S 600 . A PHYSIOLOGICAL STUDY OF THE CLI- , MATIC CONDITIONS OF MARYLAND AS MEASURED BY PLANT GROWTH (A second contribution from data obtained under the auspices of the Maryland State Weather Service, in 1914) Dissertation submitted to the Board of University Studies of the Johns Hopkins University in conformity with the requirements for the degree of Doctor of Philosophy By F. MERRILL HILDEBRANDT BALTIMORE June, 1917 [Ruprintap rrom Puystonogican Rasparcuns, 2: 341-405. 1921) A PHYSIOLOGICAL STUDY OF THE CLI- ~ MATIC CONDITIONS OF MARYLAND AS MEASURED BY PLANT GROWTH (A second contribution from data obtained under the auspices of the Maryland State Weather Service, in 1914) Dissertation submitted to the Board of University Studies of the Johns Hopkins University in conformity with the requirements for the degree of Doctor of Philosophy By F. MERRILL HILDEBRANDT I/ BALTIMORE June, 1917 {REPRINTED FROM PHysIoLoaicaL RESEARCHES, 2: 341-405. 1921) has Hid, ‘ tu ‘Gast #4 Ihe Bb Paid ere ut ‘ nee T fy * Wy y 3 i fi Gift f » Universit, MAY [8 21 : t “ . at) le Oe wae dw J ' ie GQ =} sly, <—/- f{- A PHYSIOLOGICAL STUDY OF THE CLIMATIC CONDITIONS OF MARYLAND, AS MEASURED BY PLANT GROWTH! A Srconp ContTRIBUTION FROM DaTA OBTAINED UNDER THE AUSPICES OF THE MARYLAND STATE WEATHER SERVICE, IN 1914 F. MERRILL HILDEBRANDT ABSTRACT? The present paper presents the results obtained from a study of a series of obser- vations on the climatic complexes for nine different stations in Maryland for the sum- mer of 1914, as the effectiveness of each complex was automatically integrated by soy- bean plants grown for a period of 4 weeks from the seed, new seeds being planted every 2 weeks. Corresponding instrumental observations were also studied. The field work was carried out by Dr. Forman T. McLean, under the joint auspices of the Maryland State Weather Service and the Laboratory of Plant physiology of the Johns Hopkins University. Mclean® has presented an account of the plan and methods by which the observational data were obtained and he also made a thorough study of the growth data for soy-bean, for the stations at Oakland and Easton, his main results and conclusions having been set forth in his publications. The study with which the present paper deals involved all the climatic and soy-bean data obtained by McLean; it thus included data for nine different localities well dis- tributed throughout the state. The plants for all stations and for all periods were treated practically alike, excepting for the climatic conditions of the several localities. The same soil was used for all cultures. The values obtained from the plant measure- ments, made after 2 and after 4 weeks of growth, therefore constitute comparative and quantitative descriptions of the total influence exerted by all the climatic conditions upon the plants, and they exhibit the seasonal march of this climatic resultant in terms of the responses of the different sets of cultures. The plant measurements themselves, and the values derived from them, form perhaps the most important contribution in the present paper. The graphs of these values depict the seasonal march of each cli- matic complex, not after the manner of the instrumental readings usually employed in the study of climate, but as measures of the effectiveness of the climatic complex to favor or retard the growth of the standard plant, the latter being employed as a “‘living instrument”’ or indicator of climatic effectiveness.* The plant measurements employed are: (1) stem height, (2) leaf area, (3) leaf- product (length multiplied by width), (4) dry weight. The climatic values used are: (1) air temperature, (2) the evaporating power of the air, (8) sunlight intensity and duration. Values to represent the temperature efficiency for growth are derived from the temperature records by means of the Livingston physiological temperature indices 1 Botanical contribution from the Johns Hopkins Hospital University, no. 60. This work was practically completed in 1917, but war conditions delayed its publication. 2 This abstract was preprinted, without change, from these types and was issued as Physiological Researches Preliminary Abstracts, vol. 2, no. 8, May, 1921. ~ 3 McLean, Forman T. A preliminary study of climatic conditions in Maryland, as related to plant growth Physiol. Res. 2: 129-208. 1917. A brief account had appeared earlier:—Idem. Relation of climate to plant growth in Maryland. Monthly Weather Rev. 43: 65-72. 1915. 4 Livingston, B. E., and McLean, F. T. A living climatological instrument. Science 43: 362. 1916. 341 PHYSIOLOGICAL RESEARCHES, VOL. 2, No. 8 SERIAL NO. 18, May, 1921 342 F. Merritt HinpEBRANDT All of the measurements, both plant and climatic, are expressed relatively, so that they may be compared for different stations and for different periods. Considering the soy-bean plant (as here employed) as a standard plant or indicator for the measurement of climatic efficiency to produce plant growth, it appears that, for the season of 1914, the climatic complexes for some culture periods were more efficient in this sense than might be expected from an attempt to interpret the corresponding climatic data, while the complexes for other periods were actually less efficient to pro- duce soy-bean growth than might be surmised from the corresponding climatic values. No successful method has yet been brought forward by which the value of a climatic complex, to produce growth in any plant form, may be deduced from instrumental data, and the plant measurements of this study furnish a means by which climatic efficiency as a whole may be directly compared for different periods and for different stations. The seasonal means derived from the plant data give relatively low efficiency values for the climatic complexes of the three western stations (Oakland, Chewsville and Monrovia), high values for the complexes of Baltimore and Darlington, and inter- mediate values for the complexes of the remaining stations (College, Coleman, Easton, and Princess Anne). : If indices for total seasonal climatic efficiency are derived by multiplying the seasonal average growth rate per day by the normal length (days) of the growing season for the station in question, these indices have the following values for the several stations: Oakland, 9009; Chewsville, 12480; College, 16867; Easton, 17688; Princess Anne, 19005; Coleman, 21115; Darlington, 23688; Baltimore, 25422. In considering these relative climatic indices it should be emphasized that the data of this study do not involve pre- cipitation as an influential climatic feature; the culture plants were automatically irrigated so that they never suffered from lack of soil moisture. A study was made of the interrelations of the different kinds of plant measurements, dealing therefore with some aspects of growth correlation in soy-bean. It appears that the rate of stem elongation was greater than the rate of leaf expansion when both were relatively small, while the former rate was the smaller of the two when both were relatively large. The rate of production of dry weight appears to have been nearly pro- portional to the rate of increase of leaf surface; the relative values of these two growth eriteria are generally about equal numerically. The statements just made apply to the data for plants quite openly exposed, but some observations on cultures somewhat protected above by glass were available, and also a set of observations on cultures in forest at Baltimore. These all indicate that the height rate was relatively greater than the rate of leaf expansion for these more or less shaded conditions, while the rate of dry-weight production was smaller than the corresponding rate of leaf expansion. The outcome of this part of the study may throw some light on the general problem as to what sort of plant measurements may be best suited to quantitative comparisons of the efficiencies of different climatic complexes. An interesting, and probably valuable result of this study is that the calculated leaf- product (length times breadth, which can be obtained without injury to the plants, if that is desirable) is generally proportional to the leaf area; of course for these soy-bean plants. ; The climatic data themselves showed a pronounced general agreement between the graph for sunlight and the corresponding one for evaporation (standardized white cylindrical porous-cup atmometer), this being probably due to the relatively great importance of solar radiation in determining the evaporation rate. The climatic values indicate a general seasonal march, which is very evident for temperature, less so for sunlight and rather obscure for evaporation. For details regarding the climatic values, as well as for the plant values, reference must be made to the tables and graphs and to the text of the paper. Crimatic ConpiTions or Maryianp 343 CONTENTS TM NROCKLOUTO. 55 ob Socaos es0e GOOD Gad ODO OUCC DAG OSdE 000005 UNDO CAHUDSEOO COU DUREOO TCO 344 The observational data and the averages derived from them Aa EM Y MACERWMAMNOMIS, 6550 soodoanaodoonndendAsbonoc acco DRESSES D 3 OF 349 The climatic measurements................ EP ter eree OTTERS stage toielcleless ofa: cleave ie 352 TD TSE 5 ooneicosnoddcd OH DOMOd Deo ce ase CRO OUND aon Sos Gon aaenoOCOBOUGOOK 353 IDFA Ot 5 06 0 bind oct co oo be 5/0 Gt Oho eto cL MRE a oir bis Aisa en A eer can oR 357 DY AIOHMHOMN. gaodupoadadoossonagogsnoououonSEocuRS joudbnavgdocogcodopooDepDE 309 Results and discussion TanROCMCHONYS oocpobs cco eans Cobist.cadacriég Ce emnenbH Macceaoo Goce aaoporGUdnoounen 360 Results from stations in the open The 2-week values The 2-week plant data for stations in the open Correlations between the 2-week plant graphs..............-.--..000-- co Gl Trends of the 2-week plant values and their seasonal ranges for the sey- CLALAS TA tLON Seppe etree rete aos Chertsey eussts cocoa’) elec sicieiars visiess onal es/ set she 376 The 2-week climatic data for stations in the open...................+e0es 377 shher—-weekabemperatuUnerd at aecmassristte ciesieciiereieie = + nieisleicisc|efelererereraiedelele 378 ChewsvallevandalVionnaviasmeaseererirn rece: aieleiteisi-i- arseistel-feleietetsiejiin' 378 Baltimore, Darlington and Coleman.......... sila SUS DORE LCE ie dae ane 379 ASO Aingl JPaIMNGAgs ATAOs..g0000ncacaoodondnenos boos anodoODdadO0DOS 379 Colle sempre wits cetera rs ser st cin cpohsssiors Aeterna atcha ale ies) ateshale's widherdy 9 eats 379 (0) eDicVeiina Cl aeeperegetcteeeersvrer eevee arose enc eer esac Pee NP eT eae sie oathave eLadevevenstavel oceds 379 hero enenalize dupa less rete teke eee eeien se srsteke tere eta (er sker citi) oye ieicvels wie kere sear 380 Light and the evaporating power of the air, 2-week data................. 380 Variability of temperature and evaporation values.....................-- 382 Correlation of the 2-week plant and climatic values.......................- 383 UNn@ 4 Gals VAIS ao acess cunres oc as ao Eee RGU abe coe CA ee oe Ee aome cron 385 The 4-week plant data for stations in the open...................2.+eeeee 386 The 4-week climatic data for stations in the open.....................-055- 390 Results for the three covered stations IhnROCKEA KORA G on cdaonlo boop Ona oo noaa kad SrnicS conto cence a eren eon emeds cannes 391 Mhesplant datamcoveredustavlonsessers cicada tts ei l-iciaelte elecielersietelor= 391 ithe. @akiancducovercdus tatioreecetseresreetereteteveiclavievshe ioi-ior-IelsteVetepsverereteloeeres 393 ithe BalltimoreycoveredystauLommert ri sete cr crt y ters eile lore isl eyelctaiel iter Te ee juLy ULY OULY UULY AUG AUG SEPT SEPT WAY RAY GUAE GUNES ‘Sires eres ety) Wr 30) Wa" er es EASTON 4-Week periods Expooed Station A=weeh periods UN JULY AUS AUS AUS SEPT, EOS 17 St a SepTOcT TAY HAY JUNESURE JOLY JULY AUG AUG SEPESEFESERT ES ae er ge We Te cee DARLINGTON 4—week periods Exposed Station BALTIMORE 4-week periods Exposed Station CRAY HAY JUNEGUNE JULY ALY AUG AUG SEPESEPTOCT 4 09 10 85 9 es 6 so 3 9 Ft 40 ny Ray SURES 15) 30 “is ze 10 ea Fie. 4. Graphs of 4-week data for exposed stations as named (continued). (Lines as in 4-week graphs of fig. 3.) 388 F. Merritt HinpEBRANDT entire season. The College graphs show close agreement, with dry weight above leaf area during the first part of the season. For Baltimore, the rela- tive leaf-area value differs considerably from the value for relative dry weight for the periods beginning August 6 and October 1, but the remaining periods show close agreement. The two Darlington graphs show close agree- ment for all periods. For Coleman, dry weight and leaf area agree well for all periods, except those beginning August 5 and August 19. For Easton, no large differences between these two rates occur for any of the cultures. For Princess Anne, the period beginning August 4 is the only one showing a difference of considerable magnitude between the relative leaf-area value and the dry-weight value. This property or characteristic of soy-bean renders possible the use of the - leaf area of the plant as an index of the dry weight of the tops, and appears to render soy-bean particularly promising as a standard plant for climatic investigations, as has been pointed out in a previous paper,!® from which the following paragraph is taken. “Tf the method proposed by Livingston and McLean (1916), of employing the growth rates of standard plants as indices for the comparison of different climates as these influence plant growth in general, is to be of value, it is of course necessary that suitable plant characteristics be chosen for measure- ment in determining the growth rates, and it is desirable that the measure- ments be such as may be made from time to time without injury to the plants. The most generally accepted criterion of plant growth, dry weight of tops, can be obtained but once for any individual plant, since the plant is destroyed during the determination. Also, the accurate determination of leaf area is very difficult unless the plants are destroyed. On the other hand, as McLean has emphasized, leaf dimensions may be obtained repeatedly during the development of the plant, without serious danger of inflicting injury. It may therefore be of considerable importance if leaf area, and even dry weight can be satisfactorily estimated for soy-bean by the employment of the leaf-product as an index.” Dry weight and actual leaf area were both determined only for the 4-week periods, the plants being then destroyed, but the lengths and breadths of all leaflets were obtained for both the 2-week and the 4-week periods. Con- sequently, to study the correlation between total leaf area and total Jeaf- product per plant, only the 4-week data are available and these are the ones here considered. Since soy-bean leaflets are approximately elliptical in form and since the area of an ellipse is proportional to the product of its axes, the leaflet-product (length times breadth) of any leaflet should be nearly proportional to the area of that leaflet. Whether this relation may hold during the growth of 19 Hildebrandt, F. M. Leaf product as an index of growth in soy-bean. Johns Hopkins Univ. Circ., March, 1917. P. 202-205. : Cimatic ConpiTIons or MaryLANpD 389 the leaflet under different sets of climatic conditions depends upon how nearly the elliptical form is retained. The sum of the individual leaflet-products of any plant, which is the total leaf-product for that plant, should be approxi- mately proportional to the total leaf area of the plant, if the relation given above holds. In the discussion that follows it will be shown that such an approximate proportionality does exist in the case of the 4-week soy-bean plants. In order to find out whether the actual areas of the leaves in these cultures were proportional to the corresponding leaf-products, the ratio of the two quantities was worked out for a number of the stations. It was found that the leaf-product divided by the leaf area gives a number that varies only slightly from the value 1.28. In other words, if we measure the two diame- ters of the leaflets of a 4-week soy-bean plant, multiply these two numbers for each leaflet, and add the products, a number is obtained which, when divided by 1.28, closely approximates the actual leaf area of that plant. Instead of using the sum of the products of length and breadth as an index of the area per plant we may use the sum of the squares of the lengths of the leaflets or the sum of the squares of the breadths of the leaflets. The numbers thus secured do not, however, bear as nearly constant a ratio to the actual leaf area as does the total leaf-product, and hence neither is as satisfactory an index of the area as is the leaf-product itself. One of the most interesting properties of the 4-week soy bean plant is that the dry weight of stem and leaves is approximately proportional to the total leaf area. Having, therefore, a means by which the leaf area may be con- veniently estimated, it is possible to calculate the dry weight of the plant approximately, by multiplying the leaf-area by the proper constant. The proportionality between the weight of the plant and its leaf area is not quite so constant as that between leaf area and leaf-product, but in the great majority of cases the variation in the ratio of dry weight to leaf area, from a constant value, is less than 10 per cent. The relations given hold over a very wide range of climatic conditions and for plants varying in height from 2 or 3 cm. to 18 or 20 cm. Since none of the plants in these experi- ments were grown to maturity, it is impossible to say whether this relation holds up to that time. From the foregoing facts it may be concluded that the dry weight and leaf area of soy-beans 4 weeks old from the seed can be determined approxi- mately from their leaflet dimensions. Soy-bean should therefore be very suitable for use as a standard plant for the measurement of climate in the manner suggested by Livingston and McLean, since the rate of its growth can be approximately determined from easily obtained leaf measurements. Also, the properties of soy-bean given above should make it a useful plant for any piece of physiological research in which it is desired to know approxi- mately the dry weight of the plant used, at various stages of its development. 390 FF. Merritt HInpEBRANDT The 4-week climatic data for stations in the open (see fig. 4, red lines) Tt will be remembered that the cultures were started every two weeks and that each grew for a period of four weeks. The 4-week periods thus over- lap, and attention has been called to the fact that averages of the climatic factors for these over-lapping periods form a smoother graph than averages for the 2-week periods. The 4-week graphs, therefore, show the general seasonal march of the index values for various stations better than do the 2-weck ones, while the latter show the details of the seasonal march better than the former. This fact will be brought out by a brief reference to the graphs at this point. The values of the physiological temperature indices for the 4-week periods show the seasonal marches of this condition for the various stations, from low values in May to high midsummer values, and then to low values again in the last part of the season. The graphs for all of the stations except Oakland show a steeper slope after the midsummer maximum has been passed than for the periods during which the temperature was rising to this maximum. The two maxima that were present in most of the 2-week graphs are eliminated in the 4-week averages and the graphs of temperature values show instead a period of about 6 weeks during which this condition remains approximately constant. The 4-week evaporation and light data show the general characteristics of the seasonal marches of these conditions previously noted as exhibited by the 2-week data. It will be seen, in the first place, that both graphs exhibit a downward slope from the beginning to the end of the season; and, in the second place, that both graphs show, in addition to their high primary maxi- mum in the early part of the season, one or more secondary maxima later. In some cases the secondary maxima of the evaporation graphs coincide, as to time of occurrence, with temperature maxima. Both of these general char- _acteristics shown in the 4-week graphs of evaporation and light are shown by the 2-week graphs but since small variations are eliminated by averaging the over-lapping periods, there are fewer secondary maxima in the 4-week graphs. In the case of evaporation, there is usually one secondary maximum occurring in or near the 4-week period including the last 2 weeks of July and the first 2 weeks of August. In the case of all stations this is one of the three 4-week periods showing high temperature values. ‘The 4-week climatic graphs need not be discussed further here. The method by which the 4-week data were derived from the 2-week data amounts to the same thing as smooth- ing the 2-week graphs and only the more pronounced characteristics of the graphs remain after averaging. Interest in the 4-week climatic data thus lies mainly in their relation to the plant growth rates. Cumatic ConpiTions or MARYLAND 391 RESULTS FOR THE THREE CoverEeD STATIONS INTRODUCTORY All of the data discussed up to this point were obtained for the open, with no covering other than a screen of wire netting of large mesh, to protect the plants from injury. At three of the stations, Oakland, Baltimore and Easton, as has been noted, a series of cultures was also grown under glazed cold-frame sash, supported three feet above the ground, these cultures being designated as the Oakland, Baltimore and Easton covered stations. 'The behavior of the plants grown under glass was very different from the behavior of those grown in the open, and the results for the covered stations will be considered in this section. The covered cultures were placed near the exposed cultures at each of the three places mentioned, so that the climatic conditions for the two would be practically the same, except as modified by the glass. THE PLANT DATA, COVERED STATIONS (See figs. & and 6, black lines) The effect of the glass cover was shown by the plants in two ways: (1) growth was always greater for the covered stations than for the exposed, and (2) the plants of the covered stations showed a marked difference in manner of growth from the plants of the exposed stations. The greater growth of the covered plants was shown in some cases by one, in some cases by two, or even by all three of the growth measurements taken. Not only did the plants show greater growth, but the maxima in the graphs of the various growth measurements for the covered plants do not usually occur at the same times as do the maxima in the corresponding graphs for the plants grown in the open. ‘The principal effect of the covering on the way in which the plants grew is shown by a disturbance of the relation between dry weight and leaf area. In previous discussion of this relation for the exposed plants it was noted that the relative dry-weight and leaf-area values are approxi- mately the same for the 4-week plants. In the case of the covered stations, on the other hand, every culture shows relative leaf area, as higher (usually very much higher) than relative dry weight. Stem height. for the covered cultures usually shows high values as compared to the corresponding exposed cultures. The tendency morc in previous discussion for this growth rate to fall off relatively, as the plants become larger, seems to be only slightly in evidence here. The following consideration of the covered cultures in detail will bring out these features. It should be noted that the culture periods for the covered stations each agree in length, to within a day or two, with those for the corresponding exposed stations. Such slight differences as exist in A-Week periods Covered Station BALTIMORE 4-week periods _ Forest Station DALTINORE 2-week periods Forest ation BALTIMORE ‘ BALTIMORE Z-ween periods i] 4-weeh periods Covered Station He Covered Station + FIAY DUNEGUME DULY DULY AUGAUS 3 oo 10 29: 236 20 Fig. 5. Graphs of 2-week and 4-week data for covered and forest stations, as named. Black, as in figs. 1 and 3. Red, Evaporation index. 392 CuimaTic ConpITIONS OF MARYLAND 393 the lengths of the culture periods do not in any degree account for the differ- ences in the plant measurements nor interfere with the general comparisons here made. In comparing growth for the exposed and covered cultures, no attempt will be made to account in detail for the differences between the two sets of plants in terms of climatic conditions, since the climatic influencs acting on the covered plants are not even so well known as in the case of the exposed stations, and it has already become clear that a really satisfactory interpretation of growth rates by means of such climatic measurements as are here employed is nearly hopeless at present. After the peculiarities of the covered plants have been pointed out, however, some suggestions as to .the probable causes of these peculiarities will be brought forward. EASTON 2-week period COVERED STATION. EASTON {so A-week period COVERED STATION; (40 ey JUNE JUNE JULY UULY AUG. AUG. AUG SEPT. & 242 © 20 eS C wo = MAY JUNEJUNE JULYQULY AUG: AUG, AUG. SEPT, ra 14 25 0 22 G@ 20 3 3) 14 7 Fic. 6. Graphs of 2-week and 4-week data for Easton covered station. (Lines as in fig. 5.) The Oakland covered station—The covered and exposed cultures for Oak- land differ less than do the corresponding sets for Baltimore and Easton, but they show the general features outlined above. The plants of the 2-week covered cultures for Oakland exhibit a much higher value of the leaf-product than do the corresponding exposed cultures, for the periods beginning June 18, July 2, and July 15, and the stem-height value is greater for the covered station than for the exposed station, for the periods beginning June 4, July 2 and July 15. The highest value of leaf-product occurs for the period begin- ning July 15 for the covered, and in the period beginning July 16 for the 394 F. Merritt HinpEBRANDT exposed 2-week plants. Each set of cultures show two seasonal maxima in the plant graphs, but these are much higher in the case of the covered plants than in the case of the exposed. In the 4-week graphs for the covered plants, leaf area is higher than dry weight for the whole season, while the exposed- station graph for leaf area is well below that for dry weight, from the period beginning May 23 to the period beginning July 16, inclusive. The maximum for all the growth measurements of the 4-week exposed plants occurs for the period beginning June 19, while the maximum for the covered station occurs for the period beginning July 2. Also, the graphs for the 4-week plants all exhibit higher values than do the graphs for the 2-week plants for most of the culture periods of the season. This is especially true of leaf area. Covering. the plants with glass seems to have produced a relatively high rate of leaf expansion, in spite of the fact that the evaporation value is somewhat higher. for the covered than for the exposed plants. The Baltimore covered station—The 2-week plant data for the covered sta- tion at Baltimore are plotted to a scale one-half asgreat as the scale used in plotting the exposed plant values, on account of the high values of stem height and leaf-product shown by the covered culture beginning July 9. The values of both leaf-product and stem height for this station are both uniformly above the corresponding values for the exposed station. Also, the tendency of the covered plants to elongate relatively more rapidly than did the exposed plants is shown by the stem-height values for Baltimore covered station for both the 2- and 4-week periods. It is interesting to note that the covered plants do not show specially high values of the plant growth rates for the period beginning August 6, as do the exposed plants. The 4-week graphs for the covered plants show very well the tendency of leaf area to reach values relatively higher than those for dry weight, the leaf-area graph being well above the dry-weight graph for the entire season. The Easton covered station—For Easton the covered plants, as compared with the exposed, show the general tendencies noted above. The 2-week growth rates of the covered plants, especially for stem height, are higher than the corresponding rates of the exposed plants. It will be observed that the maximum growth for the season, in both the covered and exposed 2-week cultures occurs for the period beginning August 3. The 4-week plants of the covered cultures show leaf area relatively higher than dry weight. The values for the culture period beginning June 22 are relatively low for the covered as well as for the exposed cultures. THE CLIMATIC CONDITIONS, COVERED STATIONS (See figs. § and 6, red lines) Of the three climatic factors generally dealt with in this study, evaporation alone was measured for the covered stations, so that the climatic data are much less satisfactory in this case than in the case of the exposed stations. Cuimatic ConpiITIoNs oF MaryLANnp 395 It is safe to suppose that the climatic conditions under the glass differed from those for the corresponding exposed stations in certain definite ways, period by period. The rate of evaporation for the covered stations was considerably greater than for the exposed as will be seen by comparing the values given in the tables. We may be certain, also, that some of the incident light was absorbed by the glass and that the light intensity under the cover was thus less than the intensity of the light falling on the exposed plants. Also, we may be reasonably sure that the air temperature under the glass was some- what higher than that outside, especially on quiet days when circulation of air was slight, and there was little tendency toward equalization of air tem- peratures. In considering the behavior of the covered cultures as related to climatic conditions, it may be mentioned that evaporation is known to have been more. intense and light intensity lower for these than for the corre- sponding exposed stations and periods, while air temperature was probably higher for the covered than for the corresponding exposed stations. The differences between the behavior of the plants under glass and that of the plants in the open seems to be primarily attributable to differences in light conditions for the two sets of cultures. The more rapid stem elongation occurring under glass is exactly what would be expected if the air temperature was higher and the light intensity was lower than in the case of the corre- sponding exposed cultures. The fact that leaf area is relatively high for the covered plants, as compared with their final dry weight, may possibly be related to a smaller amount of dry matter produced by photosynthesis per unit of leaf area in the covered cultures. Such a difference might be expected if the light energy available for photosynthesis were cut down by interposing between the plant and the light source a screen that absorbed a part of the light. Whatever may be the true explanation of the behavior of these plants under glass (and the true explanation will surely be much more complicated than is here suggested), the facts indicate very clearly that the growth of the plants under glass was quite different from the corresponding growth in the open. This point must be important in physiological experiments con- ducted in greenhouses. RESULTS FOR THE BALTIMORE FOREST STATION (See fig. 5) The Baltimore Forest Station was located about 150 yards from the exposed and covered stations at that place. Evaporation was the only climatic feature measured for this station. The sunshine intensity was of course very low, due to the shading and screening effect of the leaves of the trees above the experimental plants. Air temperature was also probably 396 F. Merritt HItpDEBRANDT considerably lower than that experienced by the exposed and covered plants.. The modification of growth habit in the case of the forest plants is very striking, as can be seen by an inspection of the plant graph for this station. The soy-beans were short erect growers in the open, and were erect with long stems under the glass of the covered station, but were runners in the forest. This effect on stem growth, which obviously cannot be explained as an effect of temperature alone in the case of these cultures, is relatively very great, the highest 2-week value for stem elongation being over four and a half times as great as the seasonal average for all periods and stations, and the highest 4-week value was a liltle less than four and a half times the seasonal average. As compared with plants grown in the open, the 4-week forest plants also show the same reversal in the relative positions of the leaf-area and dry-weight graphs as was shown by the covered plants. The leaf-area graph is above the dry-weight graph for the entire season in the forest. These cultures are thus more like the covered ones than they are like the exposed ones. This may possibly be accounted for by supposing that the similarity in the behavior of the plants in the covered and forest stations at Baltimore was related to a corresponding similarity in the light conditions for these two sets of cultures, but the problem is doubtless very complex. THE PLANT DATA AS MEASURES OF THE CLIMATIC EFFICIENCY FOR GROWTH OF THE STANDARD PLANTS INTRODUCTORY As has been stated, the investigation of which this study is a part was planned with the idea of obtaining some quantitative measures of the cli- matic complex for each of the various stations, in terms of plant activity. Since the soil used was the same, since its moisture content was kept high enough to support good growth at all times, for all stations and for all peri- ods, and since seeds of the same lot were used in all cases, it is supposed that the differences in the growth rates for the various periods and stations must have been due to effective environmental differences other than those of soil conditions. On account of the auto-irrigation of the cultures, precipitation was practically without direct influence upon the cultures of the exposed and forest stations, and it was of course quite without direct influence upon the cultures of the covered stations. The influential environmental conditions that differed from station to station and from period to period in these tests were those usually considered as climatic, with the omission of precipitation. The plant data, as set forth in the tables and graphs, may therefore be re- garded as approximate measures of the integrated non-precipitation condi- tions of the several climatic complexes under which the plants grew. These Cuimatic ConpITIONS OF MARYLAND 397 measures of course refer specifically to this particular variety of soy-bean plant and to the particular set of soil conditions that was common to all cul- tures. With another soil, or with another kind of plant, the plant values would of course have been more or less different from those here recorded. It remains to be found out whether or not soy-bean is a suitable standard plant for use in this sort of climatic integration when the needs of agriculture, forestry and general ecology are primarily considered. From what has been said in the preceding sections it appears, however, that soy-bean is at least especially well suited to preliminary and pioneer studies like the present one.”° From this point of view, each of the graphs of the plant values (shown by the black lines in figures 2-6) may be regarded as a representation of the sea- sonal march of the non-precipitation portion of the climatic environment for the particular station in question, the graphs for the exposed stations rep- resenting the ‘“‘natural’”’ conditions, while those for the covered and forest stations refer to the more or less modified climates experienced by these cultures. Some of the more outstanding features of these plant graphs have been mentioned in the preceding sections of this paper, and other features will become evident from a careful study of the graphs themselves, or of the tables from whose data the graphs were constructed. Much more might be said in this connection than has been said, but the newness of the present point of view, together wth the obvious complexity of the numerical results here presented, make it undesirable to attempt a careful study of these data at the present time. The tables and graphs of this paper render the numer- ical values available for future study, when this aspect of climatology and ecology shall have begun to attract more general and appreciative attention than it now enjoys. It should be emphasized that the plants have automatically weighted and integrated all the fluctuating and differing conditions for the several culture or exposure periods, and that the final summation is given in terms of the amount of growth produced in 2 weeks or 4 weeks from the seed. Dividing this final summation by the number of days in the corresponding period gives the average plant producing power of the non-precipitation part of the climatic complex for the given period and station. It has been noted that these plant values generally show a seasonal march for each station, the growth index being relatively low for periods near the beginning and end of the season, and relatively high for midsummer periods, and it has been suggested that temperature may be considered as the main controlling condition in the bringing about of these seasonal marches, various modifications being superimposed upon the temperature influence by other climatic conditions such as the intensity, duration and seasonal distribution of light, and the intensity and seasonal distribution of evaporation. 20 A study somewhat similar to this one, using wheat, pea and brome-grass as standard plants, was carried out by Sampson assisted by the author. See: Sampson, A. W. Climate and plant growth in certain vege- tative associations. U.S. Dept. Agric. Bull. 700. 72 p., 37 fig. Govt. Printing Office: Washington, 1918. 398 F. Merritt HinpEBRANDT SEASONAL AVERAGES OF MEAN DAILY INTENSITY VALUES FOR THE SEVERAL STATIONS Aside from the characteristics of the seasonal marches of the climatic con- ditions in question (which are best seen in the seasonal graphs themselves, figs. 2-6), it is of interest to average all the corresponding plant-index values for the season for each station, thus obtaining a seasonal average or mean daily plant-producing power, as a single index for each growth criterion for each station. This has been done for all the stations, for the 2-week and for the 4-week periods and for each growth criterion, and the resulting seasonal TABLE X Relative seasonal daily means for the scveral stations, by each of the fwe growth criteria. The letter H denotes high values; M, intermediate values; and L, low values. (The covered station and the forest station are included for completeness.) NUMBER | 2-WEEK | 2-WEEK | 4-WEEK | 4-WEEK | 4-WEBK STATION NAME OF STEM LEAF STEM LEAF DRY DAYS HEIGHT PRODUCT HEIGHT AREA WEIGHT OMalarrcdlx lear ratte seu a aera EE 125 L8&6 L77 L71 L71 L79 Chewsyillewee ee see eee 154 L87 L84 L75 L74 L78 Monrovia senna otis 154 L79 L80 L66 L711 L79 Colleges mrcime ys claceiy. Genie 154 M95 | M101 Ms0 | M110} M119 Baltimore weer: so hee hie ene 153 H125 H119 H104 | M115 | M105 Darling tone. aavs-ccn sa sce eee 154 H113 H118 |} H106|} H148 |} H144 Coleman tec eee ee 168 M96 | M107 Ms0 | M117) M113 Bastoni Vaca ae yet ae Pe ate 171 M95 | M105 M75 L82 L83 BrincesswAnne de tree ee 169 M106 M96 M92 | M117} M116 Oakland eoveredee- eee eae: _ M92 L78 Ms1 |} M118 M93 Baltimore, covered............... — H172 | H125| H186}] H154 M95 Easton, covered.................. — H145 H142 H116 H137 | M109 Baltimore, forest................. — |HH271 | LL*62 |HH*256 L69 | LL*41 * The doubling of a letter indicates an extreme condition; HH means very high, etc. means are shown in the last column of each of the data tables (tables I— VIII), where the corresponding seasonal averages for the climatic data are also given. It is to be remembered that the values are all relative, each one being stated in terms of the corresponding average for all stations and all periods, this unit being considered as 100. The seasonal averages for the various exposed sitions are brought together in table X and are shown graphically in figures 7 and 8, the former figure dealing with the 2-week and the latter with the 4-week plant data. The abscissas of these graphs are not quantitative; the vertical lines are equally spaced and each one represents one of the exposed stations. The stations are arranged in the order of their geographical locations, as far as this is possible in a linear series. The ordinates of these graphs represent the seasonal means. 399 Ciimatic ConpiITIONS oF MARYLAND *su01}8}s posodxo “yup YyooM-z JO sosvaoAv [eUOstag *y “DI\T NOL1GVS HOLONIVVG S5O3A1I109. ATMWASM3SHS ANNY SSAWIYd SxaowiiWwsa VIAOYUNOW UNV TVG os o9 (oye fo} = 3 O6 SUOILDIC pacodyy Gpollad Yyaom 2 CIOWAIAY TWNOCT IC 400 F. Merritt HitDEBRANDT The 2-week seasonal averages of the two growth measurements taken for the nine exposed stations, represented graphically in figure 7, show that the - plant-producing power of the climatic complex is about the same whether it is measured by stem height or leaf-product. The range of variation for stem elongation is from 79 (Monrovia) to 125 (Baltimore). In terms of this growth measurement, the average intensity of the Monrovia climatic com- plex is 63 per cent of that of the corresponding Baltimore complex. Similarly, the leaf-product mean varies from a minimum of 77 (Oakland) to a maxi- mum. of 119 (Baltimore); as measured by leaf-product, the mean intensity of the Oakland climate is 65 per cent as efficient as the corresponding mean for the Baltimore climate. Precipitation is of course left out of account here, as in the other considerations of this paper. The nine stations fall into three groups, according to these mean values: Oakland, Chewsville and Monrovia have low relative values, Baltimore and Darlington have high values, and College, Coleman, Easton and Princess Anne have intermediate and similar values. (See the letters L, H and M in table X.) Turning to the 4-week seasonal averages, as shown in figure 8, it is seen thai the graph for stem height agrees very well with the two 2-week graphs just considered. It is also seen that the 4-week graphs for leaf area and dry weight agree in a satisfactory manner. According to these two graphs, the nine stations fall into the following three groups: Oakland, Chewsville, Mon- rovia and Easton constitute the groups with low values, Darlington is alone in the group with high values, and College, Baltimore, Coleman and Princess Anne make up the group with intermediate and similar values. (See the letters of table X.) It is to be remembered that the two series of data (2-week and 4-week) refer to the same total time interval. The plants of one series were register- ing the same climatic conditions as those of the other series; indeed, they were the same plants, for the 4-week measurements were obtained from the same plants as those from which the corresponding 2-week measurements had been secured. The fact that the seasonal averages of the leaf area values (4-week) do not show the same grouping of the stations as do the leaf- product values (2-week) is to be referred to the fact that the plant alters its internal conditions with growth and age. A soy-bean plant exposed two weeks is an entirely different instrument (as far as measuring environmental efficiency is concerned) from the same plant exposed 4 weeks. For this reason it seems desirable that such studies as the present one should be car- ried out with as short periods of exposure of the standard plants as is feasible. Tt is somewhat as though the instrument wore out and altered its character- istics with too long exposure. Since it is obviously impracticable to obtain a large number of plants that are approximately alike, excepting as seeds, it seems desirable to begin each observation with new seed (as was done in this investigation), and to take the final readings before the internal condi- 401 Cuimatic Conpirions or MARYLAND NOLSV43 WOLDNITH VG Sa53A1105 AINWNY SCIDNING *suoljt}s posodxe ‘eyep yoodA-f fo sesvaroae [BUOSvag Nv wWwatos ayowi iia CUOILOLE Pacodyy cpoilad Yaa + CHOVAIAY WNOSWIS "2 ‘DIT VIAOYUNOW SATMAGMIAHD OGNVINVO 402 : F. Merritt HinpEBRANDT tions of the plants have been too seriously altered through age and the approach toward maturity. At the same time, the standard plants must of course be allowed to grow long enough so as to be influenced by the fluctuat- ing environmental conditions and long enough to give easily-obtained meas- urements. McLean (1917) has given some attention to the difference between the behavior of the soy-bean plant during the first and second two weeks of its growth from the seed, under the same set of climatic conditions and fluctuations, pointing out that the plant becomes more sensitive to evapora- tion conditions as it grows older (since its leaf surface becomes larger). In the use of standard plants as indicators of climatic efficiency the length of time chosen for the exposure period is clearly very important. It may be added that future studies may bring out certain advantages for a 3-week or 4-week exposure of soy-bean plants, as compared with a 2-week exposure, but—as has been pointed out elsewhere in this paper—details will be more apparent when the periods are relatively short, and the principles upon which this sort of work is based are more nearly fulfilled with short periods. To summarize this discussion, the nine exposed stations arrange themselves in three groups by every one of the five criteria, the grouping is identical by three of the criteria (2-week stem height, 2-week leaf-product and 4-week stem height), it is identical by the two remaining criteria (4-week leaf area and 4-week dry weight), but is it somewhat different by these two separate series of criteria. The differences are: that the second series of criteria place Baltimore in the intermediate instead of in the high group, and Easton in the low instead of in the intermediate group. It is a striking fact that all five growth criteria agree in placing Oakland, Chewsville and Monrovia in the group for low mean daily values, in giving Darlington high values, and in giving College, Coleman and Princess Anne intermediate values. Only for Baltimore and Easton, among the exposed stations, are there discrepancies. If the five seasonal values are averaged for each exposed station, the result places Oakland (77), Chewsville (80) and Monrovia (75) in the group for low averages, gives intermediate values for College (101), Coleman (103), Easton (88) and Princess Anne (105), and gives high values for Baltimore (114) and Darlington (126). These average values are shown in the third column of table XI. The average data for the covered and forest stations, also shown in table X, emphasize the influence of the glass covers and of the forest shade, etc. It is perhaps important to emphasize that the criterion of stem elongation gives the same grouping of the exposed stations by the 4-week as by the 2-week values. The ratio of the 2-week seasonal mean to the corresponding 4-week mean is shown for each exposed station below. CuimatTic ConpiITIONS oF MARYLAND 403 Oaklands are srecttis ames arse aes 1.21 Marlin shone. s teeiis icra e seer ere s 1.07 Chewsvyillewe decse cts istacetee 1.16 @olemant). wrens. Maceeurinecine 1.20 IMGTAROh aE poo once Ses eEeReebedcs 1.18 Basho ayaa cocina eae oes 1.27 Wollegen ee yacicniemes ce ccw alec 1.19 Princess Anne.................- 1.15 IBALGIMOLESyesaertes cei eee aes 1.20 The average of these ratios is 1.18. If, therefore, the stem height of the soy-bean, grown as a standard plant, be used as a measure of the climatic complex, and the measurement be expressed as relative average daily incre- ments, as in this study, the 2-week readings may be approximately reduced to 4-week readings (considering these as the standard) by dividing each 2-week reading by the constant 1.18. THE TOTAL SEASONAL EFFICIENCIES FOR THE SEVERAL STATIONS The efficiency of an environmental complex, or its power to produce growth in a standard plant, is to be considered as the product of two factors, intensity and duration. If the seasonal averages of the mean daily rates of growth, TABLE XI Relative generalized climatic-efficiency products for the several stations. RELATIVE NORMAL LENGTH OF) GENERALIZED SEA- CLIMATIC STATION NAME GROWING SEASON SONAL AVERAGE EFFICIENCY PROD- (a) OF DAILY MEAN ucr (AB) INTENSITY (B) days (QaiklainGlae. Bae cones Senos ee cas 117 77 ; 9009 Ghewswallesse.tess) oe tossen nev naeed Seer ee 156 80 12480 INVOTIM OWA Bets corse ies csete nis dvene seeks — 75 = Cho Ih apie eee ee oe A 167 101 16867 Baltimore..... Cie Sasa ohare 223 114 25422 Darlington eek visio aeee ee ast oe 188 126 23688 Coleman. Style Scere: eed cep nis eee Eee 205 103 21115 IDES Ol see OME OM oa ALG aoe ha SESS Oe 201 88 17688 Brincess Ame. c sateen 181 105 19005 to which attention has thus far been confined, be taken as the intensity fac- tors for the respective stations, for the season of 1914, and if the length of the entire growing season for each station be taken as the [duration factor, or the length of time through which the corresponding intensity is consid- ered as effective, then the product of the length of the season and the corre- sponding intensity factor should give a value that may approximately repre- sent the relative efficiency of the climatic complex for the station and year in question, by the given plant criterion. Precipitation is of course neglected, as 1t was not involved in this study. The seasonal averages of the daily means for the growth rates, as used in this study (table X), may be taken to represent the relative values of the 404 F. Merritt HitpEBRANDT climatic intensities dealt with, but the lengths of the growing seasons are only approximated by the total lengths of the test periods. Rather than to employ these lengths it will perhaps be better to use the mean (normal) lengths of the growing seasons for the several stations here considered. These may be obtained from Fassig’s paper on this subject, and they are shown in table XT, along with the corresponding generalized climatic efficiency products, obtained by multiplying Fassig’s mean length of the growing season by the corresponding average climatic intensity (including all five plant eri- teria) as developed in the preceding section of this paper. It is to be empha- sized that the intensity factors are all for the summer of 1914 and that the duration factors are normal, or at least closely approximate normal values. From table XI it appears that the lowest efficiency product is for Oakland, as would be expected, while the highest is for Baltimore. The Baltimore value is nearly thrice as great as is the value for Oakland. If we regard values above 20,000 as high and those between 10,000 uae 20,000 as inter- mediate, the stations may be grouped as follows:— Low values: Oakland. Intermediate values: Chewsville, College, Easton and Princess Anne. High values: Baltimore, Darlington and Coleman. These efficiency products may be taken to represent, more or less approxi- mately, the relative values of the climatic conditions at the various stations, to produce plant growth when irrigation is resorted to, so that drought periods are avoided as far as soil moisture is concerned. While there is no reason for thinking that these values (obtained from 2-week and 4-week periods and soy- bean plants, with the particular soil used in this study) may give really quantitative information on these climates as related to plant growth in general, still the product indices here derived are perhaps more reliable than any other series of numerical values that might be readily obtained, and they illustrate a new method by which a beginning may be made aiming toward the quantitative comparison of climatic complexes. One of the aims of ecological climatology should be to evaluate climates in somewhat the same manner as water-power, mineral deposits, and other geographically restricted sources of power for the accomplishment of human purposes, may be evaluated. The importance of this aim is very great for agriculture and productive forestry, and it is not less important for the fundamental principles of ecology. The above discussion presents one of the first serious attempts to compare the plant-producing powers of several climates by means of numerical indices. 21 Fassig, O. L. The period of safe plant growth in Maryland and Delaware. Monthly Weather Rev. 42: 152-158. 1914. Cuimatic ConpITIons or MaryuANnp 405 GENERAL CONCLUSION The results and suggestions attained by the study here reported leave the problem of agricultural or ecological climatology still very far from solved, but the purpose of this investigation has been achieved if some of the more fundamental considerations that must be taken into account in this sort of inquiry have been emphasized. The main points brought out are summar- ized in the Abstract at the beginning of this paper and do not require repeti- tion here. It is clear that this aspect of climatological science required other measures and other methods of treatment than those thus far developed by meteorological climatologists, and that much physiological knowledge must be built into the structure of the new science. It appears that the use of standard plants, in some such way as the soy-bean plants were used in this investigation, and the avoiding of the immense complications due to soil conditions when the same soil is not, employed in all cases, will lead to progress in this exceedingly difficult but both fundamentally and practically important field of human advancement. If the relations that hold between climatic conditions and plant growth are to be really understood it will be necessary for the climatological student to interest himself in plant physiology in no merely superficial way, and it will be necessary for much of the science of climatology, as it is now represented in the literature, to be very lightly stressed. The point that seems in need of emphasis is that this new aspect of climatology (or of ecology) will have to deal with climatic conditions as they affect plants; it will not need to give main attention to climatic fluctua- tions and differences per se, nor to the meteorological, physical and astronomi- cal reasons for their occurrence. BoM T Pema i whe 4 i 7 BA BoA ue ES i . Ks i ae he A pareroy River as) % An * A “yy4 v | " 7 fis t r ’ Phi : my ( 1 if hed ak hea] he bee ena ys ee 5 i i} . my * r 2 * = & , - [ . oo 2 ? VITA The writer was born December 26, 1888, at Baltimore, Maryland. He entered the Baltimore Polytechnic Institute in 1903, being graduated in 1907. During the year 1908-1909 be taught in the public schools of Balti- more. In 1909 he entered the Collegiate Department of the Johns Hopkins University, receiving the degree of Bachelor of Arts in June, 1913. During the years 1914-1917 he attended the Johns Hopkins University as a graduate student in Plant Physiology, Physical Chemistry and Botany. He was engaged in research for the Maryland State Weather Service during the year 1915-1916, and carried on research for the U. 8. Forest Service at the Utah Experiment Station of the Forest Service during the summer of 1916. Ve wees forced bal ates Ves) ue (ee ert Furs os re ss ees Pe ee yess Peet ees poe at Ot tlc) CCeeenaD i) OAC ie ae Iai ried dies } i 4 La} ) 4 74 (PLnuvE HEEy Ota Taw Peek | dae ty rochad Lec ‘ 4 Be ; ‘ a) \ \ a ( = ' \ 3