatic indices. Here is a problem that is well worthy of much deeper study than we have been able to give it. We wish to suggest one possible dynamic explanation on climatic grounds. If our charts showing mean daily evaporation (plate 53, figs. 3 and 22) be once more examined, it will be recalled that the eastern sub- division of the semiarid moisture province here exhibits a great eastern lobe reaching from Oklahoma to Pennsylvania. This general phe- nomenon is shown or suggested on other moisture charts, and may be tentatively regarded as of climatic significance, until more thorough- going studies of the aerial moisture conditions become possible. Now, this penetration of semiarid conditions into the center of the great eastern area of the semihumid province suggests that the explanation of the vegetational transition before us is probably largely related to evaporation. The same conclusion is suggested by the relative air- humidity chart (plate 65, figs. 17 and 24). Just how atmometric or air-humidity data should be treated in order to obtain a moisture index that may bring this point out in a satisfactory way, if it be true, can not, of course, be predicted. In support of the general probability that evaporation is the main climatic feature to be called upon to explain this vegetational transition, it should be remarked that small local prairies were of frequent occurrence in Indiana and Ohio when these regions were still under forest, so that the tension zone between forest and prairie was apparently very broad in the region south of the Great Lakes. It should also be mentioned that the evaporation data used for these studies all refer to a single year (Russell’s data, 1887-88) and it is suggested that a normal evaporation chart may show an iso- atmic line approximating the position of the prairie-forest boundary here in question. (b) The Northeastern and Southeastern Evergreen Forest types of vegetation are well known to be very distinct, at least floristically; yet they are to be regarded (along with the Western and Northwestern Evergreen Forests) as physiologically or ecologically rather similar, being dominated by evergreen, needle-leaved trees. It is therefore important to note that the climatic conditions that seem to correspond to the Northeastern Evergreen Forest are no more continuous with those corresponding to the Southeastern Forest than is the actual dis- tribution area of the former forest itself with that of the latter. This point is clearly shown on the three charts for E, P/E, and H (plates 53, 57, and 65; figs. 2, 3, and 17, and 21, 22, and 24), and a still more CORRELATION OF DISTRIBUTIONAL FEATURES. 575 marked climatic difference is shown between these two areas on the chart for P (plate 57, figs. 16 and 23). Turning now to the more detailed vegetation charts, the area occupied by Pinus palustris (plate 6) nearly corresponds to the south- eastern humid province, as shown by P (plate 46, figs. 2 and 21). Pinus divaricata (plate 7) occupies about the same area as the north- eastern humid province, as shown by E (plate 53, figs. 3 and 22); its area also somewhat nearly corresponds to the same province on the charts for P/E and H (plates 57 and 65, figs. 16 and 23, 17 and 24). Bulbilis dactyloides (plate 11) shows an area of distribution nearly corresponding with the eastern subdivision of the semiarid province and the most arid portion of the eastern subdivision of the semihumid, as shown by P/E and H. The line from Hudson Bay to the Gulf of Mexico passes nearly through the north-south axis of its area. Pinus edulis (plate 14) occupies nearly the arid province, as shown by E. Picea sitchensis (plate 14) occupies about the northwest humid province, by P/E, and Tsuga heterophylla (plate 14) covers about the northwestern humid and semihumid provinces by the same index. Quercus falcata (plate 18) has an area of distribution nearly corre- sponding to the southeastern humid province, as shown by P. Less satisfactory agreements with this same province are exhibited by Sapindus marginatus (plate 19) and Itea virginica (plate 23). In all these cases the distribution area extends farther north in the Missis- sippi Valley than does the climatic province as shown. Populus balsamifera (plate 19) occupies the northern part of the eastern subdivision of the humid province, by E. It extends farther south than this climatic area, as shown by P/E. The climatic charts are not sufficiently detailed in the Northwest to show a correspondence to the northwestern area of the species. Decodon verticillatus (plate 23) has an area of distribution nearly conforming with that of the eastern subdivision of the humid province, together with all but the most arid portion of the eastern subdivision of the semihumid province, as shown by P/E or H. Phorodendron juniperinum (plate 29) has an area of distribution roughly corresponding to the arid province, by P/E and H. Oxybaphus nyctagineus (plate 33) occupies the more humid part of the eastern semiarid province and the more arid part of the eastern semihumid province, as shown by P/E, the distribution area of this form being much like that of Bulbilis dactyloides (plate 11). The schematic presentation on page 576 shows the relations just described, for the individual species considered. 576 CORRELATION OF DISTRIBUTIONAL FEATURES. Moisture provinces, by P, E, P/E and H. Humid. Semihumid. Semiarid. North- | South- Western. coatoun, } canton: Western.| Eastern. | Western.| Eastern. ny Picea sitchensis (C26) an anion in (en. ree is WLP SII ee Pinus palustris CED Gig ia a inset iim atelie cicveve| cava wbe'e: § <> | commmmmuemmmberal 5: ayahalecage fl ocdlace/ sales et] lS che tea hee a eee Pinus divaricata ‘et dS: 1 nee aeeecsrene Bae Me We bee | aT Re oe Pee Tsuga hetero- ne te .) | —————n S ireee ) ee eeeleeeee Quercus falcata UBdicresatota te ro lovaseihlcc a ote: o\steell« co.'e cia o.-61 et eomeocmmmmmmommsnmmnns|ot'ot/5 5 yay Mets ie ats aye al cot nee ee Populus balsam- 112) ¢- Oy Of RORSEREN CRE PR Pec es ee eee Pree eee erm Eb Decodon verti- cillatus(P/E, eel Ee ee ee es ee a Bulbilis dactyl- ) Oxybaphus nyc- tagineus(P/E, es ee ee oe ee os (ee we Phorodendron juniperinum (P PUTS ACES PI) os! 2 a cso Sif due: » con wate oho oh yatoel| eloie ase vera fell mie 'a tn, o.a ofall etnies inet | oe eee 4. TEMPERATURE-MOISTURE PROVINCES BASED ON THE PRODUCT INDEX. It will be recalled that our three moisture-temperature charts (plates 69 to 72) show a form of climatic zonation that is very similar to that shown by the chart based on the mean aqueous-vapor pressure for the period of the average frostless season. For comparison with the vege- tational areas, only the generalized chart of moisture-temperature indices using the physiological temperature summation (plate 72, figs. 18 and 25) has been employed. To avoid confusion, it is first necessary to point out that certain climatic provinces are shown as practically the same by the moisture- ratio chart (plate 58, figs. 16 and 23) and by the one here considered (plate 72, figs. 18 and 25). The arid province corresponds very well with the province of very low moisture-temperature values. The semiarid province represents much the same area as the province of low product values, but the north-south boundary of the latter province lies much farther west at its southern end (Texas), and farther east at its northern end (South Dakota). This line on the moisture-ratio chart is practically the Hudson Bay-Gulf of Mexico line, as has been noted, while the north-south line just mentioned has a very different position. Also, the eastern projection of the province of low product values is not represented on the chart of moisture-ratios; it roughly CORRELATION OF DISTRIBUTIONAL FEATURES. Bie corresponds to the northeastern portion of the humid province, as shown on the latter chart. The northwestern areas of medium and high product values correspond, to a degree, with the similar semihumid and humid provinces on the moisture chart. The eastern half of the product chart resembles a temperature chart in its zonation, as has been mentioned, and shows no clear rela- tion to the moisture-ratio chart, excepting that the southeastern area of very high moisture-temperature values may be considered as corre- sponding to the southeastern humid area in the latter case. Only this eastern half needs, therefore, to be specially compared with the vegeta- tion charts, and in this comparison the southeastern area (of very high products and of high moisture ratios) may be left out of account. The comparison brings out the fact that there are but two cases where any striking agreement in form and position of areas is to be detected. Quercus alba (plate 18) occupies, roughly, the provinces of medium, high, and very high product values, but this species does not extend nearly as far westward in the southwestern part of its area as does the province of medium moisture-temperature products. Also, this tree does not occupy peninsular Florida, which includes the highest product values. Quercus alba may be said mainly to correspond, in its dis- tributional area, with the region having moisture-temperature indices ranging from 4 to 22, but in the southwestern portion of its area it extends westward only about as far as index-value 17. This is not to be considered a very satisfactory agreement. The other case where an apparent agreement between moisture- temperature product zones and vegetation areas is to be detected is that of the cumulative distribution of southeastern deciduous trees (plate 4). In this case the agreement is more nearly perfect than for Quercus alba, but here, also, the vegetation area does not extend south in Florida far enough to include the very highest product value. These 15 trees, considered together, occupy the provinces of high and very high product indices, except the very highest, and with the further exception that they extend much farther westward in Texas than does the province of high index values. These trees may be considered as occupying the region having moisture-temperature indices ranging from 7 to 23, but they extend to index-value 2 in Texas. On the whole, we are once more led to the conclusion that the mois- ture-temperature products do not furnish a criterion of great general value, as far as the discovery of distributional correlations is concerned, at least for the vegetation areas that we have charted. Of course, many areal correlations not here mentioned are to be found between our vegetation charts and the chart here considered, but most of these represent cases where this chart agrees, in its zonation, with the mois- ture charts, and a number of such correlations have been noted in connection with those. 578 CORRELATION OF DISTRIBUTIONAL FEATURES. 5. TWO-DIMENSIONAL CLIMATIC PROVINCES. Although a number of the various vegetational areas shown on the charts of plates 2 to 33 have been shown to be more or less precisely comparable with geographically corresponding climatic areas, it never- theless appears that such satisfactory correspondence is the exception rather than the rule. The climatic conditions concomitant with the vegetation areas that fail to show such simple correlations as have been mentioned in the preceding paragraphs require a more complex mode of description, at least until the proper simple climatic indices may be discovered. The most thoroughgoing subdivision of the country into climatic provinces, which has been attempted in our studies, is that based on the two-dimensional provinces. The use of these smaller climatic areas makes it possible to describe any vegetation area not simply correlated with either moisture or temperature proy- inces alone, in terms of both moisture and temperature conditions together. Such a description is clearly climatic and may lead to further correlations, but this method soon encounters limitations, since, as has been pointed out, there are frequently several two-dimensional provinces with the same dimensions or characteristics, and these can not as yet be simply distinguished on a climatic basis alone. For the present, and in the comparisons mentioned below for illustration, it seems best to fall back upon geographical terms, in order to distinguish such climatically similar but geographically distinct areas. This method frankly begs the entire question of correlations; it furnishes, wherever it is employed, nothing more than a geographical description of the details of configuration with which it deals. It is, however, not to be resorted to until the climatic description of the vegetational area in question is as complete as possible, so that the resulting description always bears much more climatic information than would a purely geographic description. The latter sort of description is quite useless, as far as our purposes are concerned, for it merely states that the given plant or vegetation type occurs where it is. In the following paragraph we present descriptions of several vege- tational areas, following the method just outlined. For the two- dimensional climatic provinces we shall here employ only the chart formed from the length of the average frostless season and from the precipitation-evaporation ratio (fig. 19). The cases considered are set forth here simply to illustrate the use of this method of interpreta- tion; we have not yet proceeded far enough with this more complicated aspect of the subject to be able to arrive at any very promising general- ization. The distribution areas chosen for discussion here are taken from eastern forms among those that fail to show intelligible relations to the simple climatic provinces of moisture and temperature. In the case of the evergreen broad-leaved and microphyllous trees (plate 3), attention has been called to the fact that the distribution of CORRELATION OF DISTRIBUTIONAL FEATURES. 579 this group may be very satisfactorily described in terms of the tem- perature provinces alone, but neither temperature nor moisture conditions alone are adequate to bring out any climatic correlations that may suggest an explanation as to why the broad-leaved trees occur only west and east, while the microphyllous ones occur in an intermediate region. Comparing plate 3 with figure 19, it becomes at once apparent, however, that the broad-leaved trees occur mainly in the very warm humid province (Florida and Louisiana) and that they occur in smaller number of species in the warm and medium humid and in the warm and medium semthumid provinces. Especially in California they occur in the warm and medium semihumid provinces. The microphyllous trees occur mainly in the warm and medium arid, and they occur as fewer species, especially in Texas, in the warm and medium semiarid. The two-dimensional provinces are thus fairly satisfactory in corre- lating the distribution of these two groups of trees with the two primary groups of climatic conditions. The distribution of the eastern deciduous trees (plate 5) must be described, first, in geographic terms. They occur east of the line joining Hudson Bay with the Gulf of Mexico, and they are absent from all climatic provinces west of this line. In the area thus demarked their area of greatest density lies within the cool and medium semi- humid provinces. This area does not correspond to all of the area of these provinces, but it does not significantly overlap any of the humid - provinces. Itoccupiesabout theeastern half of the cool semihumid prov- ince (Kentucky to Massachusetts), and the northern marginal portion of the eastern half of the mediwm semihumid province. For the most part, these trees may be said to occur in greatest number of species with the more humid and warmer conditions of the cool semihumid province. The distributional area of Liriodendron tulipifera, one of the eastern deciduous trees, is also not possible of description in terms of our climatic provinces alone. It must first be stated that this tree occurs only in the East. Its area occupies most of the eastern half of the cool semihumid province, not reaching the boundary of the cool humid on the north and extreme northeast. It occupies about the eastern two-thirds of the medium semihumid and all of the medium humid provinces. It also occupies the eastern lobe of the warm semihumid and a portion of the warm humid (Georgia, etc.). It does not extend into the very warm temperature province. The area of greatest frequency for this tree, which may be considered as its geographic and climatic center of distribution, lies practically in the center of the distributional area just described. This smaller area may be defined as occupying the southeastern triangular lobe of the cool semihumid province and the northern half of the central por- tion of the medium semihumid, which adjoins that triangular lobe at the southwest. Thus, this center of distribution occurs with cool- 580 CORRELATION OF DISTRIBUTIONAL FEATURES. medium temperature conditions and with the more humid conditions of the semihumid moisture province. It does not extend to the bound- ary of the humid province at any point. Silphium laciniatum (plate 25) shows a clearly outlined geographical area of distribution, occupying the Missouri-Mississippi-Ohio Valley as far north as the Grand River in Michigan, as far west as western Kansas, and as far east as the Appalachian Mountains. It does not occur either in the West or east of the Appalachians. Within these geographical limits the distributional area of this plant corresponds to the following two-dimensional climatic provinces: (a) Warmer two-thirds of the cool semihumid, west of Appalachians. (b) Small portion of cool semiarid, the warmer, more humid part of this province. (c) Medium semihumid, west of Appalachians. (d) Eastern (more humid) half of medium semiarid. (e) Most of warm semihumid (all but a small area in Georgia). (f) Coolest portion of warm semiarid (Texas). (g) Western half of warm humid (Louisiana to Florida). This Silphium appears not to extend into the very warm temperature province to any considerable extent. It occupies the more arid part of the semihumid and the more humid part of the semiarid, within the warm and medium provinces and the warmer part of the cool province. Many other examples might be given showing the use of two-dimen- sional climatic provinces, supplemented by geographical data, in climatically describing vegetational areas for purposes of comparison. Indeed, any vegetational area may be so described after the requisite two-dimensional chart has been once prepared. But the four cases considered above should be sufficient to demonstrate the investiga- tional value of this general method. If the relations holding between climatic conditions and plant activity receive the attention that they deserve from ecologists and climatologists, this method, with improve- ments, should prove very useful. Especially should this be true for studies of agricultural and forest climatology, which is just beginning to attract serious attention in this country. CONCLUSION. The work presented in this publication has fallen under three heads: (1) giving the facts as to the distribution of certain types of vegetation and certain species of plants of the United States; (2) giving the data to show the intensities of the leading climatic conditions in the United States; (3) correlating these two bodies of facts in such a man- ner as to learn the exact range of conditions under which each plant or vegetation lives with respect to each of the climatic elements. The botanical facts lead to the subdivision of the vegetation into a small number of natural areas, delimited on a purely vegetational basis, to the outlining of regions in which particular ecological types are most abundant, and to the presentation of the distributional areas of a number of important species in the vegetation. The climatological data have been selected or elaborated with respect to the conditions which are of most importance to plants, with the aim, wherever possible, of devising new expressions for the climatic conditions that might be suited to the botanical problems in hand. The correlation of the dis- tribution of plants with the distribution of various numerical values or indices of the several climatic conditions has been carried out with the full realization that such correlations do not carry conclusive proof of the existence of causal connection. It is only by careful elimination of possibilities and by comparison of results, that these correlations can be used as more than a source of suggestions. The existence of a causal relation between the climatic conditions and the vegetation of any given region is so well known as to have become practically axiomatic. A relation between climate and the distribution of the common species which dominate the principal vegetations is likewise well-established fact. But the relations between climate and the distribution of the generality of individual species is indirect and is obscured by many considerations. In an investigation of the réle that is played by the various climatic conditions in determining the optimum activity of a plant or the limitation of its distribution, it is necessary to bear in mind that the conditions operate collectively and that their influences are often interdependent. The rdéle of each condition changes with the changed values of the other conditions. In attempting to determine the relative importance of several climatic conditions as determinants of a given distributional phenomenon, it is seldom possible to do more than speak in general terms. It may be possible to state, for example, that temperature conditions are more important than moisture conditions in a given case, without its being possible to determine, on the same evi- dence, which of the several aspects of temperature is most important. The problem of the réle of climatic conditions in determining plant distribution is essentially a physiological one, since it rests, in ultimate analysis, upon the influence exerted by environmental conditions on the 581 582 CONCLUSION. activities of individual plants. The attack upon this problem must, however, be made by methods quite different from those employed in purely physiological investigations. The conditions must be measured rather than controlled, and the plant material must be examined throughout its range of occurrence, much as a large series of experi- mental cultures is scrutinized for the discovery of the effect produced by controlled conditions. The methods that must be employed hinge very largely upon the interpretation of a vast series of uncontrolled experiments under the varying conditions of natural environment. It is to the geographical aspects of the problem that we must ascribe many of its complexities and much of its difficult nature. Although the results secured in this investigation are only general in their applicability, we have endeavored to develop and make use of methods which are specific and definite enough to warrant more extended use. The basic data, both as to climate and vegetation, are scanty in many cases, and the methods used could well be employed to greater advantage with fuller data, or for the investigation of similar problems in smaller areas. The presentation of vegetational data that has been given takes no account of the minor plant communities that occupy relatively small areas in all plant formations, and owe their existence to the modifica- tion of the fundamental conditions of climate through differences in what might be designated as the response of soils to the climatic condi- tions. No account has been taken of the developmental changes of vegetation in regions with rapidly shifting topography, since these changes depend mainly on differences in the character of the soil, or on changes of environment due to the plant covering. All develop- mental changes in vegetation are due to changes of environmental con- ditions, and it is only rarely, or over very long periods of time, that these changes are in the nature of definite alterations of the general climate of the region. Our presentation of climatic conditions has been limited chiefly to those elements of the climate that are commonly measured, as it is impossible to secure well-distributed series of data for other conditions, although many of these are well known to be of great importance to plants. A departure from the customary climatological procedure has been made in securing many of the data on temperature and mois- ture conditions for the period of the average frostless season as well as for the calendar year. The length of the average frostless season has been determined for each of the stations from which other climatic data have been used. The data for the summation of temperature have been worked out by three methods, in addition to the one used by Merriam. It seemed advisable to give this promising means of secur- ing additive temperature data a thorough test, and to attempt to arrive at a method with fewer objections from a physiological standpoint than could be urged against the older methods. | } " CONCLUSION. 583 The determination of the ratio of precipitation to evaporation, which was first applied to distributional problems by Transeau, has been made for the entire United States, and has been derived by three methods. So great is the importance of the ‘‘moisture ratio,” as this has been designated, that it is greatly to be hoped that our maps showing the distribution of the ratios may soon be redrawn upon the basis of much fuller evaporation data. A further attempt has been made to secure composite expressions of groups of important climatic conditions by determining the products of the moisture ratios and the summations of temperature. A cartographic method of approaching the same end has been employed for the determination of the areas included between the isoclimatic lines for the moisture ratio and those for the physiological summation of temperature. The result is a series of climatic provinces which are based upon the two expressions of climatic conditions that are probably the most fundamental ones dealt with in this work. The correlation of climate and vegetation has been carried out in three ways: The maximum and minimum values of each climatic condition have been determined for each vegetational area or for the distributional area of each species. A comparison has been made between the amplitude of the conditions in each botanical area and the amplitude in the United States as a whole, in order to discover how small or how great a portion of the whole range of climatic conditions is occupied by the vegetation or plant in question. Comparisons have been made between the positions of isoclimatic lines and the lines drawn to show the limits of botanical areas, for the purpose of discovering close correspondences. The detailed results of these methods of correlation are given in the preceding pages; they do not lend them- selves to being summarized. The parallelism that exists between the distribution of many of the closely related climatic conditions makes it difficult in some cases to determine which of the several aspects of a given condition is of the greatest importance in controlling a particular plant or vegetation. The methods used rarely fail, however, to demonstrate whether it is the temperature group of conditions or the moisture group that pos- sesses the greater importance. With respect to the generalized vegetation areas of the United States, one of the most clear-cut evidences of a fundamental correla- tion exists in the correspondence between the position of the vegeta- tional boundaries and the position of the isoclimatic lines expressing certain values of the moisture ratio for the average frostless season. The composite character of the moisture ratio, and the fact that it is derived from such an important group of climatic conditions, give it a value of the first rank in dealing with the physical conditions determin- ing the distribution of vegetation. 584 CONCLUSION. With respect to the distribution of individual species it is to be noted that most of those which are characteristic and abundant in important vegetations are, like the vegetation itself, controlled by moisture con- ditions. In fact, the distributional limits of such species frequently lie just within or without the limits of the vegetation in which they are dominant. The limits of distribution of many herbaceous and palus- trine plants lie parallel to the isoclimatic lines for temperature condi- tions. For palustrine plants the topographic conditions make the soil-moisture nearly alike at all times and in all places, and the dis- tribution of these plants is therefore subject to temperature control. The aim of our studies has been to bring forward certain types of the methods that may be employed in studying the etiology of plant distribution and to present some of the climatological data necessary to such work in the United States. Thesubject is large and complex, but it offers promising fields for further investigation, and it is to be hoped that many more workers will be attracted to it in the early future. The growth of our knowledge of plant physiology will bring with it the need of obtaining measurements of new features of the environ- mental complex, or the need of determining new phases of the climatic elements for which we already have data. The progress which is being made in the study of light and its influences upon plants may well lead to the discovery that this group of conditions plays a more important rdéle in the distribution of plants than has heretofore been suspected. The whole field of the influence of temperature on plant distribution needs to be approached with regard to the temperature requirements of each phase of the life-history of the plant. A much more detailed analysis of temperature effects is needed, and a much more elaborate system of recording temperature data. The physical conditions of the soil need much more detailed investi- gation from the point of view of their dependence on general climatic conditions. The geographical aspects of soil-moisture and soil-tem- perature conditions have been neglected by reason of the local com- plexities that they present. There is great need of the investigation of these and other soil conditions at a large number of well-distributed localities. An elucidation of the local conditions of each place would throw light on the relation of climatic conditions to the conditions of the soil, would increase our knowledge of important aspects of the soil, and would give a basis for learning the geographical range of the intensities of these conditions. The methods used in this publication and the climatic data presented may be used to investigate the controlling conditions for other plants than those we have taken up. A marked improvement in methods would doubtless follow a truly thorough investigation of the ecological distribution and controlling conditions of any one plant. To have its greatest value, such an investigation should be made with reference to the ecological center for the plant and with reference to all parts of the edge of its distributional area. CONCLUSION. 585 The work of distributional etiology must be carried on in close coordination with the work of plant physiology. A knowledge of the fundamental physiological features of a plant should go hand in hand with an effort to investigate the features of physiological and ecological _ behavior that do not lend themselves to laboratory experimentation. A substantial advance in the investigation of the etiology of plant dis- tribution would provide facts and methods of inestimable value in the practice of agriculture, horticulture, and forestry. EXPLANATION Pey Boreal ‘Region. Lower Sonoran Zone (western area). Austroriparian Zone (eastern area), | Transition Zone (western area). ri fe Transition Zone (eastern area). 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