Setar es etitttaties catieptenite reese ereresr eset Sas atone ore nemieeeee Cornell University Library Ithaca, New York BOUGHT WITH THE INCOME OF THE SAGE ENDOWMENT FUND THE GIFT OF HENRY W. SAGE 189] RETURN TO ALBERT R. MANN LIBRARY ITHACA, N. Y. Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924001794894 BRITISH PLANTS HEIR BIOLOGY AND ECOLOGY BRITISH PLANTS THEIR BIOLOGY AND ECOLOGY BY J. F. BEVIS, B.A., B.Sc. LECTURER IN HOTANY AT THE WOOLWICH POLYTECHNIC AND H. J, JEFFERY, A.R.C5c., ELS. LATELY LECTURER KY BOTANY TO THE PHARMACEUTICAL SOCIETY OF GREAT BRITAIN WITH I15 ILLUSTRATIONS SECOND EDITION, REVISED AND ENLARGED METHUEN & CO. LTD. 36 ESSEX STREET W.C, LONDON Qk a 3 Se [FLO \s26R6\ Originally Published by Messrs. Alston Rivers, Ltd., in 1911 Second Edition, Revised and Enlarged (Methuen and Co., Ltd.), 1920 PREFACE THE object of the present volume is to fill a gap which appears to the authors to exist in the literature of ele- mentary botany, and, however imperfectly this object has been attained, it is their hope that a sufficiently clear indication has been given to teachers and students alike of the lines upon which the interaction between the vital element on the one hand and the forces of Nature on the other may be read into the hard facts of morphology, and thus the ‘dry bones” of descriptive botany be “clothed with living flesh.’ The association of form with function, of fact with environment, and of effect with cause, provides undeniably the most efficient method of securing a real knowledge of any branch of Natural History, the study of which by this means becomes one of the highest educational value. Botany is the most accessible of the Natural Sciences. Flowers are everywhere. They appeal to the wonder of the child, and for older folk their study and cultivation form an unrivalled hobby. To many, however, botany appears a science of hard names and still harder facts— at least at the outset. The knowledge derived from the text-book is too often overburdened with detail and is therefore soon forgotten. Too much of it is concerned with the bodies, and too little with the lives, of the plants so minutely described. This should not be. There is romance and tragedy in the struggle of vegetable forms and races as among animal. Plants as well as animals constitute an aggregate of living things, the component races of which compete for mastery one against the other; plants no less than animals have a- history past ‘and present—a history of strife with the elements, of invasions, of colonization, of the formation of communities and associations. In the course of time they change: some perish; others give origin to new forms destined one day to displace them and, through their v vi BRITISH PLANTS superior qualities, to win a habitation which they them- selves could not retain. The book is divided into three parts. The first part deals with fundamental ecological considerations—the factors of the physical environment—water, temperature, light, air, and soil. The second part comprises a general description of British plants examined in the light of these factors and considered as an outcome and ex- pression of them. The third part is an attempt to present in an elementary manner the physiognomy of the British flora in its most conspicuous associations, to explain its origin and development, and to analyze its present distribution. The matter dealt with in the third part covers a wide field, and in the present state of our knowledge only a broad survey can Ve attempted. This department of the science is quite a recent one; but the principles of eco- logical botany are now fairly well known. It only remains for the student to apply them to the inter- pretation of the facts of the flora of his own neighbour- hood. The book is designed as a companion to the elementary text-book and the field flora. To aid the reader, the subject-matter has been treated wherever possible in a suggestive manner, so that interest may be awakened and inquiry stimulated. With this end in view, we have been sparing of technical terms, and care has been taken to explain their meaning and to give their derivation where necessity or convenience has compelled their use. In this new edition a rather large number of altera- tions and additions have been made in order to keep pace with the onward march of the subject. Some of these, it is hoped, may be of considerable interest to the reader in view of the present tendencies of economic botany and agriculture; others, on the ecological side, are due to the authors’ desire to incorporate in their book some of the most important features of the work which has been done in various parts of the world since the publication of the first edition. August, 1920. CONTENTS PART I THE ENVIRONMENT AND ITS INFLUENCE UPON VEGETATION CHAPTER INTRODUCTION: —THE ENVIRONMENT-—FUNDAMENTAL CON- SIDERATIONS - I. CLIMATE - If. THE EFFECT OF CLIMATE UPON VEGETATION—TYPES OF VEGETATION - Il. THE INFLUENCE OF WATER ON PLANT-LIFE [V. THE INFLUENCE OF WATER ON LAND-PLANTS-—XEROPHYTES —xXEROPHYTIC FACTORS AND CHARACTERS V. WATER-PLANTS - - - - VI. TROPOPHYTES - - - - - VII. LIGHT AND HEAT - - - : - . VII. THE ATMOSPHERE - IX. THE SOIL: ITS PHYSICAL AND CHEMICAL PROPERTIES . X. THE BIOLOGY OF THE SOIL - PART II THE LIFE OF THE INDIVIDUAL PLANT—PLANT BIOLOGY INTRODUCTION—-PLANT BIOLOGY XI. DIVISION OF TERRESTRIAL PLANTS ACCORDING TO THEIR LONGEVITY AND FREQUENCY OF SEEDING XII. CLASSIFICATION OF PLANTS ACCORDING TO THEIR MODE OF GROWTH - ; - - XIN, OLASSIFICATION OF PLANTS ACCORDING TO THEIR MODE OF NUTRITION - - - - - Vi PAGE 103 107 114 123 vili BRITISH PLANTS CHAPTER PAGE XIV. THE DEFENSIVE EQUIPMENT OF PLANTS - - 135 XV. THE STORAGE OF FOOD-RESERVES—ECONOMIC BOTANY - 145 XVI. REPRODUCTION—MODES OF VEGETATIVE REPRODUCTION 153 XVII. REPRODUCTION BY SHED—POLLINATION 162 XVIII. FRUITS AND SEEDS - - e ° - 183 PART III THE BRITISH FLORA: ITS EVOLUTION AND PRESENT DISTRIBUTION XIX. VARIATION AND THE EVOLUTION OF SPECIES - - 201 XX. THE ORIGIN OF THE BRITISH FLORA - 207 XXI. THE CLASSIFICATION OF PLANTS - - - 217 XXII. PLANT ASSOCIATIONS AND FORMATIONS - = 223 XXIII. AQUATIC VEGETATION - - - . 233 XXIV. VEGETATION OF THE MARSH AND BOG =~ - 240 XXV. MOORLAND ASSOCIATIONS - - - - 248 XXVI. GRASSLAND ASSOCIATIONS - . - - 257 XXVH. WOODLANDS - = 2 i a - 265 XXVIII. MARITIME ASSOCIATIONS - - - - 275 XXIX. VEGETATION OF ROCKS AND WALLS - - 286 XXX. HEDGEROWS—CULTIVATED AND WASTE LAND + - 292 APPENDIX I.—WBISMANN’S LAW OF HEREDITY, 1885 299 APPENDIX II.—THE MENDELIAN THEORY, 1865, 1901 299 APPENDIX III.—BOTANICAL PROVINCES - 302 APPENDIX IV.—BIBLIOGRAPHY - - 303 GENERAL INDEX . . 311 INDEX TO NAMES OF PLANTS IN PART IIIs - - 331 FIG. . Seedling of Mustard, showing Root-Hairs - - bom SODIA MP w LIST OF ILLUSTRATIONS Longitudinal Section of Part of a Root-Tip, saying Outer Tissues of Root and Root-Hairs, surrounded by Soil- Particles - . Epidermis, with Stomata, from the Under Side of a on - . Vertical Section of Part of a Leaf cut through a Stoma - . London-Pride, showing Rosette-Habit - Cypress, showing Concrescent Type of Leaf - - - . Stonecrop (Sedum acre), showing Crowded Succulent Leaves - . Section of Part of a Pine-Leaf, with deeply-sunk Stoma - . Transverse Section of Rolled Leaf of Erica cinerea - - . Salicornia herbacea (Glasswort), showing Succulent Stems and Minute Adpressed Leaves - - . Transverse Section of Rolled Leaf of the Marram-Grass (Psamma arenaria) = . Erica Tetralix, showing Heath Type af Leaf - . Seedling of Acacia melanoxylon, showing Transition from Ordinary Petiole to Phyllode ‘i . Cytisus scoparius (Common Broom) - - “ 2 5. Ruscus aculeatus (Butcher’s-Broom), showing Cladodes arising in Axils of Leaves and Each bearing a Flower . Lycopodium clavatum (Common Clubmoss), with Small, Crowded Leaves . Transverse Section of Subrherged Stem of Water-Violet (Hottonia palustris) : . Formation of Brood-Bud in Potamogeton crispus . Hydrocharis Morsus-rane (Frogbit), showing Broad Floating Leaves . Climbing Shoot of Ivy, showing Leaf-Mosaic - - . Flowering Shoot of Ivy, with Leaves standing out on all Sides of Stem - . Erect Shoot of Maple, with Teams standing out on all Sides of Stem - - . Horizontal Shoot of Maple, with Leaves arranged in One Planc - - - = é = ix PAGE 54 70 71 BRITISH PLANTS . Leaf of Wood-Sorrel - - - < a F . Transverse Section of Sun-Leaf of Slipcover (Vaccinium Myrtillus) - - . Transverse Section of Shade- oe of Whortlehar’y - - . Bird’s-Foot Trefoil (Lotus coutonialue), showing Root- Nodules : + . Underground Rhizome of GorfeliGiedes : - - . Underground Rhizome of Mint - - - - . Basal Part of Potato Plant - : - - - . Swede, showing Tuberous Hypocoty]l - - - . Kohlrabi, showing Tuberous Stem bearing Leaf-Scars - . False Oat-Grass (Arrhenatherum avenaceum) - - . Convolvulus (Bindweed), twining Counter-Clockwise - . Hop, twining Clockwise - - : . Passion-Flower - . Stem-Tendrils of Ampelopsis Veitch, diowing Suckers . Everlasting Pea, with Leaf-Tendrils and Winged Stem - . Apex of a Root invested with a Mycorhiza - - . Corallorhiza innata (Coral-Root Orchid) - . Lathrea squamaria (Toothwort), showing Undergound Shoot bearing Scale-Leaves, attached to the Roots of Hazel by Absorbing Suckers - - . Cuscuta (Dodder) growing on Hop . Mistletoe attached by Haustoria to Branch of a Tree, Both seen in Section - a f . Drosera longifolia (Sundew) 7 E % . Pinguicula vulgaris (Butterwort) . é . Utricularia vulgaris (Bladderwort), showing Bladders on Submerged dissected Leaves and absence of Roots - . Acorn with Hard Cupule “ . Edible Chestnut, with Three Nuts enclosed in 5 Sidings Cupule - . Stem of Barberry with Leaf-Spines . False Acacia (Robinia Pseudacacia) with Spiny Stipules . Stinging Hair of Nettle - - . Longitudinal Section of Albuminous Seed of Onion - - . Four-Seeded Drupe of Holly - . Longitudinal Section of Albuminous Seed of Poppy . Longitudinal Section of Fruit of Garden Nasturtium, showing Exalbuminous Seed z é e . Runner of the Strawberry - - S 2 . Sucker of Elm - y # . Houseleek, with Offsets - é Fs . Tulip Bulb cut longitudinally - : é . Narcissus Bulb cut longitudinally - 7 - es . Crocus Corm in Resting Condition, with Enveloping Scale- " Leaves removed to show Solid Stem > 5 PAGE 71 72 73 95 110 110 111 112 112 113 117 117 118 119 120 124 125 127 128 129 130 131 132 137 137 141 141 141 146 147 147 147 155 155 156 157 157 158 LIST OF ILLUSTRATIONS . Longitudinal Section of the Bud shown in Fig.61 —- - . Young Crocus Corm, with Thick, Fleshy, Contractile Root - - Radical Leaf of Cuckoo - Flower (Cardamine pratensis), showing Formation of New Plantlet on Terminal Leaflet . Poa alpina, showing Replacement of Flowers by Viviparous Plantlets - ‘ , . Diagram of Flower at Time of Fertilization - - Convolvulus sepium : Longitudinal Section of Flower - . Flower of Geranium, with Sepals and Petals removed to show Honey-Glands : . Petal of Buttercup with Nectary at Base = - : . Fennel: Longitudinal Section of Flower - - - - Monk’s-Hood: Longitudinal Section of Flower - - . Christmas Rose (Hedleborus) - - - - Inflorescence of Cuckoo-Pint (Arum maculatum) - . Longitudinal Section of Unfertilized Flower of Aristolochia - . Diagram of Raceme - - . . Diagram of Corymb - : - : 2 77. Diagram of Spike - 78. Willowherb: Longitudinal Section of Flower, showing Long, Stalk-like Inferior Ovary - - 79. Diagram of Cyme~ - - - ° 80. Diagram of Head (Capitulum) - - 81. Diagram of Compound Umbel - - - 82. Achene (Cypsela) of Dandelion - ° : - 83. Samara of Ash : - - . ° - 84. Nut (Acorn) of Oak-Tree - - - - - 85. Fruit of Edible Chestnut - - - - - 86. Porous Capsule of Snapdragon - : 87. Toothed Capsule of Silene - - . Capsule of Scarlet Pimpernel, splitting deaanrorealy - . Capsule of Iris, splitting longitudinally - - . Legume of Orobus, split down both Seams - - - . Lomentum of Sainfoin - . Group of Three Follicles of Monk’s-Hood, split down One Seam only . Siliqua of Mustard, showing the Two Valves split ay from Central Partition - - . Double Samara of Sycamore 95. Schizocarp of Stork’s-Bill (Hrodium), showing Mericarps splitting away from Central Pillar < 96. Schizocarp (Cremocarp) of Fennel, piggies the Two Meri- carps split apart - - & < 97. Schizocarp of Mallow - : 5 98. Drupe of Peach cut longitudinally - - - 2 99. Fruit of Gooseberry - - - 7 2 ° xl PAGE 158 159 160 161 163 169 169 170 170 171 172 175 176 179 179 180 180 181 181 182 186 187 187 187 188 188 188 189 189 189 189 190 191 191 192 192 192 193 BRITISH PLANTS . “ Pseudocarp ” of Strawberry . - Multiple Fruit of Mulberry - - . Fruit of Fig - - . Achene (Cypsela) of Thistle - - 4, Achene of Clematis with Feathery Style . Debisced Capsule of Violet . Achene of Avens (Geum), with Hooked Style . Achene (Cypsela) of Bur-Marigold (Bidens) . Section of Soil in a Wood, showing Stratification of Plants . Hydrocharis Morsus-rane (Frogbit) . Isoétes lacustris (Quillwort) . Map of a Wood, showing Distribution of Trees . The same Wood as in Fig. 111, showing Distribution of Bracken . Salicornia herbacea (Annual Glasswort): . Carex arenaria, with Rhizome near Surface of Sand - . Tuberous Stem of False Oat-Grass - PAGE 193 193 194 195 195 197 197 197 228 238 239 266 267 278 280 297 PART I THE ENVIRONMENT AND ITS INFLUENCE UPON VEGETATION BRITISH PLANTS: THEIR BIOLOGY AND ECOLOGY INTRODUCTION THE ENVIRONMENT—FUNDAMENTAL CONSIDERATIONS THE study of plants in relation to their natural sur- roundings or environment is known as Plant Ecology (Gr. otkos, a home). Every plant which lives and succeeds in reproducing itself may be regarded, on the one hand, as an efficient machine, satisfactorily performing its various physio- logical functions; and, on the other, as an organism adapted to, or in equilibrium with, its environment. Otherwise it would perish and leave no descendants behind. Again, when we examine any association of plants living together under much the same conditions, a wonderful diversity of form, habit, and growth meets our eyes; and, seeing that each plant in this association is a successful unit in the battle of life, we are driven to the conclusion that not only is it adapted to its environment, but that this adaptation is reached in many and various ways. These three considerations—namely, (1) That a successful plant is an efficient machine ; (2) That it is in equilibrium with its environment ; (3) That this adaptation is reached in many different ways —lie at the root of Plant Ecology, and form the basis upon which our knowledge of the vegetable population of the globe has been founded. 3 4 BRITISH PLANTS In dealing with environment we have three things to consider : 1. The items or factors of the environment which affect vegetation (Part I.). 2. The effect of these factors upon (a) The vegetative, (6) The reproductive parts of plants (Part IT.). 3. The results of competition among plants, for the satisfaction of their needs, as also between plants and animals in the general struggle for existence. The results are expressed in the flora just as we find it anywhere to-day (Part III). As the first part of the book is devoted to those factors of the environment which influence vegetation, we will survey them briefly first, leaving details to future chapters. A. SOLAR ENERGY. The sun is the ultimate source of all terrestrial energy ; its rays illumine as well as warm. We have therefore to consider the solar energy which is poured upon the earth, under what appears to us its two manifestations— light and heat. To the plant, however, these two mani- festations are one—it is simply energy. 1. Light. The world of green plants owes its existence to light. In the presence of light, the green cell combines the carbonic acid gas which it receives from the air with the elements of water derived from the soil to form sub- stances like starch and sugar. The process is known as photosynthesis (Gr. photos, light; synthesis, a putting together), or carbon-assimilation. The term assimilation bas rather a wide meaning in plant physiology ; but it always implies making or building up. In its wide sense it embraces not only all those processes which, as a result of vital activity, lead to the construction of food, but also those less known and even more wonderful processes by which, step by step, food itself is transformed into FUNDAMENTAL CONSIDERATIONS 5 living flesh. In botany, however, assimilation is generally used in a more restricted sense, being limited to those processes which result in the production of nutriment only. Starches and sugars belong to a group of bodies called carbohydrates, the simplest and most widely dis- tributed of the food-stuffs. In plants they form the starting-point of all the other kinds of food. When we recognize that the vegetation is the ultimate source of food for the whole of the animal kingdom, and that the starting-point of food-production in plants is carbo- hydrate, we begin to realize the importance of that kindly light without which this earth would be a dead and un- inhabited world. Classification of the Primary Food-Stuffs.—The constit- uents of food are divided into three classes : 1. Carbohydrates (e.g., starches and sugars).—These are organic compounds of rather simple chemical composi- tion, containing the elements carbon, hydrogen, and oxygen. They owe their existence to the green cell of the plant, which is the ultimate source of all the carbo- hydrates in nature. 2. Proteins.—These are substances of complex chemical composition, and, in addition to carbon, hydrogen, and oxygen, they contain nitrogen and sulphur, and, in some cases, phosphorus. The source of the carbon is a carbo- hydrate, while the nitrogen, sulphur, and phosphorus are derived from mineral salts present in the soil and absorbed by the roots in solution in water. The construction of proteins is carried on chiefly in the leaf and at the points of growth, and is not directly dependent upon light. They are utilized in growth, and form the raw material out of which the living protoplasm is made. Proteins occur in all plant and animal tissues—e.g., as albumin in white of egg, gluten in flour, casein in milk and cheese ; lean meat is a mixture of proteins. 3. Fats and Oils.—These contain carbon, hydrogen, and oxygen, but the proportion of oxygen present is less than in carbohydrates. They occur in plants chiefly as food-reserves in fruits and seeds, and are very rarely produced as the result of photosynthesis. Unlike plants, animals can make no food ; all that they receive they obtain directly or indirectly from the vege- table world. Green plants which absorb solar energy are ° 6 BRITISH PLANTS known as autotrophic, or self-nourishing ; plants which are not green, and all animals, except a few of the very lowest, are heterotrophic—that is, they derive their food from without. These external sources of food may be either living or dead. Organisms living on rotting organic matter—e.g., animal or plant remains—are called sapro- phytes; those which prey upon living bodies, parasites. In either case the ultimate source of food is the green plant. 2. Heat. Life is impossible without a certain degree of warmth. Within certain limits, the activity of all the vital or physiological functions exhibited by plants—viz., assimi- lation, respiration, absorption, transpiration, growth— increases with a rise of temperature and decreases with a fall. It is therefore through the variation in the activity of these functions that we observe the effects of heat and cold upon plants (Chapter VII.). The general character of the vegetation everywhere is largely determined by the climate ; heat is one of the fundamental factors of climate (Chapter I.). B. THE ATMOSPHERE. The air is the second great factor of the environment. Land-plants are immersed in it ; it dissolves in water, and so reaches water-plants. In dealing with the relations between plants and the atmosphere we have to deal with 1, The chemical effects of air upon plants. 2. The physical effects of air upon plants. 1. The Chemical Effects of Air. The air is a mixture practically of three gases—oxygen (20°8 per cent.), nitrogen (79:10 per cent.), and carbonic acid gas (0°035 per cent.)—all of which are, directly or indirectly, of vital importance to plants. A varying amount of water-vapour is always present, as well as dust and impurities and traces of rare gases. (a) Water-Vapour.—The other constituents of the air remain very nearly constant, but the amount of water- FUNDAMENTAL CONSIDERATIONS 7 vapour present varies considerably at different times and in different places. Atmospheric moisture plays an im- portant réle in climate (Chapter I.). (b) Carbonie Acid Gas.—This is a compound of carbon and oxygen (CO,). The amount of the gas present in the air is small, but the importance of this quantity is manifest when we consider that it forms the ultimate source not only of all food, but also, perhaps, of every other organic compound in nature. We have already referred to the role played by carbonic acid gas in photo- synthesis. In this process, carbonic acid gas is with- drawn from the air, its oxygen is liberated as a gas, and its carbon fixed in the starch which is produced. The energy required to carry out the process is obtained from sunlight. In spite of the fact that carbonic acid gas is continually being withdrawn during photosynthesis, the quantity present in the air remains constant. This is because the atmospheric supplies are always being replen- ished from other sources—e.g., respiration (Chapter VIII.). (c) Oxygen.—All living things breathe, plants and animals alike. During respiration oxygen is taken in, food is destroyed, and carbonic acid gas given out. The nutritive body most commonly destroyed in this way is starch, and in this case the process is the reverse of photosynthesis. Energy is imprisoned in food. When food is broken down by respiration, energy is liberated, and becomes available for the performance of physio- logical work. The part played by oxygen in the process is chemical ; it attacks nutritive substances and breaks them down into simpler bodies, one of which is always carbonic acid gas. Oxygen, therefore, plays the part of an energy-liberator, and, with rare exceptions, energy in living bodies is liberated in no other way. (d) Nitrogen.—Atmospheric nitrogen is of no direct use to ordinary plants, which derive their nitrogenous supplies from mineral salts present in the soil. But all the nitrogen fixed in the soil has been ultimately derived from the atmosphere. This interesting problem of nitrogenous circulation is considered in Chapter X. 8 BRITiS.i PLANTS 2. The Physical Effects of Air. Of these the most important in relation to plants are: (a) The effects of air in motion—i.e., wind (Chapter VIII.), and— (6) Its rate of diffusion through perforated membranes, such as exist in plants in the form of surfaces pierced with small holes or stomata. C. THE SOIL. The earth, or edaphic factor, is the third great factor of the environment. The soil is the medium in which the great majority of plants are fixed, and from which they draw all the materials necessary for nutrition, excepting only the carbonic acid gas required for photo- synthesis and oxygen for respiration. The soil is considered under the following heads : (a) The physical and chemical properties of the soil (Chapter IX.). (b) The relations of soil to water (Chapter IX.). (c) The decaying organic matter in the soil—humus (Chapter X.). (d) The biology of the soil (Chapter X.). Of all the factors in the environment water is the most important. It leaves a stamp upon the vegetation that no other factor does. A large portion of Part I. is concerned with the relations between plants and water. Such, briefly summarized, are the five ecological factors of the environment—light, heat, water, air, and soil. Each is treated in detail in the chapters following. CHAPTER I CLIMATE Usp in a wide sense, the term vegetation may be applied to the general appearance of an aggregate of plants as it is viewed in a natural scene. Thus conceived, the vegetation forms a constituent feature of the landscape. Most of us have derived from pictures some idea of the great differences that exist in the vegetation in different parts of the world, and it is not difficult to associate these differences with the climate. Thus the prairies are great plains covered with grass,.but the prairie is . -@-~-dry region upon which trees cannot grow. A large part of Mexico is a semi-desert, the stony soil of which is sparsely studded with queer-looking fleshy plants, called cacti. The plains of the Amazon and the Congo are covered with great tropical forests, in which the vegetation is most luxuriantly developed, but these regions are very hot and among the rainiest in the world. From these and similar examples, it is easy to infer that a close relation exists between the vegetation of a district and the climate. We will consider how close this relation is, after we have examined the various external factors which influence and determine climate. CLIMATIC FACTORS. These may all, on analysis, be expressed in terms of two factors of the environment : I. Heat. II. Humidity. Y. Heat. The only heat received by the earth which has any effect upon climate is that which is derived from the sun. 9 10 BRITISH PLANTS The effect of solar heat upon climate may be considered in two relations : 1. Latitude—As we recede from the Equator to the Poles, the sun’s rays reach the earth in an increasingly slanting direction. In consequence of this, the heat received from the sun gradually diminishes in effect, owing to the spherical shape of the earth’s surface, and the losses sustained by the rays having to pass through increasing thicknesses of air. The Tropics occupy a belt about the Equator, and within this belt the sun is directly overhead twice every year. Here, therefore, there is no alternation of seasons corresponding to our idea of summer and winter, and consequently there is no such break in the period of vegetative activity as that which is so familiar to us—when flowers are nowhere to be seen, when seeds lie dormant, and trees have discarded their summer foliage. In the Tropics the ‘‘ winter ” temperature is but a few degrees lower than the “summer” temperature, and the vegetation is always green. There may, however, be another kind of seasonal alternation, depending, not, as summer and winter, upon the variations of heat re- ceived from the sun, but upon the supply of water received in the form of rain—that is, wet and dry seasons may alternate. In some parts of the Tropics this alterna- tion occurs with great regularity, and if the dry season is long and very pronounced, a break in the vegetation may be produced, which is very similar in its effects to that which occurs in winter in northern latitudes—e.g., in the Caatinga forests of the Brazilian plateaux. In the Tropics, therefore, dearth of water has the same effect in arresting the activity of the vegetation as cold in northern winters. In regions outside the Tropics, a regular alternation of seasons occurs, and, in consequence, that break in the vegetation known as the winter-rest. In proportion as we recede from the Tropics, the winters increase in dura- tion at the expense of summer, and not only do the summers become shorter, but the sun’s power during the daytime becomes less effective, the only com- pensation for this being the increasing length of the individual day as we approach the northern limits of vegetation. CLIMATE YW 2. Altitude——The temperature falls as we rise above the sea-level just as it falls as we recede from the Tropics. In consequence of this, the succession of vegetation up a mountain is much the same as that which we observe as we proceed along the earth’s surface from the Equator to the Poles. II. Humidity. Water has a high capacity for heat ; in other words, it takes a relatively large amount of heat to raise its temperature one degree ; it is slow to get warm and slow tocool. This fact has an important bearing upon climate, for where there is a great deal of water-vapour present in the air, the variations and extremes of temperature are not so marked as in the case of a dry atmosphere. The climate is more equable, the summers being cooler and the winters milder than in drier countries ; nor does the night-temperature differ so greatly from the day. 1. Rainfall.—This is by far the most important factor in climate, and as we are here concerned with the effects of rain on vegetation, the following considerations with respect to the rainfall become very important : (a) The actual number of inches of rain falling in a year. The effect of this varies with the temperature. In a hot country a much larger rainfall is necessary to serve the needs of the vegetation than in a cold country ; for example, the minimum amount of rain necessary to maintain forest will not be the same in all latitudes. In dealing with climatic effects, heat and moisture must always be considered together. (b) The frequency of rainy days. This is more impor- tant than the actual quantity of rain which falls in a year. Occasional torrents, however heavy, influence climate, and therefore vegetation, far less than precipita- tions, which, though less in quantity, are more evenly distributed throughout the season. (c): The season of greatest rainfall. Rain falling during the season of vegetative inactivity is largely lost to vegetation. Most rain should fall when the vegetation needs it most—that is, in summer. 2. Proximity to the Sea.—Countries near the sea generally enjoy a mild and moist climate, especially when the winds that reach them come laden with moisture 12 BRITISH PLANTS picked up in their journey over a wide stretch of sea. This oceanic type of climate prevails over western Europe and the British Isles. Owing to the humidity of the atmosphere, the winters in these regions are mild and the summers cool. Regions remote from the sea have a continental type of climate, in which the dryness of the atmosphere leads to extremes of temperature, the summers being very hot and the winters very cold: such is the climate of Russia and the interior of Canada. 3. Proximity to Highlands and Mountain Ranges.— Highland masses cool the air-currents, and much of their contained moisture is consequently condensed to liquid water, and falls as rain. There is a “wet” and a “‘dry” side to mountain chains; the side facing the direction of the prevailing winds is wet, for the air- currents, following the rising gradients of the ground, become cooled and gradually damper, the excess of moisture falling as rain ; as the currents descend on the other side, they become warmer and drier. Places situated on the “lee” or dry side are said to lie in the rain-shadow of the mountains. A good example of this is Bray, a watering-place near Dublin, which lies in the rain-shadow of the Wicklow Hills. Though on the sea- coast, it is one of the driest spots in Ireland. In the same way, the desert of Atacama is a rainless region lying along the coast of Chili in the rain-shadow of the Andes. Elevated plains, especially when they are very ex- tensive, as in Spain and India, are usually very dry, and subject to wide extremes of temperature. The winds that blow over them lose most of their moisture while they are ascending the flanks. The latter, being exposed to the falling rain, are richly clothed with vegetation, whilst the plateaux themselves are grassy plains or semi-deserts. : 4. Direction of the Prevailing Winds.—If these have passed over a broad expanse of water, they will be moist ; if, on the other hand, they have travelled over a great land-surface, they will be dry. Again, winds blowing from the north are cold and dry, while those which come from the south are warm and moist. North winds blowing southwards must gradually become warmer, and, if at the same time they are travelling over a wide~land- CLIMATE 13 surface, they must also become drier. We perceive this well enough in the case of our own east winds. 5. Ocean-Currents.—These have a marked influence on climate. The Gulf Stream spreads as a drift over the surface of the North Atlantic Ocean, moving in a north- easterly direction towards the shores of Western Europe, to which it brings the stores of heat and moisture which it has drawn from tropical seas. The climatic effect of the Gulf Stream is, however, mainly attributable to the winds that accompany it in its long journey from the Tropics. Owing to the presence of these winds, the fiords of Norway are free from ice all the winter, even up to the Arctic Circle, while on the other side of the Scan- dinavian Mountains, the Baltic Sea, exposed to the dry cold winds of Northern Russia, is frozen over for several months in the year. 6. Presence of Clouds and Fogs.—Where these con- stitute a characteristic feature of the climate, the weather is raw and cold. Clouds and fogs form an effective barrier to the sun’s rays, and the soil, thus screened, loses much of the warmth that otherwise would reach it. Scotland is, for the most part, such a country. Holland, too, is cold and foggy. In Tierra del Fuego, in certain oceanic islands like the Falklands, and in the lonely island of Kerguelen in the Indian Ocean, the sun is rarely visible through the clouds. 7. The Destruction of Forests.—Even in nature, forests may occasionally be destroyed by fire, but in most cases their disappearance is due to their deliberate removal by man. As a result, the atmosphere in these localities has become drier; on sloping ground the rain runs off instead of soaking in; the soil gets washed away; and the fertility of the land is destroyed. On the other hand, afforestation tends to increase humidity and restore fertility to the whole district. Thus, between 1863 and 1878, trees were planted on 19,500 acres of barren land on the stony slopes of Ventoux, in Provence, France. These forests yielded £2,800 a year in 1911, a sum which was expected to increase to £3,600 in five years’ time. But far more important than this is the accompanying statement that “springs have reappeared, the lower lands have increased in value, and the village proprietors have found themselves suddenly enriched.” 14 BRITISH PLANTS 8. The Nature of the Soil_—From a purely climatic point of view, the soil is not important, but ecologically it is important because it forms the abode of the vegeta- tion. Climate may determine in broad outlines the geographical aspects of the vegetation, but the diversity of the flora in particular areas is the result, not of differ- ences in the climate, but of differences of interaction between soil and climate. What these interactions are will be made plain when we have considered the properties of the soil, and the relations of the various kinds of soils to the available supply of heat and moisture (Chapter 1IX.). CHAPTER II . THE EFFECT OF CLIMATE UPON VEGETATION—TYPES OF VEGETATION THE two most important factors in the plant’s environ- ment are climate and soil. In the preceding chapter we considered one of these—climate—and we showed that the type of climate anywhere prevailing may be expressed in terms of two external agencies, variously combined—heat and moisture. Types of climate operate over wide areas, and if it is true that the character of the vegetation varies with the climate, it should be possible, in some rough way at least, to distinguish broad types of vegetation, just as it is possible to distinguish broad types of climate. Let us see how this may be done. If one were able to survey the vegetation of a country as a whole, viewing it from a distance, so as to see its broad outlines without being disturbed by details, what are the types which would stand out most conspicuously ? It would not require great powers of observation to give an answer. One person might say woodland, grassland, desert. To these another might add marshes, and another heath. These are types of vegetation regarded as aspects of scenery, but how far is it possible to associate them with definite conditions of climate? A marsh is wet, wood- lands: are generally damp, and grasslands dry; heath is very dry, and deserts are almost rainless. But there are two types of grassland—one, meadow-land, which is wet ; and the other, pasture or prairie, which is dry. The term “‘ woodland ”’ is still more vague, since it includes several. types easily distinguishable from each other. For example, there are evergreen woods and deciduous 15 16 BRITISH PLANTS woods, and of evergreen woods there are several types, each characterized by a special kind of climate. Thus we have the evergreen rain-forests of the Tropics, with a climate exceedingly hot and moist; the coniferous evergreen forests of Northern Russia, where the climate is cold end dry ; and the evergreen dry-woods of Southern Europe, where the climate is dry and warm. We seo from this that the physiognomic groups into which we have divided the vegetation (woodland, grass- land, heath, etc.) are only in part associated with definite types of climate. A nearer approach to a clir atic grouping is obtained if we split up the physiog- noric groups into subdivisions founded upon climatic differences, thus : Physiognomic Divisions. Climatic Subdivisions. 1. Woodland e+ | i. Deciduous dicotyledonous woodland. (a) Wet type (oak). (b) Dry type (birch). ii. Evergreen dicotyledonous woodland. (a) Wet type (Tropics). (6) Dry type (Mediterranean). iii. Coniferous woodland. (a) Cold type (Russia). (b) Dry warm type (Mediterranean). 2. Moorland «. | (a2) Wet moorland (bogs). ‘ (b) Dry moorland (heaths). 3. Grassland o- | (a) Wet type (meadows). (b) Dry type (pastures). 4. Deserts .. ee In this country the only natural deserts we have are sea-beaches and sand-dunes. The desert is an open type of habitat, containing many bare spots where plants are practically free from competition. Artificial open habi- tats are produced in cultivated fields and waste places frequently disturbed by man. When the ground is fully occupied by plants, the habitat is said to be closed, and where this has happened, a stable community of plants or associations of plants is established. The number of physiognomic divisions into which vegetation may be divided is, of course. a matter of TYPES OF VEGETATION 17 choice ;the same is true of the climatic divisions. Neither of the divisions, however, is very satisfactory from an ecological point of view. A proper ecological grouping must be based on something more than climate. It should take into consideration not one factor of the environment, however important, but all the factors, both of soil and climate, which influence vegetation, and lead to the establishment of plant-communities in definite habitats. There is no sharp line dividing one type of vegetation from another, any more than there is a sudden change from one climate to another. Woodland, for example, gradually merges into heath, moor, or grassland, while the transition from the grassland to the desert is almost imperceptible. In temperate regions like our own, some 25 to 30 inches of rain are required annually for the maintenance of permanent natural forest, and as we approach the Tropics the amount increases. When this minimum amount is not reached—+z.e., when the soil is too dry for forest— tree-growth becomes diminished, and gradually gives way to grass. Park-land or savannah is grassland, interrupted with trees and woods, the latter occurring generally in the wetter parts, and frequently marking the line of the watercourses. With the increasing dryness of the soil, stunted and thorny bushes make their appearance, constituting in some regions (Australia, South Africa) definite com- munities of scrub or bush; tussocks of hard, wiry grasses imperfectly clothe the soil, and as the moisture further diminishes, the vegetation gradually disappears, the bare places increase, until in the “‘ desert ’’ only a few specially equipped plants are able to eke out a precarious existence, and break the uniformity of the bare earth. Again, when we ascend the mountains of Scotland or Wales, the trees vanish from every exposed spot, and the great shoulders of the uplands are covered with dreary stretches of bog, moor, and heath. The rainfall is heavy, but the soil is cold ; covering the rock-surfaces are vast accumulations of peat, which has its own characteristic vegetation, giving a definite stamp to the scenery. These illustrations give us some idea of the influence of climate upon the vegetation. The most important 2 18 BRITISH PLANTS factor is the water-supply, the next is the temperature. The amount of water available to a plant affects its nutrition, the temperature affects its vital activities. The two things together constitute the climatic surround- ings of the plant. At this stage, and especially as we are chiefly con- cerned with our own country, it is advisable to draw attention to the influence of man upon the vegetation of the land upon which he has settled. Experience has taught him from the earliest times the importance of water upon the fertility of the soil, and by regulating and controlling the water-supply—that is to say, by drainage and irrigation—he has.modified the face of the land to meet his requirements. Bogs have been drained, moor and heath reclaimed ; forests have fallen beneath his axe, and vast tracts of country, previously. incapable of yielding crops, have been brought into profitable cultivation. Man, however, modifies the vegetation only through the soil-factors. By removing or planting forests, he may modify the humidity of the atmosphere (p. 13), but otherwise he has no control over the climate. The Cultivation of Food-Products in the British Isles. This is an interesting and instructive subject, and we touch upon it briefly because it is an excellent illustration of the work of man in regulating and modifying the vegetation. The land of Great Britain may for the present purpose be divided into three main regions : 1. Uncultivated Land, including alpine regions, moors, heaths, lowland-swamps, and natural pastures. 2. Woodland. 3. Cultivated Land : (a) Arable Land—(i.) In which wheat can be grown. (ii.) In which wheat cannot be grown. (b) Pastures— (i.) Permanent pastures. (ii.) Pastures under clover and grasses in rotation. Most of the cultivated land was at one time covered by forest. Forest-soil, if properly drained, is naturally TYPES OF VEGETATION 19 fertile, because of the relatively large amount of humus— z.e., rotting plant and animal remains—present in it. Some cultivated land has, however, been reclaimed from moorland and fen, and some even from the sea. Arable land is, as the name implies, land under the plough, and is utilized for the raising of crops. The nature of the crops raised depends, in the first place, upon the climate ; secondly, upon the soil; and, lastly, upon the conventional needs of the resident community or the demands of neighbouring or distant profitable markets. By cultivation, man so modifies the soil-factors that the soil itself plays a part vastly inferior to that of climate in deciding the most suitable crop for a locality. Indeed, under the methods of modern agriculture, the nature of the soil is quite a minor matter, granted, of course, a certain minimum of fertility. For this reason the great cereal crops like wheat, maize, and oats grow on all kinds of soil within limits which are climatically determined. The water-supply dominates all other factors in cultivated soils. Agriculture and Farming in the British Isles. 1. Wheat.—In this country wheat can only be grown with profit—at least, under present agricultural methods —where the mean summer temperature during the ripening of the ear is greater than 56° F. (13° C.), and the rainfall is less than 30 inches annually. These limiting factors readily determine the range of wheat- cultivation in the British Isles. They exclude most of ‘the upland slopes over 500 feet, large parts of the West of England, all Wales, and most of Scotland and Ireland. In countries like ours, where the winter is not severe enough to destroy the seedlings, wheat is usually sown in the autumn. In Canada, on the other hand, where the ground is frozen hard for several months, it is sown in the spring, as soon as the snow has disappeared and the ground is sufficiently thawed to be broken by the plough. — Wheat flourishes best in a dry, sunny region, and the winter conditions determine whether the sowing should be before or after the frost. The great wheat-districts of the world are thus the grasslands—e.g., the steppes of 20 BRITISH PLANTS Russia, the puzstas of Hungary, the prairies of America, and the pampas of Argentina. In the British Isles the culture of wheat is limited to certain well-defined areas : in England to the Eastern and South-Eastern counties, certain central counties, and the plains east of the Pennine Chain; in Scotland to the lowlands of the Clyde and Forth, and the coastal ledges from Berwick to the Firth of Tay; in Ireland wheat is now confined to the driest and sunniest spots, such as occur in the rain-shadow of the mountains in the south-eastern parts of the island. In recent years the fall in prices, due to the importation of wheat from abroad, has contracted the limits of its cultivation at home. Much of the land has been con- verted into pastures and market-gardens, and the culture of wheat is gradually becoming restricted to the heavy clay-lands of Essex and the Wash. Barley has a much wider range than wheat. It is grown throughout the wheat-area and considerably beyond it. Much of it is converted into beer and spirits. Oats are, of all the cereal grasses, the most indifferent to climate. They are grown all over Ireland and in the damp valleys of Scotland, where they form a staple food for man and beast. 2. Pastures.—These may be natural or artificial. The former occur in elevated regions (e.g., downs), and are used mainly for grazing sheep. Artificial pastures are of two kinds : (a) Permanent Pastures, which have been reclaimed from moor or bog. They were ploughed once and sown with grass. One crop of hay is perhaps taken off them each year, after which they are abandoned to grazing. Hill-pastures are used as sheep-runs. (6) Pastures sown with Clover and Rotation-Grasses.— These occur on the richer, moister soils of the lowlands, and require periodical manuring and ploughing. They may be used either as meadows cut for hay, yielding on damp soils two crops a year, or for the grazing of cattle in the milk-districts. The dairies of England are chiefly in the west, where the rainfall is abundant and the grasses tall and succulent. 3. The Conversion of Vegetation into Meat.—The feeding of stock upon pastures results in the conversion TYPES OF VEGETATION 21 of vegetation into milk, butter, cheese, or meat. Cattle need to be pastured on rich and succulent grasses to yield good and abundant milk. Dairy-farming is thus most successful in the moister parts of oceanic regions (e.g., Brittany, Denmark, Holland, West of England, Ireland). Where cattle are reared for meat and hides only, and not ' for milk, a relatively poor grass will suffice, and such are the conditions on the great ranches of America. The large amount of food required by stock leads, on the one hand, to the utilization of large areas of pasturage, as in the case of nomadic pastoral tribes ; or, on the other hand, to the necessity for a certain amount of artificial feeding, at least during a portion of the year, when the natural herbage fails. For this reason, fodder and root- crops are raised in dairy-districts almost entirely as food for cattle. ; Climate and Vegetation in Europe. 1. Tundras.—These are a type of treeless moorland occupying the Arctic parts of Europe. In winter they are icy. wastes, in summer, morasses. The growing season is cold and very short. The vegetation is poor and dwarfed, consisting chiefly of mosses and lichens. Mosses such as Polytrichwm occupy the wet peaty parts, while lichens like Cladonia rangiferina, the reindeer moss, flourish on the higher, drier ground. The tundras, however, are not devoid of other vegetation, especially towards their southern limits, where, among the carpets of moss and lichen, occur lycopodiums, sedges, reeds, grasses (Nardus stricta, Aira fiexuosa), dwarf shrubs (Vaccinium, Calluna), and even a few stunted trees (Arctic birches and willows), but these are rarely more than a few inches high. A similar moss and lichen-flora is found on high mountains just below the snow-line. 2. Coniferous Forests.—These form a broad belt on the glacial soils south of the Tundra. The winters are cold and long and the summers short, but tall tree-growth is made possible by the absence of violent wind in winter. The forests are composed almost entirely of conifers with very small evergreen leaves (pines, firs), a single species of which often monopolizes large areas. Two deciduous trees, however, the larch and the birch, accompany these evergreens to the limits of tree-growth and even extend 22 BRITISH PLANTS beyond them. In the vertical direction this type of vegetation characterizes the higher altitudes in moun- tainous regions everywhere in Europe. The particular kind of tree dominant anywhere naturally varies. The Scots Pine, Pinus sylvestris, likes room and sun, the ‘Norway Spruce, Picea excelsa, prefers shade. The Silver Fir, Abtes pectinata, forms vast and stately forests in the ancient highlands of Southern Germany; while the Mountain Pine, Pinus montana, inhabits the high barren slopes of the Pyrenees and French Alps. The Stone Pine, Pinus Pinea, is common in the Mediterranean, where it suppresses the holm-oak, Quercus Ilex, on high, moderately weathered slopes. The leaves of conifers contain resin and rot with difficulty. This combined with a cold soil, in which there is a deficiency of nitrifying bacteria, makes it ill-suited for cultivation. The region is sparsely populated. 3. Deciduous Forests (¢.g., oak, beech, birch, ash, etc.).— These trees lose their leaves in winter. They require more moisture and warmth than conifers, and therefore have their greatest extension in Western Europe. The leaves decay quickly and as they all fall every year a deep fertile soil is gradually formed which can be efficiently cultivated when the trees are cleared away. The greater part of the lowlands of Germany, France, and Great Britain was once covered with deciduous forests. In Germany much of these still remains, but in England only fragments of the original forests survive. With the growth of settled populations these forests have fallen a sacrifice to the needs of civilization, providing timber for the builder, fuel for the smelter of iron, and ground for the tiller of the soil. 4. Grassland.—This is characteristic of the continental type of climate with wide extremes of temperature. The rainfall, though too scanty for trees, is fairly uniform, most falling in spring and early summer, that is, in the growing season when the vegetation needs it most. The largest grasslands in Europe are found in Russia and Hungary. The Ukrainian Steppes lie between Poland and the Black Sea. These are covered with a fine sandy humus-laden soil, called loess, an air-borne deposit which owes its origin to the Ice Age. Upon the ice-sheets, which at this period covered Northern Europe, lay a TYPES OF VEGETATION 23 great mass of cold, heavy air, and from these was liberated an outward-blowing wind which carried over the Southern plains the finer particles of soil and vegetable fragments that had accumulated on the surface of the ice. These steppes are amazingly fertile. The soil is black with humus, and cultivation can be carried on year after year with little or no manuring. Clover in some places grows to a height of 15 feet and hemp to 20. Such a soil is very retentive of moisture, and this is important in a region where the rainfall is so scanty. Towards the South and the East the fibrous black earth dies away and the steppes pass into dry barren pastures. 5. The Mediterranean Region.—This has a subtropical climate characterized by wet cool winters and dry warm summers. In summer cultivation is limited by drought and in many places is possible only if the soil is irrigated. The winters are so mild that there is no break in the vegetation, and most of the crops that we grow in summer can be grown there in winter. Trees are not abundant. Most of the forests have been cut down and are now represented only by their undergrowth of shrubs, the maqui, and there is much grass. Deciduous trees are rare except in daimp spots, while the chestnut is restricted to mountainous tracts, where, of course, the climate is different. The characteristic vegetation is evergreen, e.g., the olive, orange, oleander, evergreen oak, arbutus, bay-tree, yew, cypress, stone-pine, and myrtle. Aro- matic plants are common. One palm, Chamerops humilis, reaches Europe, where it occurs on the Riviera. The flora of the Mediterranean has changed within his- torical times. When the Greeks landed in Southern Italy, forests of oak and beech were common; now they are rare, the beech being confined to the highest moun- tains. This is largely owing to the gradual dessication of the whole Mediterranean region, which has been going on for ages. But man has accelerated Nature by replacing the deciduous trees which he cut down by evergreens, most of which are derived from Asia and suit the climate better. The orange was brought from the East during the Middle Ages, and since the discovery of America, the magnolia, agave, and Indian fig have been introduced from the New World. This shows how important human control is in considering present-day floras. CHAPTER ITI THE INFLUENCE OF WATER ON PLANT-LIFE Water is the most important factor in ecology. In con- junction with heat, it determines climate. It plays a dominant part in the life and well-being of every individual plant, and decides the form and character of the vegeta- tion everywhere. Where water fails, there is no vegeta- tion; where it is most abundant, there the vegetation is most luxuriant and varied. The Réle of Water in Plants. 1. Water is the medium which conveys to all parts of the plant-body the materials required for its nutrition and growth. ‘hese must be in solution, for solid particles cannot pass through the tissues of plants. These sub- stances are, for the green plant : (a) Mineral salts, absorbed by the roots from the. soil, and destined to be employed in the elaboration of food ; and ; (0) The food itself (carbohydrates, proteins, etc.), con- veyed in a watery sap to every living part of the plant —to the points of growth, to the centres of work, or to the seats of storage. 2. The body of the plant is nearly all water. In a living cell which has reached its full size, the living substance, or protoplasm, is little more than a thin skin lining the wall. The rest is sap, a watery liquid con- taining nutriment and other substances in solution. 3. Moreover, the living cell can only keep in health and perform its functions successfully while it is turgid, or stretched with water. As soon as the cells lose their turgidity, the leaves and shoots become limp and droop, 24 INFLUENCE OF WATER ON PLANT-LIFE 25 and if this condition is prolonged the plant dries up and dies. 4. The materials derived from the soil and absorbed by the roots are conveyed in a current of water which passes up the stem to the leaves. This ascending stream of sap is called the transpiration-current, and the main- tenance of its flow is necessary to the healthy life of the plant, for, like the stream of blood in animal bodies, it conveys a cargo of materials by the utilization of which The the plant is enabled to live. path of the water is through the woody tissues which constitute the greater part of the vascular system --a Fic. 2.—LoneitupmaL SEcTION oF Part oF A Root-Trr, sHowine OvTER Tissues or Root (a) anp Root-Harrs (6), SURROUNDED BY Soru-ParticuEs (c). (HicHuy Maaniriep.) Fic. L—Szrpume or inroots and stems. From the stems eee 9 SHOWING reat trunk-veins are given off to (Naronat Sra} (). the leaves, where they break up into a network of capillaries. Here the water is yielded up to the living cells, and with it the nutrient material contained in it. Root-Absorption.—Most of the higher plants live rooted in the ground. Water enters the plant by the roots. The actual entrance is effected through the root-hairs— tiny, hair-like cells which are found clothing the roots near their tips (Figs. 1 and 2). The water passes through the walls of these root-hairs by a physical process known as liquid-diffusion, or osmosis (see p. 91). 26 BRITISH PLANTS Transpiration.—The transpiration-current starts in the roots and ends in the leaves. So long as the roots absorb, this stream is fed and kept in motion. What happens, then, to the water at the end of its journey ? The leaves must get rid of the water which the plant does not require, otherwise the current would stop, and the plant would become surcharged with water and suffocated. The nutritive substances which the current carries are left in the cells, and the excess of water is evaporated away. The doors of exit are the stomata (Gr. stoma, a mouth ; plural, stomata), small pores or openings which are found in enormous number on the surface of the leaves Fic. 3.—Epmermis, witH StomatTa, FROM THE UNDER Sipz or Aa Lar. (HicHity MaGniriep.) (Fig. 3). The water escapes in the form of vapour, and not as liquid drops. In the leaf the cells are not packed closely together, but are loosely arranged, with the cavities between them filled with ajr. All these cavities communicate with one another, and ultimately focus on to the large air-spaces which occur below the stomata. The plant has the power of varying the size of these openings according to the amvunt of moisture in the atmosphere. The aperture of the stoma is bounded by two modified epidermal cells called guard-cells (Fig. 4). When these are turgid, the stoma is wide open; when, through excessive loss of water, they lose their turgidity, they fall together INFLUENCE OF WATER ON PLANT-LIFE 27 and close the opening. This controlled or regulated diffusion of water-vapour through the stomata is called trans- piration. In a few cases water is actually expelled in liquid drops from the leaves. These emergency - exits are special openings (hydathodes ; Gr. hydatos, water ; hodos, way, channel), usually in the form of large stomata, always open. Plants which grow in an atmosphere constantly moist, such as exists in a tropical rain-forest, and to a lesser degree in damp lowland valleys in this country, must get Fie. 4.—Verticat Section oF Pant oF A LEa¥ cur THROUGH 4 STOMA. (Hicuiy Macnir1ep.) a, stoma ; 6, guard-cells ; c, epidermis ; d, cuticle ; e, air-cavity ; f, chloro- phyll-tissue. rid of their surplus water in this way, for the amount transpired is very small. The young leaves of most rapidly - growing herbaceous perennials also possess hydathodes. In these cases large quantities of water must be absorbed in order to supply adequate nourish- ment, but, at the same time, the surface through which water can be transpired is small. The only method by which the whole of the excess can be got rid of is by excret- ing it in a liquid form. The small white spots on the teeth of such leaves indicate the position of these hydathodes, and if they are examined in the early morning of a moist spring day, drops of water can be seen hanging from 28 BRITISH PLANTS the tips of the teeth or in the grasses balancea upon the apex of the leaf—eg., hogweed, garden-nasturtium, creeping buttercup, etc. The Supply of Water in the Soil.—This depends upon : (a) The rainfall—that is, upon the water which enters the soil from above ; and, as we have shown in Chapter L., this has to be considered in relation to (1) its amount, (2) its frequency, and (3) the season of maximum fall. (b) The nature of the soil which receives the rain. This is described in detail in Chapter IX. (c) The water which enters the surface-soil from below. This depends upon the existence and availability of ground-water. The ultimate source of all the water in the soil is rain; but rain, sinking through the ground, sooner or later reaches a layer of clay or hard rock, through which it cannot penetrate. Upon this imperme- able bed it settles, and forms a supply of underground, or telluric (Lat. tellus, the ground), water. The presence or absence of this underground water, its depth below the surface, the power of the soil above it to suck it up, and its availability to the surface-vegetation, constitute a factor of such ecological importance that we shall deal with it at length in a later chapter (Chapter IX.). So important is water, that plants may be divided into two groups, according to whether they live in water or upon land : 1. Aquatics (Lat. aqua, water), or Water-Plants, adapted to life in water. Their vegetative parts are partly or entirely surrounded by liquid water (Chapter V.). 2. Terrestrial, or Land-Plants, adapted to existence on land. Their vegetative organs are surrounded by air (Chapters IV. and VI.). The division between the two groups is not well marked. On the border lie plants which can live either in water or air. Many marsh-plants are amphibious in this way —e.g., Polygonum amphibium, Nasturtium amphibium, and Pilularia. The land-form, however, differs more or less from the water-form, the latter generally having weaker stems and narrower leaves. Polygonum am- phibium in water has floating leaves; on muddy soil the stems creep at the base, and its leaves are often downy. Land-plants are usually divided into three classes, according to the nature of their water environment : INFLUENCE OF WATER ON PLANT-LIFE 29 1. Hygrophytes (Gr. hygros, moist; phyte, plant), plants adapted to very moist conditions. 2. Xerophytes (Gr. zeros, dry), plants adapted to live in a dry soil or under conditions unfavourable to their development, at least during a part of the year. The conditions may all be ultimately referred to the quan- -tity, availability, and usefulness of the water-supply (Chapter IV.). A wet soil is dry if the plant can absorb none of the water. 3. Mesophytes (Gr. mesos, middle, intermediate), plants occupying an intermediate position with regard to water. 1. Hygrophytes. True hygrophytes are plants which live in places always moist, and which, during the whole year and the whole of their lives, exhibit characters adapted to moist con- ditions. The extreme hygrophyte lives in an atmosphere saturated with moisture. It cannot transpire, but the transpiration-current is kept going by the elimination of water in liquid drops through hydathodes (p. 27). More- over, such plants can only live in regions where no un- favourable season intervenes to disturb their develop- ment. Winter-cold means drought even for a hygrophyte {p. 64). For this reason they are only found in the wet parts of the Tropics, where there is no dry season and no winter. They are evergreen, and live in the shade of trees or dripping rocks. Outside the Tropics they are only found in very damp, shady places where frost is unknown. In the British Isles the nearest approach to these conditions is found in the extreme south-west of Ireland, and the plants which, in their characters, most nearly resemble true hygrophytes are the Filmy Ferns. The Killarney fern, for example, is a plant with delicate evergreen leaves, and lives in wet, sheltered crannies in the rocks near the Lakes of Killarney. Elsewhere in this country they can only be reared in glass cases, which are shaded from the sun, and in which the air is kept constantly saturated with moisture. The leaves of many of our marsh and wet-meadow plants exhibit hygrophilous (Gr. hygros, moist; phileo, I love) characters in summer ; but as they perish at the 30 BRITISH PLANTS approach of winter, and are renewed in the spring, the plants to which they belong cannot be regarded as true hygrophytes (see Tropophytes, p. 57). 2. Xerophytes. Most plants have to face a deficiency of water some- times, and if there were no peculiarity of habit or structure to prevent them from suffering during these periods, their existence would be threatened, and the continuance of the race endangered. Every peculiarity of habit and every peculiarity of structure or mode of growth which enables a plant to get through a period when water is lacking either in quantity or quality is known as a werophytic character, and these characters may be few or many, slight or pronounced, permanent or temporary, according to the conditions which obtain in the normal surroundings of the plant. Just as the true hygrophyte exhibits adaptations of a permanent character towards moist conditions, so a true xerophyte shows adaptations of a permanent character towards dry. The unfavourable conditions may only come once a year, and if the plant meets this by some permanent modification in its structure it is a true xerophyte. Thus the holly is common in moist woods in the West of England. Its leaf is evergreen, thick, and shiny—characters associated with deficiency of water (p. 39). The deficiency, however, only occurs in winter, when, through the coldness of the soil, the roots become inactive and lose, more or less completely, their power of absorption. The holly therefore exhibits during the summer characters which are only really useful in winter. In other cases, unfavourable conditions occur all the year round ; during summer they may be of one kind, during winter of another. When this is so, we should naturally expect the plants to be equipped with permanent xerophytic characters. All true xerophytes are evergreen—e.g., yew, heath, pine, ling, box, laurel, many succulents, etc. Plants which assume xerophytic char- acters only at the approach of winter are tropophytes (see p. 57). INFLUENCE OF WATER ON PLANT-LIFE 31 3. Mesophytes. Some plants are pronounced hygrophytes, others pro- nounced xerophytes. Hygrophytes lie, in their relation to water, at one end of a series which is terminated at the other extremity by the highly specialized xerophytes. Between the two lie a vast host of plants, generally known as mesophytes, which possess no marked char- acters either way. Such plants live in conditions which are fairly favourable to the plant all the year round. The true mesophyte is an evergreen, and lives only in the Tropics or in regions not far removed from them. Any winter-break would entail the assumption of some more or less marked xerophytic characters. We have no true mesophytes in Great Britain because the inter- ruption of winter is too pronounced. The nearest plants we have to them are certain marsh-plants like the iris. If the winter is mild, the leaves of the iris persist to the spring. The leaves are long, band-shaped, and erect. The latter is a xerophytic character (p. 46). Amid a host of hygrophytic characters, the iris, therefore, has at least one xerophytic character—it avoids the light by turning its leaves edgewise to it. If, however, the winter is severe, the leaves all perish, and the plant dies down to an underground stem, and behaves as a pronounced xerophyte. Tropophytes (Gr. éropos, change).—The great majority of our plants are tropophytes. Unlike evergreens, they exhibit one set of characters in summer and another during winter. They provide against drought by changing their mode of life at the approach of winter (see Chapter VI.). CHAPTER IV THE INFLUENCE OF WATER ON LAND-PLANTS—XERO. PHYTES—XEROPHYTIC FACTORS AND CHARACTERS WE considered in the last chapter the transpiration- or water-current, and we pointed out how important it is that this current should be maintained in sufficient motion and strength to satisfy the needs of the plant during the various phases of its existence. The needs of the plant, of course, vary with the seasons. In spring the demand upon the water-current is greatest, because growth is then most vigorous; in winter the demand sinks to a minimum, and the transpiration-current becomes almost stationary. This stream of water starts at the roots, where it is absorbed, and after dividing into countless tributaries, ends at the leaf-surfaces, where it is transpired. It is clear, therefore, that anything which tends either to diminish the amount of water absorbed by the roots or to increase the quantity of water transpired through the stomata, must weaken the strength ‘ and flow of the transpiration-current, and in either case. the plant may suffer through lack of water. In the summer this is serious, for if the deficiency becomes too pronounced, the plant may dry up and perish. Even when means are present, whereby the loss by transpira- tion is so regulated that it does not exceed the absorption, the current runs slow, and as the materials necessary in the construction of food are contained in this current, growth is checked, and the plant suffers from lack of nutrition. To a small extent, however, every plant has control over its transpiration. The guard-cells of the stomata are self-regulating, and they adapt the width of the apertures to the state of the weather. This is manifestly advantageous to the plant. During the day 32 INFLUENCE OF WATER ON LAND-PLANTS 33 the stomata are generally open, but if the weather is very dry and hot, as it often is at midday, the guard-cells lose their turgidity and close. This safeguards the plant against the risks of excessive transpiration during the heat of the day, but it is at the expense of assimilation, for when the pores are closed, no carbonic acid gas can enter the leaf. The stomata also close at night, probably owing to the withdrawal of light, but this, again, serves the plant well, for the ground cools at night, and the roots then absorb less water. The amount of water retained in the plant may be reduced in two ways: (a) By those causes which diminish the absorption of water by the roots. (b) By those causes which increase transpiration. Causes which reduce Absorption. 1. Cold.—Just as a rise in temperature increases the activity of all the vital functions, so a fall in temperature decreases it. For this reason, the power of the roots to absorb water declines as the soil becomes cold, and this happens, even though the actual amount of water present may increase. As freezing-point is approached, the roots become extremely inactive, and when the ground is frozen no water is absorbed at all. 2. A Sour or Salt Soil produces a similar effect. The water absorbed by roots is really a very dilute solution of mineral salts, the amount of solids dissolved in the water rarely reaching 1 per cent. As this proportion of solids is exceeded, the activity of the roots declines, and when thé amount reaches 3 to 5 per cent., the roots cease absorbing altogether ; in fact, a very strong solution presented to the root-hairs will actually withdraw water from the plant, and the cells will collapse. Salt water is therefore as good as no water at all, for none is absorbed. Just as with The Ancient Mariner, there may be “Water, water everywhere, Nor any drop to drink.” We conclude from this that, as far as plants are con- cerned, there are several conditions which produce a state of dryness besides drought. Plenty of water may 3 34 BRITISH PLANTS be present, but if it is not of the right,sort it is not absorbed. This kind of dryness is called physiological dryness, and, in ecology, when we speak of dryness in external conditions, we mean not only physical dryness, but this physiological dryness as well. Causes which tend to increase Transpiration. These are, with one exception, the same as promote evaporation from the surface of any moist body exposed to the air: 1. A Dry Air, which promotes evaporation from all the water-surfaces in contact with it. When the air is very dry, evaporation is very rapid ; as the amount of water-vapour in the air increases, the rate of evapora- tion decreases, and when the air is saturated, it ceases altogether. 2. A High Temperature, which increases evaporation by increasing the amount of water-vapour the air can hold. 3. Wind.—The faster the air in contact with the evaporating surfaces is renewed, the more quickly the water is evaporated. Wet clothes dry more quickly in a wind than in a calm. 4. Rarefaction of the Atmosphere.—Evaporation of water increases with the diminution of the air-pressure on its surface. On a high plateau, water exposed in a bowl will disappear more quickly, other things being equal, than on a lowland plain. 5. Light. — Intense illumination does not increase evaporation if the temperature is unaltered, but it does increase transpiration. The phenomenon is clearly a vital or physiological one, for light only promotes loss of water from a living plant, not a dead one. The rate at which a body loses water by evaporation depends also upon the Extent of Surface exposed to the Air. Half a pint of water spread out over a table will soon dry up, but if enclosed in a jug, with only a small evaporating surface exposed, it will take a long time to disappear, even in dry weather. It is the same with a leaf. A small, thick leaf may contain as much tissue and as much water as a large, thin leaf, but the latter will lose water more quickly than the former, because the amount of exposed surface is greater. INFLUENCE OF WATER ON LAND-PLANTS 35 Summary of Xerophytic Factors. I. Causes which ieduce Abscrption, and so set up a state of phygical or physiological dryness : 1. A physically dry soil. 2. A cold soil. 3. A salt soil. 4, A sour soil. IJ. Causes which inerease Transpiration, and so set up a condition in which the loss of water tends to outrun the supply : (a) Physical factors : 1. A dry atmosphere. 2. A high temperature. 3. Wind. 4. Rarefaction of the atmosphere. (b) Physiological factor : 1. Intense ilumination. Natural Regions where Physical or Physiological Dryness prevails. 1. Deserts (lack of water, dry air, intense illumina- tion). 2. Steppes and prairies (soil dry and hot in summer, air dry, intense illumination, intense heat, especially at noon). 3. Rocks and stones (lack of water). 4. Sandy and gravelly soils (lack of water). 5. Chalk-downs (exposed to wind—chalk is porous, and therefore apt to become very dry). 6. Bark of trees (lack of water). 7. Salty seaside - soils, salt swamps and marshes (presence of salt). 8. Peat-bogs (presence of souring acids). 9. Polar regions (cold). 10. Alpine regions (cold soil, wind, intense illumination, rarefied air). 11. Wind-swept, exposed situations (drying wirds, intense illumination). 36 BRITISH PLANTS Xerophytic Characters exhibited by Plants living under Physiologically Dry Conditions. The difficulty with xerophytes is to retain within the tissues sufficient water for their needs. If, from any cause, little is absorbed, then little must be lost. To secure this, the organs on which are found the exits for the escape of water are modified. Thus leaves and shoots are modified in form and structure, curious and characteristic habits of growth are assumed, and the display of the leaves to the light is not the same as in ordinary plants. Whatever the means adopted, the end is always the same—to keep down transpiration to a minimum, in order that as much water as possible may be retained within the body of the plant. 1. Stunted Growth of Stems.—This is brought about by lack of nourishment, a condition experienced by all xerophytes whose water-supply is limited. Trees become stunted and dwarfed, assuming the low bush-form in regions where the xerophytic conditions become pro- nounced—e.g., in semi-deserts, on dry, windy plateaux, in cold alpine regions, and near the limits of tree-growth towards the Pole. Under extreme conditions, the trees may be only a few inches high, and the annual output of leaves not more than two or three. On the crests of Snowdon, the common juniper forms a great branching mat, lying prostrate on the rocks. The soil is thin, and the plants are exposed to violent desiccating winds, great heat at noon, severe cold at night, and intense illumina- tion when the sky is clear. The internodes are short and the development of buds is feeble and irregular. The mat-like growth serves to keep the plant just out of reach of the most violent wind, and at the same time keeps the soil underneath it shady and moist. This form of stunted growth is not, however, permanent, because under more genial conditions many stunted alpines will develop tall stems, while, on the other hand, plants which are several feet high on the piains shrink to a few inches on high alps and wind-swept downs (see p. 77). Although light has some effect in dwarfing plants by promoting transpiration, it has a direct effect upon growth, which is far more important. Shoots and leaves INFLUENCE OF WATER ON LAND-PLANTS 37 exposed to the sun are always smaller than those which develop in partial shade, and the dwarfing of plants in exposed situations is due, in part at least, to this effect. Another kind of stunted growth is the rosette-form, characteristic of many plants living in regions either permanently or periodically dry or cold. The rosette- form, however, is permanent; it is inherited, and there- fore independent of circumstances. In a rosette-plant— e.g., London-pride (Fig. 5), the stem, through the sup- pression of the internodes, does not elongate, but bears Fia. 5.—Lonpon-Prme, sHowine Roserre-Hasir. (NaturaL S1zz.) a number of closely-set radiating leaves close to the ground. These leaves provide a deep shade under which no other plants can grow, and in this way the plant frees itself from competitors. The soil, being shaded, tends to remain moist beneath the leaves, and, as the stomata are found on the under surface, transpiration takes place in moist shade, and is therefore not excessive. Rosette-plants are common, not only in alpine regions, but in all grassy places where the herbage is low, and tall weeds are in danger of desiccation by violent winds. The presence of rosette-plants (e.g., dandelion, daisy, plantain) in a damp meadow might appear to conflict 38 BRITISH PLANTS with this statement, but such plants are exposed to the same danger in the physiologically-dry winter. Rosette- plants have often long tap-roots which are perennial, and the stunted stems are known as root-stocks. A third form of stunted growth is the cushion-growth, a habit assumed by many plants growing in alpine situa- tions. The stems do not elongate, but they branch freely close to the ground, forming dense cushions (Silene acaulis, Saxifraga hypnoides). 2. Leaf-Modifications.—The leaf, because of its stomata, is the chief transpiring organ of the plant. Any reduction of the leaf-surface entails a diminution in the number of the stomata, and consequently a reduction in the amount of water transpired. On the other hand, the green leaf is conspicuously the seat of food-construction, and therefore any diminution in the loss of water can only be effected at the expense of assimilation. If the leaves are small, less food is made, growth is checked, and the whole plant suffers. Reduced assimilation, however, is the lesser of two evils, and most small-leaved xerophytes show manifest signs of impaired nutrition ; in fact, the diminution in the size of the leaves is evidence in itself that the plant is imperfectly nourished. The effect of a xerophytic environment is expressed in the size, form, characters, and display of the leaves more than upon any other vegetative organ. The leaf is essentially an expression of its environment. Where moisture is abundant, and there is no danger of desiccation, the leaf is generally large and thin. As the environ- ment becomes physiologically drier, the leaf tends to exhibit one or more of the characters enumerated below. This does not mean that the removal of any particular plant to drier surroundings will result in any correspond- ing modification of its leaves. The size, form, and characters of leaves are, within narrow limits, fixed for every species. What is really meant is that those plants whose leaves display xerophytic characters, do so because they are best adapted for dry situations, and such plants will consequently be found there to the exclusion of all other plants not so well equipped to contend with the perils of drought. The various forms and characters associated with the leaves of xerophytes may be looked upon as the outcome INFLUENCE OF WATER ON LAND-PLANTS 39 of a gradual evolution along certain lines or tendencies, the purpose of which is to protect the plant against physical or physiological dryness. I. External Xerophytic Tendencies. 1. Diminution of the Transpiring Surfaces—e.g., cypress (Fig. 6). 2. Increase in Bulk compared with Surface—e.g., fleshy-leaved plants, as stonecrop (Fig. 7). Fic. 6.—Cypruss, sHowmse Fic. 7.—Stonecrop (Sedum acre), sHow ConcRESCENT TYPE or LzEar. Ina CrowpED SuccuLent Lzavzs. (NaTuURAL SIZE.) (NatTuRAL Size. ArrErn SOWERBY.) 3. Increase in Longevity.—In this country land-plants which retain their leaves in winter are almost invariably evergreen xerophytes—eg., holly, ivy, heath, laurel, box, pine. By increasing the longevity of their leaves, plants are spared the necessity of making a complete set each year. This economy is imposed on most xerophytes because they are poorly nourished, and possess none too much nutriment. 4. Development of Screening Structures, such as hairs, scales, etc., which prevent the wind from removing moist air from the neighbourhood of the stomata—e.g., mullein, cudweed. 40 BRITISH PLANTS II. Internal Xerophytic Tendencies. 1. Thickening of the Cuticle-—The cuticle is a layer or membrane formed by the external walls of the epidermal cells (Fig. 8). In xerophytes this becomes thickened and strongly cuticularized. The latter condition is brought about by the deposition in the walls of a waxy substance called cutin, a body closely allied to cork, which renders the membrane impermeable to water. Fia. 8.—Section oF Part or A Pryz-LEAF, WITH DEEPLY-SUNK Stoma (a). (Hicuiy Macnirtep.) b, guard-cells ; c, stomatal pit; d, air-cavity ; e, cuticle; f, epidermis; g, thick-walled hypodermis (sclerenchyma) ; h, chlorophyll-tissue. 2. Diminution in the Volume of the Intercellular Air- Spaces.—The cells of the leaf are packed closely together, thereby impeding transpiration. 3. The Stomata are reduced in Number and placed in sheltered positions in pits and grooves, the entrance to which is often closed by hairs—eg., heaths (Fig. 9), marram-grass (Fig. 11). The stomata thus come to be in moist chambers. In most plants, however, each stoma is sunk in its own pit—e.g., pine (Fig. 8). INFLUENCE OF WATER ON LAND-PLANTS 41 4. The Leaf is thickened either by the increase of the palisade-tissue, or by the development of special water- storing cells. The former is strongly developed in sun- leaves (Fig. 22), the latter tissue is found in succulents. The storage of water in fleshy or succulent organs is very common in desert and strand plants (see p. 277). All parts of the plant exposed to the air may become succulent —e.g., the leaves in stonecrop (Fig. 7), the stems in glass- wort (Fig. 10). The water is stored in a special tissue, the cells of which are large and devoid of chlorophyll; the cell-sap is abundant, clear, but somewhat slimy through the presence of mucilage. The presence of mucilage in Fig. 9.—TRANSVERSE SECTION oF ROLLED Lear or Erica cinerea. (HicHLty MaGnirizp.) a, cuticle ; b, epidermis ; c, mucilage in the cells; d, chlorophyll-tissue ; e, vascular bundle ; /, air-space ; g, stoma ; h, hair. water makes its evaporation difficult, and this difficulty is increased by the scarcity of air-spaces. Lignified tissue in succulents is also poorly developed, and there is little cork, the retention of water within the plant being secured by other means. 5. The Augmentation of Lignified and Corky Tissues. —This serves in leaves the same end as succulence— namely, the retention of water within the plant. No large reserves of water are here stored away for future use ; in fact, the actual water-containing and water- conducting tissue is small; but what there is, is protected by masses of sclerenchyma—elongated cells with thick, 42 BRITISH PLANTS lionified walls, non-living, and containing only air. The cells are cemented together in sheets and columns, and form very effective screens between the living cells filled with water and the external air (Fig. 11). A covering of cork on stems and shoots serves a similar pur- pose. The presence of sclerenchyma in large and long-lived leaves gives them mechan- ical support, and keeps them from being easily torn and injured —e.g., New Zealand flax (Phorm- zum tenax), Aspidistra. 6. Some xerophytes con- tain oil in their tissues, especially in the leaves. Where present, it un- doubtedly serves to check evaporation. Many strand and semi-desert plants are quite remarkable for their fragrance—e.g., rosemary, bay-laurel, sage, worm- wood, etc. Xerophytic Forms of Leaf and Shoot. The most important are : 1. The Needle-Type, as in the pine. The leaf is evergreen, thick and tough, with a much reduced sur- Fra. 10.—Saticornia herbacea (Guass- face; the internal cells are WORT), SHOWING SUCOULENT STEMS AND MNuTE Appressep Leavns. packed closely together, (SLicHTLY REDUOED. AFTER the cuticle is thick, and Sowerpy.) ° the stomata are reduced in number and sunk in pits. 2. The Conerescent Type, as in many cypresses and junipers (Fig. 6). The leaves are thick and evergreen, very small, erect, and fused with the stem along nearly their whole length. There is very little internal air- space, the cuticle is thick, and the surface smooth and polished. INFLUENCE OF WATER ON LAND-PLANTS 43 3. The Heath or Ericoid Type (Fig. 12), characteristic of many heath-plants—e.g., Hrica, Empetrum, Calluna. The leaves are small and their edges are rolled under and nearly touch, forming a chamber the entrance to which is almost closed by hairs (see Fig. 9). 4. In the Myrtle-Type the leaves are thick, leathery, and evergreen, sometimes large, as in the Rhododendron, sometimes small, as in the box. So. YZ TTT « : TANS SoM Fic. 11.—Transverszt SecTIOoN or Ro1Lep Fia. 12. — Erica Lear or THE Marram-Grass (Psamma Tetralix, SHOWING arenaria). (MAGNIFIED.) Huata Type oF s Lear. (SLIGHTLY a, sclerenchyma ; b, chlorophyll-tissue ; ¢, vas- eee ) cular bundle ; d, epidermis. 5. The Reed or Juncoid Leaf. is long, smooth, and circular. The transpiring surface is small, and as the leaves are erect, the effect of the sun’s rays upon their surface is much reduced. 6. The reduction of the leaf-surface may be carried so far that modifications are rendered necessary in other parts of the plant. When the leaf-system is too small to accomplish the necessary assimilation, this work has to be carried on elsewhere, either by the leaf-stalks, which flatten out and become green and leaf-like (the phyllodes 44 BRITISH PLANTS of acacia, Fig. 13), or by shoots which function as leaves. These shoots are of two kinds : (a) The leaves are small and green or reduced to scales, and all the stems take over the functions performed by Fia. 13.—SnEepiine oF Acacia melano- aylon, SHOWING ‘TRANSITION FROM Orprinary PetioLr (b) TO PHYLLODE (d). (Asout Harr Naturau Size.) In c the petiole is slightly winged. a, cotyledons ; e, root-nodules. leaves, as in switch- plants—e.g., horsetail, broom (Fig. 14). (b) The leaves are again reduced to scales, but most of the branches are flattened, and resemble ordinary foliage - leaves — ¢.9., butcher’s - broom (Fig. 15). These flat branches, known as cladodes or phyllo- clades, are at once distinguished from true leaves by their position in the axils of the scales, and because they themselves often bear scales, and even flowers, on their upper surface or along their edges. Cladodes are distinguished from phyllodes in the same way, for, after all, the latter are only parts of leaves, while cladodes are shoots. 7. Lack of nutrition sometimes reduces shoots to thorns, and leaves partially or en- tirely tospines. Thorny plants are pronounced xerophytes, and form a con- siderable part of the bush and scrub vegetation of semi- deserts. In England, gorse is characteristic of dry heaths. Some plants even have two forms ; Ononis arvensis (rest-harrow) growing on the seashore usually develops INFLUENCE OF WATER ON LAND-PLANTS 45 thorns; in less xerophytic situations the thorns are absent. Certain peculiarities in the arrangement and display of the leaves are also to be recognized as xerophytic Fia. 14.—Cytisus scoparius (Common Broom). (SticHTLy MaGniriep.) 1. Transverse section of green assimi- lating stem. a, sclerenchyma ; b, chlorophyll - tissue; c, cortex ; d, phloem ; e, cambium ; 7, xylem; g, pith; h, epidermis possessing stomata at intervals, 2. Portion of outer tissues, magnified more highly to show thick cuticle (a), epidermis (b), and chlorophyll- tissue (c). adaptations. The hygro- phytic type of leaf spreads out its surface to catch as much light as possible, and the leaves are so arranged. Fie. 15. — Ruscus aculeatus (BurcuER’s-BRoom), SHOWING CLADODES ARISING IN AXILS or Leaves anp EacuH BEARING 4 Frowrer. (Natura Size. AFTER KmRNER.) -with respect to each other, that every avail- able portion of space upon which light falls is occupied by a leaf (see leaf-mosaic, p. 68). The leaves of xerophytes, on the other hand, are not displayed in this way. Direct sunlight promotes transpiration, and as this is 46 BRITISH PLANTS a source of danger to xerophytes, their leaves are arranged to avoid the effects of full illumination. This is accom- plished in several ways : ; 1. The Leaves are arranged in Close Ranks or Files on the stems, so that they overlap and shade one anothr —e.g., clubmosses (Fig. 16), the evergreen Veronicas grown in gardens. 2. The Leaves turn their Edges instead of their Surfaces to the Light.—The Hucalyptus is a xerophyte. During the early years of its life, when the plant is more or less screened by the trees around, the stem produces horizontal, unstalked leaves. Later on it bears long, narrow, sickle- Fie. 16.—Lycopodium clavatum (Common CLUBMoss), wiTH SMALL, CrowpEp Leaves. (SLicHTLY REDUCED.) shaped, stalked leaves, which hang pendant, with their edges turned towards the sky. Cladodes and phyllodes also are generally vertical. The same light-avoiding habit is met with even among marsh-plants. The iris, for example, has long, upright, strap-shaped leaves. Many reeds and sedges have circular leaves which are erect. Thus plants which in other respects appear to be hygrophytes, may, if their leaves are long-lived, exhibit some xerophytic adaptations for winter conditions. We reserve the consideration of the xerophytic charac- ters associated with hibernating organs (seeds, buds, bulbs, rhizomes, etc.) for Chapter VI. CHAPTER V WATER-PLANTS At the end of Chapter HI. we pointed out that the vegetation can be divided into two great series : (1) Those which live in water, and (2) those which live on land. The water-plant lives either within or upon liquid water, and the conditions by which it is surrounded are conditions that operate in water. The land-plant, on the other hand, has its leaves and stems in the air ; it is therefore exposed to the conditions that operate in air. The distinction is fundamental. The submerged aquatic differs from the land-plant in nearly all its vital relations with the outside world. Light reaches it through the water ; air comes to it from the water; it cannot tran- spire. The land-plant, as we have seen in the case of xerophytes, is equipped for the perils of the land; the water-plant has to provide against the dangers that threaten it through the water—dangers arising from its mode of life in water. How it meets them we shall see when we have compared the characters exhibited by water- plants with those exhibited by land-plants. Aquatics, or hydrophytes (Gr. hudor, water), may be free-floating or anchored in the mud, and their leaves and shoots may be floating or submerged. The Characters of Aquatic Plants compared with those of Terrestrial Pants. 1, Aquatics show an enormous increase in the amount of internal air-space (Fig. 17). So abundant is this in some organs, that the tissue is almost limited to the thin parti- tion-walls which separate the air-chambers. The large air-spaces in the leaves are continuous with the air- 47 48 BRITISH PLANTS passages of the leaf-stalks, stems, and roots, and so a free circulation of air, and therefore of oxygen, is possible throughout all parts of the plant. In xerophytes the great danger threatening the plants is lack of water ; in aquatics a danger equally serious threatens—scarcity of air. The aquatic might experience a difficulty in obtaining sufficient oxygen for respiration after the cessation of photosynthesis if all that formed during eo DH, 201g rf ange fey Fig. 17.—TRANSVERSE SECTION oF SUBMERGED STEM oF WateEr-VIOLET (Hottonia palustris). (HigHuy Macniriep.) a, epidermis, without cuticle ; b, air-space ; c, woody part (xylem) of vascular system. the latter process were allowed to escape. Much of it does escape into the water, but sufficient is always retained in the air-spaces to insure efficient aeration. The presence of air in the tissues may also be useful in another way. It serves to keep the assimilating organs at or near the surface of the water, where oxygen and light are most abundant. 2. The cuticle in aquatics is thin and devoid of wax, Water can therefore be absorbed over the whole surface. . WATER-PLANTS 49 and the presence of stomata is rendered unnecessary. For this reason, stomata are either absent on the sub- merged parts, or, if present, they do not open and close. In floating leaves—e.g., water-lilies—normal self-regu- lating stomata occur on the upper surfaces which are in contact with the air. 3. In submerged aquatics the leaves are thin, and the epidermal cells contain chlorophyll, and so take part in the work of assimilation. This is correlated with the weak light that reaches them under water. By catching it in the outermost cells, the leaves are able to utilize the light at its strongest, before it suffers further loss by penetrating tissues which do not assimilate. 4. Diminution of the Vascular System.—Since water can be absorbed over almost the whole surface, the presence of a vascular system conveying water from the roots to the leaves is not required, and, like all useless structures, it tends to disappear. The part of the vascular system which conducts water is the wood, or «zylem, and it consists of vessels or tubes whose walls have become lignified, or woody. In the oldest aquatics the wood has disappeared entirely, but the phloem—that part of the vascular system which is set apart for the conduction of food-material made in the leaves—remains as it was. The need for the distribution of water throughout the plant has disappeared, but not the need for the distribu- tion of food. Plants growing in flowing water, however, need woody tissue in the stems to enable them to with- stand the strain to which they are subjected. 5. Roots are also superfluous in aquatics, and tend to disappear. The British plants Wolffia (a duckweed), Utricularia (the bladderwort, Fig. 46), and Ceratophyllum “(the hornwort), have no roots. In other cases roots are present, but they do not act as absorbing organs ; they bear no root-hairs, and merely serve to anchor the plants in the mud. In Lemna (duckweed) they still persist, although the plant is free-floating, but the chief purpose they seem to serve is to keep the plant right side up on the surface of the water. In dealing with xerophytes, we showed that the leaf is that part of the plant which expresses in the most striking manner the nature of the environment. The same is true in aquatics. 4 50 BRITISH PLANTS Leaf-Types in Aquatie Plants. I. Submerged Leaves. 1. The Dissected Type —e.g., water-buttercup (Ranun- culus aquatilis), water-dropwort (Hnanthe Phellandrium), water-violet (Hottonia palustris), bladderwort (Utricu- laria, Fig. 46). In these plants the submerged leaves are so extensively divided that the ultimate segments are almost filamentous. Three reasons have been offered in explanation of this dissection—namely : (1) That such a finely-divided leaf offers less resistance to moving or disturbed water than an entire leaf, and so runs less risk of being damaged by tearing. (2) That it offers increased surface for the intake of the carbonic acid gas dissolved in water, and required by the plant for assimilation ; and (3) That it offers increased surface for the absorption of oxygen required in respiration. The second explana- tion is founded on the fact that, although there is more carbonic acid gas in water than in air, itis not so avail- able. Diffusion is so rapid in air, that as soon as one particle of the gas is removed by the plant, another at once takes its place; in water, however, the rate of diffusion is much slower, and the particles removed are not so quickly replaced by others as in air. The first explanation is founded upon an obvious.danger in moving water, the third upon a still more serious peril in stagnant water. 2. The Ribbon-Type.—In this the leaf is long, undivided, and band-shaped. It tends to set itself in the same direction as the current, and as it offers no resistance to it, it is not likely to be damaged—e.g., water-plantain (Alisma Plantago), Vallisneria, Zostera, Potamogeton crispus (Fig. 18). : 3. The Awl-Shaped Type, seen in the water-lobelia (Lobelia Dortmanna), the shoreweed (Littorella), in the ce ae cryptogam, Isovtes lacustris (Fig. 110), and the water-fern, Pilularia (the pillwort). These leaves are short, smooth, thick, and tapering, and contain enormous air-spaces. WATER-PLANTS 51 II. Floating Leaves. _1. The Cireular or Orbicular Type. — Leaves of this kind float on the surface of the water—e.g., water-lilies, frogbit (Hydrocharis Morsus-rane, Fig. 19), the floating leaves of the water-buttercup (Ranunculus aquatilis). In some cases the leaf has an upturned margin, which diminishes the risk of capsizing (the great Amazon water- lily, Victoria regia). The petioles of these leaves are Fic. 18.—Formation or Broop-Bup i Potamogeton crispus. (ABOUT Hatr Naturat Size. AFrrer Kerner.) To the left a bud still attached to plant, to the right the separate bud. usually elastic and very flexible, so that they can, by coiling or uncoiling, adjust themselves to variations in the depth of the water. 2. The Long Floating Ribbon-Type.—This differs in no respect from that previously described, except that it is not submerged. It is found in Sparganium natans, and in the floating manna-grass (Glyceria fluitans). Heterophylly (Gr. heteros, different; phyllon, leaf).— Some plants exhibit two types of leaves, one float- ing and the other submerged, and the two appear together on the same plant. Thus the water-buttercup has finely - dissected submerged leaves, and broadly- 52 BRITISH PLANTS orbicular floating leaves; the arrowhead (Sagittaria sagittifolia) has long ribbon-leaves under water, and arrow-shaped leaves standing out above it; the yellow water-lily (Nuphar lutewm) in deep water forms long ribbon-shaped leaves instead of the ordinary orbicular floating ones. Dangers to which Aquatics are exposed. Most of the dangers to which xerophytes are exposed may be summed up in two words—physiological drought. Aquatics live in a medium which is all water, but, like xerophytes, aquatics have their physiological troubles too, though of quite a different kind : 1. Difficulty of Transpiration.—Being surrounded by water, the submerged aquatic is able to absorb water through the whole of its surface, but it cannot transpire. The cells, however, can get rid of superfluous water by allowing it to escape into the large air-cavities that abound in the tissues. But if these happen to be filled with water instead of air, the aeration of the plant is rendered difficult, physiological disturbances of all kinds are set up, and growth ceases. The aquatic whose air- cavities are waterlogged soon dies. In the case of floating aquatics, the upper surfaces of the leaves, in contact with the air, bear stomata. Interchange of gases can, there- fore, take place directly .between the atmosphere and the plant, but transpiration is still difficult, because the air immediately above the water is always moist. 2. Searcity of Air—Land-plants are never troubled with lack of oxygen for respiration ; it is all round them. With water-plants it is different. Running water is well aerated, and aquatics living in it never suffer from lack of air to breathe. The danger of suffocation, how- ever, is a real one in the case of plants growing in stag- nant water which is strongly charged with carbonic acid gas, but contains little or no dissolved oxygen. There is a good deal of rotting material in stagnant water, and what oxygen is absorbed is at once utilized for the decom- position of this dead matter, and little is left for the living. 3. Light.—Light is considerably altered in its passage through water. A great deal is lost by reflection upon WATER-PLANTS 53 the water-surface, especially when it is disturbed, and that which enters is still further enfeebled by absorption. Beyond a certain depth no light penetrates, and vegeta- tion is impossible. The quality of the light is also altered. The red and yellow rays are gradually absorbed, and the light, as it descends, turns from white to green, then to blue, and, before it fades out altogether, to a pale ultra- marine. The deeper vegetation of the sea consists chiefly of coloured alge, and since the rays that are lost are just those which are most concerned in assimilation, it follows that these alge must be profoundly modified in order that adequate use may be made of the changed and weakened light which they receive. The difficulty is overcome by the adoption of colour-adaptations. The pigments found in the brown and red seaweeds do not, however, replace chlorophyll; they merely mask it, acting as a screen to absorb the rays which chlorophyll alone cannot utilize. The marine alge are roughly zoned out in depth, according to their colour. Near the surface, where light is abundant, the green seaweeds flourish. At a lower depth their place is taken by the brown alge, and lowest of all grow the red seaweeds, in liquid regions where only a pale blue light reigns. The coloration of seaweeds is a striking illustration of adaptation to environment. All submerged aquatics suffer more or less from diminished light. If the surface is disturbed, only a fraction of the light that reaches the water passes into it ; the rest is scattered by the broken surface and lost. Beneath this liquid screen the aquatics live in partial shade, and show, in consequence, many of the characters of shade-loving plants. For example, they have long, thin, weak stems with distant nodes, and chlorophyll is present in the epidermal cells. Propagation in Aquatics. In this country water-plants experience, like land- plants, the vicissitudes of the seasons, but life in water is more uniform than on land, and winter to the aquatic is not the same thing as it is to the terrestrial plant. With the exception of some of the alge, summer is the season of vegetative activity, and 54 BRITISH PLANTS winter a period of rest. During winter most aquatics experience a break in their vegetative development, and in many cases they pass into specialized resting forms. In a few cases the plant merely sinks to the bottom of the pond, and there it remains till spring comes, when it rises to the surface again, and goes on growing—e.g., the water-starwort (Callitriche), the hornwort (Cerato- phyllum). In other plants, special resting or brood- buds, surrounded by closely-packed leaves, are formed. These drop off and sink into the mud, where they remain T= =, \ = fan ft ee \ YAY WY Fia. 19.—Hydrocharis Morsus-rane (FROGBIT), SHOWING BROAD FLOATING Leaves. (Suicutty Repucrep. AFTER KERNER.) a, brood-bud attached to shoot ; b, same detached and descending to bottom of pond. quiescent till the spring—e.g., Myriophyllum, bladderwort (Utricularia), frogbit (Hydrocharis, Fig. 19), the water- violet (Hottonia), and the pondweed (Potamogeton crispus, Fig. 18). In the arrowhead (Sagittaria), solid corm-like buds are formed. Water-lilies die down to rhizomes embedded in the mud. Seeding among perennial aquatics is casual, and in many forms rare. Annuals must produce seed to carry on the race, and in aquatics annuals are very rare. This is not surprising when we consider howfreely most aquatics WATER-PLANTS 55 multiply by vegetative means, and how difficult and uncertain seed-formation is with them. Again, the annual, represented in winter by seed only, is clearly more suited to life on land than in water, for it is here that winter conditions are more pronounced. Among the alleged aquatic annuals found in Great Britain may be mentioned the duckweeds (Lemna), Zannichellia, and awlwort (Subularia). The Origin of Aquatics. It is generally assumed that the first forms of life originated in water, and that the earliest plants on the globe were aquatics. By a gradual process of modification some forms became fitted to live on land, and these, emerging from the water, gradually established themselves on dry soil, and became the forerunners of all subsequent land-plants. That was a long time ago, when the world was still young, and the first stratified rocks were being laid down under water. But from that day to this, through the countless ages of geologic time, plants have been changing and modifying, the better equipped races ever driving the weaker ones out of the fair and pleasant places on the soil. The flowering seed-plant is a late arrival on the scene, the highest and the most successful expression of natural adaptation in the long line of descent of land-forms. Certainly no plant could have evolved the seed-habit while it was submerged in water; the flower is a useful structure only in the air. From this we conclude that every seed-bearing plant that lives in the water to-day is not a primitive aquatic, but has been derived from ancestors that once lived on the land. In the struggle for existence, certain plants were bound to be driven off their habitats by stronger and better equipped competitors. Those among them that were able to adapt themselves to the conditions of other environ- ments, lived on ; some took to the water, others to the hills, where competition was less keen. These were preserved from extinction ; those which could not so adapt themselves perished. Our modern flowering aquatics therefore are, in every case, the descendants of plants which were thus worsted in the struggle for existence 56 BRITISH PLANTS on land, but which afterwards succeeded in making the water a place of abode. In the course of time they have become, by gradual modification, better adapted to a watery existence, but traces of their terrestrial origin always remain. The oldest aquatics are, naturally, most changed (Lemna, Elodea, Ceratophyllum), while the latest emigrants from the land differ little in organization from ordinary land-plants—e.g., water-buttercup and water- violet. Thus, when we find an aquatic whose vascular tissue has lost all trace of lignification (e.g., Ceratophyllum), which has no stomata or no roots (bladderwort), we know that we are dealing with a very ancient race which has lost most of its land-characters through its long sojourn in the water. Probably the oldest aquatics are the duck- weeds (Lemna), which seem to have lost all the characters of land-plants except their method of flowering. Of all organs the flower is the most conservative, and, except in a few cases, all water-plants still send their flowering shoots into the air, where the flowers are pollinated by the same means as their relatives on land. Some aquatics, indeed, possess large and quite showy flowers—e.g., water-lobelia, water-violet, bladderwort—all of which are pollinated by insects. Some of the very recent inhabitants of the water live equally well on land—e.g., Polygonum amphibium. These are amphibious plants (p. 28), and it is from among the amphibious plants which throng the edges of water everywhere that the aquatic vegetation is con- tinually being recruited. CHAPTER VI TROPOPHYTES Ws live in a country subject to changing seasons. Winter follows summer, but in neither case are the conditions extreme. During the summer any interruption caused by drought is only partial and transitory. The fields may become parched and the grass brown, but little else suffers. In other countries where the summer is very dry, the break begins at the onset of the greatest heat. Our winters, too, are mild, frosts being intermittent and seldom lasting long. For this reason we are never utterly without flowers. A few hardy stragglers, sur- viving in sheltered places, bloom in November (purple deadnettle, Huphorbia Peplis, Stachys arvensis), while the first pioneers of spring bloom soon after Christmas (Christmas-rose, snowdrop, winter-aconite). The gorse is found in bloom nearly all the year ; it starts flowering after Christmas, and goes on till the middle of summer ; it flowers again intheautumn. Apart from this, however, the frequently low temperature and the prevalence of strong winds, often from the east, make the winter- break, even in England, serious for the greater part of the vegetation. For four to five months there is a marked period of rest, during which most of the vegetation is in a hibernating condition. Now, there are two ways by which plants may meet the winter : 1. They may possess permanent adaptations providing against the physiological drought of winter. These are evergreen xerophytes, bearing throughout the summer an equipment which is most useful only in winter (see p. 30). 2. They may discard all or part of their summer char- acters at the close of the vegetative season, assuming a 57 ; 58 BRITISH PLANTS xerophytic form which will suffice to carry them safely through the winter. These are tropophytes (p. 31), a type of plants well suited to a country like ours, and prevalent in all regions outside the Tropics. The least that any tropophyte can discard are its summer leaves ; shoots may also fall, and the destruction of summer organs may be carried so far that all the vegetative parts above ground may perish, and only specialized underground organs survive (pp. 62, 110). The perennial parts of tropophytes are always xerophytic. In summer, tropophytes, like other plants, are variously circumstanced with regard to water. Some live in moist habitats, others in dry. If water is abundant at all times, the summer characters are hygrophytic ; if water is deficient, xerophytic characters dominate. Between the two extremes lie a host of mesophytic forms with no marked features either way ; the water-supply is adequate, and the drought is seldom sufficiently prolonged or intense to cause them much damage. According to the nature of the swmmer environment, tropophytes are divided into three groups : 1. Hygrophilous Tropophytes, which, during summer, are surrounded by constantly moist conditions; the annually-renewed foliage and shoots exhibit only hygro- phytic characters—e.g., marsh-plants. 2. Mesophilous Tropophytes.—The great majority of tropophytes are included in this group—e.g., deciduous trees and bushes, herbaceous perennials, and most annuals and. biennials. 3. Xerophilous Tropophytes.—These are tropophytes which during the vegetative season live in physically or physiologically dry places—e.g., sand-dunes, sea-beaches. They show one kind of xerophytic characters in ‘‘ sum- mer ” and another kind during “‘ winter.”’ But, however bad the summer conditions may be, the winter conditions are worse, for then cold is added to the other factors reducing absorption. If the xerophytic characters present during summer are not sufficiently pronounced to secure safety to the plant in winter, they must be discarded, and a more suitable habit adopted. The most important representatives of this group are : 1. Succulent seaside-annuals—eg., glasswort (Sali- cornia herbacea), sea-blite (Sueda maritima), etc. ZROPOPHYTES 59 2. A few prickly, succulent, or woolly-leaved deciduous perennials—e.g., sea-samphire (Crithmum maritimum), sea- holly (Lryngium maritimum, sometimes an annual), etc. In nature, of course, these groups are not sharply separated. The divisions, being based upon the water- supply available during summer, must merge insensibly one into the other. Almost any actual plant will, owing to a mixture of characters, occupy a position between the divisions. As the environment becomes drier, the xero- phytic characters of the plants become more pronounced, while if the conditions become moister, hygrophytic characters will begin to dominate. Annuals.—We have here regarded the annual as a tro- pophyte, although at the end of summer the whole plant dies, leaving only the living seeds to carry on the race during the winter. Biologically, they are tropophytes, for it is the race that matters, not the individual. The seed is, of all resting forms, the most xerophytic. It can endure, without injury, a greater degree of drought or cold than any other hibernating structure. In an annual the summer plant is only one phase ; the young plantlet embedded in the seed during the winter is the other. The nature of the water-supply determines the characters of the adult, as it does those of all tropophytes. 1. Hygrophytic Annuals.—These are few, most plants living in moist conditions being perennial. The following marsh-plants are annuals: The celery-leaved buttercup (Ranunculus sceleratus), marsh louse-wort (Pedicularis palustris), the bur-marigolds (Bidens cernua and B. tri- partita), and the toad-rush (Juncus bufonius). 2. Xerophytic Annuals.—A large number of annuals exhibit characters which are more or less xerophytic. This is not surprising when we remember that annuals are most common in dry, waste places. (a) Seaside- Annuals, generally succulent: glasswort (Salicornia herbacea), sea-rocket (Cakile maritima), salt- wort (Salsola Kal), and sea-blite (Sueda maritima). On sand-dunes the summer is the most unfavourable season. Many of the annuals that live there germinate in the autumn, form a small rosette for the winter, and flower early the next year. When the hottest part of the summer is reached, and the sand is scorched by the sun, the plants die, leaving only their seeds to meet the hardships of 60 BRITISH PLANTS the physiological winter—e.g , Cerastium semidecandrum, Trifolium arvense, Aira preecoz, Bromus mollis, Phleum arenarium, Jasione montana, and Draba verna (see p. 107). (6) Weeds of Cultivation in dry fields and waste places —e.g., cudweed (woolly), Lepidium ruderale (leaves small, plant shrubby), corn-spurrey (fleshy), Saxifraga tridactyl- ates (rosette-form), etc. 3. Mesophytic Annuals—e.g., corn-buttercup, corn- cockle, herb-Robert, black medick, fool’s-parsley, cleavers (a climber), yellow rattle, Poa annua, etc. Biennials.—Most biennials at the end of the first season die down to tuberous underground structures (p. 111). These often carry a rosette of radical leaves close to the ground (see rosette-type, p. 37)—e.g., foxglove, carrot, burdock, ox-tongue, etc. Deciduous Trees and Shrubs.—At the approach of winter, these plants lose their leaves. They are not destroyed by cold or stripped off by the wind, but are discarded, as it were, by an act of the tree itself. The deciduous leaf is essentially a summer leaf, typically large, thin, hori- zontally placed, exposed freely to the light and the wind, and richly provided with stomata on its under surface. Although these characters favour transpiration, there is little danger of excessive transpiration in summer, because the soil is warm and the supply of water is adequate. In winter, however, the possession of these leaves would be injurious, and in most cases fatal to the plant. The soil is cold, and very little water is absorbed by the roots. If, under these circumstances, transpiration could not be controlled, the plant would lose more water than it could get, and death would follow from drought. To meet this peril, the tree forms a layer of cork across the base of the leaves in autumn. The leaves, thus cut off from their water-supplies, dry up and die, and, a layer of cells just outside the cork splitting, they easily become detached and fall to the ground. And so the tree, which during the summer stood up, with its thousands of expanded leaves—a typical mesophyte— becomes now a leafless xerophyte, with bare cork-covered twigs, and the buds which will renew its foliage in the spring protected from desiccation in many and wonderful ways. Buds and Bud-Protection——In perennial plants, the assimilating organs are formed in buds, which are situated TROPOPHYTES 61 laterally in the axil of the leaves or terminally at the end of the branches. Each bud consists of a condensed shoot-axis bearing leaves. On herbaceous shoots the buds develop immediately into leafy shoots or flowers until exhaustion puts an end to growth. The leaf-buds of trees are formed in autumn and develop in the following spring. They have therefore to endure the winter. To equip them for this, various xerophytic characters and habits are assumed. The bud is primarily protected against excessive tran- spiration by the fact that a number of leaves are closely packed together in a small space. In addition, they are usually surrounded by corky scales, which allow no water or water-vapour to pass either in or out. The escape of water from within would lead to the desiccation of the bud ; the entrance of liquid water from without would cause it to rot, or at least make it more susceptible to injury during frost. Additional security against drought is provided, in some cases, by the young foliage-leaves within the bud being covered with cottony hairs, and the scales being closed and sealed by a secretion of gum. Bud-scales, cork, cotton, and gum are, without doubt, all adaptations primarily directed against drought, but they benefit the buds in other ways too. They preserve the tender parts within against rapid changes of tempera- ture, from the ravages of injurious insects, and from the attacks of disease-spreading bacteria and fungi. A certain amount of protection is also afforded to buds while they are immature and developing. This is secured by their position in the leaf-axils, where they are, to a greater or less extent, covered by the leaf-bases. In the plane, the leaf-base envelops the whole bud, so that it is not seen until the leaf falls; in the willows and roses the stipules are so placed as to ward off the wind ; in the elder and currant (Ribes) the sheathing leaf-bases act in the same way. The morphology of the bud-seales varies; they are always leaves or parts of leaves. In the honeysuckle, privet, lilac, and holly they are complete leaves and green, but somewhat modified from the ordinary foliage-leaves. In the horse-chestnut, cherry, elder, maple, ash, and plum they are leaf-bases only. In the sweet-chestnut, lime, rose, pear, elm, blackberry, birch, oak, hazel, apple, beech, poplar, willow, and hawthorn they are modified stipules. 62 BRITISH PLANTS In the spring, when the buds open, the bud-scales fall off, leaving scars on the shoots. The shoot does not elongate in the region of these scars, and if we examine an old twig we can, by noting the places where these scars are close together in a series of rings, determine how old the twig is, and how much it has grown each year. Geophilous Plants.—This term, which means earth- loving (Gr. ge, earth ; phileo, I love), is applied to those herbaceous perennials which, after flowering, die down to the ground. During winter either nothing at all shows above ground or a rosette of closely-packed radical leaves lie just on the surface of the soil. The important part is underground. The tulip hibernates in a bulb (p. 156), the crocus in a corm (p. 158), the meadow-saxifrage in bulbils (p. 160), the potato in tubers (p. 111). The fern dies down to a short, thick, unbranched, erect rhizome, or “root-stock ”’ (p. 110); the dog’s-mercury, coltsfoot, and couch-grass to long branching rhizomes (p. 110). All these perennating* organs are stem - structures, well stored with food, from whose buds arise the shoots which appear above ground the following spring. These aerial leaty shoots are hygrophytic, mesophytic, or xerophytic, according to the water-conditions which prevail during the summer : 1. Many marsh-plants are hygrophilous geophytes, dying down in the winter to rhizomes which hibernate in the mud—e.g., reeds, sedges, horsetails, reed-mace (Typha), common reed (Phragmites), etc. Plants living in moist shade like wood-sorrel and the moschatel (Adoxa) may be regarded as less pronounced hygrophilous geo- phytes. 2. The majority of our herbaceous perennials show no decided summer habit, and are therefore mesophilous geophytes: white deadnettle, toadflax, mint, chervil, tulip, crocus, etc. 3. A few are even xerophytic in the summer—e.g., Sedum Rhodiola, a large-leaved alpine stonecrop, and Psamma, the sand-dune grass, etc. Bulbs.—As an example of a pronounced geophyte, we will take a bulbous plant. If a narcissus or tulip be dug up in September, it will be seen that a new leafy shoot is packed away in the bulb in the form of a large bud * Latin, perennis = lasting. TROPOPHYTES 63 (Figs. 59, 60). The flower is also there, and so far developed that all its different parts may be quite easily recognized and separated. In this bud-like condition the plant hibernates out of harm’s way, protected from wind and weather. “As soon as the frost is gone, and the first warm days of spring arrive, the little shoots break from their bulbous cradles and grow into the air. In a short space of time—about two months—the plant has flowered, set its seeds, and died down again to the ground. To what conditions of climate is such a life as this adapted ? Clearly to one where the unfavourable season is long and the vegetative season very short. Some of our bulbous plants, both native (snowdrop) and alien (hyacinth), are the first flowers of spring. Plants which depend upon seed for their renewal require a warmer temperature for germination than buds require for growth. They must therefore wait a little longer, till the spring is more advanced. This delay is immaterial when the vegetative season is long, but when it is short it is im- portant that the plant should start on its career as soon as possible, and, like the snowdrop, take advantage of the first warm days of spring. Most of the bulbs and corms which are cultivated come from the dry and sunny parts of the world. The narcissus is a native of the Mediterranean region, lilies come from Asia Minor, tulips from Siberia, and the gladiolus from South Africa. In these countries the middle of summer, besides being dry, is very hot, and where this is followed by a long, hard winter, as in Siberia, the vegetative season is reduced to the spring. When the drought of summer comes, most of the ground-flora perishes, and among its earliest victims is the bulbous plant. Marsh-Plants.—These plants live in a soil which is always saturated with water, but the greater part of their leaves and stems is in the air. Since water is always abundant, they naturally show many of the characters of water-plants (e.g., large internal air-spaces), and most of them may be submerged for a long time without injury. On the other hand, they agree with land-plants in pos- sessing a good vascular and mechanical system, for they draw their water from the ground, and their assimilating organs are exposed to the air. The air over the marsh is sometimes dry, especially when the wind is blowing. 64 BRITISH PLANTS This does not matter in summer, when the soil is warm, for there is plenty of water, and the roots are active. In winter, however, the water is cold, and there is danger of excessive transpiration. For this reason most marsh- plants are tropophytes, and die down in winter. Those that remain evergreen show xerophytic characters (p. 31). When the water becomes sour, the marsh passes over into the bog, and the number of xerophytic forms in- creases. The same thing happens as it becomes salt, the salt-marsh vegetation becoming markedly xerophytic. BIOLOGICAL CLASSIFICATION OF PLANTS IN RELATION TO WATER. Water-plants (Chapter V.) Land or Terrestrial plants, (Aquatics, Hydrophytes). i | | Plants showing constant characters Tropophytes (Chapter VI.) all the ycar round (Chapter IIL.). | ] | | | | | Hygrophytes Mesophytes Xerophytes Hygrophilous Mesophilous Xerophilous Tropophytes Tropophytes Tropophytes CHAPTER VII LIGHT AND HEAT THE ancients worshipped the sun as the source and giver of all good things. And so it is, for without its creating and sustaining energy this world would be a dead and tenantless globe, without sunshine and shower or any green and living thing. The energy of the sun’s rays is manifested to us under two forms : 1. Light. 2. Heat. The sun is always pouring into space streams of energy. These are transmitted as movements or undulations of the ether, a medium which is supposed to fill all space and penetrate all matter. The vibrations or undulations set up by the sun in the ether travel with incredible speed— 186,000 miles per second—and in less than nine minutes’ _ traverse the vast distance (over 90,000,000 miles) which separates the sun and the earth. Light itself is invisible ; it is a sensation or form of consciousness produced in the brain by certain rays when they strike upon the sensitive nerves of the eye. Heat, on the other hand, is produced whenever the rays falling upon a material body are absorbed. What is really absorbed in this case is energy, and the rise in temperature is due to the increase of energy thus received. Since light, then, is a mode of consciousness, sunshine is only light to those that see. Plants cannot see. So far as they are concerned, the world is always in darkness, and what, in the day- time, is streaming upon the vegetation is not light, but energy. 65 5 66 BRITISH PLANTS Light. An ordinary beam of white light is a bundle of many- coloured rays ; mixed together and falling all at the same moment upon the retina, they produce upon the brain an effect which is manifested in the consciousness as white light. A beam of light may be resolved into its con- stituent coloured rays by passing it through a triangular prism of glass. Not only are the rays bent or refracted in passing through the prism, but each coloured ray is bent to a different extent, so that, if the emergin beam is caught on a screen, a multicoloured band of light is produced which is known as the solar spectrum. This spectrum contains all the colours of the rainbow, one colour fading into the next imperceptibly. Nevertheless, seven primary colours may be recognized following each other in order : red, orange, yellow, green, blue, indigo, and violet. The red rays are the least refracted by the prism, the violet rays the most. This luminous spectrum, how- ever, does not contain all the rays which pass through the prism. The band is continued at either end by rays which, though invisible, produce other effects. The dark rays beyond the red end of the visible spectrum, falling upon a body, raise its temperature (dark heat-rays) ; those beyond the violet end do not perceptibly warm, but they act powerfully upon a photographic plate, and so are chemically very active (ultra-violet or actinic rays). Now, these variously coloured rays are of different value in their influence upon plants. The manufacture of starch in the green leaf is possible only in the presence » of light, and it is easy to find out which rays are the most useful by growing plants under coloured screens. Under a red-or yellow screen a plant will be found to be as active in making starch as if it were living in ordinary white light ; in a blue or violet light little or no starch will be formed. We conclude from this that the red and yellow rays of the solar spectrum are the most effective in photosynthesis, and that the green chlorophyll present in the assimilating cells absorbs these colours from the beams, and utilizes them in the construction of carbo- hydrates. This selection of rays accounts also for the colour of green plants; the green pigment absorbs the red and yellow rays, but rejects the green ; the rejected LIGHT AND HEAT 67 rays meet the eye, and the plant appears green. Re- garded thus, even the colour of the vegetation is an expression of its needs. Now, in dealing with sunshine and its effect upon vege- tation we have to consider two things : 1. Its intensity. 2. Its duration. : The intensity of the sunlight is measured by the amount of energy falling upon any space. It varies with the lati- tude, diminishing from the Equator to the Poles. During the day it increases from dawn to noon, and decreases from noon to sunset. The value of a beam of light, so far as its energy is concerned, is proportional to the angle at which it strikes the earth. When the sun is vertically overhead, as it is twice every year within the Tropics, its effect is the greatest ; when it is on the horizon its effect is least. This fact is well appreciated in photography. The chemical salts present in the photographic film are affected chiefly by the actinic rays which lie beyond the violet end of the visible spectrum. In July the light- value for exposure at noon is six times as great as at.7 p.m., while in the latitude of London the light- value at noon in January is only one-third of that in July. The duration of light varies with the seasons. At the equinoxes, when the sun crosses the Equator (March 21 and September 22), day and night are equal throughout the world—that is, each is twelve hours long. When the sun is overhead at the Tropic of Cancer, on June 24 (Midsummer Day), we have the longest day (about sixteen and a half hours in the latitude of London), and conse- quently the shortest night. The sun is, at that time, more nearly overhead in the North Temperate Zone than at any other, and so has its greatest effect. After mid- summer daylight decreases, both in intensity and dura- tion, until midwinter, when the days are less than eight hours in length. This accounts for the great contrast between tropical and polar climate. Within the Polar Circle the sun is never far from the horizon, while in the Tropics it is always nearly overhead. Again, the polar regions in winter revolve in continuous shade, and there is no day ; 68 BRITISH PLANTS in summer they revolve in the full light of the sun, and there is no night. The long duration of the summer days in Northern regions compensates, in some measure, for the shortness of the summer and the low intensity of solar radiation. In Norway, for example, cereals ripen in a much shorter time than in England. In the Tropics there is no alternation of summer and winter, because the sun is never far from the zenith at noon; the vegetation is therefore always green, and, provided sufficient water is present, does not display the seasonal changes of activity and rest so familiar to us. Heliotropism.—The growing parts of plants are affected by certain physical forces in the environment, with the result that the different organs tend to turn towards a position of advantage and away from a position of dis- advantage. Curvatures of this kind: are known as tropisms. Perhaps the most obvious of these tropisms is that which is brought about by oblique illumination. Stems grow towards the light, and the leaves bend so as to receive as much of it as possible. The heliotropism (Gr. helios, sun ; tropos, turning) of stems and leaves is easily observed in plants grown before a window in a _dwelling-house. The same thing happens outside. Plants growing in shady situations bend towards the direction in which they can catch the greatest amount of light. The stems curve towards a position of advan- tage, and here the position of advantage is that best suited for assimilation. Leaf - Mosaic.—Closely associated with heliotropism is the phenomenon of leaf-mosaic. If the leaves of a horse- chestnut be viewed from above, it will be found that they are so arranged in regard to size and position that each leaf gets its full share of the light without let or hindrance from its fellows. On looking down upon the leaf-surface of a shoot there is little or no overlapping, and each portion of its area is occupied by a leaf. This arrangement is brought about by the bending of the leaf-stalks under heliotropic influences, and by variation in the length of the petioles and the size of the leaves—e.g., climbing plants like ivy (Figs. 20 and 21). prostrate stems like ground-ivy, rosette-plants like dandelion, and bushes with oblique or horizontal stems like the box, yew, and maple (Figs. 22 and 23). LIGHT AND HEAT 69 The Fixed Light-Position of Leaves.—Every leaf takes up a fixed position with regard to the light, and this position is that in which it receives the maximum average . Fie. 20.—Crmeine SHoor or Ivy, sHowina Lzar-Mosato. Fic. 21.—Fiowerrme SHoor or Ivy with Leaves STANDING OUT ON ALL SmpES oF STEM. amount of light. This, of course, is only true of those herbaceous forms of leaves which spread out horizontally to catch as much light as possible. In xerophytes the 70 BRITISH PLANTS tendency is to avoid the direction of strongest light (p. 46). Sleep-Movements in Leaves ‘(nyctitropism).—In some plants leaves change their position periodically in rela- tion to light. The trifoliate leaves of the wood-sorrel close together at night (Fig. 24). The three leaflets droop into a vertical position round the petiole, forming a kind of moist chamber with the lower surfaces of the leaves facing inwards. In the clover the two basal leaves rotate Tia. 22.—Eruct Saoor or Marriz, with Leaves STANDING OUT ON ALL Srwwzs oF Stem. (REDUCED. AFTER KERNER.) so as to bring their under surfaces together in a vertical plane, while the terminal leaflet comes down over the top, closing the chamber thus formed. The position assumed at night is one which tends to diminish the loss of water from the lower surfaces of the leaves. On a clear night the eazth quickly cools, and with it the plant. Absorption is therefore diminished, but this is counteracted by the night-position assumed by the leaves. The day-position is of no service to a plant at night, and so they move into LIGHT AND HEAT 71 a position which tends to check transpiration. The stimulus causing the leaflets to move is the withdrawal of light. The vertical position is also a protection against excessive radiation and possible injury by frost. Radia- tion is greater from a horizontal than a vertical surface ; Fia, 23.—HorizontaL SHoort or Marie, with LEAVES ARRANGED IN Onz Puane. (Repucep. AFTER KeRveEn.) there is therefore less risk of excessive cooling when the leaves are in the night-position than if they remained spread out as they are during the day. Similar movements take place in flowers. The flower- heads of many Composite close at night, and only open during certain hours of the day, when the weather is fine Fie. 24.—Lzar or Woop-Sorre. 1, night-position ; 2, day-position of leaflets. and warm and the pollinating insects are about. In dull, cold weather they remain closed. By this means pollen and honey are protected from the rain, as well as from marauding insécts not engaged in pollination. The Effect of Light upon Growth.—If potatoes are allowed to sprout in darkness, not only is chlorophyll 72 BRITISH PLANTS absent, but the internodes become very long and thin, and the leaves produced are small. The diminution in size of the leaves is brought about by defective nutrition, but the abnormal lengthening of the internodes is directly due to the withdrawal of light. Lilies and grasses, under similar conditions, produce elongated leaves. On the other hand, intense illumination has a retarding effect upon growth. It is partly for this reason, and partly also from malnutrition, that arctic and alpine plants are dwarfed in stature. In the shade of hedges and woods m0 Q old ) 0 0 NK 4 Fic. 25.—TransvErsE Srction or Sun-Lear or WHORTLEBERRY (Vac- cintum. Myrtillus). (HiaHty Maaniriep. a, cuticle ; 6, epidermis ; ¢, chlorophyll-tissue ; d, air-space ; e, stoma. there is enough light to keep the plants green, but it is sufficiently weak to produce long internodes. Such plants have a long, weedy appearance, approximating to that observed in plants grown in darkness (p. 119). In sun- plants the leaves are generally small and thick, with many layers of chlorophyll-containing tissue. In the shade, on the other hand, the leaves are thinner, because the light is too weak to penetrate more than one or two layers (Figs. 25 and 26). To some extent, however, this diminution in thickness is compensated by the larger size of the shade-leaf. In many trees and bushes there is a marked difference in size between the leaves on the sunny side and those on the shady. The latter are much larger than the former. LIGHT AND HEAT 73 The Colour of Young Leaves and Shoots.—In some cases young leaves are, for some time after emerging from the bud, reddish in colour. As the leaves grow older, this pigmentation gradually passes away, and the natural green colour is assumed. The pigment is contained in the sap, and is protective. The phenomenon is chiefly observed among plants in the Tropics, where the light is very strong. Certain rays of the spectrum—chiefly the blue and the actinic, which are very abundant in strong light—act injuriously upon the chlorophyll, especially in young leaves. The red sap, by absorbing these rays, Fic. 26.—TRANSVERSE SECTION oF SHaDE-LEAF OF WHORTLEBERRY. a, cuticle ; b, epidermis; c, chlorophyll-tissue ; d, air-space; e, stoma. screens the chlorophyll from damage, at the same time allowing the red rays, to which it is transparent, to pass into the assimilating cells. -Heat and Cold. But for the heating effect of solar radiation, the world would be so cold that life of any kind would be impos. sible on its surface. In the neighbourhood of freezing- point most of the vital activities manifested by plants sink to zero. A further increase in cold becomes danger- ous, and in most cases a point is soon reached when the aerial organs are killed. Some plants, however, can withstand a great deal of cold. In one case, recorded from Siberia, a plant was suddenly caught in the grip of winter while in full flower; the sap froze hard, and animation was suspended for many months. When the frost broke, the plant thawed and renewed its activity at the point where it left off, just as if nothing had occurred. 74 BRITISH PLANTS For every plant there is a certain limit of temperature below which it cannot live, and for some hibernating structures like dry seeds the temperature which they can survive is very low indeed. Of all living things, the seed is able to resist the greatest cold, and for this reason it is looked upon as the most pronounced xerophytic structure in Nature (p. 59). Geophytic structures such as bulbs, rhizomes, etc., also survive refrigeration. Herbaceous organs, on the other hand, suffer severely at low tempera- tures, and in some cases, especially when they contain a great deal of water, they are killed off at the first frost. Heat and cold, as such, seem to evoke little or no protective adaptations in plants. They act indirectly through the water-factor, and cannot be disassociated from it. Cold, for example, diminishes absorption by the roots, and therefore necessitates a control of transpiration ; cold regions are therefore regions of physiological drought, and their vegetation is xerophytic. Heat, on the other hand, increases the activity of all the functions, absorp- tion and transpiration alike ; but as the area which ab- sorbs is generally very much less than the area of the surfaces which transpire, and as a rise in temperature increases all the factors favouring transpiration, a high temperature tends to make the loss by transpiration excessive and dangerous. Hence hot regions, unless they are constantly moist, are regions of physiological drought, and the vegetation is xerophytic. Thus, extreme heat and extreme cold both lead to xerophytic adaptation— the latter always, and the former when the water-supply becomes insufficient for the increased demands. Apart from this, however, it is pretty clear that some plants are constitutionally better able to stand cold than others ; those are most delicate whose tissues contain much water: plants accustomed to alpine surroundings must naturally be better able to stand cold than the inhabitants of warmer regions. There are some common plants, also, which flourish during winter without showing in their structure any particular marks of protective adaptation. The chickweed, for example, lives throughout the winter, survives frosts, and yet shows no special adaptations by which we can explain its immunity ; even its buds are not protected. It must therefore be constitutionally hardy. LIGHT AND HEAT 75 Its protoplasm must be made of sterner stuff than most plants. Although little is known as to the physiological causes which enable some plants to withstand low temperatures, it is probable that the density and nature of the sap play an important part. It has been demonstrated that the most hardy of the magnolias possess the densest sap. Again, it has been shown in the case of some plants which resist severe frosts that the insoluble starch in the leaves becomes replaced by soluble sugar during the winter, whilst seedlings and Jeaves which have been fed with a sugar solution can withstand a lower temperature than they normally do. The reason that has been suggested for this is that, in the ordinary course, when the sap begins to freeze, water is withdrawn and the sap becomes more concentrated, with the result that the proteins in solution in the sap and protoplasm separate out, but that if sugar is present this “ salting out ” process takes place at a much lower temperature. Acclimatization of Plants.—Southern races habituated to warm conditions are very susceptible to cold, and not easily cultivated in cold countries, except in warm green- houses. Some plants, however, may, if suitable measures be taken, be gradually acclimatized. During the winter they are kept in the greenhouse ; in the spring they are not put out in the open at once, but are first transferred to a cooler house or frame till they are hardened off. In this way they are gradually inured to colder conditions before being bedded out. CHAPTER VIII THE ATMOSPHERE In the Introduction we pointed out that the air was a mixture of gases, and we referred briefly to the part played by these gases in the life of plants. This was dealing with the atmosphere from the chemical side. It remains now to say something of the physical effects of air upon plants—that is to say, of air in motion—wind. Wind.—tThe effect of wind upon vegetation is twofold : (1) It promotes evaporation, and therefore increases transpiration ; and (2) it lowers the temperature of the bodies over which it blows. The second effect is really a consequence of the first ; for a body which is losing water by evaporation is at the same time losing heat and becoming cooler. Heat is required to change any liquid into vapour ; where the heat is not imparted from without, the body from which the water is evaporating provides it, and its temperature falls. Common experience testifies in a very simple way to the truth of this. A person, exerting himself greatly, becomes hot and perspires. If he takes off his hat, a pleasant coolness accompanies the disappearance of the perspiration from his head. Again, if ether is poured upon the hand, it evaporates away in a few seconds, but heat for the purpose is drawn from the hand, which therefore experiences a sensation of cold. Wind promotes evaporation by constantly and rapidly renewing the air in contact with the evaporating surfaces. Wet clothes dry quicker when a breeze is blowing than in a calm. Plants suffer in the same way, and when the wind is strong, they are in danger of losing more water than they can afford. This danger is increased by the drying and the cooling of the soil, and the decrease in 76 THE ATMOSPHERE 77 absorption resulting from it. Plants which grow in windy situations therefore exhibit xerophytic characters, or they soon perish, and where the wind is incessant and fre- quently violent, tall growth fails altogether, giving place to a low or dwarfed vegetation. This is well seen on wind-swept downs by the sea. On Beachy Head, for example, trees are absent, and the ground is covered with short, thick grass ; what bushes there are are low, stunted, and dense, while the herbaceous weeds scarcely rise above the level of the herbage. It is curious to observe how plants which in the sheltered hollows and dips grow to their natural size of several feet, sink on the exposed downs to an inch or two, bearing a few leaves, and perhaps a single flower. But the flower is not reduced in any way; if anything, its colours are brighter than usual. Where trees occur in windy places, they are bent away from the wind, and the branches and foliage develop en- tirely on the sheltered side. This is because the buds on the exposed side do not develop, or, if they do, the young shoots arising from them are killed by the drying action of the wind. Sometimes a curious phenomenon is wit- nessed. Springing from a broad crown of foliage rises a miniature forest of short, erect, dead twigs. These were shoots which, in their upward growth, just touched a zone where the wind was stronger than their powers of endurance; they ceased to grow, dried up, and perished. Atmospherie Equilibrium.—Before leaving the subject of the air, we will refer to a phenomenor which at first sight is not altogether easy of explanation. The world is teeming with life, animal and vegetable. Gases are continually being withdrawn from the air and discharged into it, and yet it is a matter of common knowledge that the constitution of the atmosphere remains practically constant at all times and in all places. How is this equili- brium brought about and maintained? We will en- deavour to answer the question by preparing a balance- sheet of the losses and gains of the atmosphere : 78 BRITISH PLANTS 1, Oxygen. Loss to the Air. Oxygen is withdrawn from the air, and chemically fixed in other | b substances in various ways. The process is called combustion, or oxi- dation, the chief modes of which are: 1. Burning, as by fire (rapid combustion). 2. The oxidation or rusting of metals (slow combustion). 3. Respiration, or breathing, both in animals and plants (physiological combustion). 4. Putrefaction and decomposi- tion, entailing the absorption of oxygen (organic combustion). Gain to the Air. Oxygen is restored to the air pian. Green plants in the presence of light withdraw carbonic acid gas from the air, retain the carbon, which they build up into carbohydrate, and restore again to the air the oxygen not required, The gain balances the loss. 2. Carbonic Loss to. the Air. Carbonic acid gas is extracted from the air in photosynthesis. Acid Gas. Gain to the Air. It is added to the air during the destruction of organic material. All organic substances are com- pounds of carbon. They are de- stroyed by combining with oxy- gen, being broken into simpler bodies, one of which is always carbonic acid gas: 1. Destruction by slow com- bustion, as in decomposition or decay. 2. Destruction by rapid com- bustion, as in the burning of wood and coal. 3. Destruction by physiological combustion, as in the respiration of plants and animals. The gain balances the loss. THE ATMOSPHERE 79 3. Nitrogen (see p. 99). The Interdependence of Plant and Animal Life.—In the preceding section we have drawn attention to the influ- ence of the life and death of organic material upon the constitution of the atmosphere. A large part of organic matter is used as food. Food is eaten by animals for two purposes : 1. By its further elaboration it is made into new tissue, which either replaces the waste of the old or adds to growth. 2. By its destruction during the processes of respira- tion it maintains the heat of the body, and supplies to every organ the energy necessary for it to perform its functions. ; Animals all their lives consume food, but the food which they eat they do not make. The real maker of food, and the only maker, is the green plant, which manufactures from two bodies present everywhere—carbonic acid gas and water—the primary food-stuff out of which all others are-elaborated. Animals all their lives are putting car- bonic acid gas back into the air, and from this body food is again made by the vegetation. And so the cycle ever goes on, food being continually re-formed out of the elements of its destruction. Without the green plants there could be no animals, because food would not exist. If animals alone existed, their respiration would, in the course of time, make the air unbreathable, because its oxygen could not be replenished. Without the food- consuming animals the supply of carbonic acid gas would sooner or later be used up, and the green plant would vanish. The interdependence of plants and animals is complete. CHAPTER IX THE SOIL: ITS PHYSICAL AND CHEMICAL PROPERTIES THE green plant derives its carbon, in the form of carbonic acid gas, from the air. All the other materials required for growth are obtained from the soil. If we burn a plant, the organic matter is burnt off, and an ash is left the materials of which were obtained by the plant, during life, from the soil. The soil, in which the plant is fixed, and from which it draws its inorganic food, is derived in situ from the break- ing up, or “ weathering,” of the rock-masses upon which it rests ; in other cases the soil has been deposited in its present position by the sea (marine deposits), by lakes (lacustrine deposits), rivers (fluviatile deposits, alluvium), or glaciers (till, boulder-clay), or it may have been blown there by the wind, as in the case of dunes. Mingled with the rock-particles forming the soil are the rotting remains of animals and plants, which form a constituent part of all soils supporting vegetation. Agents of Denudation. The most important natural agencies which bring about the fragmentation of rocks into soil are : 1. Water, which acts in two ways : (a) Chemically.—The water which falls on and pene- trates into the rocks is not chemically pure, but contains gases in solution. The most important of these gases— carbonic acid—dissolves out certain of the mineral con- stituents, thereby causing the rest of the rock to crumble. Clay is formed in this way from granite. Granite is a rock composed principally of three minerals — quartz, felspar, and mica. Quartz is quite insoluble in water. 80 THE SOIL 81 Felspar, chemically, is a double silicate of alumina and potash or lime. Silicate of alumina is insoluble in water, but the silicates of potash and lime are soluble. Felspar, therefore, is the weak point in granite, and in the presence of water it gradually breaks down, owing to the loss of the other constituents which the water dissolves out. Just as the strength of every chain is the strength of its weakest link, so the durability of granite is measured by the durability of its felspar. The disintegration of the felspar causes the whole rock to crumble, the insoluble silicate of alumina being washed away as clay, and the quartz and mica as sand. The mica undergoes a similar series of changes as the felspar, but so much more slowly that it plays very little part in the disintegration of the rock. Water charged with carbonic acid gas also attacks limestone and chalk, dissolving out the carbonate of lime. It is for this reason that limestone hills are often honey- combed with caves and caverns, whose collapse has formed some of the most beautiful gorges in England— e.g., the Cheddar Gorge in the Mendip Hills of Somerset- shire. (6) Physiecally—Moving water carries with it solid particles. Ifthe current is swift, heavy stones are rolled along which grind against one another and the rocks of the river-bed. The destruction of sea-cliffs by the waves is partly due to the water undermining the rock chemi- cally, and partly to their bombardment by sand and stones, especially during storms. 2. Frost causes the water which has soaked into the rocks to freeze. When the water turns into ice it expands, and its expansion cracks and breaks the rocks in all directions. 3. Glaciers grind out the beds in which they move. This is not done by the ice itself, but by the angular stones and rock-fragments embedded in it. 4. Changes of Temperature.—In the daytime rocks are heated by the sun, and expand ; at night they cool and contract. The effect of these movements, if great, is to shatter the rocks into fragments. The sands of the Sahara Desert have been formed in this way. 82 BRITISH PLANTS Rocks and Soils. The rocks from which soil is derived are either igneous or aqueous in origin. Igneous rocks are those which have been erupted from volcanoes (lavas), or which have cooled down within the earth from a molten condition (granites). Aqueous rocks, as the name implies (Lat. agua, water), have been deposited under water, in seas and lakes, or upon the beds of rivers. Most soils are derived from rocks of aqueous or sedimentary origin, the chief of which are clays, shales, and slates, which give rise to clay soils; sands and sandstones, from which sandy soils are derived ; and chalk and limestone, which form calcareous soils. The physical characters of a soil depend mainly on the size of its particles and to some extent on the presence or absence of humus (see p. 83). In describing the soil- particles it is usual to fix a more or less arbitrary limit to the size. Those which range in diameter from 1 mm. to 0:05 mm. are designated “sand,” those from 0:05 mm. to 0005 mm. “ silt,” and those below 0:005 mm. “clay.” In a heavy elay soil, the small particles greatly predomi- nate, two-fifths, perhaps, being clay, one-third silt, and less than one-tenth sand. A coarse sandy soil, on the other hand, may contain as much as four-fifths of sand, only about 5 per cent. of silt, and under 2 per cent. of clay. The rest of the soil will consist mainly of humus with a certain amount of chalk and moisture. Soils intermediate in character between the heavy clay and coarse sand are termed loams; when sand predominates the soil is a sandy loam; when clay and silt, a clayey loam. A clayey loam containing chalk is called marl; if the amount of chalk exceeds 20 per cent. it is a cal- careous marl. . Clay consists mainly of kaolin (hydrated silicate of alumina), which is derived from the felspar of igneous rocks by the action of weathering. China clay is practi- cally pure kaolin; but in most clays small quantities of quartz, mica, and other minerals are also present. When wet, the particles of clay stick together, forming a wet mass, greasy to the touch, very difficult to dry, and almost impermeable to water. In drying, clay cracks into hard consolidated masses, which the roots of plants cannot penetrate. THE SOIL 83 Silt consists of fragments of quartz, felspar, mica, zircon, iron-oxides and other minerals which have resisted weathering action. Sand consists mainly of quartz, but the other minerals found in silt are also present to some extent. A funda- mental character of sand is that it falls to powder when dry and is not plastic like clay. Chalk is an amorphous form of carbonate of lime; when crystalline, it is limestone. Soils derived from chalk or limestone are naturally alkaline and rich in lime. They usually contain some sand, derived either from the siliceous skeletons of organisms from which the rocks are built up, or from overlying deposits which have been removed by denudation. Humus.—This is one of the most important constituents of fertile soil ; when it is absent, the ground is sterile and unproductive. More or less of it is present in every soil which supports vegetation, and sometimes the soil con- sists of little or nothing else. Where plants grow, humus accumulates. The conversion of dead vegetation or animal remains into humus is brought about by germs (bacteria) and fungi living in the soil. Where air is abun- dant and the ground not too cold, the organic matter in the soil is completely destroyed, the final products of its decomposition being carbonic acid gas, water, and mineral salts—substances valuable to plants as sources of food. When, through any cause, decomposition is checked, dark-coloured “‘ earth ’-acids are formed, which have a souring effect upon the soil. If lime, however, is present, it combines with these acids, rendering the humus mild and alkaline, and consequently fertile (p. 94). Enor- mous masses of humus in the form of peat have accumu- lated upon the rock-surfaces of the land, where, through lack of air, abundance of water, or extreme cold, decom- position has been arrested. In the form of coal, we dig up and burn as fuel the compacted and mineralized humus of past ages; in the form of manure, the agriculturalist renews the fertility of the fields with “artificial”? humus. At the present day peaty matter is constantly being formed on wet, cold moors, on dry, cold heaths, in mossy bogs, whilst in forests large quantities of mild humus are formed from rotting leaves and wood. 84 BRITISH PLANTS The Relations between Soil and Climate. In Chapter I. we considered the factors determining climate, and we learned that differences in the vegetation are associated with differences in climate. But climatic conditions are the same over wide areas, and yet marked and profound differences may be found in the flora. We cannot go far from our doors without seeing this. In some localities we may, in a few hours’ walk, pass by cul- tivated fields, wet meadows, rough fields, dry pastures, reedy swamps, and several kinds of woods and thickets. The climate is the same for all; it could hardly vary in the course of a short walk. The same is true when we examine the vegetation in an upward direction. Hills of the same height may carry very different kinds of plants. One may be covered with reedy bogs, another with gorse and heather, and another with grassy pastures. Wood differs from wood, and field from field. Even in the same field, when we come to details, many differences are apparent ; the vegetation on the dry rising knolls is not the same as in the damper hollows, and disturbed patches can be detected by their distinctive plants, even from a distance. What is the cause of all this variation within the limits of practically the same climatic condi- tions ? It is the soil. All soils do not react in the same way to climate. The sun’s rays will warm one kind of soil much more than another ; with the same rainfall some soils become wetter and remain wetter than others. In a short journey we may tread many kinds of soil, each bearing its own characteristic plants. Climate deter- mines the broad features of the vegetation, but the minor differences in the flora are due to the character of the soil and the way it behaves towards the external sources of heat and moisture. The relations between soil and climate must therefore be considered under two aspects : I. The reaction of the soil to water. II. The reaction of the soil to heat. I. Soil and Water.—The most important soil or edaphic factor is the amount of available water present. This clearly depends upon— THE SOIL 865 (a) The amount of water entering the soil; (6) The character of the water ; and (c) The power of the soil to retain it. (a) The Amount of Water entering the Soil depends upon— 1. The amount of water received from above—i.e., upon the rainfall, and upon the percolation of water from rivers and lakes, springs, and adjacent sheets of water. 2. The amount of water sucked up from below. This depends upon the size of soil-particles, the presence or absence of ground-water, and its depth below the surface of the soil during the vegetative season. (6) The Availability of this Water to Plants depends, as we have already shown, upon its chemical nature. The roots of plants can only absorb very weak alkaline solu- tions. For this reason, if the proportion of material in solution in the water is greater than a certain small amount (1 to 3 per cent.), or if souring acids be present, the soil, though it may be physically wet, is physiologically dry, and little water is absorbed. The soil also must be well aerated, otherwise the water present is useless. (c) The Amount of Water retained by the Soil depends upon its physical properties, and the physical properties are all directly or indirectly connected with the size of the particles forming the soil. The Physical Properties of the Soil, and their Relations to Water.—The rock-fragments forming the soil vary in size from large stones to an impalpable dust, which, when dry, remains for a long time suspended in water, and which, when wet, becomes compacted into a greasy, heavy clay. The particles, however, touch one another at a few points only, leaving between them pores, or spaces, which are normally filled with water and air. In ordinary soils the water forms a film of varying thickness round the particles, air occupying the rest of the pore-space. As the particles are in contact, the water-films surround- ing them are everywhere in continuity with one another. These films are elastic, and the withdrawal of water at any point sets up a flow of water along the adjacent films until equilibrium is restored. If a section of some coarse-grained soil containing water be examined, it will ‘be seen that the water-films increase in thickness as we 86 BRITISH PLANTS descend, until, at the level of the ground-water, all the pore-spaces are filled with water. Towards the top, the films become very thin and stretched, and if the section is sufficiently thick, the topmost layers may be perfectly dry. As the films become more attenuated, it is increas- ingly difficult to withdraw water from them, and at last a point is reached when no more water can be withdrawn without rupturing them. This last layer is very resis- tant; it is difficult to break, and constitutes’ what is known as hygroscopic water in soils apparently dry. Hygroscopic water is of no service to plants because it cannot be drawn from the films. From all films thicker than these water may be withdrawn, but in decreasing quantity as we approach the hygroscopic limit. Such water is called capillary water, and this alone is avail- able to plants. When all the pore-spaces are filled with water the soil is said to be saturated, or water- logged ; if drainage is possible and the excess allowed to drain away, the water that is left remains as films round the particles, the rest of the pore-spaces being filled with air. When during a shower water percolates through a sandy soil, it passes down to the water-table through the water-films. 1. The Capillarity of the Soil—The pore-spaces in the soil form a series of irregularly branching tubes. These tubes are everywhere in communication with one another, opening above into the air, and reaching below to the ground-water. The power of the soil to absorb and retain water depends upon the number and width of these capillary tubes. In a narrow tube the water ascends to a greater height than in a wide tube, and is more difficult to displace. In soils where the particles are very small and very close together, the capillary tubes are very minute, and much more numerous than in coarser soils, where the air-spaces between the particles are larger. The ascent of water, therefore, is greater in fine-grained soils than in coarse. The capillarity of the subsoil becomes very important when ground-water is present. With the level of the water-table at a certain depth, some soils may be able to make use of it by suction, while others may not. 2. The Water-Capacity of Soils.—There are two limits to the water-holding capacity of soils. The higher limit THE SOIL 87 is when the soil is saturated and all the air-spaces are filled with water ; the lower limit, when the water present is reduced to the hygroscopic film. The former is the maximum, and the latter the minimum water-capacity. The maximum water-capacity obviously varies as the pore-space. Coarse-grained soils have less pore-space than fine-grained soils. In the former, the individual pore-spaces are larger, but there are fewer of them, and the total pore-space is less. For this reason the water- capacity of clay is greater than that of sand in the ratio of about 7 to 5. Peat has the highest water-capacity of all soils. 3. Water-Retaining Capacity of Soils.—This is measured by the amount of water a soil can retain after the excess of water is drained off. The water-retaining capacity of coarse sand is about 13 per cent. of its volume—i.e., 100 cubic feet of sand can absorb 13 cubic feet of water, and retain it without dripping. Compared with this, the water-retaining capacity of clay is 41 per cent. of its volume ; that is to say, clay can hold and retain three times as much water as sand. 4. Permeability of Soils—This is measured by the rate at which water is able to percolate, or soak through, a certain depth of moist soil. Water runs through loose sand more quickly than through soils of closer texture. Clay is almost impermeable, but the water that is absorbed is very tenaciously held, and it takes a long time to drain or dry it out. 5. The Absorptive Power of Soils.—If equal weights of different soils be exposed, during the night, when the air near the ground is nearly saturated with water-vapour, they will be found, in the morning, to have increased in weight, but each to a different extent. The increase in weight is due to the amount of water absorbed: in one experiment it was found that 1,000 pounds of sand, exposed for twelve hours, increased in weight 1} pounds; the same amount of clay increased 35 pounds, and peat as much as 40 pounds. II. Soil and Temperature.—The sun shines on all soils alike, but some become warm more quickly than others. For example, a dark soil absorbs more heat than a light soil, and a dry soil becomes warmer than a wet soil. In 88 BRITISH PLANTS wet soils, the greater part of the heat absorbed is used up in evaporating the water, and very little remains to raise the temperature of the mass. A soil constantly damp is therefore cold. In a dry soil little water is present, and most of the heat absorbed is utilized in raising the temperature of the particles. In a wet country like ours a pure clay soil is not generally desirable, for, besides being cold and wet, it is heavy and expensive to work. In the South of France, however, where the climate is warm and dry, such a soil is valuable bec.use of its power of retaining water during long intervals of drought. Aeration of the Soil.—The presence of an adequate amount of air in the ground is very important. The underground parts of plants have the same need of oxygen for respiration as the parts growing in the air. The oxygen passes into the roots in solution in water, or, if the roots are old and woody, through the lenticels, or air-holes, upon their surface. A poorly aerated soil is a soil deficient in oxygen, and therefore unfavourable to plant-growth. Deficiency of oxygen leads to imperfect respiration, and this entails a checking or lowering of all the vital functions, root-absorption among them. Defici- ency of air in the soil also means the absence of bacteria and earthworms as well as the production of “souring”’ acids from rotting material, which has a still greater unfavourable effect upon absorption. The Chemical Properties of the Soil—Generally speak- ing, the chemical properties of the soil are far less im- portant than its physical. In two cases only does the chemical nature of the soil seem to have a marked effect on the flora : 1. Salts.—No other salt is so universally present in soils as common salt (sodium chloride). Generally, the quantity is small, but in some localities the amount is considerable, especially along the margins of the sea, and in the proximity of salt lakes and marshes. The characteristic plants in these places are halophytes (Gr. halos, salt). These are succulent (e.g., glasswort, Fig. 10), a type usually associated with desert-conditions. In a desert rain seldom falls, but when it does the plants absorb as much of it as they can, and store it up in special tissues for use during the long intervening periods of drought. The seashore is a physiological desert. Though THE SOIL 89 much water may be present, it is too salt to be of any use. Halophytes can digest salter water than most plants, but the percentage of salt in ordinary sea-water is too much even for them. Just as with plants in the physical desert, they can only get water when it rains. The fresh water entering the soil mixes with the salt water, and dilutes it sufficiently to make it available to the plants. In hot, dry climates, as in Egypt and parts of India, there is a large accumulation of alkali salts in the soil, particularly of sodium carbonate. Sometimes they occur In such quantity as to form a white crust on the surface. Their presence inhibits plant-growth to as great an extent as an excess of common salt, and the natural vegetation is typically xerophytic. 2. Chalk.—Chalk—i.e., carbonate of lime—is insoluble in pure water, but when carbonic acid is present, it is con- verted into the soluble bicarbonate. The presence of the bicarbonate makes water “ hard,” and plants growing in chalky soils absorb it. Although most plants are in- different to soils rich in lime, it seems certain that a few plants prefer them (calciphilous plants), while others avoid them altogether (calcifuges). The most pronounced calcifuges are: gorse, broom, bracken, foxglove, ling, heath, bogmoss (Sphagnum), and sundew. These are supposed to die in water containing more than 4 per cent. of carbonate of lime. But it has been proved that if no other salts are present, the lime itself does the plants no harm. It seems that here, again, the important thing is not so much the nature of the body in solution as the amount dissolved. Some plants can stand a considerable quantity ; others, like calcifuges, very little. It is also stated that the general character of the vegeta- tion living on a soil rich in lime is xerophytic. This is generally true, because most of our calcareous rocks— limestone and chalk—form hills or downs, and the rock is porous and consequently dry. If, then, on limestone hills or chalk downs the flora is xerophytic, it is probably due to physical causes—wind, drought, etc.—and not to the chemical nature of the soil, unless, of course, the total amount of dissolved salts present in the water reaches a dangerous degree of concentration. 90 BRITISH PLANTS Comparison between the Physical Properties of Sand and Clay. Sand. 1. Particles large. 2. Individual air-spaces large; total pore-space small. 3. Cohesion of the particles very small. 4, Firm and gritty to the touch. 5. Maximum _ water - capacity small. 6. Amount of hygroscopic water small. 7. Water - retaining small. 8. Very permeable to water. capacity 9. Capillarity small. 10. A dry soil. 11. A warm soil. 12. When dry, is friable and does not crack. 13. Drainage natural and easy. 14. A sterile soil, since soluble matter is easily washed out of it. 15. A light soil, easy to work. Clay. 1. Particles small. 2. Individual air-spaces small; total pore-space large. 38. Cohesion great. 4. Soft and greasy to the touch when wet. 5. Maximum water - capacity great. 6. Amount of hygroscopic water great. 7. Water - retaining great. 8. Almost water. 9. Capillarity great. 10. A wet soil. 11. A cold soil. 12. When dry, cracks. 13. Drainage difficult. 14. A fertile soil, since it is diffi- cult to wash soluble matter out of it. 15. A heavy soil, hard and ex- pensive to work. capacity impermeable to is hard and Chalky soils occupy an intermediate position—in some characters more approaching sand, in others clay. The best soil is that which, while light and warm, is fertile in plant-food, and contains plenty of water, without appear- ing wet. No one of the above soils fulfils all these con- ditions, but by suitably mixing sand, clay, and chalk together a loam is obtained which answers most of these qualifications fairly well. CHAPTER X THE BIOLOGY OF THE SOIL WE are now able to form an idea of the true nature of the soil in which plants live. In the preceding chapter we looked upon it as a mixture of rock-fragments supplied with a certain amount of water, air, and solar warmth. It is really much more than this. Upon its surface rests a covering of green, and within it dwells a living world of animals and plants which, by their activity, fit it to become an abode for the green vegetation that rises from it into the air and light. A broad view of the soil will therefore include the follow- ing considerations : 1. The earth or soil itself, its physical and chemical properties. . The water contained in it. . The air contained in it. The warmth received by it from the sun. The humus contained in it. Its living populations : - (a) Its surface covering of green. (6) Its underground inhabitants. PR 99 bo (i.) Plants—e.g., fungi and bacteria. (ii.) Animals—e.g., earthworms. Osmosis and Selective Absorption.—In the last chapter we described the mode of occurrence of water in: the soil. The plant absorbs water through its root-hairs. These are in contact with the soil-particles, and consequently with the water-films surrounding them. When water diffuses from these films into the root-hairs, water is drawn along all the adjacent films until equilibrium is 91 92 BRITISH PLANTS restored. In this way, the roots can explore for water a larger extent of soil than that actually occupied by the root-hairs, the suction extending farthest where the “capillary tubes ” are narrowest. In most cases the particles round which the water- films occur are insoluble solids, the films alone containing the soluble matters. In clay, however, the particles of which consist chiefly of silicate of alumina, the outer layers form with water a jelly-like material surrounding the grain. This jelly-like form of the silicate has a greedy attraction for water, and, when wet, swells like mucilage, blocking up the narrow pore-spaces. It is partly for this reason that wet clay is greasy to the touch, so difficult to dry, so absorbent of solutions, and almost impermeable to the passage of water. The root-hairs present on the roots of plants are merely certain surface-cells very much elongated to increase the absorbing surface of the roots. There are no actual holes for the passage of water as there are in the leaves for the interchange of gases. The water passes through the walls of the root-hairs into the interior of the cells by a physical process known as liquid-diffusion, or osmosis. To reach the cell-sap, it is clear that the water must penetrate two very different membranes : 1. The outer cell-wall, or membrane, formed of cellulose, and non-living. This membrane is permeable to water and all watery solutions of mineral salts, whatever their strength may be. The activity of the diffusion-current depends upon the strength of the solution outside the cell compared with the strength of the solution inside, and it becomes greater as the difference between the concen- tration of the two liquids increases. If the soil-water is very dilute, there is an active current of water passing inwards. If, on the other hand, the external water contains a large amount of soluble salts, and its concen- tration approaches that of the cell-sap, the quantity of water diffusing inwards is very small; and if its concen- tration exceeds that of the sap, the current is actually reversed, water is withdrawn from the cell, and the protoplasm collapses (plasmolysis—Gr. plasma, the proto- plasm ; lysis, loosening). The result to the plant is dis- astrous. It is for this reason that only very dilute solu- tions pass right through to the inside of the cell, and that THE BIOLOGY OF THE SOIL 93 absorption diminishes as the concentration of the external water increases. (Compare Halophytes, p. 88; and Chalk-Plants, p. 89.) The critical point is reached when the outside water and the cell-sap are of the same strength. Absorption then ceases entirely. 2. The second membrane through which the water has to pass to reach the central sap is the living protoplasm, which forms a film lining the inside of the cell-wall. Whereas the cellulose-membrane is permeable to all liquids of any degree of concentration, the protoplasmic membrane allows only certain solutions.to pass through it, and these only in a very dilute condition. One is a living membrane, and the other is not, and it is the living membrane which controls the intake of water, and deter- mines in what proportion the various substances present in soil-water shall enter the plant. This is known as selective absorption. Thus, if two solutions of equal strength—one a salt of soda, and the other a salt of potash —are presented to a cell, a greater proportion of potash will pass through the protoplasm into the interior than of the soda. This happens to serve the plant very well, since potash is the more valuable salt. The Qualities of Humus-Soils.—All soils supporting vegetation contain humus. Humus in quantity forms peat, and the qualities of peat depend upon : . The origin of the peat. . .The nature of the plants forming it. . The amount of water contained in it. 4. The amount of soil mixed with it. 5. The amount of air contained in it. 6. The amount of lime in it. 7. The presence or absence in it of fungi, soil-bacteria, and earthworms. The destruction of organic matter and its conversion into humus is the work of minute non-green vegetable microbes, known as bacteria. The first stage of the process, putrefaction, is effected by bacteria which are able to live without air—that is, without oxygen. These are called ana*robic bacteria (Gr. a; an, not; aér, air). The further stages of decomposition are brought about by a succession of soil-bacteria, which cannot thrive without oxygen (aérobic bacteria). The complete decomposition of organic matter by these won 94 BRITISH PLANTS aérobic forms leads to the formation of mild humus, which forms a slightly alkaline, well-aerated fertile soil. Mixed with earth, it forms mould, the most valuable of all soils—e.g., leaf-mould, peat-mould, etc., according to the origin of the humus. When decomposition is checked, raw or acid humus is produced. This takes place when any cause is present which renders the humus unfit for the existence of aérobic bacteria—e.g. : 1. Deficiency of oxygen. 2. Excess of water excluding air. 3. Too low a temperature. 4. Deficiency of lime. The acidity of raw peat is due to the fact that in the absence of sufficient oxygen and lime free “‘ earth ”’-acids are produced which sour the soil. If the causes which exclude the bacteria be removed by drainage, etc., the aérobic -bacteria resume their sway, the humous acids disappear, and a mild humus is pro- duced. The presence of vegetable fibre in soil increases its capacity for water. Peat itself is a perfect sponge; it has a greater water-capacity than any other soil. Sour peat, whether wet or dry, is poor in nutriment, for the form in which the nutriment exists is not one available to plants ; it contains many fungi, but few bacteria and no earthworms. Peaty matter is accumulating wherever vegetation is flourishing and the soil is cold or wet. Leaf- mould is forming in forests, mossy humus in bogs, fibrous peat on heaths. The Living Populations of the Soil. 1. Soil Bacteria.—Countless swarms of bacteria—a few malignant, but the most so beneficent as to be indispens- able to plant life—inhabit the upper layers of the soil. It seems likely that most of the chemical changes taking place in the soil, especially in its organic constituents, are due to their activities. ‘The most important are the following: (a) Nitrate - Bacteria. — The nitrogenous compounds present in decaying organic matter are decomposed in THE BIOLOGY OF THE SOIL 95 three stages. Each stage is effected by a different kind of bacterium. In the first, compounds of ammonia are produced, but these can only be effectively util- ized as food by fungi. If air is plentiful, these ammonium - compounds are converted into ni- trites, and finally into nitrates. In the form of nitrates, they constitute the sole source from which the green plant obtains its nitrogen. (6) Nitrogen - Bacteria. —A few bacteria, how- ever, are able to assimi- q- late the free nitrogen ak that exists in the atmo- sphere, Just 2S green yg. 27. — Brev’s - Foor Treror, plants assimilate car- (Lotus corniculatus), saowmne Roor- bonic acid gas. These Noputzs (a). bacteria are present in all well-aerated soils. By their agency the soil is always being replenished with nitrogen, for when they die their bodies are decomposed and nitrates produced. (c) Root-Nodules.—Other bacteria which utilize atmo- spheric nitrogen inhabit the bodies of green plants. They swarm in the nodules, or swellings, which occur so abun- dantly on the roots of leguminous plants—e.g., peas, beans, clover (Fig. 27). For this reason, leguminous crops may be raised in a soil from which nitrates are excluded ; they make no demands upon the soil for this salt, and if they are dug into the ground at the end of the season, they actually increase the amount of available nitrogen present in it (see Symbiosis, p. 132). 2. Soil Protozoa.—Minute unicellular animals occur in enormous numbers in the soil and play a prominent part in determining its fertility. These protozoa feed on the soil bacteria, and if present in too great abundance hinder, or even prevent, the formation of nitrogenous salts. In such cases, by partially sterilizing the soil by means of antiseptics, and particularly by heating it, its Ze sNias 96 BRITISH PLANTS fertility is improved; and this effect has been attributed to the consequent destruction of the harmful soil protozoa. 3. Earthworms.—Charles Darwin, in his classic work on Earthworms, has revealed to us the réle played by these humble creatures in Nature. They inhabit most soils containing a mild alkaline humus, which they pass through their bodies, depositing the “casts”? upon the surface. They are thus always transplanting soil, bringing it from below and exposing it to the air—ploughing, as it were, the land. Darwin estimated that several inches of soil are thus displaced in a century, and he attributed the gradual burial of stones and the sinking of old masonry to their agency. Worms also honeycomb the soil, pushing their tunnels several feet, or even yards, below the surface. This permits the free circulation of air in the deeper layers of the soil, keeping them sweet for the long exploring roots of the deeper-rooted trees. 4. Fungi.—In all moist soils rich in humus fungi abound. Not being green, these plants can make no carbohydrate. They live on rotting vegetation, much as bacteria do; but whereas bacteria are concerned chiefly with the nitrogenous compounds, fungi attack the carbo- hydrates, which they remove from the soil. Manures. 1. Natural Manure.—This consists of plant or animal remains, or animal excrement, which are added to the soil to replenish its fertility. It is usually exposed on or near the surface for a time, to allow it to rot or ferment ; afterwards it is dug into the ground, so that the soil- bacteria may get to work on it. To prevent the volatile compounds formed during the decomposition from escaping into the air, and to prevent the formation of free acids which would sour the ground, lime is added to the manure. It is important to remember that manure in itself is not plant-food ; it is only a source of plant-food. It must become rotten, and then be allowed to remain in the ground till the soil-bacteria have converted its organic material into mineral salts before plants can be said really to feed on it. In market-gardens, manure is employed not only as a source of food for plants, but also as a source of heat for raising early vegetables. For the latter pur- THE BIOLOGY OF THE SOIL 97 pose it is placed in a compact mass below the plants, that it may ferment, and the heat evolved during the putrefac- tion warms the soil above and stimulates the growth of the plants rooted in it. 2. Artificial Manures.—These- are manures applied to the soil in a form which requires no further decomposition by soil-bacteria—e.g., the nitrate, potash, soda, and phos- phatic manures of commerce. Guano, being very rich in nitrates, belongs practically to the same category. They should all be applied in very dilute solutions (p. 92). 3. Living Manures.—There have been several attempts in recent years to place on the market extracts of soil- bacteria. They are intended to make poor soils fertile, or to increase the weight of the crops taken from them. Extracts of root-nodules are regarded in some quarters with special favour. The seeds are infected with the extract before sowing, or the ground is watered with it when the plants are coming up. The Rotation of Crops. Different crops withdraw from the soil its nutrient salts in different proportions, one crop demanding more of one salt than another. For this reason, the same crop raised year after year in the same soil would soon impoverish it. To obviate this, crops are grown in rotation, each crop differing from the rest in its demands upon the soil. By this means, nutrient substances are not withdrawn from the soil more quickly than they can be replaced by the ordinary methods of manuring. As an example of the rotation of crops, we will set forth the so-called Norfolk, or Four-Year Rotation, popular on light sandy soils in this country. It will be seen that the same crop is never raised from the same field two years in succession, but only at intervals of four years : First Year. | Second Year.| Third Year. | Fourth Year. First field - R. B L. W. Second field B. L W. R Third field - ( Pr W. R. B Fourth field W. R B. L a 98 BRITISH PLANTS L.=A Leguminous Crop—e.g., clover, a deeply-rooting plant, which is sown as a catch-crop. In a wet country like ours it is not advisable to let the ground lie fallow in order to allow time for nitrates to accumulate in pre- paration for a crop demanding much of this salt, for the frequent rains wash them out almost as soon as they are formed. To obviate this, catch-crops like clover are put on the land. Clover has bacterial nodules on its roots, and if the crop is cut or eaten off the land, and not up- rooted, it enriches the soil with nitrogen, and at the same time consolidates it by its vegetable fibre and increases its water-capacity. W.=Wheat, which is generally sown in autumn as soon as the catch-crop is ploughed in. It is a deep- rooted cereal, which makes a great demand on the soil for silica and potash, as well as nitrates. It prefers a rather heavy soil. R.= Root-Crops—e.g., turnips, swedes, etc. Two out of the three preceding crops—viz., barley and wheat— make exhausting demands upon the soil, and they follow so quickly upon each other that very few opportunities are allowed to clean and prepare the ground thoroughly. Root-crops, however, are not sown before the spring, and this allows time for the land to be deeply ploughed and cleared of weeds. Root-crops also require a good deal of manure to encourage the production of large fleshy roots. This is therefore the right time to add plenty of manure to the soil, much of which will be available for succeeding crops. B.=Barley, a shallow-reoted cereal with similar de- mands to those of wheat. Being a surface-feeder, barley gets its nutriment from the upper layers of the soil, and for this reason it follows the root-crop, while the ground is still rich with manure. Wheat is a deep feeder, and naturally follows later, when the nutriment from the manures has sunk deeper in the soil. On heavy soils oats are generally substituted for barley. Different soils require different rotations, and agricul- tural practices vary even on the same soils. Moreover, it has now been shown that rotation is not really necessary to prevent the soil becoming impoverished. With proper treatment and scientific manuring the same crop may be taken year after year from the same ground without diminishing its value. THE BIOLOGY OF THE SOIL 99 The Creulation of Nitrogen. After what we have said in this chapter, we can now complete the balance-sheet of the atmosphere, which we left unfinished on p. 79. 1. The Sources of Nitrogen in the Soil.—(1) The nitro- genous compounds present in the soil are produced, as we have seen, by the agency of bacteria in the decomposition of humus. (2) During thunderstorms, in the path of the lightning, oxygen and nitrogen unite, and the resulting oxides of nitrogen are swept down by rain into the soil as nitrous acids ; these are readily oxidized and combined as nitrates. (3) Certain soil-bacteria, some living free in the soil and others located in root-nodules, are able to fix or assimilate the free nitrogen of the air. The latter is ultimately restored to the earth in the form of nitrates (p. 95). 2. The Losses of Nitrogen in the Soil.—(1) The nitrates and ammonium-compounds taken up by plants are restored to the soil when the plants decay. If the plants are eaten by animals, the ingredients are returned to the soil in their excreta, or finally in their bodies after death. The loss to the soil in this case is only temporary. (2) Nitrates and ammonium-compounds are very soluble in water, and much of what is formed in the soil is carried away by water and lost in the sea. Everywhere, and at all times, there is leakage of nitrogen from the soil. The loss is made good from the air in the ways we have described. The air, in fact, is the ultimate source of all the fixed nitrogen in Nature, just as it is the source of all its carbon. Soil-Cultivation and Fertilizers. The basis of fertility is good husbandry. Plant and soil analyses help very little in determining what a plant requires or what a soil wants. The chief need of the soil is an adequate and continuous supply of water. How important this is may be inferred from the expenditure of so many millions of pounds in regions where prolonged drought makes cultivation precarious or impossible. There is always some water in the soil and the amount 160 BRITISH PLANTS increases as we get into the deeper layers. During dry weather water is being constantly brought to the surface by capillary attraction. The advantage of hoeing the ground in dry weather depends upon the fact that by so doing we cut off the upward water-columns an inch or so from the surface and so prevent the rising water from running to waste by evaporation. It also encour- ages the plants to send down deeper roots into the soil in search of water. The principles of soil-fertility and the reaction of plants to fertilizers are as yet very imperfectly known, and much of what we do in practice is the out- come of long experience rather than the result of scientific research. In general we know that a soil deficient in plant-food is not likely to produce good crops, while a soil rich in such material will do so; also that some crops take out of the soil certain ingredients which can be re- placed by some form of manuring; and, also, that certain fertilizers benefit certain plants or have a general bene- ficial action on their growth. Thus the chief functions of the three great chemical fertilizers in common use are: 1. Nitrates, by supplying nitrogen, encourage growth of roots, stems and leaves. 2. Potash, by its action in facilitating the processes of photosynthesis, promotes the production of starches and sugars, and is therefore usefully applied to potatoes, root-crops and fruit-trees. 3. Phosphates, by stimulating the vital activities, encourage early and abundant flowering and fructifica- tion. The choice and efficiency of manures depend upon many other things than the mere putting into the ground of the chemical substances which a plant needs. The soil is, to a large extent, a living medium, a busy factory in which aerobic bacteria are preparing plant-food. The most important of these bacteria are those which convert nitrogen-yielding substances into nitrates and those which manufacture nitrates from atmospheric nitrogen (see p. 94). The rate at which this nitrification goes on in the soil may be regarded as the index of its fertility. An immense number of aerobic bacteria occur in all fertile soils, and anything that diminishes the number and activities of these bacteria diminishes the fertility of the soil. The proper nitrification of the soil is the first requisite. It THE BIOLOGY OF THE SOIL 101 varies with the climate, the season, the physical state of the soil, its humus-content, its chemical reactions, its aeration, and the presence or absence of deleterious micro-organisms or their products. Continued cropping does not necessarily exhaust a soil, and when it does it is probably due not so much to the depletion of the nutritive salts as to its failing powers of nitrification. Nitrate- bacteria can only flourish in a well-aerated and alkaline soil. Hence the importance of digging deep and liming well. Under conditions of bad culture, even with no lack of manure, the soil becomes sour and foul. The aerobic bacteria diminish, and in their place anaerobic bacteria and other denitrifying organisms make their appearance and impoverish the soil. In some cases it has been shown that certain toxic substances, acids and aldehydes, are excreted by the plants themselves; and it is alleged by some authorities that with continued cropping these may accumulate to such an extent as to thoroughly poison the ground. This has not been proved, but even if it is so good cultivation will remove them. Moreover, nitro- gen is the most easily lost of all plant-foods. Owing to the extreme solubility of most of its compounds, it is easily washed down beyond the roots, or it may be dis- sipated into the air as gas by the agency of anaerobic bacteria. To retain the nitrogen in the soil it is again necessary to use plenty of lime. Lime, and in its less active form chalk, is, in fact, the most important of all manures. It not only favours nitrification by correcting acidity, but it liberates plant-food from manures and minerals, at the same time fixing it in a form which pre- serves it in the soil and makes it available for plants. The amelioration of the soil for the production and con- servation of nitrogen likewise favours the availability of other chemical substances needed by plants, particu- larly phosphoric acid and potash. Labour and lime, together with large and regular supplies of organic matter in the form of manutes, will secure conditions of fertility in practically all soils. PART II THE LIFE OF THE INDIVIDUAL PLANT— PLANT BIOLOGY INTRODUCTION Botany may be divided into two branches : 1. Morphology (Gr. morphe, form), which deals with the form, structure, origin, and development of plant- organs and their relations with one another. This branch includes—(a).external morphology, dealing with the out- ward form of organs; and (6) internal morphology, dealing with the morphology of the minute cells and tissues of which the organs are composed. 2. Physiology (Gr. physis, nature, inherent qualities), which deals with the functions of the organs. Thus we may consider a leaf with respect to its form, size, and structure ; we may trace its origin and develop- ment, and compare it with other leaves and organs. This is examining the leaf morphologically. Or we may direct our attention to the work or functions performed by the leaf, its vital activities—assimilation, transpiration, respiration—and consider how the particular leaf under notice performs them. This is looking at the leaf on the physiological side. In dealing with the form and structure of any organ it is important to associate its form with its function. Variation of form means variation of function, if only in degree. Thus, suppose we compare two leaves—a holly- leaf with a lime. They are very different. One is thick, tough, shiny, and evergreen, and the main veins are con- tinued as spines. The other is thin and deciduous. But modification in form implies modification in function. A green leaf has three main functions : it assimilates, it transpires, it respires. Each of these functions is modified as the form is modified, but which is the most important ? On reference to p. 43, we see that the tough evergreen 193 104 BRITISH PLANTS leaf is a xerophytic form adapted to live through the winter, and that the purpose of its special characters is to reduce loss of water by transpiration within safe limits. The leaf of the lime has no such characters ; it is equipped for summer use only. Again, assimilation is not so active in the holly as in the lime. A lack of nutrition is the result, and this is actually expressed in the leaf by the spines. The spines are modified vein- structures between which the leaf-tissue has failed to develop. On well-grown plants growing in good soil the leaves tend to lose their spines because they are better nourished. Thus each leaf has written upon it functional signs that the physiologist can read, even if only imperfectly. We understand a structure when we know the reason for it—that is, when we know its function. A plant is a living thing. It lives in a certain environment, and the nature of that environment, acting through its vital functions, is expressed in its outward form and inward structure. The study of organisms as living things is Biology (Gr. bios, life). To connect form with function, and both with environment, is to make botany a biological study. With morphology alone we have little to do ; in ecology, morphology is quite dominated by biology. This biological method of looking at plants is quite recent. It requires an accurate knowledge of the main facts of physiology, and this our forefathers did not have. Physiology grew as the science of chemistry and physics developed. But our ancestors saw form clearly enough, although they knew little of the functions that underlie form, and still less of the relations between form and environment. The early botanists studied plants as they found them—in the garden, in the field, in the herbarium. They examined the outward form, marking and recording resemblances and differences. Upon these morpho- logical characters they founded their classifications and generalizations. It must be granted, however, that broad or general views can only be obtained when plants are collected into groups, and the simplest and most natural groupings are those which are founded upon the resemblances and differences of external form. Even at the present time it is found convenient for PLANT BIOLOGY 105 most purposes to rely upon morphology as the basis of classification. Our flowering plants, for example, are divided into groups, called Natural Orders, which are based entirely on morphological relations. Generally the form and characters of the flowers are taken, the vegeta- tive bodies of plants varying too much to enable us to make much use of them as a basis of classification. Biologically, however, other classifications are possible. Plants may be grouped together according as they re- semble one another biologically, and it is the biological groupings of plants and parts of plants that are important in ecology. As an example, we have already in Part I. classified plants according to their relations to water, dividing them first into two main groups, Water-Plants and Land- Plants, and then the land-plants further into Hygro- phytes, Mesophytes, and Xerophytes, according as the amount of water available is abundant, adequate, or small. In this classification form and structure were regarded merely as the expression of physiological need, and we selected water since it is the most important of all the ecological factors and lies at the base of nutrition. This division takes no account of relationships. Plants which are widely separated in descent may fall together in the same ecological grouping. Thus the Cacti and Euphorbias are plants which have no relationship at all to one another, but when growing in deserts they ap- proach one another so closely in form and characters that it is difficult to distinguish between them apart from their flowers. It is a case of parallel development. Growing under similar conditions, surrounded by the same environment, these two races have, in the course of ages, succeeded in adapting themselves to the same conditions in the same way. On the other hand, two closely - related species may be widely separated eco- logically, one being adapted to one mode of life, the other to another. One, for example, may be a hygrophyte and the other a xerophyte. The genus Senecio, to which the groundsel and ragwort belong, is remarkable in this respect. It is of world-wide distribution, and contains a huge number of species. But it includes plants of the most diverse habit. Some are annuals, others perennials ; some are alpines, some marsh-plants, some pronounced 106 BRITISH PLANTS xerophytes ; some climb, some have succulent leaves, others leaves hoary with hairs; one has leaves like ivy, another leaves like flax, another leaves like heath ; but the flowers are the same in all. ; In Part II. we have to deal chiefly with biological divisions based upon functional similarities and dis- similarities. Land-plants, for example, will be divided— 1. According to their Longevity (Chapter XI.). 2. According to their Frequency of Seeding (Chap- ter XI.). 3. According to their Mode of Growth (Chapter XII.). 4. According to their Mode of Nutrition (Chapter XIII.). CHAPTER XI DIVISION OF TERRESTRIAL PLANTS ACCORDING TO THEIR LONGEVITY AND FREQUENCY OF SEEDING Division of Plants according to their Longevity. 1. Annuals.—These plants live through one or part of one vegetative season and then perish, after providing for the continuance of the race by the production of seed— €.g., poppies, groundsel, chickweed. There are two kinds of annuals : (a) Those which germinate from seed in the spring, and perish towards the close of the vegetative season—summer- annuals—e.g., poppy. (b) Those which germinate from seed in the autumn, forming small plants with a few leaves, arranged on the rosette plan, close to the ground, and continue their growth in the following spring—autumn-annuals. They flower early, and perish before the hottest part of the summer sets in. Annuals belonging to this class must seed very freely because the fatality among the seedlings during the winter is very great. In both cases there is a season during which the plants are not seen at all. In the case of summer-annuals this season is the period of vegetative rest, winter ; in the case of autumn-annuals, which are vernal bloomers, it is the summer, when hot, dry conditions bring about a period physiologically unfavourable to the plants. The latter are found chiefly in dry steppe-regions, or in sandy and stony places which become very hot and dry in summer. They are not common in England, but a few are found on sand-dunes, a habitat specially remarked for summer drought—e.g., the vernal whitlow-grass (Draba verna), the early forget-me-not (Myosotis collina), and the small mouse-ear chickweed (Cerastium semidecandrum). 107 108 BRITISH PLANTS Gardeners obtain early spring flowers by sowing hardy annuals at the end of summer. These germinate in the autumn and bloom early in the spring, and the flowers then last longer than usual—e.g., cornflower, poppy, sweet pea. The fecundity of some annuals is so great and the lives of the individuals so short that several generations are produced in the course of the year—e.g., Poa annua, groundsel, chickweed, shepherd’s-purse. Annuals are characteristic of temperate regions, where there is a marked alternation of seasons, and the summer is sufficiently long to allow the plants to proceed from seed to seed again. The seed, in which the young plantlet hibernates, is a pronounced xerophytic structure, and can withstand the most extreme and prolonged hardships (see p. 74). The great majority of the so-called weeds of cultivation are annuals, as naturally they must be, for they are destroyed regularly at harvest-time, and their propaga- tion is dependent entirely upon the seed left behind in the ground after the crop and its weeds have been removed. There are very few annuals among alpines and aquatics. 2. Biennials.—These plants, as the name implies, live during two vegetative seasons. During the first they produce leaves and manufacture food, which is stored up in their underground parts for use the next season, when they flower, seed, and die—e.g., turnip, carrot, foxglove, burdock, mullein. The swollen underground organs found in biennials at the end of the first season are merely temporary reservoirs of food. During the formation of the seed, a steady migration of food-material takes place from these organs, which become depleted, and ultimately shrivel up. Essentially, biennials differ little from annuals, and especially from those which we have called autumn-annuals. They only seed once, and under un- favourable conditions they may even flower the first year and die, while in a few cases strong and vigorous plants, enjoying the advantages of a good soil and a happy habitat, may live for several years. The latter is fre- quently the case with wild biennials grown in gardens— e.g., wallflower, foxglove. 3. Perennials.—Perennials are plants which live for more than one year, and seed several times before they die. All parts of a perennial have not the same longevity ; some parts live longer than others. The leaves of a LONGEVITY OF PLANTS 109 deciduous tree, for example, fall at the end of summer, and must, therefore, be renewed annually. Perennials may be classified according to the longevity of their assimilating parts : (a) Evergreen Perennials.—The leaves of these plants live through the winter ; they may last several years, but sooner or later their vitality fails, and they drop. They do not, however, all fall off together, and the plants are never leafless. Winter-green plants, if land-plants, are all xerophytes outside the Tropics. They are character- istic of climates which are hot and dry during summer— e.g., the Mediterranean laurels and myrtles; where summer and winter are both inhospitable seasons— e.g., the coniferous forest ; or where the winter season is very long—e.g., alpines. (6) Deciduous Plants.—In these certain of the vegetative organs, always including the leaves, are shed annually : (i.) Deciduous Trees and Shrubs, in which the shoots are all long-lived, and only the summer-green foliage is annually shed. They fall simultaneously at the approach of winter, leaving the trees bare. In some coppiced plants, however, the leaves may not all fall together. The oak, for example, when pruned in hedges, forms the usual plate of cork across the base of the leaves at the end of summer, the leaves die, but the final rejectment by the splitting of the separation-layer is not com- pleted, so that many dead leaves remain on the tree during a large part of the winter—falling gradually, as they are blown off. A still more familiar instance of pruning interfering with leaf-fall is the privet (Ligustrum vulgare), planted everywhere in.gardens and hedges. When kept well pruned, many of the leaves remain alive and green upon the bushes till the new leaves break from the buds in spring. Pruning is an artificial way of reducing transpiration, and acts, therefore, as a xerophytic adaptation. (i.) In other plants the smaller twigs are shed annually as well as the leaves. (iii.) In others the leaves die and all the shoots except those close to the ground. These bear the renewal-buds, and in many cases leaves as well—e.g., chrysanthemum. (iv.) Herbaceous Perennials.—Here the destruction of the aerial parts is complete. The individuals, however, 110 BRITISH PLANTS remain alive beneath the soil in the form of various xerophytic structures from which new shoots arise in the spring (see Geophytes, p. 62). Underground FPerennating Organs found in herbaceous perennials : Fic. 28.—Unprercrounp RuizomEe or CoucH-Grass. 1. Rhizomes.—These are perennial underground stems bearing scales, buds, and roots (Figs. 28, 29). Rhizomes are not roots, though they are underground and carry roots. The leaves on subterranean stems are scaly when they do not come above ground, because, being in the dark, chlorophyll does not develop. The buds occur in the axils of these scales, or terminally at the extremity of Fia. 29.—UnpzereRounD RuIzomz oF Mint. the branches. Rhizomes are more or less thick and fleshy, containing a store of food-material which migrates to the brood-buds when they develop the aerial stems. When the rhizome is short, thick, and erect, as in some ferns, it is known as a root-stock. 2. Tuberous Structures.—These are fleshy, hibernating organs stored with food. The fleshy part may be either LONGEVITY OF PLANTS ill stem or root. In the potato (Fig. 30) the tubers are the swollen tips of underground stems, and the “ eyes ’’ upon them are the buds. In the dahlia some of the roots which grow from the base of the stem become tuberous. In the lesser celandine and some orchids, buds at the base of the stem develop an adventitious root which becomes large and tuberous. The fleshy tuberous roots of biennials are generally complex structures. The part from which the buds arise is stem. In the carrot and turnip the greater part of the tuber is derived from hypocotyl, that portion of the axis that lies between the primary root and the cotyledons ; the lower part, bearing rootlets, represents the primary root. In the swede Fic. 30.—Basat Part or Potato Piant. a, underground branch bearing stem-tuber 6. To the left branches of an aerial shoot are producing tubers. (Fig. 31) the stem forms a collar at the top of the tuber ; most of the rest is hypocotyl. In the kohlrabi (Fig. 32) (German, cabbage-turnip) a big round tuberous body projects from the soil, the surface of which is covered with leaf-scars; this is stem. In the dandelion and chervil the carrot-like tap-roots are perennial. In the false oat (Arrhenatherum avenaceum) the perennating function is assumed by the base of the stems, the inter- nodes of which become swollen with food, forming chains of little tubers (Fig. 33). 3. Bulbs and Corms.—These are compressed buds, or rhizomes, capable, by reason of the food they contain, of living an independent life. Corms are solid bulbs, the food being stored in a short tuberous stem which is covered 112 BRITISH PLANTS with membranous scales ; in bulbs the food is in the leaves, which are thick and fleshy, the stem being reduced to a flattened disc, bearing roots (see p. 156). Classification of Plants according to their Frequency of Seeding. This is a more scientific method than by longevity, which is governed by the seasons. We have already pointed out that there is no strict line of division between Fic. 31.—SwEpDE, sHOWING TUBER- Ria. 32. — KonwLrasi, SHOWING ous Hypocoryn. (REDUCED.) TuBEROUS STEM BEARING LEaF- a, “collar.” Scars (a). (REDUCED.) annuals and biennials, or between biennials and peren- nials, and the estimation of age by the seasons fails alto- gether in the case of plants which live in those parts of the Tropics where there is no climatic interruption in the development of the vegetation. According to the frequency of seeding, plants are divisible into two groups : 1. Monocarpie Plants (Gr. monos, once ; carpos, fruit), which produce seed once and then die, and— 2. Polyearpie Plants (Gr. poly-, many), which fruit more than once in their lifetime. FREQUENCY OF SEEDING 113 The first class includes all annuals and biennials, and those perennials, native and foreign, which flower after several seasons’ growth and then perish. All these are, biologically, annuals. The most remarkable mono- carpic perennial is the American aloe, Agave americana. This wonderful plant grows very slowly, forming a gigantic rosette of long, thick, fleshy leaves, two or three each year. At last it flowers, bearing a huge inflorescence, Fic, 33.—Fatsr-Oat Grass (Arrhenatherum avenaceum). (RECDUED.) a, erect flowering shoot; b, decayed leaf-bases ; ¢, swollen internode; d, lateral branch ; e, horizontal part of rhizome ; /, axillary bud. reaching in some-cases a height of 20 feet. It lives to a great age before flowering, sometimes a hundred years. But the act of flowering and fruiting is so exhausting that the plant is unable to survive it,and it dies. This aloe is therefore, in spite of its longevity, biologically, an annual, for, like all annuals, it seeds once and dies. To the polycarpic group are assigned all trees and shrubs, herbaceous perennials, and geophytes. CHAPTER XII CLASSIFICATION OF PLANTS ACCORDING TO THEIR MODE OF GROWTH I. Terrestrial Plants, rooted in the Soil. Tue following grouping is based upon the position of the leaves on the stem, and the habit of the stems with regard to light : 1. Plants with Cauline (7.e., stem or shoot) Leaves only : (a) Plants with erect stems. (i.) Annual herbs. (ii.) Herbaceous perennials. (iii.) Shrubs and bushes, branching freely from the ground. (iv.) Trees with trunks. (b) Plants with prostrate stems. (c) Plants with climbing stems. (d) Cushion-plants. The Cushion-Plant branches freely at the ground-level, forming a multitude of short erect stems. Most of the leaves are near the extremity of the stems, the leaves below being smaller and generally wider apart. Such plants may be looked upon as an approximation, through xerophytic conditions, to the rosette-form, by the pulling down to the ground of long-stemmed plants in such a way that the whole plant forms a mosaic of small loose rosettes raised only a few inches above the soil. Many alpines and some maritime plants form these low cushions —e.g., moss-campion (Silene acaulis), mossy saxifrage (Saxifraga hypnoides), sea-purslane (Honckenya peploides), and mountain-avens (Dryas octopetala). 114 MODES OF GROWTH 116 2. Plants with Radical Leaves only: (a) Rosette-plants—e.g., dandelion. The daisy is really a cushion-plant, but the stems have sunk below the level of the ground, producing on the surface a number of distinct rosettes. (b) Some bulbous geophytes—e.g., crocus, wild hya- cinth. 3. Plants with both Radical and Cauline Leaves : (a) Most biennials. (b) Some herbaceous perennials—eg., oxtongue (Hel- minthia echioides), rock-cress (Arabis), and some hawk- weeds. (c) Grass-like plants with tufted habit—e.g., tussock- grasses, like Aira flexuosa and Holcus lanatus, and some sedges. Except in geophytes, these differences of habit are due to the many and varied methods adopted by plants to secure for themselves an adequate amount of sunlight. Leaves, as the chief assimilating organs, seek to take the most favourable light-position. Trees effect this by lifting up, on tall trunks, a crown of foliage above the surrounding vegetation. The leaves of rosette-plants grow close to the soil and smother other plants that might shade them. Plants with prostrate trailing stems thread their devious ways among the lower vegetation, seeking out favourable spots where their leaves may catch the light. The creeping habit of long frail stems leads to the scrambling habit, in which the stems hook and catch on to other plants, scrambling over their backs into the light. From the scrambler to the climber is a shortstep. Epiphytes (see p. 121) make their light-position secure by perching on the elevated branches of trees. Climbing Plants.—These have long weak stems, and solve the problem of reaching a position of advantage by climbing up the bodies of other plants. They derive no nourishment from the supports upon which they lean, although occasionally they injure them by intercepting the light which properly belongs to them. Climbing plants were divided by Charles Darwin into the following roups : i. ‘Seramblers, or Hook-Climbers.—These are the least specialized of climbing plants, and are not far removed 116 BRITISH PLANTS from those of prostrate habit. The latter may easily, if circumstances favour, become scramblers. True scram- blers, however, are provided with special organs to aid them—hooks, prickles, barbs, or recurved bristles. These grappling structures are not to be associated with the sensitive climbing organs of true climbers, for they are not sensitive to contact, and have no movements of their own. The bramble or blackberry (Rubus fruticosus) and the dog-rose (Rosa canina) are provided with large re- curved hooks, which arise as emergences upon the stems and leaves. In the moss-rose and Rosa spinosissima the emergences are straight, and not fitted for climbing ; they serve only for defence. The goosegrass, or cleavers (Galium Aparine), scrambles up the hedges by its rough stems and leaves, which are armed with a multitude of small reflexed bristles. 2. Twiners.—The weak stems of these plants twine round supports, provided that these are not too thick nor too smooth. The growing tips are sensitive, and describe slowly and regularly circles or ellipses. This spontaneous movement is known as “ circumnutation ” (Lat. cirewm, around ; nuto, I nod). In the black bryony (Tamus communis) the apex describes one complete revolu- tion in about two and a half hours. By means of these movements the stem is enabled to come into contact with supports which it otherwise would fail to reach. On reaching the support, the stem twines round it. Some plants twine clockwise, others counter-clockwise, but for every plant the direction is fixed, the habit having become hereditary. (a) Twiners climbing counter-clockwise, the usual habit: Phaseolus vulgaris (scarlet runner), Wistaria chinensis, Convolvulus sepium (Fig. 34), Polygonum Convol- culus, Cuscuta (dodder, Fig. 42). (6) Twiners climbing clockwise: Humulus Lupulus (hop, Fig. 35), Tamus communis (black bryony), Lonicera (honeysuckle). Wistaria and hop connect this group with the scram- blers, the former having recurved spiny stipules, and the latter T-shaped stiff hairs. Solanum Dulcamara (woody nightshade) is a feeble climber, and may climb either way. In the open, where the light is strong, it is a low bush, not climbing at all. CLIMBING PLANTS 117 In the shade of hedges the stems become etiolated and weak, and the plant climbs. Polygonum dumetorum is another imperfect twiner. 3. Tendril-Climbers.—Tendrils are organs specialized for climbing. While growing, they are sensitive to con- tact, and their free ends circumnutate. On touching a support, the tendril twines round it. The free portion between the support and the plant then takes on a spiral twist, a reversal-point naturally occurring in the middle, since both ends are fixed and incapable of movement (Fig. 36). The object of this is to bring the stem closer to its support, and at the same time to increase the strength Fia. 34.—Convoluulus (BrInpwEED), Fic. 35.— Hop, rwinina Croox- TwiIntna COUNTER - CLOCKWISE. WISE. (REDUCED.) (REDUOED.) of the anchoring cable. The closer the plant is to its support and the stronger the cable, the less liable is it to be wrenched away from it by the wind and damaged. When twining has ceased, the tendril thickens and becomes woody, thus making the connection permanent and durable. The hooks and bristles of scrambling plants are oppor- tune outgrowths from the surface of the plant. They are neither leaves, roots, nor shoots. Tendrils, on the other hand, represent definite plant-organs—leaves or portions of leaves, stem-structures, or even roots. We mean by this that the rudiment of the tendril started as one of 118 BRITISH PLANTS these organs, but in the course of its development it became modified as a climbing organ, and gradually assumed the form and functions of a tendril. The Morphology of Tendrils—(a) Modified Stem or Shoot Structures.—These generally occur in the posi- tion of flowering shoots, and so may be regarded as modified floral branches. Their morphological nature may be established from the fact that they arise in the axils of leaves (passion-flower, Fig. 36), or them- selves bear leaves or the vestiges of leaves, or even Fie. 36.—Passron-Fiowrr. (REDUCED.) a, branch-tendril with reversal-point at b ; c, extra-floral nectary ; d, vegeta- tive bud ; e, flower-bud ; /, stipule. flowers (vine). They may be branched (vine) or un- branched (passion-flower.) In the virginia-creeper, Am- pelopsis Veitchii (Vitis inconstans or Veitchit), each branch of the tendril bears at its tip a small adhesive pad, which cements the tendril to the wall (Fig. 37). (6) Modified Leaves or Parts of Leaves—(i.) Leaflets.— In peas (Lathyrus) and vetches (Vicia) some of the ter- minal leaflets are modified into tendrils. In the ever- lasting pea all but two leaflets are modified, and the stem becomes winged and green (Fig. 38). In Lathyrus A phaca every leaflet becomes a tendril, and the work of assimila- CLIMBING PLANTS 119 tion is taken over by the stipules, which are large and leaf-like. (ii.) Stipules.—In Smilax (sarsaparilla), a climber fre- quently seen in greenhouses, the tendrils are in the’ posi- tion of stipules. Climbing hooks may also be present. (iii.) Leaf-Tips.—In the climbing lily (Gloriosa superba), another foreign greenhouse-plant, the tips of the leaves are prolonged into strong cord-like tendrils. (iv.) Petioles—The garden-nasturtium (Tropeolum majus) and Clematis have sensitive leaf-stalks. On reach- ing a support, they curve round it, and then become tough and indurated, thus enclosing it firmly. 4. Root-Climbers.— These climb by adventitious roots, which arise in great number on the shaded side of the stem. They act as suckers, pinning the branches to the wall or to the tree up which the plant climbs—e.g., ivy (Hedera Helix). No suckers, however, occur on the flowering shoots, which turn away from the wall and grow outwards towards the light. In many plants the morphological nature of the tendril is doubtful. In the vegetable-marrow (Cucurbita), cucumber (Cucumis), white bryony (Bryonia dioica), Ke. 878 and other Cucurbitacee, the tendrils have Te ee been variously described as modified Ampelopsis leaves, stipules, shoots, and even roots. vor ica It is possible, however, that in these (REDUCED.) cases the tendrils are not modified forms of other organs at all, but are organs, suz generis (Latin, of their own kind)—that is, organs which have arisen and have been developed for the purpose which they serve, never having been anything else but what they are. Climbing plants begin their existence as deep-shade plants, the shade being produced by the vegetation upon which they climb. In the Tropics most are inhabitants of forests ; in this country they are found in thickets and hedges. Here the partial light produces effects which are ordinarily associated with plants grown in darkness. Such plants have a long weedy appearance, with weak 120 BRITISH PLANTS trailing stems and small leaves. The long internodes are the direct result of weak light ; the small leaves are due to malnutrition, resulting from the impoverishment of the chlorophyll through lack of light. In extreme cases the chlorophyll becomes yellow or etiolated, and ceases to make starch altogether ; the plant then starves to death. 3 <4 J Fic. 38.—Everuastinc PEA witH Lerasr-TENDRILS AND WiIncED STEM. (REDUCED.) The evolution of the climbing habit may be traced in several of our common British plants. The woody night- shade shows how an ordinary plant may become a twiner. The common fumitory (Fumaria officinalis), and its near ° relation, the climbing corydal (Corydalis claviculata), give us an idea of one method by which the tendril may be evolved. These two plants are occasional climbers, with large, much-dissected leaves. The leaflets are small, and the whole leaf appears to be more or less sensitive, so EPIPHYTES 121 that the main leaf-axis and its branches may all alike take part in attaching the plant to a support. There is no doubt that a similar general sensitiveness preceded, in evolution, that localized and specialized sensitiveness which, in other climbers, is confined to the tendrils. This sensitiveness, which accompanies partial etiolation in weak light, and which makes the evolution of the tendril possible, is probably due to the fact that etiolated stems and shoots, through their rapid growth, have soft tissues with no strong woody elements during their sensitive period. Sensitive movements can only be associated with organs whose tissues consist of thin-walled cells, turgid with water. There are not many native climbers in temperate climates. In tropical rain-forests, tree-growth is so luxuriant that the forest-floor is always plunged in partial gloom. Climbers with great twisted woody stems sprawl over the undergrowth, and loop themselves from tree to tree, climbing towards the light they need so much. Among the few woody climbers (lianes) we possess are the clematis, honeysuckle, and ivy. © II. Epiphytic Plants, perched on Trees. Epiphytes are plants which grow and pass their lives perched upon the elevated parts of other plants, chiefly the branches of trees. They are anchored to their sup- ports in various ways, usually by clasping roots. As a result of their position, they have a precarious water- supply, and invariably show xerophytic characters. The most common epiphytes are found among the lower plants, —lichens, liverworts, mosses, and ferns. The most abun- dant epiphyte everywhere is the lichen, which shows wonderful adaptations to extreme conditions. Generally, however, epiphytes are only found in moist shady places, and this is especially true of the flowering epiphyte, which is rare in the temperate regions of the world. In the Tropics, on the other hand, they form a feature of the vegetation of the dripping forests, the home of the epiphytic orchids and bromeliads. In this country we have no true flowering epiphyte. Accidentally, however, many plants are epiphytic. In humid mountain glens we may often see plants growing on other plants—ashes 122 BRITISH PLANTS on oaks, stonecrop on the aged branches of trees, and various ferns, especially the polypody. The true epi- phyte, however, is a xerophyte amid vegetation markedly hygrophytic. It has a special mode of seed-dispersal, for the seeds have to be lifted and deposited in places far above the ground-level. They are exceedingly small and light, as in orchids, or they are enclosed in fleshy and sticky fruits, which birds carry and drop among the branches of the trees. Climbing plants and epiphytes which use other plants for support are said to form guilds with them. Without their aid climbers would fail to develop. Epiphytes, on the other hand, can and do grow in soil, if there is no vegetation around to obscure the light. CHAPTER XIII CLASSIFICATION OF PLANTS ACCORDING TO THEIR MODE OF NUTRITION 1. Green plants. 2. Non-green plants— (a) Saprophytes, (6) Parasites. 3. Insectivorous plants. 4. Symbiotic plants. 1. Green Plants.—These plants are autotrophic, or self- nourishing. They are able, by means of the chlorophyll present in their green cells, to form, during sunlight, carbo- hydrate from carbonic acid gas. The nitrogen required for the synthesis of proteins is obtained, not from the air, but from mineral salts present in the soil. 2. Non-Green Plants.—These, not possessing chloro- phyll, are unable to make carbohydrate for themselves. They must therefore obtain it from sources outside their own bodies—i.e., they are heterotrophic. Not needing light for photosynthesis, they generally live in darkness. According to the nature of the source from which they obtain their food, non-green plants are divided into two groups—Saprophytes and Parasites. (a) Saprophytes (Gr. sapros, rotten).—These are colour- less—that is, non-green—plants which obtain their carbo- hydrate from the rotting products of vegetable or animal remains. Animal excrement, since it contains organic material exposed to decay, is a minor source. The majority of fungi are saprophytes ; not being able to make sugar for themselves, they get it from the humus in which they grow. Soil-bacteria are saprophytes, and so are all those bacteria which initiate putrefaction or bring 123 124 BRITISH PLANTS about destructive changes of any kind in organic matter. All plants, green or not, if supplied with sugar and the proper mineral salts, can build up proteins, so there is no reason why the proteins required by saprophytes for their nutrition should be obtained in the same way as the carbohydrates. Saprophytes, as a rule, make their own proteins, but most of the nitrogen required is ab- sorbed in the form of ammonium-compounds, in which humus is richer than in nitrates. At the same time, there is little doubt that proteins, if present, are also absorbed and utilized in the economy of the plant. (i.) Total Saprophytes.—There are very few total saprophytes among the higher flowering plants in this country, and a curious feature in them all is that they do not absorb the products of decay from the humus directly, but through the intervention of a mycorhiza, a sub- ject-fungus which inhabits the roots or stems, and replaces the root-hairs of the higher plant (Fig. 39). The British flowering saprophytes are: Neottia Nidus-avis, the bird’s-nest orchid, a leafless saprophyte, fre- quently occurring in the humus of moist woods, especially under old beeches and in hazel-copses. Fic. 39.—APEX oF A Monotropa Hypopitys, the yellow ee ee (Mag. bird’s-nest, a saprophyte with scaly NIVIED.) leaves found occasionally in the humus of fir, beech, and birch woods. Corallorhiza innata, the coral-root, a very rare sapro- phytic orchid found in boggy woods. It has a much- branched fleshy rhizome, but no roots (Fig. 40). Epipogum aphyllum, a yellow orchidaceous plant, possessing neither leaves nor roots. It has been found only once in this country. Saprophytes are all descended from green ancestors, but owing to their mode of life the presence of chlorophyll is rendered unnecessary, and it has disappeared ; the leaves, not being required for purposes of nutrition, have degenerated into scales or vanished altogether. The chief factor in their degeneration has, no doubt, been the SAPROPHYTES 125 mycorhiza, associated with their roots. The saprophyte at first made use of it merely to obtain water, but in the course of time it increased its demands, and at last came to rely upon it for all its food. Since light is unnecessary, saprophytes can escape competition by living in the densest shade of forests, where a green plant could not exist. (ii.) Partial Saprophytes.—Many green plants also possess a mycor- hiza, and indirectly, therefore, live on humus. This is the case with many of our forest-trees, which in- habit a soil rich in a somewhat acid humus—e.g., pine—and with plants living in peat—e.g., heath. The mycorhiza is at first external on the roots and functions merely as root-hairs, but in those cases where it penetrates deeply in the tissues, its value to its partner increases, and it passes on to it something more than water; in fact, condi- tions are being established which, in the course of time, may lead to the degeneration of the green plant into a colourless saprophyte. (0) Parasites (Gr. parasites, one who sups at another’s table).— These obtain the materials for their nutrition—carbohydrate and pro- : téin—from living hosts, animals or "0. 40. Gon fea a plants. Among the lower plants OrcurD). many fungi are parasitic, and some : ‘hat ate Eaensp hits are capable Steal of becoming so, if circumstances absence of roots. allow. Swarms of bacteria are parasites, and though some are harmless, or even beneficent, others are malignant and give rise to disease. These lower plants are usually internal parasites, in- habiting the tissues of animals or plants. Higher plants which are parasitic attach themselves extern- ally to the bodies of other plants, and by means of special sucking organs — haustoria — penetrating the tissues, absorb nutriment from them. The depend- 126 BRITISH PLANTS ence of the parasité upon the host may be partial or complete : (i.) Partial or Hemi-Parasites.—These possess chloro- phyll, and so can make carbohydrate for themselves. They are generally found attached to the roots of other plants, which they tap for water ; but, like other para- sites, they take all they can get, and though water is their chief need, food may be absorbed as well. The British partial parasites, with the exception of Thesiwm humi- fusum (bastard-toadflax), belong to the Natural Order Scrophulariacez, and include the following plants, which live attached to the roots of grasses by means of haus- toria: Huphrasia (eyebright), Rhinanthus (yellow rattle), Pedicularis (lousewort), Melampyrum (cow-wheat), and Barisia. All these plants live in grass-communities. Grass-roots form a turf so thick that no other shallow- rooted plant has a chance in competing with them for water. Daisies, dandelions, and other plants growing in meadows have long roots which penetrate the turf, and so they are not really in competition with the grasses. These hemi-parasites, however, are shallow-rooted. Being forced, then, to compete with the grass-roots, they solve the difficulty by constraining their rivals to their service. They fix their own roots upon theirs, and tap them for water. But that they take something more than water is proved by the fact that, in a field where yellow rattle is abundant, the grass is poor and sickly, and much of it dies before the season when it should be mown for hay. (ii.) Total Parasites—These are devoid of chlorophyll, and depend entirely upon their hosts for food. In the British flora the following are total parasites among the flowering plants : Lathrea (toothwort), Orobanche (broom- rape), Cuscuta (dodder). The first two belong to the Orobanchacee, a Natural Order- which differs very little from the Scrophulariacez, in which most of the partial parasites are included. (1) Lathrea TurovcHouT the country the term ‘“ moor” is applied to almost any stretch of treeless, barren land. It may be the wet peat-bogs of the uplands, the heather-clad regions of hill-slopes, or the dry heaths of the lowlands. The word, although used in apparently so wide a sense, is, nevertheless, a good one, for all these areas have some- thing in common. They all occur on soil poor in nutri- tive salts, and contain more or less humous acids. Peat is present in all cases, varying in thickness from a few inches on the dry grass-heath to 30 feet or more on the wet cotton-grass bogs. It is the relative abundance of peat which determines the character of the vegetation of the moorland ; and the amount of peat in turn depends upon the character of the underlying rock, the altitude, and the rainfall. We saw in Chapter X. that vegetable remains are decom- posed by bacteria, and, if the conditions are unfavourable for their growth, peat will accumulate. The following regions are those unsuited for the work of bacteria, and it is in these places that peat is formed and moorland- associations develop : 1. In the lowlands where drainage is very bad, and where the water becomes stagnant. This means a lack of oxygen in the soil, whilst the evaporation from the surface keeps it cold. 2. At high altitudes where the temperature is low for the greater part of the year. If this is combined with a very heavy rainfall, bad drainage, and the accumula- tion of stagnant water, the conditions for peat-formation are intensified. It is under these circumstances that the cotton-grass bog reaches its greatest development. 248 MOORLAND ASSOCIATIONS 249 3. At any altitude where lime is quite absent from the soil, as on dry, sandy, or pebbly heaths, where any lime at one time present has been washed into the subsoil. Peat seldom accumulates directly on limestone or chalk, for here the conditions are all in favour of bacterial action. Plenty of oxygen and lime is present, and the soil is warm, except at very high altitudes. When peat does form in a limestone district, it can usually be traced to the presence of a capping of some other material on the limestone. The great bogs of Ireland, for example, lie chiefly on limestone covered with boulder-clay. The plant-remains in moorland-peat, in contrast to those in marshy peat, are well preserved. A section through a thick deposit will often show a well-marked stratification of the material from which the peat is formed. At one level Sphagnum may predominate, at another cotton-grass, whilst remains of birch and pine are frequent. From these different layers we can gain, not only an idea of the former vegetation, but also of the changes in climate which the district has undergone. Humous acids are formed in all peaty soils, and, if present in large quantities, the absorption of water is rendered difficult, and the vegetation is of a pronounced xerophytic type, as on damp heather-moors and cotton- grass bogs. Here the dominant plants are either heath- like plants with small evergreen rolled leaves (Figs. 9, 12), or grass-like plants with erect cylindrical leaves. Mineral food is scarce, especially the nitrates and phosphates of lime, potash, and magnesia, and plants which can obtain food from other sources than the mineral substances in the soil have a big advantage over their neighbours. Carniv- orous plants, for example, which obtain a large pro- portion of their food from the bodies of insects, are common in the wetter parts ; whilst others—e.g., all the heaths, cranberries, and some grasses—obtain organic food through the agency of fungi (mycorhiza) which become attached to their roots. The various associations of moorland-plants may be summarized as follows : 1. On dry, poor soils, drainage good : (a) Grass-Heath, peat thin. (b) Calluna-Heath, peat thicker. (c) Vaceinium-Moor, alpine. 250 BRITISH PLANTS 2. On moist soils : (d) Heather-Moor, peat deep. 3. On wet soils : (e) Molinia cerulea-Bog, a modification of the grass-heath. (f) Erica Tetralix- Moor, a modification of the heather-moor. (g) Myrica Gale-Bog, intermediate between the marsh and the heather-moor. (h) Eriophorum-Moor (cotton-grass moor), on flat summits where rainfall is high. (*) Sphagnum-Bog, on constantly wet rocks, etc., where water is not very rich in humous acids. 1. Grass-Heath. On well-drained soils possessing a thin covering of peaty material a grass-community becomes established. This grass-heath is similar in many respects to the natural pasture, but is at once distinguished by the presence of Calluna vulgaris (heather) and other heath-plants. It occurs on the steeper slopes of mountains, and occa- sionally on gravelly soil in the lowlands. Where the slope of the ground is more gradual, peat accumulates to a greater extent, and Calluna competes successfully with the grasses, with the result that an association intermediate between the grass-heath and the Calluna- heath is formed. A similar intermediate association may develop through the artificial drainage of a heather- moor. The grass-heath in some cases arises as a sub- stituted association when a Calluna-heath is used as grazing-land. The heather is destroyed to a greater or less extent, and grasses take its place ; but it soon reverts to its original condition if cattle are kept off the land. Festuca-Agrostis-Anthoxanthum Association—On the richer soils the dominant grasses are those of the natural pasture: Festuca ovina, Agrostis vulgaris, Anthoxanthum odoratum, and Aira flexuosa. Calluna is always very abundant, but in exposed places is usually dwarf. Ulex europeus (gorse) is also occasionally very abundant. Of typical heath-plants associated with the foregoing, Vac- cinium Myrtillus (bilberry, whortleberry), Hrica cinerea MOORLAND ASSOCIATIONS 251 (usually dwarf), Salix repens, Polygala serpyllacea (heath- milkwort), Pedicularis sylvatica (lousewort), and Luzula campestris are frequently found. Other characteristic plants are: Potentilla Tormentilla, Galium saxatile, Pteris aquilina, Thymus Serpyllum, Achillea Millefolium, Hier- acium Pilosella, Campanula rotundifolia, Veronica offi- cinalis, Huphrasia officinalis, Teucrium Scorodonia, Rumex Acetosella, and various species of Rubus. Nardus stricta-Association.—On poorer soils, especially in the sub-alpine zone (1,000 to 2,000 feet), Nardus stricta (matgrass), which is found sparingly at lower levels, becomes dominant. Festuca ovina, Aira flexuosa, and Agrostis vulgaris also occur, with Molinia cerulea in the wetter parts. Cailuna is usually less frequent than in the preceding association, but Vaccinium Myrtillus is more abundant. Potentilla Tormentilla, Galiwm saxatile, Rumea Acetosella, and Luzula campestris are still very common, and mixed with these are plants characteristic of high altitudes—e.g., Juncus squarrosus, Lycopodium Selago, and L. clavatum. Molinia-Bog.—Where the drainage is very bad and the soil wet, Nardus stricta is displaced by Molinia cerulea. Associated with the latter are many bog-plants—e.g., Juncus effusus, J. conglomeratus, Eriophorum vaginatum (cotton-grass), Aira cespitosa, Narthecium ossifragum, Hydrocotyle vulgaris, Vaccinium Oxycoccus, Erica Tetralix, Rhynchospora alba, Carex (C. Goodenowit and other species), Sphagnum, etc. Above 2,000 feet the Molinia-bog passes over into cotton-grass moor. Summit-Heath.—At high altitudes (1,500 to 2,500 feet) a summit-heath, intermediate in character between the alpine-pasture and the Vaccinium-moor, is developed. It consists chiefly of Nardus stricta and Juncus squarrosus, with Vaccinium Myrtillus sometimes sharing dominance. A few plants found at lower levels still persist—e.g., Aira flexuosa, Potentilla Tormentilla, Galium saxatile, Huphrasia officinalis, and Luzula campestris—but alpine plants are frequent—e.g., Alchemilla alpina, Lycopodium Selago, L. clavatum, Empetrum nigrum (crowberry), Vaccinium Vitis-idea (cowberry), Antennaria dioica, and in the wetter parts Rubus Chamemorus, Vaccinium uliginosum (mountain blaeberry), and Hriophorum. 252 BRITISH PLANTS 2. Calluna-Heath. This association occurs in similar situations to the grass-heath, but on poorer soil, covered with a deeper layer of peat. The peat is composed of mosses charac- teristic of dry soils—e.g., Polytrichum and Dicranum. Calluna vulgaris is almost exclusively dominant, with occasional areas in which Hrica cinerea or Ulex europeus is most abundant. Grasses are common—eg., Festuca ovina, Aira flexuosa, and Agrostis vulgaris. This abun- dance of grasses distinguishes the Calluna-heath from the heather-moor. The remaining vegetation is that typical of dry soils, and includes most of the grass-heath plants. Rare plants found include Erica ciliaris and EH. vagans, the latter confined to Cornwall, the former almost so, and Dabecia polifolia (St. Dabeoc’s-heath), found only in Connemara (see p. 213). Although most common on siliceous soils, this associa- tion is not confined to them, being found occasionally on limestone—e.g., Mendip Hills and Peak District. In such cases many plants of the limestone-pasture occur, in addition to the ordinary forms. 3. Heather-Moor. The heather-moor occurs where the peat is deeper (4 to 5 feet) and wetter than that of the Calluna-heath, and is found chiefly on the more gentle slopes of moun- tain-sides. The peat is made up almost entirely of Sphagnum ; in fact, the heather-moor sometimes arises on top of a Sphagnum-bog. In other cases it is formed by the drainage of a cotton-grass moor. The dominant plant, as a rule, is Calluna vulgaris, but in the wettest parts it is displaced by Erica Tetralix. The heather grows more luxuriantly here than on the Calluna-heath, and sometimes reaches a height of several feet. The thick dense growth forms excellent covert, and most of the “shooting moors ”’ belong to this asso- ciation. The associates of the heather are the same as on the Calluna-heath, but they are much less abundant. Erica cinerea, however, is usually very common, and moisture-loving plants are frequent. MOORLAND ASSOCIATIONS 253 Erica Tetralix-Association.—In the wettest parts Erica Tetraliz is dominant, and forms a distinct association within the heather-moor, characterized by the presence of a number of bog-plants—e.g., Drosera rotundifolia, Pinguicula vulgaris, Andromeda polifolia, Narthecium ossifragui.r, Rubus Chamemorus, Vaccinium Oxycoccus, Myrica Gale (sweet gale), Salix repens, Sphagnum, Molinia cerulea, and various species of Juncus, Carex, and Hriophorum. Rare plants occurring in this associa- tion are: Hrica ciliaris, more common here than on the dry Calluna-heath, and Hrica Mackati and EH. mediter- ranea, confined to a few districts in western Ireland (see p. 213). In many parts the Hrica Tetraliz-association merges gradually into the Sphagnum-bog or Eriophorum-moor. 4. Myrica-Bog. In parts of the wet heather-moor, where water is very abundant, and sweeter than elsewhere, Myrica becomes dominant. A similar association is met with in low- lying districts where drainage is bad, and on the wet margins of the highland type of lake. This association is intermediate between the wet heather-moor end the marsh, many of the plants of the latter being present—e.g., Ranunculus Flammula, Viola palustris, and Anagallis tenella. Very abundant plants are: Molinia cerulea and Erica Tetraliz. Common or frequent plants, in addition to the above marsh-plants, are: Drosera, Pinguicula, Narthe- cium, Juncus species. Eriophorum vaginatum, Carex species, and Sphagnum. Calluna vulgaris occurs, but is not common. 5. Sphagnum-Bog. Sphagnum is a peculiar moss with a big system of water-storing cells in its leaves, so that the plant holds water like a sponge. The stem grows continuously at the apex; the lower part dies away, and gradually be- comes converted into peat. The stem is clothed with living green leaves for about 6 to 8 inches below the tip, and as it is choked with water inside, it cannot 254 BRITISH PLANTS live completely submerged, on account of the diffi- culty of obtaining sufficient air. The plant can only grow in quantity when the rainfall is heavy and the atmosphere very humid. Sphagnum-bogs develop com- monly on wet, sloping rocks in mountainous regions. Here plenty of water is available at all times, but the slope of the rock allows the surplus water to drain away. When the plant is growing luxuriantly, it forms a deep, loose covering in which other plants find a difficulty in taking root, and, further, only those plants which can keep pace with the upward growth of the moss can exist. All other plants will be smothered. The most frequent plants are those with long straggling stems, rooted in the firmer peat below—e.g., Vaccinium Oxycoccus, Rubus Chamemorus, Empetrum nigrum, and monocotyledons with long narrow leaves, which can grow up between the moss—e.g., Narthecium, Eriophorum, sedges, and rushes. Erica Tetralix occurs occasionally ; also Drosera rotundi- folia, Pinguicula, and small creeping plants—e.g., Ana- gallis tenella, Campanula hederacea, and Selaginella selaginoides. 6. Eriophorum-Moor. On the flat summits of moorland (1,250 to 2,000 feet), especially in the Pennines, vast areas are covered with an association of cotton-grass, locally known as moss- moors. The peat is always thick, sometimes reaching a depth of 30 feet, and saturated with the sourest water. The development of this association over large areas is dependent on very heavy and continuous rainfall—at least 40 inches per annum, since the only water which reaches it comes from above. At similar altitudes where the rainfall is less a heather-moor develops. The Eriophorum- moor is also found at lower levels where water is abundant —eg., the levels of Somerset. A very constant feature of these moss-moors is their shape. They are higher in the centre than at their edges, so that the general appearance is that of an inverted saucer. Bare peat : very common where water has denuded the sur- ace. Eriophorum vaginatum is usually dominant, sometimes MOORLAND ASSOCIATIONS 255 E. angustifolium, and, in places, Empetrum nigrum. Rubus Chamemorus and Vaccinium Myrtillus are abun- dant. Occasional plants are: Erica Tetraliz, Calluna vulgaris, Drosera rotundifolia, Narthecium, Lycopodium Selago, Selaginella selaginoides, Carex canescens, and Scir- pus cespitosus. Sphagnum is quite subordinate. On the drier edges Calluna and Erica Tetralix become more common, and on the slopes the moor often passes over into a heather-moor. 7. Vaccinium-Moor. Associations dominated by Vaccinium Myrtillus (bil- berry) are in most cases essentially alpine (2,000 to 3,000 feet), but in Yorkshire the Vaccinium-moor is not deter- mined by altitude alone; it invariably forms the sky- limit of the moor where it is exposed to all weathers. The peat is much shallower than in the Eriophorum- moor. Two types of Vaccinium-moor may be distinguished— one a dry form, a climatic variety of the grass-heath or alpine pasture, and the other wetter, a climatic variety of the heather-moor, or Eriophorum-moor. In either case alpine plants are frequent. The dry type occurs on shallow peat liable to drought, and includes a large number of grasses—e.g., Festuca ovina, Aira flexuosa, Nardus stricta, etc., and other plants of the grass- heath or alpine pasture. Alpine plants are: Alche- milla alpina, Empetrum nigrum, Phleum alpinum, club- mosses, etc. * In the climatic variety of the heather-moor, Calluna is abundant, and in that of the Eriophorum-moor, the cotton-grass. Other plants found are: Erica Tetralix, Rubus Chamemorus, Arctostaphylos Uva-ursi (bearberry) ; Betula nana (dwarf birch), and Azalea procumbens, con- fined to Scottish highlands; Trientalis europea and Tofieldia palustris (Scottish asphodel), confined to North Britain. Cornus suecica (dwarf dogwood) is one of the most constant associates of Vaccinium in the alpine zone of North Britain. , The relation between the various moorland associations, or groups of associations, may be expressed in the follow- 256 BRITISH PLANTS ing way. Variations due chiefly to altitude are indicated vertically ; to moisture, horizontally; the wet types to the right, the dry to the left. Vaccinium-moor. ne Summit-heath. Oe moor. 2 bog. Heather-moor——Erica . Ss 3 = | 3 Calluna-heath. 8 g use % | Grass-heath on ie is: S poor soil = aa aces a a or sedge-bog Myrica-bog. Grass-heath on (Juncus, Carex, “a a : rich soil (Festuca-Agrostis association). Ce“ ~ ~ Ss —+ Increasing moisture. CHAPTER XXVI GRASSLAND ASSOCIATIONS 1. Natural Pasture. On the more gentle slopes of hills and mountains the drainage is good, and the soil consequently dry. If an abundance of plant-food is available in the soil, grasses become established, and produce areas of natural pasture. Grassland of this kind is found at all altitudes from sea- level almost to the summits of the highest mountains, more especially where the rocks are igneous or calcareous. Sandstone hills rarely support a natural pasture, for plant-food is scarce, and the slopes become covered. with grass-heath or calluna-heath (pp. 250, 252). The vegetation has to rely for its water-supply almost entirely on atmospheric precipitations—rain or dew. Periods of drought are therefore frequent, whilst strong drying winds prevail for a large part of the year. These factors tend to favour the development of a xerophytic type of vegetation, which becomes more pronounced. at high altitudes. ‘The natural pasture forms the great sheep-runs of all hilly districts, and the continual grazing has a certain influence on the vegetation, many of the taller plants being cut down to a few inches, and prevented from producing flowers and seed. The turf is usually dense and compact, formed of the fibrous shallow roots of the grasses, which monopolize the water and food in the upper layers of the soil. The only herbs which can obtain a footing are perennial plants, whose long roots can explore the soil beneath the turf, or hemi-parasites (p. 126), which become fixed to the roots of the grasses and absorb much of their food from them—e.g., Huphrasia officinalis ieyebnen® and 257 1 258 BRITISH PLANTS Thesium humifusum (bastard toadflax). On very poor soils, or exceptionally dry ones, the turf is less dense, and a few annuals find room to grow—e.g., Linum catharticum (purging flax) and Arenaria serpyllifolia (thyme-leaved sandwort) on the dry chalk-downs. Most of the peren- nial herbs are rosette-plants—e.g., Hieraciwm Pulosella and Rumex Acetosella—or have prostrate stems—e.g., Galium saxatile and wild thyme. Not only are these plants protected from drying winds by placing their leaves close to the ground, but the growth of other plants in their immediate neighbourhood is to a large extent prevented. Mole-heaps, earth thrown out from rabbit-burrows, and other areas of disturbed ground, pos- sess quite a different flora from the surrounding pasture, weeds of cultivation, both annual and perennial, being common. The constitution of the flora of a natural pasture varies with the chemical nature of the soil and the altitude. On the very rich soils formed by the disintegration of igneous and metamorphic rocks the dominant grasses are much the same at all levels, but in the alpine regions the commonest plants associated with them are alpines not found in the lower regions. Lowland and sub-alpine pastures on rich soil occur up to an altitude of about 2,000 fect, but their vertical range depends on the amount of water present in the soil ; on very dry soils they do not extend so high. The dominant grasses are: Anthoxanthum odoratum (sweet vernal-grass), Agrostis vulgaris (fine bent-grass), Festuca ovina (sheep’s-fescue), and Aira flexuosa (waved hair- grass). The latter two grasses are well adapted for dry conditions, their leaves being rolled inwards, so that the lower surface, bearing stomata, are protected from the wind (p. 43). Nardus stricta, found occasionally here, but more commonly at higher levels, has similar rolled leaves. Other grasses frequently found are: Poa pra- tensis (smooth meadow-grass) and Triodia decumbens (heath-grass). Of rosette-plants, the commonest are: Hieracitum Pilosella (mouse-ear hawkweed), Achillea Millefolium (yarrow, milfoil), Rumex Acetosella (sheep’s- sorrel), Carlina vulgaris (carline- thistle), and Luzula campestris (field wood-rush). In many plants the leaves close to the ground are large, those on the erect stems GRASSLAND: ASSOCIATIONS 259 small or narrow, so that the plants approach very closely to the rosette-plants in habit—eg., Scabiosa Succisa (devil’s-bit scabious), Campanula rotundifolia (hairbell), and Pimpinella Saxifraga (small burnet-saxifrage). The most abundant creeping plants are: J'rifolium repens (Dutch clover), 7. dubiwm (small yellow trefoil), Lotus corniculatus (bird’s-foot trefoil), Galium saxatile (heath- bedstraw), G. verum (lady’s-bedstraw), and Thymus Serpyllum (wild thyme). Where the soil is calcareous, lime-loving plants are common—e.g., Linum catharticum, Polygala vulgaris (milkwort), Viola hirta (hairy dog- violet), and Helianthemum Chamecistus (rock-rose). On dry soils heath-plants may be abundant—eg., Ulex europeus (gorse, furze) and Pieris aquilina (bracken), together with scattered dwarfed plants of Vaccinium Myrtillus (whortleberry) and Calluna vulgaris (heather, ling). When these plants are abundant, the natural pasture passes over into a grass-heath. Alpine Pasture.—At an altitude of about 2,000 feet and over, a natural pasture may occur on rich soils. The dominant grasses are those found at lower levels, but Nardus stricta is more abundant, and may share dominance with the others, and the viviparous variety of Festuca ovina is common. But, above all, the alpine pasture is distinguished by the presence of alpine plants, of which Alchemilla alpina (alpine lady’s-mantle) is a constant and abundant representative. Most of the herbs occurring at lower levels are found here, and, in addition, the following alpines: Poa alpina, Potentilla Crantzii (alpestris, alpine cinquefoil), Cerastium alpinum (alpine mouse-ear chickweed), Lycopodium Selago, L. alpina, L. clavatum (club-mosses), and—confined to Scot- land or North Britain — Sagina Linnei (mountain- spurrey), Ozxytropis uralensis, Gentiana nivalis, and Kobresia caricina. Limestone-Pasture and Chalk-Downs.—The soil formed on a hill composed of limestone or chalk is invariably dry, for the rock allows the water to drain away very rapidly—the chalk to a much greater extent than the limestone. The rock itself is very soluble in water con- taining carbonic acid gas, and the soil-water consequently contains a large percentage of bicarbonate of lime. Many plants prefer a dry soil, and others a chalky one 260 BRITISH PLANTS (see p. 89), and so we find the vegetation of limestone and chalk hills is made up of a mixture of these two types. The herbage on the windward side of the hill is short, the plants seldom rising more than an inch or two above the ground, but in the sheltered parts the plants grow more luxuriantly, and it is here that most of the calciphilous plants mentioned below are found. The dominant grasses are Festuca ovina, NSesleria cerulea (in northern counties), Keleria cristata, and Poa pratensis ; common or frequent grasses are Agrostis vulgaris, Cynosurus cristatus (crested dog’s-tail), Brachy- podium pinnatum, and Briza media (quaking-grass). Azra flezuosa and Nardus stricta, so common on all other dry soils, are never found where lime is present—that is, they are typical calcifuges (see p. 89). The herbs found on the limestone-pasture and chalk-down include most of those occurring in the pasture on rich soil (p. 258). Other plants, associated with a dry soil, commonly found are: Potentilla Anserina (silver- weed), Veronica officinalis (common speedwell), Huphrasia officinalis, Carduus acaulis (stemless thistle), and Daucus Carota (carrot). The calci- philous plants may be grouped according to their demand for lime as follows (those marked with an asterisk are more or less rare plants found only on chalk-downs in this country, although on the Continent, where they are more abundant, they may occur on other soils as well): 1. Confined to calcareous soil : Hippocrepis comosa (horse-shoe vetch), Aceras anthro- pophora (green man-orchid), and Polygala calcarea (chalk- milkwort, found only on the chalk-downs of South England). 2. Almost exclusively found on calcareous soil : Anthyllis Vulneraria (lady’s-fingers), Ophrys muscifera (fly-orchid, growing usually at the top of downs under the shade of bushes), Campanula glomerata (clustered bell- flower), and Buxus sempervirens (box-tree). 3. Prefer calcareous soil : *A nemone Pulsatilla (pasque-flower), Helianthemum Cha- meecistus (rock-rose), Polygala vulgaris (milkwort), Linum catharticum (purging flax), L. usitatissimum (common flax, linseed), L. perenne (perennial flax), Poterium Sangutsorba (lesser burnet), Viola hirta (hairy dog-violet), Asperula cynanchica (squinancy-wort), Centaurea Calcitrapa (star- GRASSLAND ASSOCIATIONS 261 thistle), Pécris hieractoides (hawkweed oxtongue), Sca- biosa Columbaria (small scabious), Carduus acaulis (stem- less thistle), *Phytewma orbiculare (round-leaved rampion), Chlora werfoliata (yellow-wort), Erythrea Centaurium (centaury), Verbascum Thapsus and V. nigrum (mulleins), Inula Conyza (ploughman’s-spikenard), Reseda lutea (wild mignonette), R. Luteola (dyer’s-weed), Salvia Verbenaca (clary), Ajuga Chamepitys (ground-pine), Plantago media (hoary plantain), *Thesium humifusum (bastard toad- flax), Spirea Filipendula (dropwort), Spiranthes autum- nalis (lady’s-tresses), *Orchis hircina (lizard-orchid, ex- tremely rare, found only in Kent), O. pyramidalis (pyra- midal orchid), O. ustulata (dwarf orchid), Ophrys apifera (bee-orchid), *O. Arachnites (late spider-orchid, very rare, in Kent only), O. aranifera (spider-orchid), Habenaria conopsea (fragrant orchid), Carex flacca, Juniperus com- munis (common juniper). The following table exhibits the relationships of the natural pasture : Vaccinium moor. Alpine pasture. Sub-alpine and low- Calluna-heath. land natural pasture. Geass eae: | Farmland. 2. Artificial Pasture and Meadow. In the region of deciduous trees, large areas of grassland exist in this country, on soil very rich in plant-food and with a high water-content. Most of this grassland is re- claimed woodland, and if allowed to fall out of cultivation would return to its natural condition, and once again become clothed with forest. In other cases marshes or bogs have been drained and converted into good pasture- land or meadow. Artificial grassland may be divided into two groups, according to the agricultural practices to which the land is subjected. Meadow-land is essentially hay-producing, one or two crops being removed annually, and only after the last crop has been cut are cattle allowed on the land. 262 BRITISH PLANTS Pasture, on the other hand, is used only for grazing (see p. 20). The continual removal of hay from the meadow tends to impoverish the soil, and fresh supplies of plant-food must be added in the form of manure, if profitable crops of hay are to be obtained. The manuring of the ground has a big influence on the growth of all the plants, both grasses and weeds, but in a different way. The grasses grow more luxuriantly, and form such a dense shade that many of the weeds are killed outright, whilst others are so injured by the lack of light that the seed produced is poor in quality and small in quantity, and in time these, too, disappear. At the experimental station at Rotham- sted plots of meadow-land have received the same treat- ment for over fifty years, and the influence of manuring is strikingly shown by the relative proportion of grasses and weeds in the hay obtained from each plot. On un- manured ground less than half the hay was graminaceous herbage, over 39 per cent. useless weeds, and the remainder leguminous herbage—e.g., clover and bird’s-foot trefoil, which has a fairly high dietetic value. On well-manured ground over 99 per cent. of the hay consisted of grasses and less than 1 per cent. of weeds. The age of the meadow also is an important factor in determining the abundance and character of the weeds. In temporary meadows, laid down to grass for one year only, a large proportion of the weeds are annuals—relics of the previous year’s cultivation; but in permanent meadows they gradually disappear as time goes on, and in old meadows they are as rare as in the natural pasture. The grasses required to produce good hay must be quick-growing, and this can only be when an abundance of water is present in the soil, and the climate is warm and humid. Meadows are consequently chiefly found in valleys and on low-lying ground. On hill-slopes the con- ditions are unsuited to the rapid growth of the herbage, and the land is laid down as pasture. This artificial pasture forms much better grazing-ground than the natural pasture, for the land is specially prepared, and the grasses are more luxuriant and selected for the purpose in view. The artificial pasture is usually kept for milch- cows, the natural pasture for sheep. & The most common grasses selected for laying down GRASSLAND ASSOCIATIONS 263 grassland are: Lolium perenne (perennial rye-grass), Anthoxanthum odoratum (sweet vernal-grass), Dactylis glomerata (cock’s-foot), Poa pratensis and P. trivialis (smooth and rough meadow-grasses), Alopecurus pratensis (fox-tail), Phlewm pratense (Timothy-grass), Agrostis alba (white bent-grass), Holcus lanatus (Yorkshire-fog), Cyno- surus cristatus (crested dog’s-tail), Festuca pratensis (meadow-fescue), and F'. ovina (sheep’s-fescue). With these grasses are mixed intentionally certain leguminous plants—e.g., clovers—and on dry pastures these form a large percentage of the seed sown. Weeds of Meadow-Land.—The character of the weeds in the meadow depends largely on the chemical and physical nature of the soil. The plants are tall-growing and, protected by the surrounding grasses, are mesophytic, with large thin leaves. Transpiration is often difficult, owing to the humid atmosphere, and many of the plants possess hydathodes (p. 27), by means of which the water is got rid of in a liquid form. Sawifraga granulata ex- hibits such a character in a pronounced degree; the water which is excreted contains a large amount of calcium carbonate, which is left behind on the leaf as a white chalky deposit when the water evaporates. They are for this reason often called chalk-glands. As in the natural pasture, competition is very keen, and all the weeds are perennial or hemi-parasites—e.g., Rhinanthus and Bartsia. On damp, heavy soil the commonest weeds are: Rumex Acetosa (sorrel), R. crispus (dock), Ranunculus repens (creeping buttercup), R. bulbosus (bulbous buttercup), Plantago lanceolata (ribwort-plantain), Carduus palustris (marsh-thistle), Senecio vulgaris (groundsel), S. Jacobea (ragwort), Orchis mascula (early purple orchid), Traga- pogon pratensis (goat’s-beard), Prunella vulgaris (self-heal), Lapsana communis (nipplewort), Lathyrus pratensis (yellow meadow-vetchling), Cardamine pratensis (cuckoo-flower), Geranium pratense (meadow crane’s-bill), Spiraea Ulmaria (meadow-sweet), Sazifraga granulata (meadow-saxifrage) ; and the rare Crocus vernus, Fritillaria Meleagris (snake’s- head), and Colchicum autumnale (autumn-crocus, meadow- saffron). Rhinanthus Crista-galli (yellow rattle) is found in almost all meadows on clay land. : In low-lying meadows near rivers the water-level is 264 BRITISH PLANTS often above the surface for part of the year, and many marsh-plants occur. On drier land, as in chalk-districts, the characteristic weeds are: Chrysanthemum Leucanthemum (ox-eye daisy), Lotus corniculatus (bird’s-foot trefoil), Silene Cucubalus (bladder-campion), Plantago media (hoary plantain), Anthyllis Vulneraria (lady’s-fingers), Onobrychis sativa (sainfoin), Poterium Sanguisorba (lesser burnet), and Scabiosa Columbaria (small scabious). All of these are found on the damper soil, but more sparingly than on the chalk. Many orchids also occur—e.g., Habenaria conop- sea, Orchis pyramidalis, O. maculata, Aceras anthropo- phora, Ophrys muscifera, and Listera ovata (twayblade). Weeds of the Pasture.—The weeds of the artificial pasture vary only to a slight extent from those of the natural pasture, but they are usually less numerous. Rosette-plants and forms with prostrate stems are com- monest—in one pasture only nine out of the forty-six species present had tall, erect stems. Of these the thistles (Carduus arvensis, C. lanceolata, and C. palustris) escape destruction by grazing animals owing to the spiny nature of their leaves, whilst buttercups (Ranunculus acris, R. bulbosus) possess a very acrid juice which warns off the cattle. A number of weeds are very common in the damp artificial pastures, but absent or uncommon in the drier natural pastures—e.g., daisy, dandelion, hawkweeds (Hieracium), hawkbits (Leontodon), hawkbeards (Crepis), cat’s-ear (Hypocheris radicata), and cowslip (Primula verts). CHAPTER XXVII WOODLANDS In these islands the rainfall is everywhere sufficient to maintain woods below a certain altitude (varying from 1,500 to 2,000 feet in different parts of the country). Only on moors and in bogs, where the cold, sour, unaerated peat is unfavourable to tree-growth, are woods impossible. Yet even here on the borders where the moor merges into the wood, one is ever advancing upon the other. It seems certain that many of our upland moors were onco wooded, but as the peat accumulated the woods were gradually submerged by the moor; in fact, remains of birch and pine have been discovered in peat far above the present limits of tree-growth (see p. 249). In other places where the moor becomes drier and air permeates the soil, trees gain upon the moor, following in the track of the invading ling and heath. Woods are of two kinds—(a) deciduous, (6) evergreen. Evergreen trees are xerophytic, their leaves being retained during the winter, but evergreen woods are very poorly represented in our flora, and these have always been planted, except some of the pine-woods of Scotland. DECIDUOUS WOODS. Deciduous trees shed their leaves at the commencement of the cold season, and are leafless xerophytes in winter (see p. 60). The soil is generally rich in humus, well aerated, and well supplied with earthworms, soil-bacteria, and fungi. As soon as the humus so accumulates that it becomes sour, the earthworms and soil-bacteria disappear, and the wood is in danger of perishing and giving way to moor or heath. Our native forest-trees are few in number, and of these very few indeed are ever dominant (oak, beech, ash, birch). 265 266 BRITISH PLAN1IS The undergrowth present depends upon : 1. The amount of light coming from above. Some trees intercept much light and cast a deep shade— e.g., beech and sycamore—and under these the vegetation is scanty or absent ; others allow a considerable amount of light to penetrate through their loose crowns—e.g., oak, ash, birch—and this supports a rich and varied flora, both bushes and herbs. This is well seen in the accom- panying maps (Figs. 111, 112), taken from Woodhead’s paper on The Ecology of Woodland Plants, which show clearly that the distribution of the bracken is determined solely by the amount of light present, the soil varying in different parts of the wood from a stiff clay to a light sand. Fig. 111.—Map or a Woop, sHowiInG DIsTRIBUTION OF TREES. (AFTER WoopHEAD.) shade-trees, Acer, Ulmus, Fagus. = Ci Ulmus montana. AA Acer Pseudoplatanus. § & conifers. I oak dominant. OO Fagus sylvatica. 2. The nature of the soil, the presence of mild or sour humus, and the amount of water present. The soil-factor usually determines the kind of plant which is found, and the light-factor decides its abundance. The herbaceous or ground-vegetation in woods consists chiefly of shade-plants, their stems being tall or elongated, and their leaves often large. They are nearly all peren- nials, hibernating by rhizomes, tubers, or bulbs ; annuals are rare. In the humus of moist, shady woods a few colourless saprophytes grow (Neottia, Monotropa), but epi- WOODLANDS 267 phytes, apart from a few ferns (e.g., Polypodiwm vulgare), mosses, liverworts, and lichens, are mostly accidental. Lianes, or woody climbers, which form such a character- istic feature of tropical forests, are merely represented in our woods by the honeysuckle, ivy, and clematis. Many of our forest- trees, living in soil rich in humus, are partial saprophytes, their roots being clothed with a mycorhizal fungus instead of root-hairs (see p. 124). Natural and Artificial Woods. These islands were once far more extensively wooded than they are now, although it is still a well-wooded country. Most of the huge forests which once covered Fig. 112.—Tue Same Woop as In Fic. 111, spowrne Distrisution oF Bracken. (ArrER WOODHEAD.) hy Pteris aquilina (bracken). the land have been cut down for economic purposes, or cleared to make way for cultivation and pasture. Much of what remains has been seriously interfered with and modified by man, by periodic cutting and replanting, and in many places new woods have been planted on arable land and pasture. Artificial woods are not always easy to distinguish from natural woods, especially when they are old-established, and felled portions have been re- planted with trees native to the soil and common to the district. Natural woods regenerate themselves by seed, and in them trees are seen in all stages of development ; 268 BRITISH PLANTS seedlings, saplings, and great aged individuals are all mixed together, competing freely with one another for room and light. In plantations it is different ; many, if not all, of the trees are of the same age; aliens are common —e.g., chestnuts, sycamores, elms, and firs—and trees not common to the district or soil—e.g., pine in most parts of the country, and the beech in the west ; and if the planta- tion is recent the ground-vegetation is not typically wood- land at all. In the course of time, however, these out- of-place inhabitants of the undergrowth disappear, and true woodland forms take their place. Copses. Cutting or coppicing also modifies woods, and most of our woods are cut periodically, either gradually or en masse. In woods trees are generally felled one here and one there, and new saplings put in their place. In copses, on the other hand, the whole wood is periodically cut down, and then allowed to regenerate itself from the boles left in the ground. In the latter case the shade of the wood is banished, and with it most of the shade- loving woodland plants. Sun-loving forms come in from the adjacent pastures and heaths, and maintain their position until a new wood springs up from the ruins of the old, and increasing shade drives them out. From what we have said, it is clear that although natural woods are not common in this country, many artificial woods—-especially those which have been derived from ancient ones, or old plantations of native trees, and even coppiced woods before the trees are cut—stand very near natural woods, and give us a good idea of what the primitive woodland-vegetation of these islands was like. Thickets. In rough, uncultivated, and especially hilly places, and on the rocky sides of river-valleys, bushes and shrubs become more abundant than trees. In these places the soil is very shallow, and tree-growth is often dwarfed or stopped altogether. The most abundant bush in this country is the hazel, which in former times was extensively planted in woods for economic purposes, especially among oaks and ashes, which were periodically coppiced. WOODLANDS 269 The scar-woods of the limestone-dales of the Pennines are hazel-thickets, with ash as an occasional or frequent associate. Mountain-ash, hawthorn, holly, sloe, buck- thorn, dogwood, elder, wayfaring-tree, and privet also occur, the first three being the most common. Plenty of light penetrates to the soil, and the ground-vegetation is rich and varied. Many plants of the limestone-cliff are present (see p. 290), as well as shade-loving forms. The latter include Sanicula europea (wood-sanicle), Asperula odorata (sweet woodruft), Lathrea squamaria (toothwort), Mercurialis perennis (dog’s-mercury), Paris quadrifolia (herb-paris), Scilla nutans (wild-hyacinth), and several very rare plants—e.g., Polemonium ceruleum (Jacob’s- ladder), Cypripedium Calceolus (lady’s-slipper orchid, now almost extinct), Polygonatum officonale (Solomon’s-seal), and Convallaria majalis (lily-of-the-valley). The following classification of deciduous woods is that adopted by Moss, Rankin, and Tansley.* A. The Alder-Willow Series. Dominant trees: Alnus glutinosa, Salix cinerea, and S. Caprea. Found in very wet places, by marshes, streams, and in fens (see p. 243). B. Woods on Non-Caleareous Soils. _ I. Oak-Woods—(a) Lowland Type.—The damp oak- wood occurs on clays, loams, moist sands and gravels, and clay-with-flints (covering large parts of the chalk- downs of south-eastern England), up to about 600 feet. It is most common in valleys and on alluvial plains rich in humus. The trees are more luxuriant than on the up- lands, and they cast a deeper shade. The herbaceous ground-vegetation consists, therefore, either of shade-plants or early flowering perennials, which bloom before the trees are laden with foliage. Although the oak is domi- nant, many other trees are present, and in some places compete with the oak for dominance—e.g., ash, birch, hornbeam. The ash is typically a tree of the limestone, but it is common in damp lowlands as well, especially on * “The Woodlands of England,” New Phytologist, vol. ix., 1910, p. 113. 270 BRITISH PLANTS deep marly soils. The birch is also common in the lowlands, but in the uplands it beats all trees, and ulti- mately entirely supplants the oak, ash, and beech at the higher altitudes of tree-growth on all kinds of soils. From the great admixture of other trees competing with the oak for dominance, these woods are often called mixed deciduous woods. The hazel is the most common shrub of the undergrowth, and in woods periodically cut oak- hazel coppices are formed. Vegetation of the Damp Oak-Wood—1. Trees.—Quercus Robur (= pedunculata, with stalked acorns) is the dominant tree on deep soils, Q. sessiliflora (with sessile acorns) on shallow soils; ash, birch, beech, hornbeam; and of trees commonly planted—sweet chestnut, sycamore, poplars, lime, and elm. 2. Shrubs.—Hazel (most abundant), maple, sloe, haw- thorn, elder, bramble, wild rose, honeysuckle, sallow- willow (Salix Caprea), holly, dogwood, and -guelder-rose. 3. Herbaceous Undergrowth.—Anemone nemorosa, Pri- mula vulgaris, Scilla nutans (on loose soils), Mercurialis perennis, Ranunculus Ficaria, Euphorbia amygdaloides (wood -spurge), Stellaria Holostea (greater stitchwort), Oxalis Acetosella, Viola sylvatica, Sanicula europea, Geranium Robertianum, Epilobium montanum, Lamium Galeobdolon, Lysimachia nemorum (yellow pimpernel), Nepeta Glechoma, Lychnis dioica, Ajuga reptans, Prunella vulgaris ; woodland - grasses: Miliwm effusum, Bromus giganteus, Brachypodium sylvaticum, Melica uniftora, etc. ; ferns: Aspidium Filiz-mas, Athyrium Filix-femina, and in light soils Pteris aquilina. (6) Upiand Type.—A dry oak-wood is found on dry, non-calcareous soils at an altitude of 500 to 1,000 feet. The wood is more open than on the lowlands, and this favours a thick shrubby undergrowth and a luxuriant ground-vegetation. But this diminishes as the soil becomes drier and the altitude increases. The soil is deficient in humus, and the undergrowth includes a small proportion of heath-plants (ling, gorse, bracken, etc.) exhibiting xerophytic characters. As the altitude in- creases birches gradually succeed in dominating the oak, and at about 1,000 feet the oaks disappear altogether, while the birches continue up to the limit of tree-growth (1,500 feet in England). The ash is absent, and likewise WOODLANDS 271 the hazel, sallow-willow, dogwood, and guelder-rose, heath-shrubs taking their place. Characteristic plants are, in addition to those mentioned: Holly, Scilla nutans, Teucrium Scorodonia, brambles, wild rose, Potentilla Tormentilla, Rumex Acetosella, Anemone, Digitalis pur- purea, Galium saxatile, Solidago Virgaurea, Hieracium species, the typical heath grass Aira flexuosa, and Holcus mollis. II. Oak-Birch-Heath Association. — On dry, coarse, sandy soils or dry peaty soils in the lowlands, the birch and heath-plants which we found were present in the dry oak-wood become so numerous as to constitute a distinct association. This type is common round London on the dry heaths and commons—e.g., Bostall Heath and Keston Common. Silver birches are abundant, but the woods are very open, long stretches of ground being covered. with grass (Aira flecuosa chiefly) or shrubs (ling, gorse, whortleberry, honeysuckle, and bramble). The characteristic shrubs and small trees are holly, mountain- ash, hawthorn, blackthorn, alder-buckthorn (Rhamnus Frangula), juniper, and white beam. Herbs characteristic of dry ground are common—e.g., Teucrium Scorodonia, Galium saxatile, and Potentilla Tormentilla. This association is generally regarded as a stage in the degeneration of oak-wood into heath, brought about by a gradual increase in the dryness of the soil. The next stage in its deterioration is seen in the rough common dominated by bracken, gorse, and bramble, described in Chapter XXX. III. Birch-Wood.—At high altitudes (above 1,000 feet) the dry oak-wood changes into a birch-wood, and thcre seems no doubt that the change is due entirely to climatic conditions. The oak cannot grow at these high levels, and the birch, freed from competition, becomes the dominant tree. The undergrowth is much the same as in the oak-birch-heath association, but mosses are very abundant. In the north of England, and more especially in Scotland, many of the plants characteristic of pine- woods (p. 274) are found in the birch-wood. 272 BRITISH PLANTS C. Woods on Caleareous Soils (e.g., chalk, limestone, marls, and other soils rich in lime). I. Ash-Oakwood Association.— On deep calcareous clay a wood intermediate between the damp oak-wood and the ash-wood is found in many parts. The oak and ash share dominance. and the ground-vegetation is very similar to the damp oak-wood, but the wayfaring-tree, spindle-tree, and clematis are common here, yet rare in the oak-wood. The dogwood, privet, maple, sloe, and hawthorn also are far more abundant in this type of wood. Of the herbaceous undergrowth, the following are char- acteristic : Paris quadrifolia, Colchicum autumnale, Iris fetidissima, Epipactis media, E. purpurata, and Cam- panula Trachelium. II. Ash-Woods.—These are the typical woods on lime- stone, where the soil is dry and poor in humus. The ash does not cast much shade, and the undergrowth is conse- quently rich and varied. The most frequent trees other than ash are the wych-elm (Ulmus montana) and haw- thorn. Many lime-loving species are present—e.g., white beam (Pyrus Aria), wayfaring-tree, yew, Inula Conyza, Scabiosa Columbaria, Carduus eriophorus (woolly-headed thistle), Origanum vulgare, etc. Heath-plants are absent, except where the limestone is covered with glacial clay or marl from which the lime has been washed out. This type of wood gradually merges into the ash- oakwood at low altitudes, as the soil becomes damper, and at high altitudes birches replace the ash. III. Beech-Woods.—The beech-wood is confined almost entirely to the chalk of South-East England, where it occurs as a zone on the borders of the damp oak-wood which covers the clay-with-flints on the top of the Downs. It also occurs to the west of England on the oolitic lime- stone of the Cotswold Hills. ‘The branches of the beech are placed horizontally, and the leaves being situated in one plane, cast a very deep shade, which prevents the develop- ment of all undergrowth except a few mosses—e.g., Leuco- bryum. Every autumn the ground receives an enormous harvest of falling leaves, and as these decay slowly they form another unfavourable factor for the production of ground-vegetation. The trees and shrubs present in the WOODLANDS 273 more open parts and on the outskirts of the wood are those of the ash-wood—e.g., hawthorn, white beam, way- faring-tree, buckthorn, spindle-tree, dogwood, sloe, hazel, maple, elder, juniper, and yew. The herbaceous under- growth includes, in the more open parts, wood-sanicle, wood - violet, dog’s - mercury, enchanter’s - nightshade, Helleborus viridis, and in the deepest shade the orchid Cephalanthera pallens, and the colourless saprophytes Neottia Nidus-avis and Monotropa Hypopitys. Also grow- ing in the more shady parts are Daphne Laureola (spurge- laurel), Ruscus aculeatus (butcher’s-broom), Atropa Bella- donna (deadly nightshade), and Bunium flecuosum (pignut). _ The relationship between the various deciduous woods is shown in the following diagram : Birch-wood, Dry heath. "Peres. Oak-birch-heath Ash-birchwood. association. Limestone- pasture. Dry oak-wood. Ash-wood. Beech-wood. | es Damp oak-wood. Ash-oakwood. Alder-willow thickwi. EVERGREEN WOODS. The trees which constitute the evergreen wood in this country are Conifers (pines and firs), and it will be con- venient to include with them another Conifer—the larch— which is deciduous. The pine, the most common of the Conifers, is a most accommodating tree in regard to its soil requirements. It will grow in any situation which is dry, whether physi- cally or physiologically. For this reason pine-woods and plantations are found on sandy plains at low altitudes, 18 274 BRITISH PLANTS partially-drained peat-bogs, dry rocky crags and slopes along mountain valleys, and on heather-moors at high altitudes (1,500 to 2,000 feet). In most cases the pine- woods are artificial, but natural pine-woods occur in the Highlands of Scotland. The tree had a much wider range in former times, for remains have been found in the peat of the Pennines at an altitude of 2,400 feet, although at the present time the upper limit in that district is but 1,750 feet. The spruce, Douglas-fir, and larch, which are frequently planted, reach higher levels than the pine. In the Pen- nines they occur up to an altitude of 2,015 feet. Above 1,800 feet the plants become dwarfed, and at the upper limits the spruce assumes a dense shrubby habit, 2 to 3 feet high, and often forming a low mat close to the ground. In Forfar a larch-wood extends up to 2,500 feet. The pine produces as much shade as the beech when growing in close canopy, and this dense shade affects the plant itself as well as the undergrowth. The lower branches die away, leaving a long straight stem crowned by a mass of foliage. On the edge of the wood the branches persist right to the ground. Where the shade is most dense only a few mosses occur, and occasionally Monotropa Hypopitys. As in the beech-wood, the ground is covered with a thick layer of slowly decaying leaves, which likewise prevents the undergrowth from developing. In more open parts the chief woody plants are an occa- sional mountain-ash and birch, and invaders from the neighbouring heath—e.g., ling, whortleberry, and black- berry. Common herbs are Potentilla Tormentilla, Galium saxatile, Veronica Chamedrys, and Oxakis Acetosella. A number of plants are found in the primitive woods or ancient plantations in Scotland and North England, and in many cases the relative age of a plantation can be determined by their presence or abundance. These plants include : Linnea borealis, Pyrola minor, P. media, P. rotundifolia, P. secunda, and P. uniflora (winter- greens), T'rientalis europea (chickweed winter-green), Listera cordata (small twayblade), and the colourless saprophyte Corallorhiza innata (coral-root orchid). CHAPTER XXVIII MARITIME ASSOCIATIONS On the seashore and the margin of estuaries the soil varies considerably ; it may be sandy, shingly, rocky, or muddy, and the vegetation exhibits a corresponding diversity. The flora of a mud-bank is quite distinct from that of a sand-dune, and this in turn differs from the flora of rocks and cliffs. In all cases the land-plants are typical xerophytes, but the xerophytic character may be ‘ due to a variety of causes. It may be an adaptation to physiological dryness, as in the plants of a mud-bank where the water is strongly saline ; to physical dryness, as in sand-dune plants where the soil contains very little water; or to the action of strong drying winds, as in those of cliffs. In this chapter, then, a number of distinct formations will be described. They are brought together merely for convenience, and not necessarily because they are related ecologically. As is the case with other plants, the maritime plants group themselves naturally in two divisions—the aquatic vegetation and the terrestrial. J. Maritime Aquatic Vegetation. On the open seashore and in estuaries, where the water-level is alte/ing with each tide, the aquatic vegeta- tion is almost restricted to one group of plants—the sea- weeds, or alge. The absence of other plants is due chiefly to the difficulty of bringing about fertilization. The periodic rising and falling of the water makes it necessary that fertilization should be effected under water. Aerial flowering stems would be of no use, for the flowers are submerged at high tide and ordinary pollen destroyed ; 275 276 BRITISH PLANTS even if the flowers could adjust themselves to different levels, the waves and surf would keep them perpetually wet. The pollen-grains of a marine aquatic, therefore, must either be adapted for dispersal under water or the flowers must be cleistogamic (p. 165). At the same time the plant has to adjust its absorbing structures to suit the salt water. The only genus of British flowering plants which has succeeded in adapting itself to this mode of life is Zostera (grass-wrack), of which two species have been found in this country—Z. marina and Z. nana. They occur all round the coast in muddy estuaries of rivers, often growing where they are left uncovered by the receding tide. The pollen-grains are thread-like, and have the same specific gravity as sea-water, so they can float at any depth in the water, and be carried to the large stigmas. The plant is rooted in the mud, and its long, narrow, strap-shaped leaves offer little resistance to currents. : The reproductive cells of the alge, on the other hand, are always adapted for life under water. They possess, as a rule, small hair-like structures, which by their move- ment propel the cells through the water. Tn the brackish water which collects in ditches on salt- marshes the following aquatic flowering plants are found : Ruppia maritima (sassled pondweed), which has pollen very similar to Zostera ; Zannichellia pedunculata ; and a form of the water-crowfoot, Ranunculus Baudotit. Seaweeds can only grow in abundance on a rocky coast. They are attached to the substratum by a small flat disc, and if a plant became fixed to loose sand or mud the first wave which came along would wash away plant and sand together. Almost the only exception to this is Chorda filum, which possesses a very long, thin, cord-like frond, the lower part of which may become embedded sufficiently deep in sand or mud to prevent the plant from being torn away. We haveseen in Chapter V., p. 53, that the depth at which seaweeds grow is dependent on the presence of colouring matter in their fronds—the red seaweeds growing in deep water, the brown in shallower water, and the green ones quite near the surface. The brown seaweed is usually the dominant form, and these exhibit a zoning amongst themselves. The depth at which they grow depends on a MARITIME ASSOCIATIONS 277 number of factors, the chief of which are: their power of resisting desiccation both during their germination and vegetative growth ; their rate of growth, the more quickly- growing plants forcing others to retire to higher levels; and on considerations connected with their reproduction. In the deeper water, never exposed to the air even at the lowest spring-tides, Laminarias are dominant. Above this may be a zone of Halidrys siliquosa, only exposed for a short time at low water during spring-tides ; then one of Fucus serratus, uncovered at low water, but during neap- tides for a short time only ; higher still are zones of Fucus vesiculosus and Ascophyllum nodosum, exposed for longer periods ;.and then Fucus platycarpus, only covered by the highest tides. The spray-washed rocks, seldom sub- merged at all, are covered with the xerophytic fronds of Pelvetia caniculata. In this situation the plant is exposed to drying winds and the heat of the sun throughout the day, and excessive evaporation of water is prevented by the presence of a hard, thick, external layer almost impermeable to water. II. Maritime Terrestrial Vegetation. The associations of terrestrial maritime plants may be grouped according to the nature of the soil and elevation above sea-level as follows : 1. Muddy banks of estuaries, salt-marshes, etc. 2. Sandy seashore. 3. Pebbly seashore and shingle. 4. Rocks and cliffs. 5. Exposed slopes facing the sea. The associations—with the exception of those of the last group, which are usually pasture-associations—are of an open type ; competition is not keen, and almost every plant, adapted by its structure for life in such surround- ings, has a chance of surviving. One association is often rapidly replaced by another, as on sand-dunes, and changes in the distribution of the plants are always going on. As in other open associations, annuals are very abundant, both in individuals and species, and in some cases constitute the sole flora—e.g., Salicornia herbacea and strand-associations. 278 BRITISH PLANTS 1. Flora of Mud-Flats and Salt-Marshes.—On the muddy margins of estuaries, and in low-lying ground liable to periodic floods from the sea, the soil is saturated with salt- water, and the plants are typical halophytes, with fleshy leaves or stems (p. 88). Usually submerged in the water of the estuary is a zone of Zostera, and above this, on the flat reaches, wholly or partially submerged at high tide, the only vegeta- tion consists of Salicornia herbacea (annual glass- wort), or in some parts S. radicans (perennial). These may form a thick sward if the mud is only just covered at high tide, but when the water is deeper the plants stand far apart. The Salicornias have fleshy green stems and minute, adpressed, succulent leaves (Fig. 113) ; the cell-sap is highly con- centrated, and in every way the plant is excellently adapted for life in this extreme xerophytic en- vironment. Salicornia is the first inhabitant of the mud - flat, and sediment brought down by the river is caught at the base of Eo? the plants. As the mud Fig, uS—Salconie, perbacea accumulates, the flat be- SoWERBY.) comes higher and drier, and other plants now begin to colonize the ground. The earliest of these new-comers are Glyceria maritima and Triglochin maritimum, which sometimes form a distinct zone above the Salicornia. Out of reach of the highest tides the general salt-marsh flora develops. The vegetation is frequently of two types. In the wetter parts a salt reed-swamp develops, characterized by monocotyledons with erect, narrow leaves, MARITIME ASSOCIATIONS 279 tee and constituting an edaphic modification of the ordinary reed-swamp. The dominant plants are Juncus Gerardi, Scirpus maritimus, Glyceria maritima, and in some parts Phragmites communis also. Where the soil is drier the plants are not so tall-growing as in the reed-swamp ; this type may be called a salt-meadow. Characteristic salt-marsh plants are: Aster Tripolium (sea-aster), Statice Limonium (sea-lavender), Armeria mari- tema (thrift), Plantago maritima (sea-plantain), Salicornia herbacea, Spergularia salina (sea-spurrey), Sueda maritima (sea-blite), Atriplex portulacoides (sea-purslane), Artemisia maritima (sea-wormwood), Beta maritima (sea-beet), Triglochin maritimum (seaside arrow-grass), Cochlearia officinalis, C. anglica, and C. danica (scurvy-grasses), Glaux maritima (sea-milkwort), Atriplex littoralis (grass- leaved orach), Scirpus triqueter, S. rufus, Hordeum mari- timum (sea-barley). 2. Flora of Sandy Shores.—On sandy shores the con- ditions are quite different from the salt-marsh. Except within reach of the tides, the amount of salt present in the soil-water is comparatively small. The porous soil allows rain to drain away very rapidly, and the plants are sub- jected to long periods of drought. The xerophytic character of the plant, therefore, is not due to physiological dryness, but to physical. Yet the adaptations are the same in both cases—succulent plants are as common here as in the marsh ; indeed, some are found in both situations —e.g., thrift, scurvy-grass, sea-plantain, and beet. Where strong winds prevail in a direction at right angles to the coast the sand is blown inland, and accumulates in long sand-hills or dunes running parallel to the shore. Unless the dune is completely covered with vegetation— and this is rarely so—the sand is carried still farther inland, and a second or even third line of dunes arises on the landward side. The flora of the dune varies according to the compactness of the sand, and to some extent according to its chemical composition. Most of the sand consists of quartz-particles, and the soil is consequently very sterile, but when the remains of marine shells are present the soil is richer, and many calciphilous plants occur. On the sand immediately above the mean high-water mark, and only covered at very high tides, a narrow zone of strand-vegetation occurs. The water within reach of 280 BRITISH PLANTS the plants is heavily laden with salt, and they are therefore halophytes. The most frequent plants are low-growing annuals, which have no power of growing upwards when covered with sand, and this zone is consequently absent where the sand is advancing on the sea. Perennials are very rare, and occur sporadically, for the plants may be uprooted during storms, and only those which produce a large quantity of seed each year, as in annuals, can persist. The most common member of this association is Cakile maritima (sea-rocket), a fleshy-leaved plant belonging to the Wallflower-family. Several plants belonging to the Chenopod-family usually occur—e.g., Salsola Kali (salt- wort), with short prickly leaves, Atriplex patula, A. Fic. 114.—Carex arenaria, wira RuIZOME NEAR SURFACE oF Sax hastata, A. Babingtonii (oraches), Chenopodium rubrum, and C. album (goosefoots). Arenaria peploides (sea-purslane) is the only perennial which is found at all constantly in this zone, and then only very sparingly. Above the strand-vegetation the sand is usually loose. The first plant to colonize this drifting sand is Agropyron junceum (sea couch-grass); which possesses long under- ground rhizomes bearing tufts of leaves at intervals. The sand is held at the base of the leaves, and as it increases in amount so the leaves grow upwards, and retain yet more sand. In this way a low embryonic dune is built up. Carex arenaria (Fig. 114), a plant of similar habit to the sea couch-grass, is sometimes associated with that grass in forming these dunes. When the sand has been MARITIME ASSOCIATIONS 281 consolidated to some extent, other plants colonize the ground. Chief among these are Arenaria peploides, Eryngium maritimum (sea-holly), Glaucium luteum (horned poppy), Zuphorbia Paralias, Cakile maritima, Convolvulus Soldanella (sea-convolvulus), and various species of Atriplex. Psamma arenaria (marram-grass) occurs spar+ ingly in this zone, but once it gets a footing the sand collects very rapidly, for the grass is very quick-growing, and the incipient dune is converted into a high one. The Agropyron disappears, for it cannot keep pace with the growing dune. ; Shifting Dunes.—The first line of high dunes is very characteristic in appearance. The seaward side is bare of vegetation, and rises at a sharp angle to the crest, where Psamma is abundant, and then slopes more gradually down on the landward side. The sand is being con- tinually blown along, and for this reason the dunes are often called ‘‘ shifting dunes.”’ The term white dune is also used, referring to the fact that so much bare sand is exposed. The binding power of Psamma is much greater than the plants of the embryonic dune, and for this reason it is often planted in places where it does not grow naturally, in order to prevent the dunes from travelling inland. It is also frequently planted on sand-bunkers on golf-links. The plants of the shifting dune are subjected to a number of factors which increase transpiration. The wind is strong, the light very intense, and the white sand reflects nearly all the heat of the sun, rendering the air extremely hot and dry. The surface-layer of sand is heated rapidly, and the water quickly driven off, so that both in regard to the soil and the air above it the dune is a true desert. Yet beneath this dry layer there may be an abundance of water, as is shown by the presence of fresh- water marshes in many of the hollows. The adaptations to these xerophytic conditions take the form of erect cylindrical rolled leaves (Fig. 11), as in all the grasses ex- cept the lyme-grass ; the development of a surface-coating of wax, as in the latter plant, sea-holly, and horned poppy ; succulent leaves, as in most of the plants other than grasses ; the formation of thorns or spines, as in rest- harrow and sea-buckthorn ; and in all cases the presence of very long roots which can explore the moister soil Missing Page Missing Page 284 BRITISH PLANTS during storms may be washed with spray approximates very closely to that of a salt-marsh. Indeed, many plants are common to both. As in the salt-marsh, succulence is a very common character. The plants usually grow on ledges where the soil is thin and water scarce, whilst the light is often as intense and the exposure as great as on a sand-dune. The most frequent plants met with in this situation are: Crithmum maritimum (samphire), Inula crithmoides (golden samphire), Statice auriculefolia (sea-lavender), Armeria maritima (sea-pink, thrift), Plantago maritima, P. Coronopus, Cochleartia danica, Sagina maritima (sea- spurrey), Spergularia rupestris, Sedum anglicum, and Asplenium marinum (sea-spleenwort). Of rarer plants, Cotoneaster vulgaris grows only on Great Orme’s Head, Peonia corallina on Steep Holme off the coast of Somer- set, Matthiola incana (sea-stock) in the Isle of Wight and at Ramsgate, Brassica oleracea (the wild type from which the cabbage, cauliflower, broccoli, kale, etc., have been derived) in a few places on the south coast of England, Lavatera arborea (tree-mallow) chiefly on the south and west coasts of England and Ireland, Ligusticum scoticum (lovage) confined to the coast of Scotland, North-East England, and North Ireland. 5. Exposed Slopes facing the Sea.—On the wind-swept cliff-tops and slopes facing the sea the vegetation is greatly affected by the strong drying winds. Pasture- land usually covers the slopes, but the grasses are dwarfed and seldom flower. Any plant which rises above the level of the grass is cut down by the wind and destroyed. Consequently the plants which do persist must be as short as the grass. Plants which are 1 to 3 feet high in sheltered spots are reduced to 1 or 2 inches, or even less, on the wind-swept slopes. The flowers are not altered conspicuously in size, but they are considerably reduced in number. and hidden on minute stalks in a little nest of leaves. On the more exposed parts of Beachy Head the following plants are thus dwarfed, the figures in brackets indicating the height of the plants in more favourable situations : Phyteuma orbiculare (4 to 18 inches), Erythrea Centaurium (6 to 18 inches), Scabiosa Columbaria (1 to 2 feet), Hchium vulgare (1 to 3 feet), Centaurea nigra (4 to 3 feet), Carduus acaulis (3 to 12 inches), Hwphrasia MARITIME ASSOCIATIONS 285 officinalis (1 to 12 inches). Anthyllis Vulneraria, Lotus corniculatus and Thymus Serpyllum are prostrate plants which never rise from the soil. In many parts of western Ireland a similar type of vegetation is produced, but the dominant plants are not grasses, but plantains—e.g., Plantago maritima and P. Coronopus. Where the ground slopes down nearer the sea the plants are liable to be covered with spray in the time of storm, and here many of the plants of exposed rock intermingle with those of the pasture. The latter invariably possess leaves much more succulent than when they grow inland. The following selected list of plants found on exposed slopes on the west coast of Cornwall is typical of such situations : Maritime Species : Inula crithmoides, Statice auricule- folia, Armeria maritima, Plantago maritima, Cochlearia danica, Spergularia rupestris, Huphorbia portlandica. Inland Species: Anthyllis Vulneraria, Lotus corniculatus, Centaurea Scabiosa, Erythrea Centaurium, Hieracvoum species, Leontodon species, Polygala vulgaris, Thymus Serpyllum, Potentilla Tormentilla, Daucus Carota, Calluna vulgaris, Erica cinerea, and Ulex europeus ; the last three very dwarf. Geranium sanguineum (bloody crane’s-bill) is almost confined to these exposed slopes on calcareous soil. CHAPTER XXIX VEGETATION OF ROCKS AND WALLS I. Alpine Rocks. Tue alpine zone in this country extends from the limit of tree-growth (usually about 2,000 feet) to the summits of the highest mountains. It is on the sheltered rock-ledges in this region that the typical alpine plant, whose char- acters have been described in Chapter IV., reaches its greatest development. Some are restricted to this zone, and all have their headquarters here. In the west of Ireland, however, their distribution is different : some— eg., the mountain-avens (Dryas octopetala), rose-root (Sedum Rhodiola), and yellow mountain-saxifrage (Sawi- fraga aizoides)—grow on rocks at the sea-level, whilst others never reach the 2,000-feet line. It is very difficult to understand why this should be so, for the climate is milder and more humid than any other part of the British Isles. The strong winds and gales which prevail for the greater part of the year, however, prevent the growth of many lowland forms, and alpines, naturally adapted by their tufted or rosette habit for an environment similar to this, might live there free from competition. Although this may account for their presence at sea-level, it sheds no light on their absence from the higher zones, for compe- tition does not drive the alpine up a mountain-side ; it merely restricts its distribution to the higher levels. Alpine plants occur in all situations—in exposed, wind- swept places, on the dry upper slopes (pp. 255, 259), and on wet peaty soil (p. 253)—but all the plants of these habitats, except those of the wettest bogs, grow also on rocky ledges and clefts. Here the surface-soil is very thin, but the cracks and crevices are filled with soil, and into this 286 VEGETATION OF ROCKS AND WALLS — 287 the plants send their exceptionally long roots in search of food and water. The rainfall is heavy on the mountain- top, and out of reach of the wind the plants suffer little from lack of water. But they are subjected to intense cold during the night and brilliant illumination in the day- time, whilst the air is rarefied. The first factor hinders absorption, and the others favour transpiration, so the plants are typical xerophytes, with rosettes of leaves or cushion-habit. Geophytes are very rare, owing to the absence of sufficient soil in which the plants could hiber- nate, whilst the short vegetative period prevents the establishment of most annuals. Indeed, there is no true alpine annual in this country, although lowland forms may exist at the highest levels—e.g., Huphrasia officinalis and Poa annua, at 3,980 feet in Perthshire. Many alpines are found in the neighbourhood of alpine streams and rills. Here plenty of water is available at all times, but the climatic factors mentioned above are as evident as on the rock-ledges, and the habit of the plants is the same. Some of the rarest British plants, last survivors of an arctic climate (see p. 210), are found at high altitudes. The Highlands of Scotland are specially noteworthy in this respect. The following plants are found in a few localities only in Scotland : Arabis alpina (Skye), Draba rupestris (Ben Lawers, Perthshire, 3,000 to 3,980 feet), Arenaria rubella (Perth, 2,700 to 3,800), Sagina nivalis (Perth, 3,100 to 3,900), S. Boydii (Braemar), Astra- galus alpinus, Oxytropis campestris (Perth), Saxifraga cernua (Ben Lawers, 3,800), S. rivularis (Ben Nevis, Ben Lawers, Cairn Gorm, etc., 3,500 to 3,900), Hrigeron alpinum (Perth, 2,500 to 3,500), Gnaphaliwm norvegicum (Perth, Forfar, and Aberdeen), Lactuca alpina (Aberdeen), Menziesia cerulea (Perth, 2,350 to 2,460), Gentiana nivalis (Perth, 2,400 to 3,450), Myosoteis pyrenaica (Perth, 2,400 to 3,450), Veronica fruticans (Perth, 1,200 to 3,600); dwarf species of willow, $ to 2 feet high—Salix Arbuscula and S. lanata (Perth); Luzula arcuata (summits of several mountains—e.g., 4,290, Ben Macdhui, Aberdeen), Juncus biglumis, Carex alpina, C. rupestris, C. atrofusca (Perth, 2,600), Alopecurus alpinus, Phleum alpinum, Poa laxa. Other alpines confined to Scotland, but more widely spread than the foregoing, are: Cherleria sedoides, Sagina 288 BRITISH PLANTS Linnei, Oxytropis uralensis, Gnaphalium supinum, Vero- nica alpina, Juncus trifidus, Carex vaginata, Salix Myrsinites, S. reticulata, Athyrium alpestre, Azalea pro- ens: Arctostaphylos alpina, Betula nana (1 to 2 feet igh). Only one alpine is confined to south Britain—Lloydia serotina—tfound in one or two places on the Snowdon range. Many species of Saxifraga are found on the mountains of west or south-west Ireland, and nowhere else in the British Isles. Of these S. wmbrosa (London-pride), widely scattered, and S. Geum, confined to Kerry and Cork, are members of the Lusitanian flora (see p. 213); S. hirsuta is found only in Kerry and Cork, S. elegans only in Kerry ; S. decipiens extends from western Ireland to western Scotland and north Wales; S. cespitosa occurs very rarely in Kerry, Carnarvonshire, Westmorland, and Aberdeen. Arenaria ciliata has its only British station in the Ben Bulben range, west Ireland. Other alpines found more or less abundantly in most mountainous districts throughout the British Isles are: Thalictrum alpinum, Draba incana, Cochlearia alpina, Thiaspi alpestre (not in Ireland), Silene acaulis, Cerastium alpinum (not in Ireland), Arenaria verna, Dryas octopetala, Alchemilla alpina, Saxifraga aizoides (vills), S. oppositt- folia, 8. hypnoides, S. stellaris (rills), Sedum Rhodiola (the only succulent alpine plant in this country), Anten- naria dioica, Saussurea alpina, Polygonum viviparum, Oxyria reniformis, Salix herbacea, Empetrum nigrum, Poa alpina, Juniperus communis var. nana, Asplenium viride, Dryopteris montana, Cystopteris fragilis. Confined to northern England and Scotland are: Potentilla Sibbaldi, Epilobium alsinefolium, EH. alpinum, Linnea borealis, Galium boreale, Salix Lapponum. An interesting feature of the alpine vegetation is the presence of a number of plants found also on the seashore. Thus, Cochlearia grenlandica, Silene maritima, Armeria maritima, and Plantago maritima are almost as common on damp alpine ledges as on the wind-swept face of a sea- cliff. In other cases the plant is represented by a variety, and not the type—e.g., Sagina maritima var. alpina. Plants whose home is in the mountain may extend down to the sea-coast—e.g., Draba incana and Oxytropis ura- VEGETATION OF ROCKS AND WALLS lensis, or a variety of the alpine form—e.g., Arenaria verna var. Gerardt. Many lowland plants grow in the more sheltered places, even to the summits of the highest mountains. They are often dwarfed, and some—e.g., Poa annua and Festuca ovina—seldom produce flowers, but multiply vegetatively (see p. 160). The following list, compiled largely from Williams’s High Alpine Flora of Britain,* shows the highest range of the plants, usually in the Scottish Highlands : 289 Viola palustris, 4,000 feet. Galium saxatile, 4,000 feet. Euphrasia officinalis, 3,980 feet. Rumex Acetosa, 3,980 feet. R. Acetosella, 3,980 feet. Ranunculus acris, 3,980 feet. Achillea Millefolium, 3,980 feet. Poa annua, 3,980 feet. Cardamine flexuosa, 3,900 feet. Taraxacum officinale, 3,900 feet. Solidago Virgaurea, 3,900 feet. Campanula rotundtfolia, 3,800 feet. Cardamine hirsuta, 3,800 feet. Festuca ovina, 3,770 feet. Thymus Serpyllum, 3,700 feet. Caltha palustris, 3,600 feet. Veronica serpyllifolia, 3,500 feet. Adoxa Moschatellina, 3,500 feet. Lychnis dioica, 3,500 feet. Tussilago Far fara, 3,500 feet. Oxalis Acetosella, 3,480 feet. Viola lutea, 3,450 feet. Agrostis canina, 3,400 feet (Carran- tual, Ireland). Ce eeenens oppositifolia, 3,400 eet. Heracleum Sphondylium, feet. Stellaria uliginosa, 3,300 feet. Potentilla Tormentilla, 3,300 feet. Mercurialis perennis, 3,300 feet. Sagina procumbens, 3,290 feet. Viola Riviniana, 3,000 feet (Car- rantual, Ireland). Lotus corniculatus, 2,800 feet. Anemone nemorosa, 2,750 feet. 3,300 On dry exposed rocks the only vegetation consists of close-growing lichens and minute mosses ; there is neither sufficient water nor nutriment to support flowering plants. On dry rocky summits, too, vegetation is scarce. Lichens —e.g., Cetraria islandica and species of Cladonia—and the woolly fringe-moss (Rhacomitrium lanuginosum) are usually common. The latter occasionally forms a thin layer of peat in which a few starved and stunted flowering plants become established—e.g., Empetrum, Vaccinium, Lyco- podium Selago, Potentilla Sibbaldi, Gnaphalium supinum, Azalea procumbens, Salix herbacea, Juncus trifidus, Carex rigida, Festuca ovina. All except the last three are low- lying plants, which form a mat close to the ground, and so escape the full force of the wind. * In Annals of Scottish Natural History, 1908-1910, - 19 290 BRITISH PLANTS II. Sub-Alpine and Lowland Rocks. Below the alpine region exposed rocks and cliffs are frequent, especially in limestone districts. Where the rock is protected from the wind and supplied with plenty of water, shrubs and low trees’‘are common, in some cases forming thickets or woods, as in the scar-woods of the Pennines. But in exposed situations on vertical cliffs only xerophytic herbs become established. The general habit of the plants is similar to those of alpine rocks, for the environment is much the same. The cold, however, is not so severe, and the plants can grow in drier situations. Owing to this often extreme dryness, succulents—e.g., Sedum—are more abundant, but rosette-plants are still common. Competition is not severe, and many annuals are found, among: them several autumn-annuals which flower early in spring, before the hot sun has parched the soil—e.g., Draba verna, Hutchinsia petrea, Myosotis collina (see p. 107). On limestone rocks the following plants may occur, in addition to the three annuals mentioned : Rosette-Plants : Arabis hirsuta, A. stricta, Draba muralis, D. incana, Thlaspi alpestre, Saxifraga tridactylites, Hiera- cium Pilosella. Succulents: Sedum Telephium, S. album, S. acre, S. reflecum, S. rupestre, S. anglicum. Other Plants : Thalictrum minus var. calcareum, Heli- anthemum Chamecistus, Dianthus cesius (Cheddar-pink, found only in the Cheddar Gorge), Arenaria serpyllifolia, A. verna, Geranium sanguineum, G. lucidum, Lactuca muralis, Parietaria officinalis, Festuca ovina, Asplenium Adiantum-nigrum, A. Trichomanes, A. Ruta-muraria, Cystopteris fragilis. III. Walls. The vegetation of a wall is in many of its features similar to that of ordinary rocks. The substratum is dry, and, as a rule, poor in nutritive material. The extent and variety of its flora will depend on the material of which the wall is composed, whether held together by mud or by mortar, and on its age. A brick wall is first tenanted by minute alge, lichens, and mosses, which assist in the disintegration of the mortar, and so prepare VEGETATION OF ROCKS AND WALLS 291 the way for the germination of the seeds of flowering plants. The most common of these are Sedum acre, Sagina procumbens, Saxifraga tridactylites, Poa annua, Asplenium Ruta-muraria, and Cardamine hirsuta, and on old walls where more soil has collected, Parietaria offict- nalis, Linaria Cymbalaria, Lactuca muralis, Diplotaxis muralis, Epilobium lanceolatum, etc. On the wall-top mosses are abundant, and growing amongst them many annuals and a few perennials characteristic of dry soils— eg., Capsella Bursa-pastoris, Cerastium vulgatum, C. semidecandrum, Erodium cicutarium, Valerianella olitoria, Filago germanica, Myosotis collina, Veronica agrestis, V. arvensis, V. serpyllifolia, Thymus Serpyllum, Antirrhinum majus, Cheiranthus cheirt, Arenaria serpyllifolia, Medicago lupulina, Sempervivum tectorum, Hieracitum Pilosella, Rumex Acetosella, Centranthus ruber, many small grasses, and very frequently Polypodiwm vulgare. The walls which bear the richest flora, however, are those built up of rough blocks of stones held together with mud. These are frequently built in rocky upland districts to separate fields, and represent the hedgerow of the lowlands. These walls become the home of many plants from the surrounding fields, and in addition to those in the preceding list the following are commonly met with: Jasione montana, Cotyledon Umbilicus, Lepi- dium Smithit, Geranium molle, G. lucidum, Sedum species, Erica cinerea, Calluna vulgaris, Genista pilosa, and many ferns, including Adiantum Capillus-Veneris, Asplenium laneeolatum, A. Adiantum-nigrum, A. Trichomanes, Cete- rach officinarum, etc. CHAPTER XXX HEDGEROWS—CULTIVATED AND WASTE LAND Wirain the area of cultivation the hedgerow forms one of the most striking features of the country. They are artificially made, and where not cleared and trimmed too scrupulously, and not ruined by dust, they form a rich and varied community of the waifs and strays of all kinds of associations. The hedge itself is made of trees and bushes, and at the top of the hedge-bank the soil is dry and shady. The deep shade prevents many plants from becoming established, but those with climbing stems or much-divided leaves are common. The former can reach the light by climbing to the top of the hedge, whilst the finely-cut leaf allows what little light there is to reach all parts of the plant. From the hedge slopes down the bank, which is warm and sunny on the south side if not overhung by trees. The north, shady side harbours a much richer flora than the other, and the plants are taller and more luxuriant. The light strikes the bank obliquely, and prostrate plants or rosette-plants, which place their leaves at right angles to the incident light, are common on the drier side. The effect of the lateral light is seen in the way some of the leaves, especially of seedlings, turn away from hedges (see Heliotropism, p. 68). At the bottom of the bank may run the ditch, which was formed when the hedge-bank was built up. The flowers there are immigrants from the damp meadow or stragglers from the marsh, and if the ditch is always filled with water a mixed assemblage of aquatics will flourish (see p. 236). Then beyond this, bordering the road, may stretch a rough piece of grass, a place of refuge for outcasts from the pasture, or, if bare spots occur, from the cultivated field. The vegetation of the hedgerow is therefore very 292 HEDGEROWS 293 mixed and varied, and each division must be studied separately. The influence of man is seen perhaps more in the hedge- row than anywhere else. Not only does the frequent clipping of the bushes and trees affect the vegetation underneath by letting in more light, but the woody plants themselves are affected—e.g., beeches and oaks which are periodically pollarded usually retain their leaves on the branches in a dried state during winter (see p. 109). The Hedge itself.—The most common trees are: oak, elm, and ash ; horse-chestnut, willow, lime, poplar, horn- beam, pear, and wild plum are frequent ; white beam is common on chalk soils. Of bushes or shrubs, the hawthorn, hazel, elder, maple, and sloe (blackthorn) are most abundant. More or less common species are: privet, cherry-laurel, gorse, pollard beech and oak, dogwood, wayfaring-tree (Viburnum Lantana), and buckthorn (Rhamnus catharticus), the last three especially on chalk soils. Climbers.—Scramblers : Rose, bramble, cleavers (Galium Aparine), crosswort (G. Cruciata), hedge-bedstraw (G. Mollugo). Twiners : Honeysuckle, Convolvulus sepium, hop, black bryony (Zamus communis), woody nightshade (Solanum Dulcamara). Tendril-climbers : Vetches (Vicia sepium and V. Cracca, leaf-tendrils), yellow meadow-vetchling (Lathyrus pra- tensis, leaf-tendrils), Clematis Vitalba (sensitive petioles), white bryony (Bryonta dioica). Root-climber : Ivy, attached to tree-trunks, or just as frequently trailing along the ground. Herbs.—The following list includes most of the com- moner hedgerow plants. Where the habitat is not stated, the plant grows equally well in the shade or in the more open parts. Achillea Millefolium (yarrow, milfoil), dry, open; divided leaf. Adoxa Moschatellina (moschatel), shade ; divided leaf. Agrimonia Eupatoria (agrimony), dry, open; divided leaf. Ajuga reptans (bugle), damp, shade. Alliaria officinalis (Jack-by-the-hedge). Allium ursinum (ramsons), deep shade. Arctium Lappa (burdock). Arenaria trinervia (sandwort), shade ; prostrate, si Artemisia vulgaris (mugwort), divided leaf. 294 BRITISH PLANTS Arum maculatum (cuckoo-pint), shade. Ballota nigra (black horehound). Bellis perennis (daisy). Calamintha Clinopodium (wild basil), dry, shade. C. officinalis (calamint), dry, shade. Campanula Trachelium (nettle-leaved bell-flower), shade. Capsella Bursa-pastoris (shepherd’s-purse). Cardamine hirsuta (hairy bitter-cress), open, dry. Carduus arvensis (field-thistle), open, dry. C. lanceolatus (spear-thistle), open, dry. Caucalis Anthriscus (hedge-parsley), shade; divided leaf. Cerastium vulgatum (mouse-ear chickweed), open, dry; prostrate. Cherophyllum sylvestre (wild chervil), shade ; divided leaf. C. temulum (rough chervil), shade; divided leaf. Chelidonium majus (greater celandine), shade; divided leaf ; found only in neighbourhood of dwellings. Circea lutetiana (enchanter’s-nightshade), shade. Conium maculatum (hemlock), divided leaf. Digitalis purpurea (foxglove), shade. Dipsacus sylvestris (teazle). Epilobium angustifolium (rose-bay willowherb), damp, shade. Eupatorium cannabinum (hemp-agrimony), moist ; divided leaf. Fragaria vesca (strawberry), divided leaf. Geranium molle (dove’s-foot crane’s-bill), dry ; dissected leaf. G. Robertianum (herb-Robert), shade; dissected leaf. G. rotundifolium (round-leaved crane’s-bill), dry. Geum urbanum (avens), moist; divided leaf. Helminthia echioides (oxtongue), dry. Heracleum Sphondylium (hogweed), divided leaf. Hypericum per foratum (St. John’s-wort), dry. Inula dysenterica (fleabane), wet ditch. Lamium album (white deadnettle). L. Galeobdolon (yellow deadnettle, archangel), shade. L. purpureum (red deadnettle). Lapsana communis (nipplewort), dry. Lathrea squamaria (toothwort), deep shade; parasitic on roots of hazel, elm, etc. Linaria vulgaris (toadflax), dry. Lithospermum officinale (gromwell), dry. Lychnis alba (white campion), L. dioica (red campion), shade. L. Flos-cuculi (ragged Robin), moist. Matva sylvestris (mallow), dry. Mercurialis perennis (dog’s-mercury), shade. Nepeta Cataria (catmint), shade. N. Glechoma (ground-ivy), shade ; creeping. Pastinaca sativa (parsnip), shade ; divided leaf. Plantago lanceolata (ribwort plantain), open, dry. Potentilla anserina (silverweed), open, dry ; divided leaf. P. Fragariastrum (barren strawberry), divided leaf. Primula vulgaris (primrose), shade. Ranunculus acris (acrid buttercup), open; divided leaf. R. auricomus (goldilocks), shade ; divided leaf. R. Ficaria (lesser celandine), wet ditch. HEDGEROWS 295 BR. repens (creeping buttercup), open; divided leaf. Rumer crispus (dock), open, dry. R. obtusifolius (broad-leaved dock), open, dry. Sagina procumbens (pearlwort), open, dry. Scrophularia nodosa (figwort), damp, shade. Sedum Telephium (live-long), dry, shade. Senecio erucifolius (hoary ragwort), dry ; divided leaf, WS. Jacobea (common ragwort), dry ; divided leaf. S. vulgaris (groundsel). Sonchus arvensis (sow-thistle), dry. Spirea Ulmaria (meadow-sweet), moist, shade; divided leaf. Stellaria graminea, shade. S. Holostea (greater stitchwort), shade. S. media (chickweed), prostrate. Symphytum officinale (comfrey), moist, shade. Urtica dioica (nettle), dry. Veronica Chameedrys (germander-speedwell), shade, V. officinalis (common speedwell), dry, open. Viola odorata (sweet violet), shade. V. sylvatica (dog-violet), shade. Among grasses the most common are: Bromus sterilis (barren brome), B. mollis (soft brome), Arrhenatherum avenaceum (false oat), Avena fatua (wild oat), A. sativa (cultivated oat), Festuca Myuros (wall-fescue), Briza media (quaking grass), Brachypodium sylvaticum (false brome), Triticum repens (couch-grass), Agrostis vulgaris (fine bent-grass), Hordeum murinum (barley), Poa annua, and Aira flexuosa. Where the climate is moist, ferns occupy an important position in the vegetation of the hedgerow. The most frequent are: Scolopendrium vulgare (hart’s-tongue), Aspi- dium Filiz-mas (male-fern), A. aculeatum (prickly shield- fern), Athyrium Filix-femina (lady-fern) ; where the soil. is sandy: Blechnum Spicant (hard fern), Asplenium Adiantum-nigrum (black spleenwort) and Polypodiwm vulgare (common polypody). In west Britain and Ireland Osmunda regalis (royal fern) is often common in the hedgerow-ditch. The strip of waste ground between the hedge and the road is tenanted by a very mixed vegetation, especially if the soil is frequently disturbed. The first plants to appear are annuals—e.g., Poa annua, shepherd’s-purse, and groundsel-—but if left undisturbed the soil becomes covered with a grass-community, and many of the weeds disappear. In addition to many plants of the dry hedge- bank, the following may occur : clovers, Medicago lupulina (black medick), Plantago major (great plantain), Melilotus 296 BRITISH PLANTS officinalis (melilot), Ononis arvensis (rest-harrow), Filago germanica (upright cudweed), Matricaria inodora (scent- less may-weed), Bartsia Odontites (hemi-parasitic on grasses), Malva rotundifolia (dwarf mallow), Potentilla reptans (creeping cinquefoil), Polygonum aviculare (knot- grass), many species of Chenopodium (goosefoot). Commons.—In many parts of the country, within the region of cultivation, large stretches of waste land exist, on which the dominant plants are bracken, furze, and bramble. The soil is always very poor and dry, either sandy or stony. In many cases these commons are relics of cultivation. They are usually situated near towns, where land is valuable, and where every part of it is as far as possible utilized for raising crops, or used as pasture- land. Woods on dry soils have been destroyed, and the land experimented with in this way, but it has often turned out unprofitable, and the land has been allowed to go to waste. Rough grasses and herbs are the first to obtain a hold on the soil, and then larger plants, including the three which are now dominant. The association is a very mixed one, and of an open character. Birches, and often oaks, are more or less abundant, and it seems prob- able that in the course of time a Birch-Oakwood associa- tion will once more occupy the soil (see p. 271). The most abundant plants growing with the three dominant ones are dry-loving grasses—e.g., Festuca ovina, Aira flexuosa, Aira precox, Brachypodium pinnatum, Nardus stricta, and Agrostis vulgaris. Calluna vulgaris and Erica cinerea are often abundant, and many typical heath-plants—e.g., Thymus Serpyllum, Huphrasia offici- nalis, Lotus corniculatus, Hypericum pulchrum, Galium saxatile, Teucrium WScorodonia, Potentilla Tormentilla, P. repians, Hieracium Pilosella, Rumex Acetosella, Veronica officinalis, Campanula rotundifolia, Stellaria graminea, Hrodium cicutarium, Viola sylvatica, etc. Of woody plants other than those mentioned, the hawthorn, broom, and sloe are the most common. Many of the wider strips of roadside-waste are of this character. Cultivated Ground.—Where the land is periodically ploughed to receive new crops, weeds are very abundant. When the crop is in its seedling-stage, and earlier, the weeds have nothing to compete with, and can grow apace. HEDGEROWS 297 Indeed, if the land is not properly cleaned, the crop itself is likely to succumb. Root-crops especially are liable to suffer in this way, for they are low-growing, and the weeds can grow up above them into the light and air. But the danger is not so great in a cornfield, for the cereals are quick-growing plants which effectually smother most others, and by the time the ears are ripe there are very few strong, healthy weeds left. The most frequent weeds, therefore, are annuals, which reach maturity early and produce an abundance of seed before the crop is big enough to injure the plants. Perennials are rare in the cornfield, for they not only have little opportunity of forming good seed, but their underground parts are cut up and destroyed by the plough. The only ones which are abundant are those like’ the false oat (Fig. 115) or the couch-grass, which have sub- terranean stems stored with food. When these are cut up by the plough they are not destroyed, but actually in- creased in numbers, for any one of the little tubers of the false oat, or any portion of the couch-grass rhizome, will give rise to a new individual. We have already seen in = Chapter XX. that the true = ee eee a sca weeds of cultivation—di.e., those which are found only in ground disturbed by man—are aliens; but many plants of our native flora occur as well. Those which do best as weeds are those with a good mechanism for the dispersal of the fruits or seeds—as fast as they are killed off in the field so they arrive again from their natural habitat. The most common of our native weeds are the following, those marked with an asterisk being annuals: *Myosurus minimus (mouse-tail), Ranunculus repens, *R. parviflorus, *Cardamine hirsuta, *Sisymbrium Thalianum (thale-cress), 298 BRITISH PLANTS Silene Cucubalus (bladder-campion), Cerastiwm vulgatum, *Stellaria media, *Arenaria serpyllifolia, Sagina procum- bens, *Spergula arvensis, *Geranium dissectum, *G. molle, *Medicago lupulina. *Trifolium pratense, *T'. repens, *T7'. procumbens, *Alchemilla arvensis, *Caucalis nodosa, *Valerianella olitoria (corn-salad, a doubtful native), *Knautia arvensis (field-scabious), *Matricaria inodora, *Senecio vulgaris, Carduus lanceolatus, C. arvensis, *Ana- gallis arvensis, *Myosotis arvensis, *Convolvulus arvensis, *Ouscuta (dodder, parasitic), *Linaria minor, *Mentha arvensis, *Calamintha Acinos, *Stachys arvensis, *Galeopsis Tetrahit, *G. Ladanum, Plantago lanceolata, P. major, *Polygonum Convolvulus, Rumex species, Festuca rubra (with rhizome). APPENDIX I. Weismann’s Law of Heredity, 1885.—This is based upon the almost absolute distinction in every plant and animal between the physical body or soma and the germ- cell, male or female, which the soma encloses, and which, when fertilised, becomes the germ of a new individual. The essential part of the theory is that the environment does not directly influence the germ-cells but only the soma, and that none of the characters acquired by the soma during its life-time alters the essential constitution of the germ-cell and is transmitted through it to the next generation. The germ-cell is therefore sacrosanct and race is everything; training and environment may im- prove or debase the individual but not the stock. This theory is directly opposed to the old conception of heredity, enunciated by Lamarck in 1809, according to which acquired characters may be transmitted as such from generation to generation. Though Weismann may have pushed his arguments too far, yet his theory in its broad bearings appears to be true and it has certainly stimulated subsequent research along healthy and fertile lines. APPENDIX II. The Mendelian Theory, 1865, 1901.—This theory of transmission fits in well with the Mutation or Saltation theory of variation and Weismann’s theory of heredity. Gregor Mendel (1822-1884), the son of a peasant in Austrian Silesia, was a priest and afterwards abbot of an Augustine monastery in Briinn, Moravia, and his great discoveries in heredity form one of the most romantic chapters in modern science. His researches on pea- hybrids were published in 1865 in an obscure local period- ical. They were neglected by the learned in his life- time and for many years ae out of knowledge until 2 3006 BRITISH PLANTS de Vries unearthed them in 1896 and published them in 1901. Mendel selected the pea for two reasons: (1) be- cause the flowers are self-fertilized and therefore under experimental control; (2) because they exhibit numerous characters which breed true—an almost idea] state of things for the purpose. Mendel marked out for experi- ment a certain number of differentiating characters and investigated their inheritance separately. Crosses were made between tall and dwarf varieties, coloured seed- coats and white, white and purple flowers, yellow and green-coated seeds, smooth and wrinkled seeds, etc. However the cross was made it was found that all the plants obtained by sowing these hybrid-seeds exhibited one only of each pair of contrasted qualities. The one quality so exhibited he called Dominant, and the other, which was apparently suppressed, the Recessive. Thus on crossing tall plants with dwarf, all the subsequent plants were tall. In the hybrids, therefore, tallness is dominant and dwarfness recessive. The seeds of this generation, produced by self-fertilization, were sown and both tallsand dwarfs appeared among the plants obtained. The latent character, dwarfness, had thus reappeared in the second generation, and that in the proportion of 1 to 3. No intermediate forms occurred. On sowing the seeds of the second generation, obtained as before by self-fertilization, the seeds from the Recessive (here dwarf) plants produced only dwarfs. and these were found to breed true from generation to generation. On the other hand, the seeds from the tall or Dominant plants, when sown, again produced both talls and dwarfs, the dwarfs always breeding true to type and the talls only true in the proportion of 1 to 2. The talls which breed true are called pure Dominants; those that do not are hybrids or impure Dominants. Thus on sowing the seeds of any generation the result is always: Dominants : Recessives :: 3 : 1. ‘The Recessives breed true, but only one-third of the Dominants; so that the real constitution of any generation, only to be ascertained by sowing its seeds, is: Pure a : Impure Dominants or Hybrids: Recessives —, ; 2 . APPENDIX IT 301 This is Mendel’s Law of Inheritance of unit-characters. It reveals: 1. That when unit-characters only are selected a hybrid is always like one or other of the parents. 2. That we can only ascertain that such a cross is a hybrid by sowing its seeds, when the latent quality will reappear among the individuals of the next generation. 3. That when sufficient individuals are taken, a certain mathematical ratio is found to exist between the Hybrids, pure Dominants and Recessives of any of the succeeding generations. Now this ratio, experimentally obtained and confirmed again and again, admits of a very simple interpretation. In the process of sexual reproduction the total number of possible gametic fusions, male (3) and female (?), which can be obtained by crossing a Dominant form D with a Recessive form £ is obviously the number of com- binations which can be formed by associating together the following couples: BOL GR) & tines g¢R x gR = 1 pure R. Thus the Mendelian Law has a physiological basis, the constitution of the hybrid being explained by the prin- ciple of what is known as the Segregation of the Gametes in the generations that follow. When two or more unit characters are combined the resulting ratios are more complicated, but several of these have been worked out mathematically and demonstrated practically. An enormous amount of work has been accomplished during recent years on the transmission of all kinds of unit-characters in plants and animals, and in spite of much slipshod and misleading work which has been published as bearing upon Mendelian inheritance, many reliable results have been obtained, some of which have turned out to be of great economic value. No modern horticul- turist or animal-breeder can now afford to be ignorant of this important discovery in the realm of heredity. It is even important in investigating the transmission of many human characters. 302 BRITISH PLANTS Nevertheless, the law, unfortunately, cannot be uni- versally applied. Some qualities blend in the hybrids, and there the law, as at present expressed, fails. Sooner or later it may be shown that Mendel’s Law is only.a special case of a wider generalization. When this is found the whole problem of hereditary transmission is within sight of solution. APPENDIX III. Botanical Provinees.—At the present time the flora of the world is divided into a number of separate Botan- ical Provinces or Floral Regions, just as the world of living men is divided ethnologically into distinct race- divisions. Each Botanical Province is distinguished by the presence of certain groups of plants, species, genera or natural orders, which differ as we pass from one pro- vince to another, although the conditions of soil and climate may be similar. Different authorities divide up the earth differently, but the classification usually followed is that of Drude, who distinguishes fourteen Floral Regions. Europe is divided into only two Botan- ical Provinces: (1) the Northern, and (2) the Southern or Mediterranean Province, the boundary being formed by the Pyrenees and the Alps. The Northern Province extends from Europe through Siberia to the Pacific and also includes the northern portion of the American conti- nent. The uniformity of the flora throughout the Northern Temperate regions of the world suggests, by itself, a former union between the Old and the New Worlds, either across the Northern Atlantic or Behring’s Strait or both, an inference which is supported by several other considerations. This lack of uniformity in the present flora of the world was not always so. As far as we know it dates only from the Glacial Period. Before then there seems to have prevailed throughout geological history, with one excep- tion, a great uniformity of climate and consequently of flora also. This one exception was the Permo-Carboni- ferous Period, when there were two marked Botanical Provinces in the world, the Northern Province, north of the Equator, and the Southern or Gondwanaland Province APPENDIX III 303 inthe Southern Hemisphere. The latter was characterized by the dominance of a certain ancient and, of course, now extinct fern, Glossopteris, which is unknown among the fossil remains of the Northern flora. The problem of uniform climates and world-wide floras and faunas is one of the most interesting and puzzling in the whole range of science. APPENDIX IV. BIBLIOGRAPHY. General Works. Avebury, Lord : “ British Flowering Plants.” London, 1905. Boulger, G. S.: ‘Plant Geography.” London, 1912. Bower, F.O. : “ Botany of the Living Plant.”” London, 1919. Goebel, K.: ‘“ Organography of Plants.” Engl. ed. by I. B. Balfour. Oxford, 1900-1905. Gray, Asa: “ Structural Botany.” London, 1900. Jackson, B. D.: “ A Glossary of Botanic Terms, with their Derivation and Accent.” London, 1916. Jost, L.: “ Lectures on Plant Physiology.” Trans. by R. J. Harvey Gibson. Oxford, 1907. (Suppl., 1913). Kerner, A., von Marilaun : “ The Natural History of Plants.” Trans. and ed. by F. W. Oliver, with the assistance of M. Busk and M. Ewart. London, 1898. Pfeffer, W.: ““The Physiology of Plants.” Trans. and ed. by A. J. Ewart. Oxford, 1900-1903-1906. Schimper, A. F. W.: ‘Plant Geography upon a Physiological Basis.”’ Trans. by W. R. Fisher, rev. and ed. by P. Groom and I. B. Bal- four. Oxford, 1903. Strasburger, E.; “A Text-Book of Botany.” Engl. ed. rev. by W.H. Lang. London, 1912. es Warming, Eug.: “ Gicology of Plants. An Introduction to Plant- Communities.” Engl. ed. by P. Groom and I. B. Balfour. Oxford, 1909. Willis, J. C.: “A Dictionary of the Flowering Plants and Ferns.” Cambridge, 1919. The Soil. Darwin, C.: “The Formation of Vegetable Mould through the Action of Worms, with Observations on their Habits.’’ London, 1881. Hall, A. D.: J. “ The Soil.”’ London, 1920. 2. ‘ Fertilisers and Manures.” London, 1910. 38. ‘‘ The Feeding of Crops and Stock.” London, 1911. 304 BRITISH PLANTS Biology of Shoots, Flowers, and Fruits. Belt, T.: ‘The Naturalist in Nicaragua.” London, 1888, reprinted in Dent’s Everyman Series. (Myrmecology). Darwin, C.: 1. “ The Movements and Habits of Climbing Plants.’ London, 1865 (1875). 2. “Insectivorous Plants.” London, 1875. 3. “The Various Contrivances by which Orchids are Fertilised by Insects.”” London, 1862 (1877). 4. “The Effects of Cross and Self Fertilisation in the Vegetable Kingdom.” London, 1876 (1878). 5. “ The Different Forms of Flowers on Plants of the same Species.” London, 1877 (1880). Guppy, H. B.: 1. “ Observations of a Naturalist in the Pacific between 1896 and 1899.” Vol. ii. Plant Dispersal. London, 1906. 2. ‘Plants, Seeds and Currents in the West Indies and Azores.”’ London, 1917. Kerner, A., von Marilaun: “ Flowers and their Unbidden Guests.”’ Trans. by W. Ogle. London, 1878. Knuth, P. E. O. W.: “ Handbook of Flower Pollination.” Trans. by J. R. Ainsworth Davis. Oxford, 1906. Inbbock, Sir John: 1. “ On Buds and Stipules.” London, 1899. 2. “On British Wild Flowers considered in Relation to Insects.” London, 1875. 3. ‘ Flowers, Fruits, and Leaves.’ London, 1888. Miller, H.: “The Fertilisation of Flowers.” Trans. by d’Arcy W- Thompson. London, 1883. Ward, H. Marshall.: “ Trees, A Handbook of Forest-Botany for the Woodlands and the Laboratory.” Vol. i. Buds and Twigs: Vol. iv. Fruits. Cambridge, 1904, 1908. Economic Botany. Freeman, W. G., and Chandler, S. E.: ‘“‘The World’s Commercial Products. A Descriptive Account of the Economic Plants of the World and of their Commercial Uses.”” London, 1907. Holland, J. H.: 1. “The Useful Plants of Nigeria.” Kew Bulletin, Additional Series, ix. 1908-1911-1915 (in progress). 2. “ Food and Fodder Plants.” Kew Bulletin, 1919, Nos. 1 and 2, pp. 1-84. Kew Gardens: “ Official Guide to the Museums of Economic Botany. No. 1, Dicotyledons. No. 2, Monocotyledons and Cryptogams.” London, 1907, 1895. Lindley, J.,and Moore, 7. (eds.): “ The Treasury of Botany.” London? 1889. APPENDIX IV 305 Prain, Sir David (ed.): “ A list of Economic Plants Native or Suitable for Cultivation in the British Empire.” Intro. by A. B. Rendle. Kew Bulletin, 1917, Nos. 7 and 8, pp. 241-296. Watt, Sir George : ‘The Commercial Products of India.”” London, 1908. Willis, J. C.: “A Dictionary of Flowering Plants and Ferns.” Cam- bridge, 1919. Variation and the Evolution of Species. ‘ Bateson, W.: ‘‘ Mendel’s Principles of Heredity.” Cambridge, 1913. Darbishire, A. D.: 1. “ Recent Advances in the Study of Heredity.” New Phytologist. vols. viii. and ix., 1909, 1910. 2. “‘ Breeding and the Mendelian Discovery.” London, 1911. Darwin, C.: 1. “ The Origin of Species by means of Natural Selection.” London, 1859 (1872). 2. “ The Variation of Animals and Plants under Domestication.” London, 1867 (1875). Doncaster, L.: “Heredity in the Light of Recent Research.” Cam- bridge, 1910. Lock, R. H.: “ Recent Progress in the Study of Variation, Heredity ‘and Evolution.” Rev. by L. Doncaster. London, 1916. Punnett, R. C.: ‘“‘Mendelism.” London, 1919. Reid, G. Archdall : ‘“‘ The Laws of Heredity.” London, 1910. Seward, A. C. (ed.): “ Darwin and Modern Science, Essays in Com- memoration of the Centenary of the Birth of Charles Darwin and of the Fiftieth Anniversary of the Publication of the Origin of Species.”” Cambridge, 1909. Thomson, J. A.: “Heredity.” London, 1919. Vries, H. de: 1. “ Plant Breeding.” London, 1907. 2. “The Mutation Theory. Experiments and Observations on the Origin of Species in the Vegetable Kingdom.” Trans. by J.B. Farmer and A. D. Darbishire. London, 1910-1911. Wallace, A. W.: “‘ Darwinism.” London, 1889. Weismann, A.: 1. ‘‘ Essays upon Heredity and Kindred Subjects.” Trans. by E. B. Poulton, etc. Oxford, 1891-1892. 2. ‘“‘The Germ-Plasm: A Theory of Heredity.” Trans. by W. N. Parker, etc. London, 1892. 3. “The Evolution Theory.” Trans. by J. A. Thomson. London, 1904. The Origin of the British Flora. Dunn, S. T.: “ Alien Flora of Britain.” London, 1905. Forbes, Edward: ‘‘On the Connexion between the Distribution of the Existing Fauna and Flora of the British Isles, and the Geological Changes which have affected their area, especially during the Epoch of the Northern Drift.” Mem. Geol. Surv. Great Britain, vol. i., 1846, pp. 336-432. 20 306 BRITISH PLANTS Praeger, R. Ll.: “ A Tourist’s Flora of the West of Ireland.” Dublin, 1909. Praeger, R. Ll.: “Clare Island Survey, Part X.: Phanerogamia and Pteridophyta.” (Origin of the Flora.) Proc. Roy. Irish Acad., . Vol. xxxi., 1911, pp. 1054-96. Reid, C.: “ The Origin of the British Flora.” London, 1889. Reid, C., Stapf, O., and others: “The Relation of the Present Plant Population of the British Isles to the Glacial Period.” Report, British Association, Portsmouth, 1911, p. 573. Stapf, O.: “A Cartographic Study of the Southern Element in the British Flora.” Proc. Linn. Soc., 1916-17, pp. 81-92. The Classification of Plants. Rendle, A. B.: “The Classification of Flowering Plants, Vol. i.” Cam- bridge, 1904. Sachs, J. von: “ History of Botany, 1530-1860.” Trans. by H. E. F. Garnsey, rev. by I. B. Balfour. Oxford, 1890. (For Species, Hybrids, Variation, see under “ Variation and the Evolution of Species.”’) Plant Associations and Formations. I. Floras, etc. Babington, C. C.: “Manual of British Botany.” Ed. by H. and J. Groves. London, 1904. Bentham, G., and Hooker, Sir J. D.: “ Handbook of the British Flora.” London, 1892. Druce, G. C.: “ List of British Plants.” Oxford, 1908. Fitch, W. H., and Smith, W. G..: ‘Illustrations of the British Flora.”’ London, 1916. Hanbury, F. J. (ed.): ‘The London Catalogue of British Plants, 10th edition.”’ London, 1908. “ Hayward’s Botanist’s Pocket-Book.” Revised and enlarged by G. C. Druce. London, 1909. Hooker, Sir J. D.: “Student’s Flora of the British Isles.” London, 1884. Moss, C. E.: “The Cambridge Pritish Flora.” Cambridge, 1914 (in progress). Praeger, R. Li. : “A Tourist’s Flora of the West of Ireland.’ Dublin, 1909. Rendle, A. B., and Britten, J. : “ List of British Seed-Plants and Ferns.” London, 1907. Watts, H. M.: ‘‘ A School Flora.” ‘London, 1915. Williams, F. N.: “The High Alpine Flora of Britain.” Ann. of Scot. Nat. Hist,, 1908, 1909, 1910. APPENDIX IV 307 Il. General Works on Plant Associations and Formations. Baker, 8. M.: ‘Zoning of the Brown Seaweeds on the Sea-shore,” New Phytologist, vol. viii., 1909, p. 195; vol. ix., 1910, p. 54. Brenchley, W. E., and Adam, H.: “ Recolonisation of Cultivated Land allowed to revert to Natural Conditions.” Journal of Ecology, vol. iii., 1915, pp. 193-210. Clements, F. H. : “‘ Research Methods in Ecology.” Lincoln, Nebraska, U.S.A., 1905. Clements, F. E.: ‘‘ Plant Physiology and Ecology.” London, 1907. Clements, F. H.; “Plant Succession: An Analysis of the Development of Vegetation.” Publication 242, Carnegie Institution, Washing- ton, 1916. Jefferies, T. A.: “Ecology of the Purple Heath-grass (Molinia caerulea).”” Journ. Ecol., vol. iii., 1915, pp. 93-109. Moss, C. E.: ‘‘ The Fundamental Units of Vegetation.” New Phyto- logist, vol. ix., 1910, p. 18. Moss, C. H., Rankin, W. M., and Tansley, A. G.: “The Woodlands of England.” New Phyt., vol. ix., 1910, p. 113. Oliver, F. W.: “The Shingle Beach as a Plant Habitat.” New Phyto- logist, vol. xi., 1912, p. 73. Oliver, F.W., and Salisbury, H. J.: “ Vegetation and Mobile Ground as illustrated by Sueda fruticosa on Shingle.” Journ. Ecol., vol. i., 1913, pp. 249-272. Pearsall, W. H.: ‘On the Classification of Aquatic Plant Communi- ties.” Journ. Ecol., vol. vi., 1918, pp. 75-84. Smith, W. G.: “The Origin and Development of Heather Moorland.” Scot. Geogr. Mag., vol. xviii., 1902, pp. 587-597. Smith, W.G.: “The Distribution of Nardus stricta in Relation to Peat.” Journ. Ecol., vol. vi., 1918, pp. 1-13. Smith, W. G.. and Crampton, C. B.. “ Grassland in Britain.” Journ. Agric. Sct., vol. vi., 1914, pp. 1-17. Tansley, A. G@. (ed.): “ Types of British Vegetation.” By Members of the Central Committee for the Survey and Study of British Vegeta- tion.” Cambridge, 1911. Watson, W.: “The Bryophytes and Lichens of Calcareous Soil.’ Journ. Ecol., vol. vi., 1918, pp. 189-198. Watson, W.: “The Bryophytes and Lichens of Fresh Water.” Journ. Ecol., vol. vii., 1919, pp. 71-83. Yapp, R. H.: “ Stratification in the Vegetation of a Marsh.” Annals of Botany, vol. xxiii., 1909, p. 275. Yapp, R. H.: “ Spiraea Ulmaria, L., and its Bearing on the Problem of Xeromorphy in Marsh Plants.” Ann. Bof., vol. xxvi., 1912, pp. 815-872. 308 BRITISH PLANTS Til. Plant Associations and Formations of Particular Regions of the British Isles. Adamson, R. S.: “ An Ecological Study of a Cambridgeshire Woodland.” Journ. Linn. Soc., Botany, vol. xl., 1912, pp. 339-384. Adamson, R. S.: ‘“‘On the Relationships of some Associations of the Southern Pennines.” Journ. Ecol., vol. vi., 1918, pp. 97-109. Armitage, H.: “Vegetation of the Wye Gorge’ at Symonds Yat.” Journ. Ecol., vol. ii., 1914, pp. 98-109. Brown, G.: “Survey of the Vegetation of the Parish of Shotts, Lanark- shire.” Trans. Bot. Soc. Edinb., vol. xxvi., 1913, pp. 101-118. Cotton, A. D.;: “Clare Island Survey, Part XV.: Marine Alge.” Proc. Roy. Irish Acad., vol. xxxi., 1912. Crampton, C. B.: “ The Vegetation of Caithness considered in relation to the Geology.”’ Edinburgh, 1911. Crampton, C. B., and Macgregor, M.: ‘The Plant Ecology of Ben Armine, Sutherlandshire.” With Vegetation Map. Scot. Geogr. Journ., vol. xxix., 1913, pp. 169-192, 256-266. Farrow, H. P.: “On the Ecology of the Vegetation of Breckland.’’ Journ. Ecol., vol. iii., 1915, pp. 211-228; vol. iv., 1916, pp. 57-64; vol. v., 1917, pp. 1-18, 104-113, 155-172; vol. vi., 1918, pp. 144-152; vol. vii., 1919, pp. 55-64. Fritsch, F. E., and Parker, W.: “‘The Heath Association on Hindhead Common.” New Phyt., vol. xii., 1913, pp. 148-163. Hardy, M.: “ Esquisse de la Géographie et de la Végétation des High- lands d’Ecosse.”” With Vegetation Map. Paris, 1905. Hardy, M.: “ Botanical Survey of Scotland. A General Map of the Highlands with a Sketch of the History and Methods.” With Vegetation Map. Scot. Geogr. Mag., vol. xxii., 1906, pp. 229-241. Harrison, J. W. Heslop : ‘‘ A Survey of the Lower Tees Marshes and of the Reclaimed Areas adjoining them.” Trans. Nat. Hist. Soc. Northumberland, Durham and Newcastle-on-Tyne, New Ser., vol. v., 1918, pp. 89-140. Jeffreys, H.: ‘On the Vegetation of Four Durham Coal-Measure Fells.” Journ. Ecol., vol. iv., 1916, pp. 174-195; vol. v., 1917, pp. 129-154. Lewis, F. J.: “Geographical Distribution of Vegetation of the Basins of the Rivers Eden, Tees, Wear, and Tyne.” With Vegetation Maps. Geogr. Journ., vol. xxiii., 1904, pp. 313-331; vol. xxiv. 1904, pp. 267-285. Lewis, F. J.: “The History of the Scottish Peat Mosses and their Relation to the Glacial Period.” Scot. Geogr. Mag., vol. xxii., 1906, pp. 241-252. Marsh, A.S. : “The Maritime Ecology of Holme-next-the-Sea, Norfolk.” Journ. Ecol., vol. iii., 1915, pp. 65-93. Moss, C. l’.:; “ Peat-Moors of the Pennines: Their Age, Origin, and Utilization.” Geogr. Journ., vol. xxiii., 1904, pp. 660-671. Moss, C. H.: “ Geographical Distribution of Vegetation in Somerset.”’ With Vegetation Map. London, 1907. APPENDIX IV 309 Moss, C. H.: “ Vegetation of the Peak District.” With Vegetation Maps. Cambridge, 1913. Newman, L. F.,and Walworth, @. : “ A Preliminary Note on the Ecology of Part of the South Lincolnshire Coast.” Journ. Ecol., vol. vii., 1919, pp. 204-210. Oliver, F. W., and Salisbury, EZ. J.: “Topography and Vegetation of Blakeney Point, Norfolk.” London, 1913. Pearsall, W. H.: “The Aquatic and Marsh Vegetation of Esthwaite Water.” Journ. Ecol., vol. v., 1917, pp. 180-202; vol. vi., 1918, pp. 53-74. Pethybridge, G. H., and Praeger, R. L'.: “The Vegetation of the District lying South of Dublin.” With Vegetation Map. Proc. Roy. Irish Acad., vol. xxv., B., 1905, pp. 124-180. Praeger, R. Li.: “ A Tourist’s Flora of the West of Ireland.’’ Dublin, 1909. Praeger, R. Id.: “Clare Island Survey, Part X.: Phanerogamia and Pteridophyta.” With Vegetation Map. Proc. Roy. Irish Acad., vol. xxxi., 1911. Salisbury, EZ. J.: “The Oak-Hornbeam Woods of Hertfordshire.” Journ. Ecol., vol. iv., 1916, pp. 83-117; vol. vi., 1918, pp. 1452. Salisbury, B. J.: “The Ecology of Scrub in Hertfordshire: a Study in Colonisation.” Trans. Herts. Nat. Hist. Soc., vol. xvii., 1918, pp. 53-64. ‘ Smith, R.: “Plant Associations of the Tay Basin.” Proc. Perthshire Soc. of Nat. Sci., vol. ii., 1898; vol. iii., 1900. ‘ Smith, R.: “Botanical Survey of Scotland: I. Edinburgh District.” With Vegetation Map. Scot. Geogr. Mag., vol. xvi., 1900, pp. 385-416. Smith, R.: “Botanical Survey of Scotland: II. Northern Perthshire District.” With Vegetation Map. Scot. Geogr. Mag., vol. xvi., 1900, pp. 441-467. Smith, W.G. : “ Botanical Survey of Scotland: III. and IV. Forfar, and Fife.” With Vegetation Maps. Scot. Geogr. Mag., vol. xx., 1904; pp. 617-628; vol. xxi., 1905, pp. 4-23, pp. 57-83, pp. 117-126. Smith, W. G. and Moss, C. H.: “ Geographical Distribution of Vegeta- tion in Yorkshire. Part I.: Leeds and Halifax District.” With Vegetation Map. Geogr. Journ., vol. xxi., 1903, pp. 375-401. Smith, W. G., and Rankin, W. M.: “Geographical Distribution of Vegetation in Yorkshire. Part II.: Harrogate and Skipton District.” With Vegetation Map. Geogr. Journ., vol. xxii., 1903, pp. 149-178. Stevenson, E. H.: “‘ Notes on the Vegetation of Weston Bay, Somerset.” Journ. Ecol., vol. i., 1913, pp. 162-166. Tansley, A. G. (ed.): “ Types of British Vegetation.” By Members of the Central Committee for the Survey and Study of British Vegeta- tion.” Cambridge, 1911. Tansley, A. G.: “The Vegetation of Hampstead Heath and the Neigh- bouring Woods,” in “ Hampstead Heath, its Geology and Natural History,” prepared under the auspices of the Hampstead Scientific Society. London, 1913. 310 BRITISH PLANTS Tansley, A. G.and Adamson, R. G. : “ Reconnaissance in the Cotteswolds and the Forest of Dean.” Journ. Ecol., vol. i., 1913, pp. 81-89. Watson, W.: “The Distribution of Bryophytes in the Woodlands of Somerset.” New Phyt., vol. viii., 1909, p. 90. Watson, W. : “‘ Cryptogamic Vegetation of the Sand-dunes of the West Coast of England.” Journ. Heol., vol. vi., 1918, pp. 126-143. West, George: “A Comparative Study of the dominant Phaneroga nic and Higher Cryptogamic Flora of Aquatic Habit in Three Lake Areas of Scotland.” Proc. Roy. Soc. Edinb., vol. xxv., 1905, pp. 967-1023. We:t, George: ‘‘ A Further Contribution to a Comparative Study of the dominant Phanerogamic and Higher Cryptogamic Flora in Scottish Lakes.” Proc. Roy. Soc. Edinb., vol. xxx., 1910, pp. 65-182. Woodhead, T. W.: “‘ Ecology of Woodland Plants in the Neighbourhood of Huddersfield.” Journ. Linn. Soc., Botany, vol. xxxvii., 1906, pp. 333. Yapp, R. H.: “ Wicken Fen.” New Phyt., vol. vii., 1908, p. 61. Yapp, R. H., Johns, D., and Jones, O. T.: “The Salt Marshes of the Dovey Estuary.” Journ. Ecol., vol. iv., 1916, pp. 27-42; vol. v., 1917, pp. 65-103. GENERAL INDEX _ Contractions employed: chars. = characters; fl. = flower; fr. = fruit ; infl. inflorescence ; pl.=plant ; repr.=reproduction ; var.=variety; veg. =vegetation. Signs employed : * illustration ; derivation given. Absorption of water : by roots, 25,33, 74, 91; causes reducing, 32, 33, 35 ; selective, 91; by soils, 87 Acacia, phyllodes, 44; cornigera, 133; false (see Robinia pseudacacia) Aecclimatization of plants, 75 Achenes, 137, *}186, *195, *197 Acids in plants, 143 Aconitin, 143 Aconitum Napellus, 143 : Acorn: fr., *187; seed, 146; pro- tection of fr., 136, *137 Actinic rays, 65, 67, 73 Adoxa, 62 Aigopodium Podagraria, 216 Aeration : of soil, 88 ; by earthworms, 96 ; of water, 52 Aérobic bacteria, 93 Aiithusa cynapium, 216 Afforestation and climate, 13 Agave: source of alcohol, 151; americana, 113 Agriculture in British Isles, 19 Agrimony fr., 197 Air: chemical effects of, 6; in aquatics, 48, 52 ; composition of, 6 ; dry, 34, 35 ; physical effects of, 8, 76 ; rarefied, 34, 35 Aira flexuosa : habit, 115 ; preecozx, 60 Air-spaces: in aquatics, 47, *48 ; in leaves, *26, *27; in marsh-plants, 63; in xerophytes, 40 Albumin, 5 Albuminous secds, *146, *147 Alcohol, 151 Alder, infl., 180 Alder-thickets, 229, 243, 269 Aleurone-grains, 146 ; -layer, 147 Alga, colour of, 53 31] Aliens, 215 Alisma Plantago, leaf-form, 50 Alkaloids, 143 Allium vineale, bulbils of, 160 Alluvium, 80 Almonds, bitter, seeds of, 138 Aloe, American. See Agave ameri- cana Alopecurus agrestis, 216 Alpine: flora, 210; pasture, 230, 251; plants, 36, 37, 38, 72, 74, of Treland, 286, 288, Scotland, 287, British Isles, 288; regions, 35; rocks, 286; vegetation : summit- heath, 251, vaccinium-moor, 255, pasture, 259, rocks, 286 Altitude: influence on climate, 11; on veg., 21 Alumina, silicate of, 87 Ammonium compounds produced by bacteria, 95, 124 Ampeopsis Veitchii tendrils, 118, *119 Amphibious plants, 28, 56 Anagrobie bacteria, +93 Andrena, 171 Anemophilous fis., 166 Angiosperms, +163, 183 Animals : food of, 5,79; and plants, 79 ; and seeds, 196 Aniseed oil, 151 Annuals, 107, 145 ; aquatic, 54, 55, 236; autumn-, 59, 107; garden-, 108; hygrophytic, 59; marsh-, 59, 240 ; mesophytic, 60 ; seasice-, 58, 59, 277, 280; summer-, 107; tropophytic nature of, 59 ; weeds of cultivation, 108, 215 Anther, 162 312 BRITISH Ants and flowers, 172; symbiosis, 133 Apocarpous ovaries, +183 Apple, 149; bud-scales, 61; polli- nation, 174; fr., 143, 184, 193, 194 ; suckers, 155 Apricot suckers, 155 Aquatic associations: in slowly- flowing water, 235; standing water, 236 ; swiftly-flowing water, 234 Aquatics, {28 ; aeration of, 52; air- spaces in, 47 ; annual, 55 ; brood- buds in, 54; chars. of, 47; chlorophyll in, 49 ; colour of, 53 ; cuticle in, 48; dangers to which exposed, 52; distribution, 233; free-floating, 237; leaf-form in, 50; in relation to light, 52; origin of, 55; pollination, 166, 275 ; propagation, 53 ; respiration in, 48, 50, 52 ; rooted, 237 ; roots, 49; seeding of, 54, 275; trans- Piration of, 52; vascular system of, 49 ; zoning of, 236 Aquatie vegetation: fresh - water, 230, 231, 233 ; maritime, 231, 275 Aqueous rocks, +82 Arabis, 115 Arable land, 18, 19 Arbutus Unedo, 23, 211, 212, 213; berries, 149 Arctic plants, 72 Arenaria ciliata, 213; 218 Aril, 196, 197 Aristolochia (Birthwort or Dutch- man’s-pipe), fly-trap, 176 Arrack, 151 Arrowhead. See Sagittaria sagitti- folia Arrowroot, 150 Artichoke, 150 Artificial habitats, 16, 215, 216; manures, 97; pastures, 20, 261, 264 Arum maculatum berries, 149; polli- nation, *175 Ash, 22, 269; bud-scales, 61; fr., 186, *187; fruit-dispersal, 195; weeping, origin of, 154 Ash of plants, 80 Ash-oakwood association, 229, 272 Ashwood-association, 229, 272 Asparagus, 150 Aspidistra: leaf-type, 42; pollina- tion, 176 peploides, PLANTS Assimilation, 4; in aquatics, 53; light-rays necessary for, 66; of nitrogen by bacteria, 95, 99; in xerophytes, 38 Associations, 224, 227; alpine, 251, 255, 259 ; aquatic, 234 ; bog-, 246, 251, 252, 253; classification of, 229 ; grassland-, 257 ; herbaceous, 230; influence of man on, 226; maritime, 275; marshland-, 241, 244; methods of studying, 226; moorland-, 225, 248 ; strand-, 277 ; woodland-, 265 Atmosphere, 6, 34, 76 Atmospheric equilibrium, 77 Atriplex portulacoides, 218 Atropa Belladonna : poison, 136, 143; pollination, 174 Atropin, 143 Aucuba, 154 Autogamy, +164 Autotrophic pls., 6, 123 Avens See Geum; mountain-. See Dryas octopetala Awl-type of leaf in aquatics, 50 Awlwort. See Subuwaria Bacteria: action on dead organic matter, 83 ; injurious, 61 ; nitrate, 95; nitrogen-, 95, 99; parasitic, 125 ; saprophytic, 123 ; soil-, 83, 93, 95, 96, 97, 123 Ballota nigra, 216. See Horehound Balsam. See Impatiens Noli-me- tangere Bamboo, 212 Banana, 149. See Musa Sapientum Barberry : honey, 172 ; root-suckers, 155; spines, 140, *141. See Berberis vulgaris Barley : alcohol from, 151; crops, 98; cultivation of, 20; food, 147; starch, 146 Bartsia: parasitic nature of, 126; water-glands of, 127 Bastard - toadflax. See Thestum humifusum Bay-laurel, 42 Beach, 16, 58 ; veg. of pebbly beach, 283 ; of sandy beach, 279 Beachy Head, 77, 284 Bean : climber, 116 ; protein of, 147; root-nodules, 95 ; seed, 146; dis- persal of seed, 195 Bedstraw, 217. See Galium Beech, 22; bud-scales, 61; fr., 187; insects on leaves of, 144; GENERAL INDEX pollination, 167; shade cast by, 272 Beechwood-association, 229, 272 Beer, 20, 151 Bees, 172, 173, 174 aa fleshy root, 150; sugar in, 46 Beetles, pollination by, 172, 174 Begonia, vivipary, 160 Belladonnin, 143 Belt on myrmecophily, 133 Berberis vulgaris spines, 140, *141. See Barbe: Berry, 136, 192, 193, 196 Beverages, 151 Bibliography, 339 Bidens, 59 ; fr., *197 Biennials, 58, 60, 108 ; leaves, 115; tuberous roots, 1; xerophytic chars. of, 60 Bilberry, acids in, 143 Bindweed, 217 Biological classifications, 105 ; habi- tats, 223, 225 Biology, +104, 184 Birch, 22; bud-scales, 61; com- petition ‘with the oak, 270, 271; infl., 180 ; pollination, 167 Birchwood-association, 229, 271 Birds : migratory, 198; as pollinators, 176; as seed-distributors, 196, 198, 209, 211, 213, 233; water-, 233 Bird’s-foot trefoil, ules, *95. See latus Bird’s-nest orchid. Bird’s-nest, yellow. Hypopitys Birthwort, 218. See Aristolochia Bitter principles in pls., 136, 139, 142 217; root-nod- Lotus cornicu- See Neottia See Monotropa Blackberry : bud-scales, 61; fr., 192. See Rubus fruticosus and Bramble Black bryony. See ZYamus com- munis Black horehound. See Badlota nigra Black medick. See Medicago lupu- lina Blackthorn. See Prunus spinosa Bladderwort. See Utricwaria Bluebell, 228 ; fr., 187 Bog-associations, 246; Carex, 231; Juncus, 231; Molinia caerulea, 231, 250, 251; Myrica, 231, 250, 253 ; Sphagnum, 231, 250 Bog-moss. See Sphagnum 313 Bogs, 240, 246 ; insectivorous pls. in, 131; xerophytic factors of, 35 Boulder-clay, 80 Box, 39 ; leaf-form, 43 ; leaf-mosaic, 68 ; pollination, 176 Bracken, 89, 228 Braets, 181, 194 Bramble, 220, 221; honey, 173; prickles, 141. See Rubus fruti- cosus and Blackberry Brandy, 151 Brassica, 216 Brazil-nut, 146 ; oil in, 143 British Isles: agriculture, 19; cli- mate, 12 Broccoli, 150 Bromus arvensis, 216 ; mollis, 60 Brood-buds, 161 ; in aquatics, 54 Broom, 44, *45, 89, 152,220 ; honey, 171; pollination, 169, 171. See Cytisus scoparius Broomrape. See Orobanche Brussels-sprouts, 150 Bryonia dioica, 214; berries, 149; pollen-flower, 171 ; tendrils, 119 Bryony: black (see Tamus com- munis); white (see Bryonia dioica) Bucktftorn, 152 Bud-detachment, 161; 60 ; -scales, 61; winter-buds, 60 Bulbils, 62 ; repr. by, 160 Bulbs, 62, 111, 115, 143, 161; multiplication by, 156 Bull’s-horn acacia. See Acacia cornigera Burdock, 60, 108 ; fr., 197 Bur-marigold. See Bidens Bush-communities, 17 ; -swamp, 243 Butcher’s-broom, 214 ; phylloclades, 44, *45 Buttercup, 218 ; fr., 187 ; honey, 172; hydathodes, 28; nectaries, 169, *170 ; seed, 146 Butterflies, 170, 173, 174 -protection, -variation, 153 ; Caatinga forests, 10 Cabbage (Brassica oleracea), 150 Cacti, 9, 165 Cakile maritima, 59 Calcareous marl, 82; rocks, 82; soils : pastures on, 272, woods on, 272 Calceolaria pollination, 171 Calcifuges, 89, 260 Calciphilous pls., 89 ; list of, 260 Calcium oxalate, 142, 143 314 Callitriche, 54 Calluna; relation to chalk, 89; distribution of, 224 ; honey, 173; leaf, 43. See Heather and Ling Calluna-heath, 230, 249, 250, 252 Callus, 140 Calyx, 162 Cambium, *45 Campanula hybrida, 216 Camphor, 143 Campion, moss-. Canada, 12, 19 Canadian fleabane, 216 ; pondweed, ' 234 (see Elodea canadensis) Canals, veg. of, 236 Canterbury-bell pollination, 173 Capers, 151 Capillarity, 86 Capillary water, 86 Capitulum, 178, *181 Capsella ; autogamy, 165; fecundity of, 108 ; fr., 188. Sce Shepherd’s- purse Capsular fruits, 187 Capsules, 189 Caraway oil, 151 Carbohydrate, 5 ; in seeds, 146 Carbon-assimilation, 4. See“Photo- synthesis Carbonate of lime, 89 Carbonic acid : agent of denudation, 80 ; gas, 4, 7, 50, 78 Cardamine pratensis: buds on leaves, *160 ; seeds, 196 Carex-bogs, 231, 247 Carlina spines, 140 Carnivorous pls., 246, 249. See Insectivorous pls. Carpels, 183 Carrot, 60, 108 ; fleshy root, 108, 150 Caryopsis, 186 Cascades, veg. of, 234 Casein, 5 Castor-oil, 148 ; seeds, 143, 147 Casuals, 215 Catch-crops, 98 Catkin, 178, 180 ; pollination, 167 Cat’s-tail grass, 217 Cattle, 20, 21 Cauliflower, 150 Cauline leaves, 114, 115 Celandine: greater (see Chelidonium majus) ; lesser, tubers, 111. See Ranunculus Ficaria Celery, 150 Cell-sap, 24, 92 Cellulose, 92 ; as food-reserve, 146 See Silene acaulis BRITISH PLANTS Cell-wall, 92 Centaurea Calcitrapa spines, 140; Cyanus, 216 Cerastium arvense, 216; andrum, 60, 107 Ceratophyllum, 49, 54, 56 Cereal seeds, 146 Cetraria, 133 Chalk, 82; disintegration of, 81; effect on pls., 89; glands, 263; plants, 260 Chalk-downs : xerophytic factors of, 35 ; xerophytic nature of pls. on, 89 ; veg. of, 258, 259 Chalk-soil : meadows on, 264 ; physi- cal properties of, 90 Chamerops, 23 Chamomile, 152, 216 ; oil, 151 Charlock, 216. See Sinapis ar- vensis Chelidonium majus latex, 142 Chemical action of water, 80, 81; properties of soils, 88 Chenopodium, 216. See Goosefoot Cherry : bud-scales, 61; citric acid in, 148 ; -laurel, 152 Chervil, 62 ; root, 111 Chestnut, edible (sweet): bud-scales, ue ; forests of, 23 ; fr., 136, *137, 187 semidec- Chestnut, horse: bud-scales, 61 ; fr., 190 ; infl., 179 Chicory, 216 Chickweed : fecundity of, 108, 165; pollination, 165; resistance to cold, 74. See Stelaria media Chillies, 151 Chlorophyll: as energy-absorber, 66 ; effect of intense light on, 73; in mistletoe, 128 Chlorophyll-tissue : in aquatics, 49 ; in sun and shade leaves, *72, *73 ; in xerophytes, *41, *45 Christmas-rose, 57, 177. Seo Helle- borus Chrysanthemum, 109 ; segetum, 216 Cichoriex latex, 142 Cinchona, 142, 152 Cinnamon, 151 Circea fr., 197 Circumnutation, +116 Citric acid, 143 Cladodes, 44, *45, 46 Classification of plants, 217; arti- ficial system, 219 ; natural system, 219 ; biological, 105 ; climatic, 16 ; ecological, 223; physiognomic, 16; GENERAL INDEX according to: frequency of seed- ing, 112, longevity, 107, mode of growth, 114, mode of nutrition, 123, water, 28, 64 Clay, 82; absorptive power, 87; origin, 81; permeability, 87; physical properties of, 82, 90; relation to water, 92; water- capacity of, 87; water-retaining capacity of, 87 Clayey loam, 82 Claytonia, 215 Cleavers, 60, A parine Cleistogamous fis., 165, 275 Clematis Vitalba, 214, 217 ; fr., ¥195; pollen-flower, 168 ; tendrils, 119 Cliffs, veg. of, 283 Climate, 9 ; effect on veg., 15 ; types of, 12 ; relation of soil to, 84 Climatic factors, 9 ; types of veg., 16 Climbers, root-, 119; stem-, 114; tendril-, 120 Climbing habit, evolution of, 120 Climbing lily. See Gloriosa superba Climbing plants, 115; habitat of, 119; leaf-mosaic, 68; of the hedgerow, 293; of woods, 267. See also Cuscuta, 128 Clouds, effect on climate, 13 Clover, 20 ; infl., 179 ; root-nodules, 95; sleep-movement of leaves, 70 ; white, insect-visitor, 171, 174. See Trifolium repens Cloves, 151 Clubmoss, *46 Coal, 83 Coccus, 190 Cock’s-foot, 217 Cocoa, 151 Coco-butter, 148 Coconut, source of alcohol, 151 ; oil, 147, 148 Coffee, 151 Colchicum autumnale, 159 Cold : effect on absorption of water, 33 ; effect on pls., 73; effect on formation of humus, 83 Colonists, 215 Colonization: of islands, 208; of British Isles, 209 ; of Ireland, 214 Colour-adaptations, in alge, 53 Colour of young leaves and shoots, 73 Coltsfoot, 62 Columbine, position of honey, 173 Colza-oil, 148 197. See Galium 315 Combustion, 78 Commons, 271, 296 ; veg. of, 296 Competition among pls., 4, €5, 203, 208, 210 : Composit : infl., 181; self-pollina- tion in, 177 ; sleop-movements in flower-heads, 71; unisexual fls., 176 ; insect-visitors, 170 Conerescent type of leaf, *39, 42 Condiments, 151 Cones of pine, 184 Coniferous forest-belt, 22; woods, 250 Conifers: pollen, 167; seeds, 183; dispersal of seeds, 196 Continental climate, 12, 23 ; islands, 207 Convolvulus: nectaries, *169; se- pium, climbing plant, 116, * 117, insect-visitor, 171 Copper-beech, origin of, 205 Coppicing : effect on leaf-fall, 109 ; on veg. of woods, 268 Copses, 268 Corallorhiza innata, saprophyte, 124, ¥*125 Coral-root orchid, 217. See Coral- lorhiza Cork, 41, 60, 61, 140 Corms, 62, 63, 111; multiplication by, 161 Corn-bluebottle, 216; -buttercup, 60 ; -cockle, 216 ; -marigold, 216 ; -spurrey, 60 Corolla, 162 Cortex, *45 Corydalis claviculata, evolution of tendril, 120 Corymb, *179 Cotton-grass, 217. See Eriophorum Cotton-grass association, 232 Cotton-grass (Eriophorum) moor, 931, 250, 251, 252, 253, 254, 255 Cotton-seed oil, 148 Cotyledons, storage of food in, 146 Couch-grass, 62; rhizomes, *110, 154 Cow-wheat. See Medampyrum Cranberry, citric acid in, 143 Crane’s-bill. See Geraniwm Crategus Oxyacantha, thorns, 140. See Hawthorn Creeping Jenny, 156 ; stems, 156 Crithmum maritimum, 59 Crocus, 62, 115, 215; structure of corm, *158; autumn- (see Col- chicum autumnale) 316 Crops : in Great Britain, 19 ; catch-, 98; root-, 98; rotation of, 97; variation in, 222 Cross-fertilization, 164, 165; -polli- nation, 164, 166 Crucifer® (Crucifers), fr., 188; honey, 172 ; infl., 179 Cryptogams, 7183 Cuckoo-flower, 196 (see Cardamine pratensis); -pint, 175 (see Arum maculatum) Cucumber (see Cucumis) ; squirting, 196 Cucumis, tendrils, 119 Cucurbita, tendrils, 119 Cudweed, 39, 60 Cultivated fruits, 148, 149, 154; ground: weeds of, 296 (list), annuals of, 60, 216 (list); land, 18, 19 Cultivation: annual weeds of, 108 ; of food-products, 18 Cupule, *187 Currant, 61; acids in, 143. See Ribes Current-leaves, 235 Cuscuta : twiner, 116, *128 ; germi- nation of seed, 128; parasitic nature of, *128 Cushion-growth, 38 ; -plants, 114 Cuticle, *40, *45, 140; in aquatics, 48 Cutin, 40 Cyclamen, 215 Cyme, 178, 180, *181 Cypress, *39, 42 Cypsela, *186, *195, *197 Cytisus scoparius, 220. See Broom Dabecia, 213 Dahlia, tubers, 111 Dairy-farming, 20, 21 Daisy, 126, 217 ; rosette-habit, 115 Dandelion, 37, 126, 152; fr., 195; infl., 181 ; latex, 142 ; leaf-mosaic, 68; pollination, 170; rosette- habit, 115; root, 111. See Tarax- acum Darkness, effect on growth, 71 Darwin : on climbing pls., 115; cross- fertilization, 164; dispersal of seeds, 197 ; earthworms, 96 ; evo- lution, 202, 204 ; primrose, 177 Date, seed, 146 Deadly nightshade, 217. See Atropa Belladonna Deadnettle, 30, 62, 216 ; honey, 173 Deal, 152 BRITISH PLANTS Deciduous trees and shrubs, 60, 109 ; woods, 265, 270 (mixed) ; wood- land, belt in Europe, 22 Decomposition, 78, 93 Defensive equipment of pls., 135 Dehiscence of fruits, 187 Denizens, 215 Denudation, agents of, 80 Deserts, 16, 17, 35, 86, 88, 281 De Vries, 204, 205, 221 Dhurra. See Sorghum vulgare Dichogamy, +176 Dicotyledonous woodland, 16 Diffusion, 8, 25, 50, 92 Digitalin, 143 Digitalis purpurea, 143. See Fox- glove Dimorphism, +177 Dicecious fls., {176 Dionea, 132 Disease-producing enemies of pls., 1 Dissected leaves in aquatics, 50 Dodder. See Cuscuta Dog-rose. See Rosa canina Dog’s-mereury, 62 ; pollination, 176; rhizome, 154. See Mercurialis perennis Dog’s-tail grass, 217 Dominant pls., 227 Draba verna, 60, 107, 218 Drosera, 89 ; insectivorous pl., 129, *130; See Sundew Drought, effect on blooming of annuals, 107 Drugs, 152 Drupe: dispersal, 196; of holly, *147; protection by bloom, 136; structure, *192 Dryas octopetala, 114 Dryness, physiological, 34 Dry side of mountains, 12 Duckweed, 49. See Lemna Dune-marsh, veg. of, 283 Dune-ridges, veg. of, 231, 281 Dunes, 80; fixed, 230, 282; grey, 282 ; shifting, 281; white, 281 Dutchman’s-pipe, 176, 217. See Aristolochia Dwarfing of plants, 36; by wind, 77, 284, 286 Earth-acids, 83, 88, 94 Earthworms, 88, 96, 265 Ecological groupings : climatic, 16; physiognomic, 16; by habitat, 223 ; by plant-associations, 224 GENERAL INDEX Economie botany, 152 Edaphie factors, 8, 84 Eel-worm, 135 Egg-cell, 162 Elder : berries, 149; bud-protection, 61; bud-scales, 61 Elementary species, 205, 221 Elm: bud-scales, 61; pollination, 167 ; seed, 154 ; suckers, *155 Elodea, 56, 215, 234 Embryo-sae, *163 Embryo, storage of food in, 146 Empetrum, 43 Enchanter’s-nightshade, 197. See Circea Endemic, 208 Endocarp, 192 Endosperm, *146, *147 Enemies of plants, 135 Energy, 7 ; solar, 4, 65 Engler, 219 Entomophilous flowers, 167 ; classi- fication of, 172 Environment : adaptation to, 3, 105, 203 ; factors of, 4 Epicarp, 192 Epidermis, *26, *27 Epiphytes, 121; seeds of, 122; of woods, 266 ; xerophytic nature of, 121 Epipogum aphyllum, 124 Erica, 39, leaf, 43; stomata, 40, ciliaris, 218, cinerea, *41; Mackayi, 213 ; mediterranea, 213 ; Tetralix, *43, pollination, 173; vagans, 213. See Heath Erica Tetralix-moor, 231, 250, 252 Ericoid type of leaf, *43 Erigeron canadense, 216 Eriocaulon, 214 Eriophorum. See Cotton-grass Eriophorum (cotton-grass) moor, 231, 250, 251, 253, 254, 255 Erodium: dispersal of fr., 195; structure of fr., 190, *191 Eryngium maritimum, 59 Estuarine marshes, 231 Eucalyptus leaves, 46 Euphorbia, 105 ; latex, 142 ; hiberna, 213; Peplis, 57 Euphrasia, 220; parasitic nature, 126 Europe, climate and veg. of, 21 Evaporation, 34, 76 Evening-primrose: honey, 173; infl., 180 ; time of opening, 174 saprophyte, 317 Evergreen coniferous woods, 22, 265, Evergreen dicotyledonous woods, 16, ae Evergreen oak, 23, 212 ; perennials 109; plants, 10, 16, 29, 30, 31; xerophytes, 39 Everlasting pea. folia Evolution, 201; theory, 202; of aquatics, 55 ; flowers, 168 ; thorns, 139 Eyobright, 220. See Huphrasia See Lathyrus latte False fruits, 193 False oat, swollen stem, 111, *113 Farming in British Isles, 19 Fats and oils, 5 ; storage in seeds, 147 Felspar, 81 Fennel, nectaries, *170 Ferns, 62 ; Filmy, 29 ; Killarney, 29, 214 Fertilization, 164 Festuca - Agrostis - Anthoxanthum association, 250 Festuca ovina, viviparous, 160 Ficus. See Fig Fig, 212; fr., 193, *194; symbiosis with insects, 133 Figwort : fls., 177 ; pollination, 174, See Scrophularia Filament, 162 Fir, 22 ; resin of, 142 Flax, 152 Fleabane, 218 Fleshy roots, 150 ; of xerophytes, 41 Flies, visiting fls., 172, 173, 174 Floating leaves, 51; manna-grass (see Glyceria fluitans) Flora, Alpine, 210; discontinuous, 211; of Great Britain, 209 ; Irish, 214; Lusitanian, 211; Pre-glacial, 212 Floras, list of, 342 Flour, 148 Flowers: anemophilous, 166 bee-, 174; butterfly-, 174; cleisto- gamous, $165; dimorphic, 177; entomophilous, 167 ; honey-, 169 ; and insects, 167, 178 ; moth-, 174; pollen-, 168; protandrous, 177; protogynous, 177 ; self-pollinated, 164, 165; sleep-movements in. 71; structure of, 162, *163 ; time of opening, 174; trap-, 175; tri- morphic, 177; unisexual, 176; wasp-, 174 ; water-nollinated, 166 318 Fluviatile deposits, 80 Fly-catchers, 132; -traps, 175, 177 Fodder, 21 ; -plants, 150 Follicle, 188, *189 Food, 24, 79, 145; -products, culti- vation of, in British Isles, 18; -reserves, 62, 145, 161 ; -stufis, 5 Fool’s-parsley, 60, 216. See Hthusa cynapium Forests: destruction of, 13, 22; maintenance of, 11, 17. Sce Woodland, Woods, Coniferous, Deciduous, Dicotyledonous, Ever- green, and Rain-forests Forget-me-not, 217; early (see Myposotis collina) Form : in relation to function, 103 ; to environment, 105 Formations, 225, 226, 227 Foxglove, 60, 108, 152; calcifuge, 89; infl., 179; pollination, 173, 174. See Digitalis Fox-tail grass (Alopecurus), 216 Freaks, origin of, 153 Frogbit. See Hydrocharis Frost, 81 Fruits : achenial, 186 ; capsular, 187 ; classification of, 184, 194; culti- vated, 148; propagation of, 149, 154; sterility of seeds in, 149; defensive equipment, 135, 136, *137; development of, 183; dis- persal of, 195, by animals, 196, birds, 196, 198, water, 197, wind, 196, 198; dry, 186; explosive, 196; false, 185, 193; feathered, 195; poisonous, 149; storage of food in, 148 ; succulent, 136, 148, 192 ; true, 185 ; winged, 195 Fumaria officinalis, tendrils, 120 Fumitory, 216. See Fumaria Fungi, disease-producing, 61, 135, 138 ; parasitic, 125 ; saprophytic, 128 ; soil-, 96, 133 Galium, 60; honey, 172; Aparine (cleavers), climbing habit, 116; fr., 197 Garden-annuals, 108 Geitonogamy, +166 Gentian, 152 Genus, 218 Geophilous plants, +82 Geophytes, 62, 74, 118, 115 Geranium: fr., 190; honey, 173; nectaries, *169; seeds, 196; pros- ratum, 221; pusillum, 216; Robert- BRITISH PLANTS tanum, 60; sanguineum, 221; garden-, origin of vars., 154 Germination of seeds, 137, 138 Geum, fr., * 197 Ginger, 150, 151 Glacial Epoch, 209, 210, 212 Glaciers, 81 Gladiolus, 63 Glasswort. See Salicornia Gloriosa superba, tendrils, 119 Glossopteris, 339 Glyceria fluitans, 51 Goat’s-beard, 217. See Tragapogon Gooseberry, 215 ; fr., *193, 195 Goosegrass. Sce Galium Aparine Gorse: fl., 169; flowering of, 57; honey, 171; seeds, 196; thorns, 44, 139, 140. See Ulex Goutweed, 216, 218. See Agopo- dium Granite, 80 Grape: fr., 136, 184, 193, 195; source of alcohol, 151 Grasses : effect of shade on, 72; fis., 177; fr., 186; hay-producing, 262 ; hemi-parasites on, 126; infl., 179, 180; pollination, 167 Grass-heath, 230, 249, 250; -land, 15, 16, 17, 19, 28, 261; -wrack (see Zostera) ; of Parnassus, mock- glands, 171 Ground-ivy : creeping stems of, 156; leaf-mosaic, 68 Groundsel, 108. See Senecio vul- garis Ground-water, 28, 85, 86 Growth: classification by mode of, 114; influenced by light, 71 stunted, 36 Guano, 97 Guard-cells, *26, *27 Guilds, 122 Gulf Stream, 13 Gums, 142 Gymnosperms : seeds, 183 pollination, 163 Habenaria intacta, 213 Habitat : biological, 223 ; closed, 16; open, 16 Hairs, 39, 61, 136 Halophytes, +88, 278, 279 Haptera, 235 Hart’s-tongue, 217 Haustoria of parasites, 125, *129 Haw, 149, 184. 193 Hawkmoth, 170, 171, 174 GENERAL INDEX Hawkweed, Hieracium Hawthorn : bud-scales, 61 ; fr., 193; suckers, 155; thorns, 140. See Crategus Oxyacantha Hay, 20, 262 Hazel: bud-scales, 61; fls., 176; fr., 187; infl., 180; oil of nut, 147; See aap 167; -thickets, 269. ee Corylus Heartsease, 217 Heat : as a climatic factor, 9 ; effect on pls., 73; influence on vital functions, 6 ; rays, 65 Heath, 39 ; calcifuge, 89 ; mycorhiza of, 125 ; stomata, 40 ; type of leaf, 43. See Hrica Heath, 15, 16; Calluna-, 230, 249, 250, 252; grass-, 230, 249, 250; summit-, 251 Heather. See Calluna Heselerstador, 230, 250, 252, 253, 115, 220, 221. See Hedera Helix, climbing roots, 119. See Ivy Hedge-mustard, honey, 172 Hedgerow plants: effect of weak light on, 72; list of, 292 Helianthemum Breweri, 209 ; gutta- tum, 213 eee 68 ; in the hedgerow, Helleborus, honey-leaf, 172 Helminthia echioides, 60, 115. See Oxtongue Hemi-parasites: of meadow-land, 263 ; of natural pasture, 257 Hemlock, 152 Henbane, fr., 187 Heracleum Sphondylium, pollination, 170. See Hogweed Herbaceous perennials, 58, 62, 109, 113, 115, 145 Herbals, 217 Herb-Christopher, 217 ; -paris, 214; -Robert, 60, 217 , Heterophylly, {51 Heterotrophie plants, 6, 123 Hieracium, 220 ; latex, 142 ; iricum, 209 Highlands, 12, 17, 21 High-moor, 246 Hip, 149, 184, 195; citric acid in, 143 Hogweed : fis., 181’; hydathodes, 28 ; pollination, 170. See Herac’eum Holcus lanatus, 115 319 Holly: bud-scales, 61; fr., *147; leaf, relation between form and function, 103; spines, 104, 139, 140 ; xerophytic chars. of, 30, 39 Hollyhock, fr., 191 Honckenya peploides, 114 Honey: a source of alcohol, 151; con- cealment in fls., 170, 172 ; position in fls., 172, 173 ; protection of, 172 Honey-dew, 134; -fls., 169; -glands, *169, *170 Honeysuckle : bud-scales, 61 ; direc- tion of climbing, 116 ; honey, 173 ; visits of insects to, 170. See Lonicera Hook-climbers, 115 Hop, 214; climbing pl., 116, *117; fr., 194; pollination, 167. See Humulus Hornbeam, fr., 194 Hornwort. See Ceratophyllum Horse-chestnut : bud-scales, 61; fr., 190 ; infl., 179 ; leaf-mosaic, 68 Horse-radish, 151 Horse-tail, 44, 62 Hottonia, 56 ; brood-buds, 54 ; leaf- form, 50; stem-structure, *48, See Water-violet. Houwseleek, 155, *156 Humble bees, 171, 173 Humidity, 9, 11. Sce Moisture Humous acids, 88, 94, 240, 244, 246, 249 Humulus Lupulus, direction of twin- ing, 116, *117 Humus, 19, 83, 93 ; “‘ artificial,” 83 ; mild, 94; of woods, 265; -soils, properties of, 93 Hyacinth, 63, 115; repr. by bulbs, ’ 157; Roman, 143 Hybrids, 221 Hydathodes, 727, 29, 263 Hydrocharis, 51, *54 Hygrophilous pls., 29; phytes, 58, 62 Hydrophytes, ¢47 Hygrophytic annuals, 59 Hygroscopie water, 86 Hymenoptera, 173 Hypocotyl, 146; swollen, 111 tropo- Ice Age, 22, 209, 210, 212, 213 Iceland moss. See Cetraria Igneous rocks, 82 ; Impatiens Noli-me-tangere : fr., 189; seeds, 196; fuli, 215 In-breeding, 165 320 Indian corn. See Zea Mais Indigenous plants, 215 Inflorescences, 178 Insectivorous plants, 129, 131 Insect-pollination, 167 Insects, adaptation to fls., 168, 178 Integument, *163 Interdependence of pl. and animal life, 79 Intoxieating drinks, 151 Inula salicina, 214 Ireland : alpine veg. of west of, 286, 288 ; flora of, 213, 214 ; pls. absent from, 214 Iris : aril (fcetid iris), 196; fr., 187, *189; honey, 172; pollination, 177; rhizome, 154; seed, 146; mesophytic chars. of, 31; xero- phytic chars, of, 31, 46 Irrigation, 18 Islands : flora of, 207 ; origin of, 207 Isoétes, 50 Ivy, 39; climbing pl., 119; honey, 172; leaf-mosaic, 68, *69. See Hedera Helix Ivy-leaved toadflax, 216 Jacob’s-ladder, 217 Jasione montana, 60 ; insect-visitors, 170 Juneoid type of leaf, 43 Juncus-bog, 231, 246 Juncus bufonius, 59 Juniper, 36 ; leaf, 42 ; seeds, 196 Key, 186, 190 Killarney fern, 29, 214 Knight, Robert, on self-fertilization, 165 Kohlrabi, 111, *112 Labiate, fr., 191 ; honey, 173 Laburnum seeds, 138 Lactuca, latex, 142. See Lettuce Lacustrine deposits, 80 Lady’s-fingers, 217; -smock, 217; -tresses, 217 Lakes : veg. of highland-, 238 ; veg. of lowland-, 236 Land-plants, 28; growth-forms of, 114 Larch, 22 ; -wood, 274 Larkspur: fr., 188, *189; pollina- tion, 174 Latex, 142 Lathrea: parasitic habit of, 126, *127 ; water-glands of, 127 BRITISH PLANTS Lathyrus tendrils, 118, *120 Latitude, 10 Laurel, 23, 39, 212; bay-, 42; cherry-, extra-floral nectaries, 173 Lavender, essential oil, 143, 151 Law of heredity, 335 Leaf: -fall, 60, influenced by pruning, 109, and coppicing, 109; erect-, 46; -modifications, 38; -mosaic, 45, 68; position in xerophytes, 46; -scars, 62; shade-, 72, *72; -spines, 140, *141; sun-, *72; -types in aquatics, 50, in xero- phytes, 42 e Leaves: colour of young, 73; cur- rent-, 235; edible, 150; fixed light-position of, 69 ; floating-, 49, 51; heliotropism of, 68; sleep- movements in, 70, *71 Legume, 188, *189 Leguminos® : fr., 188 ; honey, 173 Leguminous crops, 98 ; pls. : protein of, 147, root-nodules of, 95 Lemna, 55, 56 ; roots, 49 Lemon, 143, 151 Lenticels, 140 Lentils, 147 Lepidium ruderale, 60 Lepidoptera, 170, 174 Lepidopterous flowers, 170 Lettuce, 150. See Lactuca Lianes, 121, 267 Lichen-flora, 22 Lichens : epiphytic, 121 ; habitat of, 133 ; symbiosis of, 132 Light: in relation to aquatics, 52; composition of, 66; source of energy, 4, 65; effect on growth, 36, 71, 72, 115, 120; effect of intensity on veg. of woods, 266, 269, 270, 272, 274; -position of leaves, fixed, 69, in xerophytes, 46 ; effect on transpiration, 34, 45 Lignified tissue, 41 Lilac: bud-scales, 61; infl., 179; suckers, 155 Lily, 63, 72; repr., 15; tiger-, repr. by bulbils, 160 ; of the Valley, 214 Lime-tree : bud-scales, 61 ; fr., 194; honey-dew, 134 ; insects on leaves of, 144; leaf, relation between form and function, 103 Lime: bicarbonate, 89; carbonate, 89 ; silicate, 81 Limestone, 82; disintegration of, 81; -hills, 81; -pasture, 230; GENERAL INDEX -tocks, veg. of, 290; woods on, 272 Ling. See Calluna Linseed oil, 143, 147, 148 Liquid-diffusion, 25, 92 LItriodendron, 212 Littorella, 50 Loam, 82 hands Dortmanna, 50, 56; urens, Lochs, highland, veg. of, 238 Lomentum, 188, *189 London-pride, 213; rosette-plant, +37. fraga umbrosa Longevity : classification of pls. ac- cording to, 107; of leaves of xerophytes, 39 Lonicera. See Honeysuckle Loosestrife: purple (see Lythrum Salicaria) ; yellow (see Lysimachia vulgaris) Lousewort. See Pedicularis Lowland-pastures, 230, 258 ; -plants found in alpine regions, 289; -rocks, veg. of, 290 Low-moor, 246 Lungwort, 217 Lusitanian Flora, 211 geey fl., 174; fr., 187; honey, Lycopsis arvensis, 216 ane vulgaris, insect-visitor, 1 Lythrum Salicaria, fls., 177 offsets, 155; See Sazi- Macroyis, 171 Magnolia, 212 Maize, sugar of, 146. See Zea Mais Malic acid, 143 Mallow, 216 ; fr., 191, *192 Malva sylvestris, 216 Man, influence on veg., 13, 18, 19, 198, 227, 293 Mandioe (Manihot), tapioca from, 150 Mangold, 150 Manna-grass. See Glyceria fluitans Manures, 96; artificial, 97; a source of humus, 83; living, 97; natural, 96; effect on veg. of meadows, 262 Maple : bud-scales, 61; fr., 190, 195; honey-dew, 134; leaf- -mosaic, 68, *70, *71; origin of variegated vaTs., 154 Maqui, 23 Marigold, fr., 137 321 Marine deposits, 80 Maritime; aquatic veg., 275; as- sociations, 275; pls. growing in a regions, "288, adaptations or, Marl, 82 Marram-grass : leaf, 40, *43; asso- ciation of, 281 ; binding power of, 281. See Psamma Marsh: -annuals, 59, 240; dune-, 283 ; estuarine-, veg. of, 231, 278 ; freshwater-, 230, 240, 283 ; -mari- gold, 168; -plants: habit com- pared with bog pls., 245, chars. of, 240, geophilous, 62, hygrophilous, 29, 58; position of leaves with regard to light in, 46, tropophytes, 58, xerophytic chars. of, 58 ; salt-, veg. of, 231, 278 ; true, 244, 253 Mead, 151 Meadow-land, 261 ; weeds of, 263 Meadows, 15; on damp soils, 263 ; on dry chalk soils, 264 ; salt-, 279 Meadow-saxifrage, 62, 160. See Saxifraga granulata Meadow-sweet, 217 Mealies. See Zea Mais Meat, 5 ; transformation of veg. into, 20 Medicago, 60 ; fr., 188 ; infl., 179 Medick. See Medicago Mediterranean region, 16, 23 Melampyrum, parasitic nature, 126 Melilot, 216 Mericarp, 190 Mesoearp, 192 Mesophilous tropophytes, 58, 62 Mesophytes, $29, 31 Mesophytie annuals, 60 Mica, disintegration of, 81 Micropyle, 163 Mignonette, insect-visitor, 174 Millet, 147, 148 Milk, 20, 21; -thistle, 216, 217 Mimiery, 137, 138 Mineral salts, 24, 33, 123 Mint, 62; honey, 173; rhizome, *110; repr. by rhizomes, 154; volatile oil, 143 Mistletoe, 214; berries, 149; para- sitic nature of, 128, *129 Mock-orange, suckers, 155 Moisture, 11, 34. See Humidity Mole-heaps, 258 Molinia cw@rulea-bog, 231, 250, 251 Monkshood, 217 ; honey-leaf, * 171 Monoearpic plants, 7112 21 322 Moneecious flowers, +176 Monotropa, saprophytic nature of, 124 Montbretia, 159 Moor: cotton-grass (Eriophorum), 231, 250, 251, 252, 253, 254, 255 ; Erica Tetralix-, 231, 250, 252; heather-, 230, 250, 252, 253, 254 ; high-, 246; low-, 246; moss-, 225, 254 ; shooting-, 252 ; Vaccinium-, 231, 249, 251 Moorland, 16, 19, 248; pls., chars. of, 249 Morphia, 143, 152 Morphology, +103, 104, 184 Moschatel. See Adoxa Moss-campion (see Silene acaulis) : -moor, 225, 254; -rose, prickles, 116 Mossy saxifrage. See Sazifraga hypnoides Moths, 170, 173, 174 Mould, 94 Mountain-ash berries, 149; -avens (see Dryas octopetala) Mountains: light, 35, 36; rainfall, 12, 17 ; temperature, 11, 17 ; veg., 22, 36; lowland pls. growing on, 289 Mountain-torrents, veg. of, 234 Mouse-ear chickweed. See Ceras- tium Mouse-tail. See Myosurus Mucilage, *41 Mulberry, fr., 192, *193 Mullein, 39, 108 Musa Sapientum, 149 Mustard : fr., *190; source of, 148, 151; and cress, 150 Mutation Theory, 204 Mycorhiza: of bogand moorland pls., 246, 249; of forest-trees, 267 ; of partial saprophytes, 125 ; of sapro- phytes, *128 ; symbiosis, 133 Myosotis collina, 107 Myrica-bog, 231, 250, 253 Uyriophyllum : brood-buds, 64 ; pol- lination, 167 Myrmecophily, +133 Myrtle, 23 ; leaf-type, 43 Narcissus, 62, 63; bulb, *157; fr., 186, 189; infl., 181; nectaries, 169 Nardus stricta-association, 251 Nasturtium amphibium, 28 ; garden- : hydathodes, 28; tendrils, 119; fr. and seed, *147 BRITISH PLANTS Natural manures, 96 ; Orders, 105 ; regions of dryness, 35 ; Selection, 139, 203 : Nectaries, 169; extra-floral, *118, 134, 173; sham, 17% Nectarine, origin of, 205 Needle-type of leaf, 42 Nepenthes, 132 5 Neottia, saprophytic nature, 124 Nettle, 152. See Urtica New Zealand flax, 42 Nightshade: common, 216; deadly (see Atropa Belladonna) ; enchan- ter’s, 197 (see Circwa); woody, 152, sham-nectaries, 171 Nitrate-bacteria, 95 Nitrates produced by bacteria, 95 Nitrites produced by bacteria, 95 Nitrogen, 7; -bacteria, 95; circu- lation of, 99 ; fixation of, by bac- teria, 99; losses of, in soil, 99; sources of, in soil, 99; utilization of, by pls., 95 Nitrogenous food-reserves, 146 Nomenelature of pls., 217 Norfolk Rotation, 97 Nuphar, 52 Nutmeg, 151 Nutrition: of green pls., 123; in sectivorous pls., 129; non-green pls., 123; parasites, 125; sapro- phytes, 123 ; effect of xerophytic conditions on, 36, 38, 139 Nuts, *187, 196; oil, 146; tection against animals, 136 Nux-vomica, 138 Nyetitropism, 70 pro- Oak, 22; bud-scales, 61; competi- tion with the birch, 270, 271; effect of coppicing, 109 ; enemies of, 138; galls, 138; pollination, 167 ; tannin of, 142 Oak-birch-heath association, 229, 271, 296 Oakwood-associations, 229 Oak-woods: damp, 269; dry, 270; lowland-, 269 ; upland-, 270 Oats, cultivation of, 20, 98 ; food, 147 Ocean-currents, 13 Susans climate, 12, 21, 22 ; islands, 0 Gnanthe fluviatilis, 209; Phellan- drium, 50 Gnothera biennis (see Evening-prim- rose) ; Lamarckiana, 295 Offsets, 155 GENERAL INDEX Oils, 5 ; antiseptic, 136 ; fixed, 142; as a food-reserve, 146, in seeds, 147 ; volatile, 143, 151; in xero- phytes, 42 Oily seeds, 148 Old man’s-beard, 217. Vitalba Olive, 23 Onion: bulb, 150; bulbils, 160; infl., 181 ; seeds, *146 ; sugar, 146, 150 Ononis arv-nsis, spines, 44 Open associations, 277 ; habitats, 16 Opium, 143, 152 Orange, 23 Orchid: bird’s-nest (see Neottia) ; coral-root (see Corallorhiza) ; fr., See Clematis 189; honey, 173; infl., 180; pollination, 177; seed, 195; tubers, 111 Origin of species, 202 Orobanche, 127 Orobus, fr., *189 Osier-beds, 244 Osmosis, 25, 91 Ovary, 162, 183 Ovule, 162 Oxalis, 62 ; aril, 196 ; cleistogamous fis., 165; fr., 189 ; oxalic acid in, 143; rhizome, 154; seeds, 196; sleep-movements of leaves, 70, *71. See Sorrel Oxtongue, 60, 217. See Helminthia echioides Oxygen, 6, 7; in air, 78, 79; in water, 48, 50; in soil, 88 Peony, 168 Palisade-tissue, 41, *72 Palm, 23 ; -oil, 148 Pampas, 20 Paniele, 178, 179 ; Papaver somniferum, alkaloid of, 143, 152 Paper-making materials, 152 Pappus, 195 Parasites, 6, 125, 206 ; partial, 126 ; total, 126; hemi-parasites of meadow-land, 263, of natural pasture, 257 Parallel development, 105 Parietaria officinalis, fls., 177 Paris quadrifolia, 214 Park-land, 17, 126 3 Parsley, 181 Parsnip, 150 Partial parasites, 126 ; saprophytes, 125 323 Passion-flower, tendrils, *118 Pastures, 15, 18, 20; alpine, 230, 251, 259 ; artificial, 20, 261, weeds of, 264; limestone-, 230, 259; lowland-, 230, 258 ; natural, 257; sub-alpine, 230, 258 Pea: fr., 188; pollination, 165; protein of, 147 ; root-nodules, 95 ; seeds, 146, 195; tendrils, 118. See Lathyrus Peach ; fr., *192 ; origin of, 205 Pear : bud-scales, 61; cultivation of, 149 Peat, 17, 83, 93, 94, 244, 248, 254; absorptive power of, 87; -bogs, 246, 248, xerophytice factors of, 85; moorland-, constitution of, 249, comparison with marsh-peat, 249, conditions necessary for formation of, 248 ; water-capacity of, 87. See Humus Pedicularis, 59; parasitic nature, 126 Pelargonium : fr., 190, 195; infi., 181 Pellitory. See Parietaria officinalis Pennyeress, 216 Pepper, 151; -mint, 151 Percolation in soils, 87 Perennating organs, 62, 109; struc- tures, [62 Perennials, 108; evergreen, 109; herbaceous, 109, 115, 145 ; tropo- phytic, 59 ; xerophytic, 59 Perianth, +162 Pericarp, 183 Permeability of soils, 87 Peruvian bark. See Cinchona Petals, 162 Petioles : of Acacia, 43, *44; of float- ing aquatic leaves, 51 Phaseolus vulgaris, twiner, 116. See Scarlet runner. Phleum arenarium, 60 Phlem, *45 ; in aquatics, 49 Phormium tenax, 42 Photosynthesis, +4, 66, 78 Phragmites, 62; -associations, 242, 245 Phylloclade, 44, *45 Phyllode, 43, *44, 46 Physical action of water on rocks, 81; properties of soil, 85 Physiognomic groups, 16 Physiological dryness, 34, 85 Physiology, +103 Pillwort. Ses Pilularia Pilularia, 28, 50 324 Pimpernel, fr., 187, *188 Pine, 22, 39, 272; leaf, 42; myco- rhiza, 125; pollination, 163, 167; former range of, 273 ; resin, 142 ; seeds, 183, 195; shade cast by, 273 ; stomata, *40 Pineapple, 194 Pin-eyed fils. of primrose, 177 Pine-woods, 273 Pinguicula: insectivorous pl., 130, *131; grandiflora, 218 ; lusitanica, 213 ; vulgaris, 130 Pinks, infl., 180 Pistil, 162 Pitcher-plants, 132 Pith, *45 Plains, 12 Plane, bud-protection, 61 Plantain, 37; fis. 177; fr, 187; infl., 180 ; pollination, 167 Plant-associations, 224 Plasmolysis, +92 Plateaux: climate of, 12; growth- form of pls. on, 36 Plum: bud-scales, 61; cultivation of, 149; fr., 184, 192, 194; extra- floral nectaries, 173 ; suckers, 155 Poa alpina: fecundity of, 108 ; vivi- pary, 160, *161; annua, 60 Pod, i88 Poisonous fruits, 149; pls, 152; seeds, 138 Polar regions : duration of sunlight, 67; growth-form in, 36; xero- phytic factors of, 35 Polien, 162 ; -bread, 168 ; -fls., 168 ; -grains, 162, 163; -tube, *163; of wind-pollinated fls., 166; of Zostera, 166 Pollination, 163, 164 ; agents of, 166 ; by birds, 176 ; cross-, 164, 166 ; by insects, 167; self-, 164, effect on the stock, 165 ; prevention of, 176 ; by slugs, 176 ; a form of symbiosis, 133 ; by water, 166 ; by wind, 166 Polyearpic plants, +112 Polygonum amphibium, 28, 56; Convolvulus, direction of twining, 1 : ;dumetorum, imperfect twiner, Il Polypody, epiphytic, 121 Pome, 193 Ponds, lowland-, veg. of, 236 Pondweeds, 233. See Potamogeton Pools, highland-, veg. of, 238 Poor man’s weather-glass, 217. See Anagallis BRITISH PLANTS Poplar: bud-scales, 61; fis., 176 pollination, 167, seeds, 195; suckers, 155 Poppy, 216; fr., 187; opium-, 152 ; seeds, 146, *147, 195; summer- annual, 107 Pore-space, 85 Potamogeton crispus, brood-buds, *51, 54; leaf-form, 50, *51 Potash, silicate of, 81 : Potato, 62 ; a source of alcohol, 151 ; food-reserve in tubers, 150; “seed ’-, 149; starch, 145; tubers, *111; repr. by tubers, 149, 159 Potato, sweet, 150 Potentil, honey, 172 Poterium, pollination, 167 Prairies, 9, 15, 23, 35 Prickles,139, 140 Primrose : dimorphism, 177; honey, 173 . Privet : berries, 149; bud-scales, 61 : origin of variegated vars., 154; effect of pruning, 109 | Prostrate stems, pls. with, 114 Protandrous flowers, $177 Protein, 5, 124 ; in seeds, 146, 147 Protogynous flowers, 177 Protoplasm, 92, 93 Protozoa, soil, 96 Prunus spinosa, thorns, 140 Psamma, 62; -association, 281; leaf, 40, *43; binding power of, 281 Pseudocarp, 185, 193 Putrefaction, 78, 93 Puzstas, 20 Pyrus torminalis, 214 Quartz, 80 Quinine, 142, 152 Raceme, 178, *179 Radical and cecauline leaves, pls. with, 115 Radish, 216; fr., 188 ; fleshy root, 150 Rainfall, 11 Rain-forests, 9, 16, 23, 27, 121; -shadow, 12 Ranunculaces, nectaries, 169 Ranunculus aquatilis, 56, 220, hete- rophylly, 51, leaf-form, 50, 51; arvensis, 60 ; Ficaria, tubers, 154, 159 ; sceleratus, 59 Rape: -oil, 148 ; -seed, 148 GENERAL INDEX Rare plants. See Aliens, Alpines, Discontinuous, and Lusitanian floras Raspberry: fr., 192; acid in, 143; suckers, 155 Receptacle, *163 Reduction in structure, 206 Reed: common (see Phragmites) ; -mace (see 7'ypha) Reeds, 62; position of leaves in yaad to light, 46 ; pollination, Reed-swamp, 225, 230, 241; zoning of, 242 Reed-type of leaf, 43 Reindeer-moss, 22 Reproduction : of maritime aquatics, 275; by seed, 153, 162; vegeta- tive, 153, 154 Resin, 142 Respiration, 6, 7, 78; of aquatics, 48, 50, 52 ; of roots, 88 Rest-harrow, thorns, 44 Retrogression, 206 Reversal-point in tendrils, 115 Ba: 60; parasitic nature, Rhizomes, 62, *110; food-reserves in, 150; multiplication by, 161 Rhododendron, leaf-form, 43 Rhubarb, 152; calcium oxalate in, 143 ; culinary, 150 Ribbon-type of leaf in aquatics, 50, 51 Ribes, 215 ; bud-protection, 61 Rice : food, 147 ; -starch, 146 River-banks, veg. of, 243 Rivers, veg. of, 235 Robinia spines, 140, *141 Rock-cress (see Arabis) ; -rose, 169 Rocks : alpine, veg. of, 286 ; classifi- cation of, 82; denudation of, 80 ; limestone-, veg. of, 290 ; lowland-, veg. of, 290; maritime, 231, veg. of, 283 ; sub-alpine, veg. of, 290 ; xerophytic factors of, 35 Rocky summits, veg. of, 289 Rolled-leaves, 40, *41, *43 Root-absorption, 25 ; -climbers, 119; -crops, 21, 98; -hairs, *25, 49, 91; -nodules, *44, *95, 97, 98, symbiosis, 133; -stock, 38, 62, 110 ; -tubers, 111 Rootless plants, 49, ¥*132; Corallorhiza, 124 Roots: of aquatics, 49; clasping, 121; fleshy, 150 131, 325 Rosa canina, climber, 116; spinos- issima, prickles, 116 Rose, 220 ; bud-protection, 61 ; bud- scales, 61; fl, 168; fr., 193; moss-, 116; prickles, 116, 141; suckers, 155 Rosemary oil, 42 Hosablech wilt, *37, *62; pls., 60, 15 Rotation : of crops, 97 ; -grasses, 20 Rubber, 142, 152 Rubus, 220; scrambler, 116. See Bramble Rum, 151 Rumez, pollination, 167 Runners, 155, 161 Rupture-wort, 218 Russia, 12, 21, 22 Rye, 147 Sage oil, 42, 143 Sagittaria, heterophylly, 52; winter- uds, Sago-palm, food, 150 Sainfoin, fr., 188, *189 St. Dabeoe’s-heath, 213, 217 St. John’s-wort, 217 ; fl., 169 Salicornia, 41, 58, 59, 88, 278; -agsociation, 277, 278 Salix, 220. See Willow Salsola Kali, 59 Salt : common, 88 ; effect on absorp- tion, 33, 88 ; -marsh, veg. of, 231, 278; -meadow, 279; -swamps and marshes, xerophytic factors of, 35 Saltation, 204 Saltwort. See Salsola Kali Salty seaside soils, 35 Samara, 186, *187, 190 Sand, 82; absorptive power, 87; origin from granite, 81; per- meability, 87 ; physical properties, 81, 90 ; water-capacity, 87 ; water- retaining capacity, 87 Sand-dunes, 16, 82, 90; formation of, 279 ; fixed, veg. of, 230 ; ridges, veg. of, 231; veg. of, 58, 59, 107, 225 Sandstone-hills, veg. of, 257 Sandy loam, 82 ; soils, 35 Saprophytes, 6, *123, 206; partial, 125 ; total, 124; of woods, 266 Sarsaparilla. See Smilax Savannah, 17. See Park-land Saxifraga Geum, 213, 221; granu- lata, 62, bulbils, 160 ; hirsuta, 213; 326 BRITISH hypnotdes, 38, 114; tridactylites, 60; wmbrosa (London-pride), 37, 155, 213, 221 Saxifrage: honey, 172 ; meadow- (see Saxifraga granulata) ; mossy (see 8. hypnoides) Seabious, 218; infl., 181; insect- visitors, 170 Scale-leaves of bulbs, 156 Seales, bud-, morphology of, 61 Scarlet runner, 116 Sear-woods, 269 Scent of pls., 143 Schizocarps, +190 Selerenchyma, 41, *43 Scotland : climate, 13 ; alpine veg. of Highlands, 287; veg. of mountains, 17, 287 Seramblers, 115 Scrophularia, insect-visitor, 174 Scrophulariacex : hemi-parasites of, 126 ; honey, 173 Serub-communities, 17 Seurvy-grass, 218 Sea-blite (see Sueda maritima) ; couch - grass association, 280; -holly (see Eryngium) ; -purslane, 218; -rocket (see Cakile mari- tima); -samphire (see Crithmum maritimum) Seashore: condition of soil-water, 88 ; veg. of, 275 Seaside-annuals, 58, 59 Seasons, effect on veg.: summer and winter, 10, 31, 57, wet and dry, 10 Seaweeds : colour of, 53 ; zoning of, 278, 283 Sedges, 46, 62, 115; pollination, 167 Sedum acre, *39 ; Rhodiola, 62 Seedlings, *25; of Acacia, *44; protection of, 138 Seed-plants, 163, 183 Seeds : in aquatics, 54; albuminous, *146, *147; defensive equip- ment, 135, 137; dispersal: by animals, 196, birds, 196, man, 198, water, 197, wind, 195; of epi- phytes, 122; exalbuminous, 146, *147; explosive, 196; formation of, 162, 183 ; hairy, 195 ; oily, 148, 196 ; repr. by, 153, 162 ; resistance to cold, 74; starchy, 147 ; sterile, 149 ; storage of food in, 108, 146 ; winged, 195 ; xerophytic chars. of, 59, 74, 137 Selective absorption, 91 PLANTS Self-fertilization, 165; -pollination, 164, effect on stock, 165, preven- tion of, 176 Semi-deserts, 9, 12, 36 Senecio, 105; Jacobea, insect-visi- tors, 170 ; vulgaris, autogamy, 165 Sepals, 162 Service-tree. See Pyrus torminalis Shade-plants, 72, *73 Sheep, 20; fat-tailed, 145; -runs, 257 Shepherd’s-needle, 216, 217; -purse (see Capsella) Shirley poppy, origin of, 153 Shoots: colour of young, 73; xero- phytic forms of, 42 Shore, sandy, veg. of, 279 Shoreweed. Sce Littorella Shrubs, deciduous, 60, 109 Silene: fr., 187, *188 ; honey, 173; acaulis, 38, 114 ; nutans, 174 Silica, 82, 98 Silicate of alumina, 81; lime, 81; potash, 81 Silicula, 188 Siliqua, 188, *190 Silverweed, 217 Simethis, 214 Sinapis, 148 ; arvensis, 216 Sisyrinchium, 214 Sleep-movements: in fis., 71; in leaves, 70, *71 Sloe. See Prunus spinosa Slopes, exposed maritime, veg. of, 284 Slugs, 176 Smilax, 212 ; tendrils, 119 Snapdragon: fr., 187, *188 ; honey. Ava) Snowdrop, 57, 63, 214, 215 Societies, plant, 225 Soil, 8, 14, 80, 91; acids in, 88, 94; aeration of, 87; aldehydes, 101; animals in, 96; -bacteria, 83, 88, 95, 96, 123; biology of, 91; capil- larity, 86; chemical properties, 80, 88; and climate, 14, 84; dry, 85, 87; -factors, 8, 18, 19, influence on flora, 21, 84; forest-, 18; formation of, 80; -fungi, 96, 133; nutriment in, 24, 33, 83, 90, 93; percolation of, 87; per- meability of, 87; living popula- tions of, 94; physical properties, 80, 85, 90; protozoa, 95; salt-, 33; sour, 33, 35, 88, 94; and temperature, 87; influence on GENERAL INDEX veg., 21, 84; warm, 87; water in, 28, 84 ; water-capacity, 86 ; water- retaining capacity, 87 ; xerophytio factors of, 35 Solanum Dulcamara, _ incipient climber, 116 ; nigrum, 216 Solar energy, 4, 9, 65 ; spectrum, 66 Sonchus, latex, 216 ; arvensis, 216 Sorghum vulgare, 148 Sorrel, wood. See Oxalis Sow-thistle. See Sonchus Spadix, *1'75, 178 Sparganium natans, leaf, 51 Spathe, *175, 178 Species, 218, 219, 224; elementary, 205, 221 ; origin of, 202 Spectrum, solar, 66 Spermaphytes, 183 Sphagnum, 89, 253 Sphagnum-bog, 231, 250, 252, 253 Spices, 150, 151 Spike, 178, *180 Spinach (Spinacea oleracea), 150 Spindle-tree : aril, 196; honey, 172 ; structure of seed, 146 Spines : of holly, 104; origin of, 139 ; as a protection, 136, 140, *141; in xerophytes, 44 Spirea, pollen-fi., 169 tranthes Rom J Spirits, 20, 151 Sports, horticultural, 153 Spurge, 216, 218 ; latex, 142 Stachys arvensis, 57, 216 Stamen, 162 Starch: formation of, 5, 66 ; reserve-, 145 ; Starchy seeds, 147 Stellaria media, fertilization, 165 Stems : assimilating, 44, *45 ; edible, 150; stunted growth of, 36; -tubers, 111 Steppes, 19, 23, 35 Stigma, 162 ; of wind-pollinated fls., 167 Stinging hairs, *141; nettle, 167, 216 Stipules : as bud-scales, 61; hollow, 133; as spines, 140, *141; as tendrils, *118 Stitehwort, 217; cleistogamous fis. 165 Stolons, 155 Stomata, *{26, *27; in aquatics, 49; regulating transpiration, 32 ; in xerophytes. 38, 39, 40, *41, *43 , 214 327 Stoneerop, *39, 41, 218; honey, 172 Stones, xerophytic factors of, 35 Storage of food-reserves, 145 Stork’s-bill, 217. See Hrodium Strand-vegetation, 277, 279 Stratification of plants in woods, 228 Strawberry : acids in, 143 ; fr., 184, 187, *193 ; stolons of, *155 Strawherry-tree. See Arbutus Unedo Streams, veg. of, 235 Struggle for existence, 203, 208, 210 Strychnin, 143, 148 Strychnos Nux-vomica, 148, 148 Stunted growth of stems, 36 Style, 162 Sub-alpine pastures, 230 ; rocks, veg. of, 290 Subdominant plants, 227 Submerged leaves, 50 Subularia, 55 Succulent plants, *39, *41,*42, 58,88 (see Halophytes) ; fruits, 136, 192 Suckers, 155, 161 Sueda maritima, 58, 59 Sugar, 5, 146; -cane (Saccharum officinarum), 150, 151; -maple (Acer saccharum), 150 Sumach, 212 Summer-annuals, 107 Summit-heath, 251 Sun, the source of energy, 4, 65 Sundew, 89. See Drosera Sunflower : fr., 187 ; seed, 146 Sunlight, 4, 65 ; duration of, 10, 67 ; influence on veg., 67 ; intensity of, 67. See Light Sun-plants, *72 Survey-maps, ecological, list of, 344 Surveys, ecological, 229 Survival of the fittest, 203 Swamp-cypress, 212 Swamps: bush-, 243; reed-, 241. See Bogs and Marshes Swede, 111, *112, 150 Sweet chestnut, 23, 61, 136, *137, *187 ; cicely, 216 ; potato, 150 Switch-plants, 44, *45 Syeamore, fr., 190, *191; dispersal of fr., 195 Symbiotic insects, 133, 144 ; pls., 132 Symbiosis, 132 Syncarpous ovaries, +183 Tamus communis, 214 ; berries, 149; climbing pl., 116. See Black Bryony 328 Tannin, 150 Tapioca, 150 Taraxacum. See Dandelion Tares, 218 Taxodium, 212 Tea, 151 Tellurie water, {28, 85, 86 Temperature, 6, 10, 11, 18 ; effect on pls., 74, rocks, 81, soil, 87 ; effect on transpiration, 34, 74. See Heat and Cold Tendril-climbers, 117 Tendrils: leaf-, 118; leaf-tip, 119; morphology of, 118; petiole-, 119; stipule-, 119; stem- or shoot-, 118 Terrestrial plants, 28, 114 Thalictrum minus, pollination, 167 Thesium humifusum, parasitic nature of, 126 Thickets, 268; alder- and willow-, 229, 243, 269 ; hazel-, 269 Thistle, 198; fr., *195; 141 Thorny plants, 17, 44, 139 Thorns : origin of, 139 ; as a protec- tion, 140 ; in xerophytes, 17, 44 Thrift, infl., 181 Thrum-eyed fis. of primrose, 177 Tiger-lily, bulbils, 160 Till, 80 Timber, 22, 152 Toadflax (Linaria), 62, honey, 170, pollination, 173; bastard- (see Thesium) Tobacco, 152; -plant: fl., 174, honey, 178, pollination, 173 Toddy, 151 Toothwort, 126, 217. See Lathrea Touch-me-not, 217. See Impatiens Noli-me-tangere Tragapogon, latex, 142 Transpiration, 26, 32; in aquatics, 52 ; causes which increase, 34, 35 ; regulated by stomata, 32; in- fluenced by wind, 76 ; in winter, 60 ; in xerophytes, 32, 34, 35, 36, 40 prickles, Transpiration-current, 25, 32; in hygrophytes, 29; in xerophytes, 32 Traveller’s-joy, 214. See Clematis Vitalba Treacle-mustard, 216 Tree-growth : limit of, in England, 270 ; effect of wind on, 77 ; effect of xerophytic conditions on, 36 BRITISH PLANTS Trees, deciduous, 60, 109 Trichomanes radicans. See Killar- ney fern ; Trifolium arvense, 60 ; repens (white clover), 171, 174 Trimorphie flowers, 177 Tropeolum majus : hydathodes, 28 ; tendrils, 119 ; fr. and seed, *147 Tropical rain-forests, 9, 16, 23, 27, 121 Tropies: climate, 10; duration of sunlight, 67 ; veg., 10, 23 Tropisms, 68 Tropophytes, 30, 31, 57; hygro- philous, 58; mesophilous, 58; xerophilous, 58 Tuberous roots, 111 Tubers, 60, 62, 110 ; food-reserves in, 150 ; multiplication by, 159, 161 ; root-, 111; stem-, 111 Tufted habit, 115 Tulip, 62, 63 ; bulb, 156, *157 Tulip-tree, 212 Tundras, 21 Turgidity, 24 Turnip, 108, 111, 150 Turpentine, 142 Twiners, 116 Typha, 62 Ulex europeus (see Gorse); minor, 214 Ulira-violet rays, 65 Umbel, 178, 181, *182 Umbellifere : fis., 176 ; fr., 186, 191, *192; honey, 172; infl., 181; insect-visitors, 170; nectaries, 169, *170 ; seed, 146 Uncultivated land, 18 Underground perennating organs, 109 Undergrowth of woods, 266 Units of classification. See Variety, Species, Genus, Association, For- mation Urtica : stinging hairs, *141 ; dioica, 216 ; wrens, 216 UViricularia: brood-buds, 54; in- sectivorous habit, 131, *132; leaf-form, 50; pollination, 56; absence of roots, 49 Valerian, 152; infl., 180; pollina- tion, 170 Vaecinium-moor, 231, 249, 251, 255 Vallisneria, 50 Variation, 210, 219, 220; bud-, 153 GENERAL INDEX Variegation in leaves, 154 Varieties, 219, 221 Vegetable-marrow, 119 Vegetables, culinary, 150 Vegetation: relation to climate, 9, 15 ; climatic subdivisions of, 16 ; of Europe, 21; physiognomic divisions of, 16; relation to soil, 14, 21, 84, 88, 91 ; types of, 15 Vegetative reproduction, 149, 153 Venus’ fly-trap, 132 Verbena officinalis, 216 Vernal whitlow-grass. verna Veronica: evergreen, 46; honey, 173 ; agrestis, 216 ; arvensis, 216 Vetch, 216, 218. See Vicia Viburnum Lantana, 214 Vicia, tendrils, 118; sativa, 216 Victoria regia, 51 Vine, tendrils, 118, 119 Viola: cleistogamous fls., 165; fr., See Draba 187, *197; insect-visitors, 171, 174; nectaries, 169; seed-dis- persal, 196 Violet, 218. See Viola Virginia-creeper, tendrils, 118 Viscum album, parasitic nature of, 128, *129 Vitis, tendrils, 118, *119 Viviparous pls., 160 Wales, veg. of mountains, 17 Wallflower, 108 ; fr., 137, 187 Walls, veg. of, 290 Walnut, 212 ; oil, 143, 147 Wasps, 172, 174 Waste ground, veg. of, 295 Water : absorption of, by roots, 25, 33, 35, 74, 91; causes reducing absorption, 32, 33, 35; effect of cold upon absorption, 74 ; amount available to pls, 85; amount entering the soil, 85; amount retained by the soil, 85 ; capacity for heat, 11 ; -capacity of soils, 86 ; capillary-, 86 ; chemical action on rocks, 80; classification of pls. according to, 64; as an agent of denudation, 80; evaporation of, 34, 76 ; excretion of, 27, 29, 263 ; -films, 85; -glands, 127; ground or telluric-, 28, 85, 86; hygro- scopic, 86 ; light in, 52 ; path of, in stems, 25; physical action on rocks, 81; percolation of, in soil, 87: influence on pl. life, 24, 32; 329 role in pls., 24; an agent of pol- lination, 166 ; -retaining capacity of soils, 87; agent of seed dis- persal, 197; soil-, 28; stomata, 27; -storing tissue, 41; -supply, 18, 19, 28 ; -vapour, 6, 26 Water-buttercup (see Ranunculus aquatilis) ; -cress, 151 ; -dropwort (see G@nanthe); -lily: leaf, 51, seeds, 197, stomata, 49, winter- condition, 54 ; -lobelia (see Lobelia Dortmanna) ; -plantain (see Alisma Plantago) ; -plants (see Aquatics) ; -starwort (see Callitriche) ; -violet (see Hottonia) Waterfalls, veg. of, 234 Wax, 136 Wayfaring - tree. Lantana Weeds: of cultivated ground, 296 ; of cultivation, 60, 108, 215, 216 (list), 227, 297 (list); of the meadow, 263; of the natural pasture, 264 Weathering of rocks, 80 Weeping ash, 154 ; willow, 154 Weismann’s law of heredity, 335 Wet meadow-plants, hygrophilous chars. of, 29 Wheat: albuminous seed of, 146; aleurone-layer of, 147; as a food, 147; -crops, 98; cultivation of, 18, 19; flour of, 148 Whisky, 151 Willow, 220, 221; bud-protection, 61; bud-scales, 61; fis, 176; seeds, 195; -thiclets, 229, 243, 269 ; weeping-, 154 Willowherb: fr., 187, 189; honey, 173 ; infl., 180; seeds, 195, 197 Wind, 12 ; effect on pls., 36, 37, 76, 284, 286; agent of pollination, 166 ; effect on transpiration, 34; agent of seed-dispersal, 195, 198 ; forming sand-dunes, 279 Wine, 151 Winter, 10; -rest in aquatics, 54; -rest in tropophytes, 57 Winter-aconite, fl., 57 Wistaria: climbing pl., 116; spiny stipules, 116 Wolffia, absence of roots, 49 Woodland, 15, 16, 17, 265; belt of deciduous-, in Europe, 22; swamp-, 243, 269 Woods: artificial, 267; ash-, 229, 272; ash-oak, 229, 272; beech-, See Viburnum 330 BRITISH PLANTS 229, 272s birch, 229, 271; de- ciduous, 265, 270; evergreen coniferous, 230, 265, 273 ; ground- veg. of, 266 ; larch-, 274 ; natural, 267 ; oak-, 229, 269; pine, 273; scar-, 269 Wood-sorrel. See Oxalis Wormwood, 42 Wounds, 140 Xerophilous tropophytes, 58, 62 Xerophytes, 29, 30, 36, 74, 77, 89; air-spaces in, 40; palisade-tissue in, 41, *72; cuticle of, 40; development of hairs and scales in, 39, of sclerenchyma in, 41, *43 ; evergreen-, 39 ; mucilage in, *41; position of leaves in, 46; size of leaves in, *39; oil in, 42; water-storing tissue in, 41 Xerophytie annuals, 59; chars., 30, 36, 58, 60, 61; conditions, 33, effect on form of leaf, 38, of stern, 36 ; factors, 32, 35; forms of leaf and shoot, 42; structures in leaves, 39, 40 ; regions, 35, 77, 89 ; tendencies, 39 Xylem, *45 ; in aquatics, 49 Yellow bird’s-nest (see Monotropa) ; rattle (see Rhinanthus) ; water-lily (see Nuphar) Yew: leaf-mosaic, 68; succulent seeds, 196 ; Irish, origin of, 205 Yucca, 133 Zannichellia, 55 Zea Mais, 147, 148 Zones, 226 Zoning: of aquatics, 236; of the reed-swamp, 242 ; of seashore pls., 278, 283 ; of seaweeds, 276 Zostera: leaf, 50; pollination, 166, 276 INDEX TO NAMES OF PLANTS IN PART III For common names of grasses and ferns, see under these headings. Abies (fir, spruce), 268, 273, 274 Acer campestre (maple), 270, 272, 273, 293; Pseudo-platanus (syca- more), 266, 270 Aceras anthropophora (green man- orchid), 260, 264 Achillea Millefolium (milfoil, yar- row), 251, 258, 289, 293 Acorus Calamus (sweet sedge), 242 Adiantum Capillus-V eneris (maiden- hair fern), 291 Adoxa Moschatellina (moschatel), 289, 293 AZigopodium Podagraria (gout-weed), 21 6 Aisculus Hippocastanum (horse- chestnut), 293 Aithusa cynapium (fool’s-parsley), 216 Agrimonia Lupatoria (common agrimony), 293 Agrimony, common — Agrimonia Eupatoria ; hemp — Eupatorium cannabinum Agropyron junceum (sea couch- grass), 280, 281, 282 Agrostis alba (white bent-grass), 263; canina (brown b.), 289; vulgaris (fine b.), 250, 251, 252, 258, 260, 295, 296 Aira cespitosa (tufted hair-grass), 244, 251; flexuosa (waved h.), 250, 251, 252, 255, 258, 260, 271, 295, 296 ; preecox (early h.), 296 Ajuga Chamepitys (ground-pine, yellow bugle), 261 ; reptans (com- mon bugle), 270, 293 Alchemilla alpina (alpine lady’s- mantle), 251, 255, 259, 288; arvensis, 298 Alder-buckthorn — Rhamnus Fran- gua ; common—Alnus glutinosa Alge, marine, 275 ahs Plantago (water-plantain), 4 Alkanet, field—Lycopsis arvensis Alliaria officinalis (Jack-by-the- hedge, hedge-garlic), 293 Allium ursinum (ramsons), 293 ane glutinosa (alder), 243, 244, 69 Alopecurus agrestis (slender fox-tail), 216; alpinus (alpine f.), 287; geniculatus, 242; pratensis (meadow-f.), 263 Althea officinalis (marsh-mallow), 243 Anagallis arvensis (scarlet pimper- nel), 282, 298; tenella (bog-p.), 245, 247, 253, 254 Andromeda polifolia, 247, 252 Anemone nemorosa (wood-anemone, wind-flower), 270, 271, 289; Pulsatilla (pasque-flower), 260 Antennaria dioica (mountain cud- weed), 251, 288 Anthemis nobilis (camomile), 216 - Anthoxanthum odoratum (sweet ver- nal grass), 250, 258, 263 Anthyllis Vulneraria (lady’s-fingers, kidney-vetch), 260, 264, 285 Antirrhinum majus (snapdragon), 291 Apium inundatum (least marsh- wort), 240, 242; nodiflorum (marshwort), 242 Arabis alpina (alpine rock-cress), 287; hirsuta (hairy r.), 290; stricta (Bristol r.), 290 Arbutus Unedo (strawberry-tree), 211, 212, 213 331 BRITISH Arctium Lappa (burdock), 293 332 Arctostaphylos alpina (alpine bear- berry), 288 ; Uva-ursi (bear- berry), 255 Arenaria ciliata (ciliated sandwort), 213, 288 ; peploides (sea-purslane), 280, 281, 283; rubella (mountain 8.), 287; serpyllifolia (thyme- leaved s.), 258, 282, 290, 291, 298 ; trinervia (three-nerved s.), 293; verna (spring s.), 288, 290, var. Gerardi, 289 Armeria maritima (thrift), 279, 284, 285, 288 Arrhenatherum avenaceum (false oat- grass), 295 Arrow - grass— Triglochin ehead—Sagiitaria sagittifolia Artemisia maritima (sea - worm- wood), 279; vulgaris (mugwort), 293 Arum maculatum (cuckoo-pint), 294 Ascophyllum nodosum, 277 Ash—Fraxinus excelsior Asperula cynanchica (squinancy- wort), 260 ; odorata (sweet wood- ruff), 269 Asphodel, bog — Narthecium ossi- fragum ; Scottish—Tofiedia pa- lustris Aspidium aculeatum (prickly shield- fern), 295 ; Filix-mas (male fern), 270, 295 Asplenium Adiantum-nigrum (black spleenwort), 290, 291, 295 ; lanceo- latum (lanceolate s.), 291; mar- inum (sea-s.), 284; Ruta-muraria (wall-rue), 290, 291 ; Trichomanes (maiden-hair s.), 290, 291 ; viride (green s.), 288 Aster Tripolium (sea-aster), 279 Astragalus alpinus (alpine milk- vetch), 287 Athyrium alpesire, 288; Filix-fa- mina (lady-fern), 270, 295 Atriplex spp. (orachs), 281, 283; Babingtonii, 280; hastata, 280, 283 ; littoralis, 279; patula, 280 ; portulacoides (sea-purslane), 279 Atropa Belladonna (deadly night- shade), 273 Autumn crocus—Colchicum autum- nale Avena fatua (wild oat), 295; sativa (cultivated oat), 295 Avens, common—Geum urbanum ; mountain—Dryas octopetala SPP. 5 PLANTS Awlwort—Subularia aquatica Azalea procumbens (mountain- azalea) 255, 288, 289 Azolla caroliniana 238 Ballota nigra (black horehound), 216, 294 Balsam, orange - flowered — Impa- tiens fulva Barley—Hordeum spp. Barren strawberry—Potentilla Fra- gariastrum Bartsia Odontites, 263, 296 ; viscosa, 241 Basil, wild—Calamintha Clinopo- dium ; -thyme—C. Acinos Bastard toadflax—Thesium humi- fusum Beak-rush—Rhynchospora alba Bearberry—Arctostaphylos spp. Bedstraw—Galium, spp. Bee-orchid—Ophrys apifera Beech—Fagus sylvatica Beet, sea—Betla maritima Bell - flower — Campanula, spp. 3 -heather—Erica Tetralix Bellis perennis (daisy), 264, 294 Beta maritima (sea-beet), 279, 283 Betony—Stachys, spp. Betula alba (birch), 243, 265, 266, 269, 270, 271, 274; nana (dwarf b.), 255, 288 Bidens cernua (nodding bur-mari- geld), 241; tripartita (tripartite -), 241 Bilberry—Vaccinium Myrtillus Bindweed — Convolvulus arvensis ; black—Polygonum Convolvulus Birch—Betula, spp. Bird’s-foot trefoil—Lotus cornicu- latus Bird’s-nest, yellow—Monotropa Hy- popitys ; orchid—Neottia Nidus- avis Bittercress—Cardamine, spp. Bittersweet—Solanum Dulcamara Black-berry — Rubus fruticosus ; -bindweed—Polygonum Convolvu- lus ; -bryony—T'amus communis ; -horehound—Ballota nigra ; -thorn —Prunus spinosa Bladder - campion — Silene inflata (Cucubalus) ; -wort—Utricularia, spp. ; -wrack—Fucus vesiculosus Blaeberry, mountain,— Vaccinium uliginosum Blechnum Spicani (hard-fern), 295 INDEX TO NAMES OF PLANTS Blinks—Montia fontana Blite, sea—Sueda maritima Bluebell— Scilla nutans ; of Scot- land—Campanula rotundifolia Bog-asphodel—Narthecium ossifra- gum ; -bean—Menyanthes trifoli- ata ; -moss—Sphagnum ; -myrtle —Myrica Gale; -pimpernel — Anagallis tenella Box-tree—Buzrus sempervirens Brachypodium pinnatum, 260, 296 ; sylvaticum (false brome), 270, 295 Bracken—Pteris aquilina Bramble—Rubus fruticosus Brassica, spp. (mustards), oleracea (wild cabbage), 284 sik media (quaking grass), 260, Bromus arvensis (field-brome), 216 ; giganteus, 270; mollis (soft b.), 295 ; sterilis (barren b.), 295 Brook-lime—Veronica Beccabunga ; -weed—Samolus Valerandi Broom, butcher’s — Ruscus leatus ; common—Cytisus parius ; hairy—Genista pilosa Bryonia dioica (white bryony), 214, 293 216; acu- 8co- Bryony, black—Tamus communis ; white—Bryonia dioica Buckthorn, alder—Rhamnus Fran- gula ; common—R. catharticus ; sea—Hippophaé rhamnoides Bugle—Ajuga spp. Bugloss, viper’s—LHchium vulgare Bulbush — Scirpus lacustris and Typha latifolia Buniam flexuosum (pignut), 273 Burdock—Arctium Lappa Bur-marigold—Bidens spp. Burnet, lesser—Poterium Sangut- sorba ; -saxifrage — Pimpinella Sazxifraga Bur-reed—Sparganium spp. Butcher’s-broom—Ruseus aculeatus Butomus umbellatus (flowering rush), 242 Buttercup—Ranunculus spp. Butterwort—Pinguicula spp. Buxus sempervirens (box-tree), 260 Cabbage, wild—Brassica oleracea Cakile maritima (sea-rocket), 280, 281 Calamagrostis Epigeios (wood-reed), 4.4. 2 Calamint—Calamintha officinalis 333 Calamintha Acinos (basil-thyme), 298; Clinopodium (wild basil), 294 ; officinalis (calamint), 294 Callitriche aquatica (water-starwort), 237 Calluna vulgaris (heather, ling), 250, 251, 252, 253, 255, 259, 270, 271, 274, 285, 291, 296 Caltha palustris (marsh-marigold), 244, 245, 289 Camomile—Anthemis nobilis Campanula glomerata (clustered bell- flower), 260; hederacea {ivy-leaved b.), 245,254 ; hybrida( =Specularva hybrida, Venus’ looking-glass). 216; rotundifolia (hair-bell, blue- bell), 251, 259, 289, 296 ;) T'rache- lium (nettle-leaved b.), 272, 294 Campion, bladder—Silene inflata (=Cucubalus) ; moss—S. acaulis ; red—Lychnis dioica (=diurna); sea—Silene maritima; white— Lychnis alba ( =vespertina) Canadian pondweed—HElodeu cana- densis : Capsella Bursa-pastoris (shepherd’s- purse), 216, 291, 294, 295 Cardamine flexuosa (wood bitter- cress), 289; hirsuta (hairy b.), 289, 291, 294, 297; pratensie (cuckoo - flower, lady’s - smock), 244, 263 Carduus acaulis (stemless thistle), 260, 261, 284, 294; arvensis (field-t.), 264, 298; ertophorus (woolly-headed t.), 272; lanceo- latus (spear-t.), 264, 294, 298 ; Marianus (milk-t.), 216; palus- tris (marsh-t.), 263, 264; pycno- cephalus (slender t.), 282 Carex spp. (sedges), 242, 245, 247, 251, 253, 254; alpina, 287; are- naria (sand-s. ), 280,282 ; atrofusca, 287 ; canescens, 255 ; flacca (glau- cous 8.), 261 ; Goodenovit (common s.), 251; rigida, 289; rupesiris (rock-s.), 287 ; vaginata, 288 Carlina vulgaris (carline-thistle), 258, 282 Carpinus Betulus (hornbeam), 269, 270, 293 Carrot, wild—Daucus Carota Castanea vesca (sweet chestnut), 268 Catabrosa aquatica (whorl-grass), 242 Catch-fly—Silene spp. Cat-mint—Nepeta Cataria Cat’s-ear—Hypocheris radicata 334 BRITISH Caucalis Anthriscus (hedge-parsley), 294 ; nodosa, 298 Celandine, greater — Chelidonium majus ; lesser—Ranunculus . Fi- caria Centaurea Calcitrapa (star-thistle), 260; Cyanus (cornflower), 216; nigra (bardhead, knapweed), 284 ; Scabiosa (great knapweed), 285 Centaury—Hrythrea Centaurium Centranthus ruber (red valerian), 291 Cephalanthera pallens (large white helleborine), 273 Cerastium alpinum, 259, 288; ar- vense, 216; semidecandrum, 282, 291; vulgatum (mouse-ear chick- weed), 291, 294, 298 Ceratophyllum demersum wort), 237 Ceterach officinarum (scaly spleen- wort), 291 Cetraria islandica (Iceland-moss), 289 Cherophyllum sylvestre (wild cher- vil), 294 ; temulum (rough c.), 294 Chara (stonewort), 237, 239 Charlock—Sinapis arvensis Cheddar-pink—Dianthus cesius Cheiranthus Cheiri (wallflower), 291 Chelidonium majus (greater celan- dine), 294 Chenopodium spp. (goosefoots), 216, 296 ; album, 280 ; rubrum, 280 Cherleria sedoides (cyphel), 287 Cherry-laurel— Prunus Lauro-cera- (horn- sus Chervil—Cherophyllum spp. Chestnut, horse—Aisculus Hippo- castanum ; sweet—Castanea vesea Chicory—Cichorium Intybus Chickweed, common — Stellaria media ; mouse-ear — Cerastium spp.; winter-green—T'rientalis europea Chlora perfoliata (yellow-wort), 261 Chorda filum, 276 Chrysanth L th (ox- eye daisy), 264; segetum (corn- marigold), 216 Chrysosplenium oppositifolium (golden saxifrage), 244, 289 Cicely, sweet—Myrrhis odorata Cichorium Intybus (chicory), 216 Cinquefoil—Potentilla spp. ; marsh —Comarum palustre Cirewa lutetiana (enchanter’s-night- shade), 273, 294 3 PLANTS Cladium Mariscus, 245 Cladonia, 289 Clary—Salvia Verbenaca : Claytonia alsinoides, 215 ; perfoliata, 215 Cleavers—Galium A parine Clematis Vitalba (traveller’s-joy), 214, 267, 272, 293 Cloudberry—Rubus Chamemorus Clover—T'rifolium spp. Club - moss — Lycopodium -rush—Scirpus spp. Cochlearia alpina, 288; anglica, 279; danica, 279, 284, 285; grenlandica, 288; officinalis (scurvy-grass), 279 Colchicum autumnale (autumn cro- cus), 263, 272 Colt’s-foot—Tussilago Farfara Comarum palustre (marsh-cinque- foil), 245, 247 Comfrey—Symphytum officinale Conium maculatum (hemlock), 294 Convallaria majalis (lily- of - the- valley), 214, 269 Convolvulus arvensis (bindweed), 298; sepium (hedge-b.), 293; Soldanella (sea-b.), 281, 282 Corallorhiza innata (coral-root or- chid,) 274 Coral-root orchid—Corallorhiza in- SPP. 5 nata Corn-bluebottle—Centaurea Cyanus; -campanula— Campanula hy- brida ; -cockle—Lychnis Githago ; -flower—Centaurea Cyanus,; -ma- rigold—Chrysanth getum 5 -salad—Valerianella olitoria Cornish heath—Frica vagans Cornus sanguinea (dogwood), 269, 270, 271, 272, 273, 293; swecica (dwarf d.), 255 Corydalis claviculata (climbing fumi- tory), 293 Corylus Avellana (hazel), 268, 270, 271, 278, 293 Cotoneaster vulgaris, 284 Cotton-grass or sedge—Hriophorum spp. Cotyledon Umbilicus (navelwort),291 Cowberry— Vaccinium Vitis-Idea Cow-parsnip—Heracleum Sphondy- lium Cowslip—Primula veris Crack-willow—Salix fragilis Crambe maritima (sea-kale), 283 Cranberry—Vaccinium Oxycoccus a INDEX TO NAMES OF PLANTS Cranesbill—Geranium spp. Crategus Oxyacantha (hawthorn, whitethorn), 269, 270, 271, 272, 273, 293, 296 Crepis spp. (hawk’s-beard), 264 Cress, great water- —Nasturtium amphibium ; hairy bitter-—Carda- mine hirsuta; penny- —Thlaspi arvense ; rock. —Arabis spp. and Draba spp. ; thale- —Sisymbrium Thalianum ; water- —Nasturtium officinale Criihmum maritimum (samphire), 283, 284 Crocus, autumn—Colchicum autum- nale Crocus vernus, 215, 263 Crosswort—Galium Cruciate Crowberry—Empetrum nigrum Crowfoot—Ranunculus spp. Cuckoo-flower — Cardamine pra- tensis ; pint—Arum maculatum Cudweed, alpine—@naphalium nor- vegicum ; dwarf—G. supinum ; mountain—Antennaria dioica ; upright—Filago germanica Currant—Ribes spp. Cuscuta spp. (dodder), 298 Cynoglossum officinale (hound’s- tongue), 282 Cynosurus cristatus (crested dog’s- tail), 260, 263 Cyphel—Cherleria sedoides Cypripedium Calceolus (lady’s-slip- per orchid), 269 Cystopterts fragilis (brittle-fern), 288, 290 Cytisus scopartus (broom), 296 Dabecia olifolia (St. heath), 213, 252 Dactylis _ glomerata grass), 263 Daisy, common—Bellis perennis ; ox-eye—Chrysanthemum Leucan- themum Damasonium stellatum (star-fruit), 242 Dandelion—Taraxacum officinale Daphne Laureola (spurge-laurel), 273 Daucus Carota (wild carrot), 260, 283, 285 Dead-nettle—Lamium spp. Devil’s-bit scabious—Scabiosa Suc- cisa, Dianthus cesius (Cheddar-pink), 290 ; prolifer, 283 Dabeoo’s (cock’s-foot 335 Dicranum, 252 Digitalis purpurea (foxglove), 271, 294 Diplotazxis muralis (wall-rocket), 291 Dipsacus sylvestris (teazle), 294 Dock—Rumex spp. Dodder—Cuseuta spp. Dog-rose—Rosa canina ; -violet-— jola canina and Riviniana ; -wood—Cornus spp. Dog’s-mercury, Mercurialis perennis Dove’s-foot crane’s-bill—Geranium molle Draba incana (hoary rock-cress), 288, 290; muralig (wall r.), 290 ; rupestris (rock-cress), 287; verna (whitlow-grass), 282, 290 Dropwort—Spirea Filipendula ; water—@nanthe fistulosa Drosera spp. (sundews), 247, 253 ; rotundifolia (round-leaved s.), 253, 254, 255 Dryas octopetala (mountain-avens), 286, 288 Dryopteris montana (mountain-fern), Duckweed—Lemna spp. Dyer’s-weed—eseda Luteola Earthnut—Bunium flexuosum Echium vulgare (viper’s - bugloss), 283, 284 Elder —Sambucus nigra Elm—Ulmus spp. Elodea canadensis (Canadian pond- weed), 215, 233, 237 Elymus arenarius (lyme-grass), 281, 282 Empetrum nigrum (crowberry), 251, 254, 255, 288, 289 Enchanter’s - nightshade — Circea lutetiana Epilobium spp. (willowherbs), 243, 245; alpinum (alpine w.), 288; adlsinefolium (mountain w.), 288 ; angustifolium (rose-bay w.), 294 ; hirsutum (hairy w.), 244; lanceo- latum (spear-leaved w.), 291; montanum (broad smooth-leaved w.), 270 Equisetum spp. (horsetails), 241, 245 ; limosum, 242 Erica ciliaris (ciliate heath), 213, 252, 253; cinerea (common h.), 250, 252, 285, 291, 296 ; Mackayt (Mackay’s h.), 213, 253 ; mediter- ranea (Mediterranean h.), 213, 336 BRITISH 253; Tetralix (cross-leaved h., bell-heather), 247, 251, 252, 253, 254, 255; vagans (Cornish h.), 213, 252 Erigeron alpinum (alpine flea-bane), 287 ; canadense (Canadian f.), 216 Eriocaulon septangulare (pipewort), 214 Eriophorum spp. (cotton-grasses or sedges), 245, 251, 253, 254; angus- tifolium, 255; vaginatum, 251, 253, 254 Erodium cicutarium (stork’s-bill), 282, 291, 296 Eryngium maritimum (sea-holly, 281, 282, 283 Erysimum cheiranthoides (treacle- mustard), 216 Erythrea Centaurium (centaury), 261, 282, 284, 285 Euonymus europeus (spindle-tree), 272, 273 Eupatorium cannabinum (hemp- agrimony, 243, 294 Euphorbia spp. (spurges), 216; amygdaloides (wood -s.), 270; hiberna (Irish s.), 213; Paralias (sea-s.), 281, 282; portlandica (Portland s.), 282, 285 Euphrasia officinalis (eyebright), 251, 257, 260, 284, 287, 289, 296 Eyebright—Zuphrasia officinalis Fagus sylvatica (beech), 265, 266, 268, 270, 293 Fern, brake—Pteris aquilina; brittle — Cystopteris fragilis ; filmy—Trichomanes radicans ; hard—Blechnum Spicant ; hart’s- tongue—Scolopendrium vulgare ; Killarney—Z'richomanes radi- cans; lady—Athyrium Filiz-fe- mina; maiden-hair—Adiantum Capillus-Veneris; male—Aspi- dium Filiz-mas ; prickly shield— A. aculeatum ; royal—Osmunda regalis Festuca Myuros (mouse-tail grass), 295 ; ovina (sheep’s-fescue), 250, 251, 252, 255, 258, 259, 260, 263, 289, 290, 296 ; pratensis (meadow f.), 263; rubra (red f.), 298, var. arenaria, 282 Field-madder—Sherardia arvensis Figwort—Scrophularia spp. Fulago germantca (upright cud weed), 282, 291, 296 PLANTS Fir, Douglas—Abies Douglasié ; Scotch—-Pinus sylvestris Flag, yellow—ZJris Pseudacorus Flax—Linum spp. ; toad- —Linaria spp. Flea-bane—Hrigeron spp. ; common —Inula dysenterica Flowering rush—Butomus umbella- tus Fly-orchid—Ophrys muscifera Fontinalis antipyretica, 234 Fool’s-parlsey—Avthusa cynapium Forget-me-not—Myosotis palustris Foxglove—Digitalis purpurea Fragaria vesca (strawberry), 294 Fraxinus excelsior (ash), 244, 263, 266, 268, 269, 272, 293 Fritillaria Meleagris (snake’s-head fritillary), 263 Fritillary, snake’s-head—Fritilaria Meleagris Frog-bit—Hydrocharis Morsus-rane Fucus platyecarpus, 277; serratus, Be vesiculosus (bladder-wrack), 77 Fumaria spp. (fumitory), 216 Fumitory—Fumaria spp. ; climbing —Corydalis claviculata Furze— Ulex spp. Galanthus nivalis (snowdrop), 214, 215 : Gale, sweet—Myrica Gale Galeopsis Ladanum (red hemp- nettle), 298, var. canescens, 283 ; Tetrahtt (common hemp-nettle), 298 Galium Aparine (cleavers, goose- rass), 283, 293; boreale (northern edstraw), 288; Cruciata (cross- wort), 293; Mollugo (great hedge- b.), 283, 293 ; saxatile (heath-b.), 251, 258, 259, 271, 274, 289, 296; verum (yellow or lady’s-b.), 259, 282 Garlic, bear’s—Allium ursinum; hedge—Alliaria officinalis Genista pilosa (hairy broom), 291 Gentiana nivalis (small mountain- gentian), 259, 287 Geranium dissectum (cut-leaved crane’s-bill), 298 ; luctdum (shin- img c.), 290, 291; molle (dove’s- foot c.), 291, 294, 298; pratense (meadow c.), 263 ; pusillum (small c.), 216; Robertianum (herb- Robert), 270, 294, var. purpu- INDEX TO NAMES OF PLANTS reum, 283 ; rotundifolium (round- leaved c.), 294; sanguinewm (bloody c.), 285, 290 Germander - speedwell — Veronica Chamedrys Geum urbanum (avens), 294 Gipsy-wort—Lysopus europeus Glasswort—Salicornia herbacea Glaucium luteum (sea or horned poppy), 281, 282, 282 Glaux maritima (sea-milkwort), 279 Glyceria aquatica (reed manna- grass), 237, 242; maritima, 278, 279 Gnaphalium norvegicum (alpine cud- weed), 287; supinum (dwarf c.), 288, 289 Goat’s-beard—Tragapogon pratensis Golden-rod—Solidago Virgaurea ; saxiflage—Chrysosplenium oppo- sitifolium ; samphire — Inula crithmoides Goldilocks— Ranunculus auricomus Goose - foot — Chenopodium spp. ; -grass—Galium A parine Gorse— Ulex spp. Gout - weed — Aigopodium Poda- graria. Grass, arrow —T'riglochin spp. ; bent—Agrostis vulgaris ; brome— Bromus spp.; cat’s-tail—Phlewm pratense; cock’s-foot—Dactylis glomerata ; cotton—Eriophorum spp.; couch—Triticum repens ; crested dog’s-tail—Cynosurus cris- tatus; false brome—Brachypo- dium spp.; false oat—Arrhena- therum avenaceum; fescue— Festuca spp.; fox-tail—Alope- curus pratensis ; goose—Galiwm Aparine ; hair—Aira spp. ; heath —Triodia decumbens; knot— Polygonum aviculare; lyme— Elymus arenarius; marram— Psamma arenaria; melick— Me- lica unifora; millet—Milium effusum ; moor—NSesleria cerulea ; mouse-tail—Festuca Myuros ; of Parnassus—Parnassia palustiis ; purple moor—WMolinia carulea ; quaking—Briza media; reed— Phragmites vulgaris ; rye—Lolium. perenne ; scorpion— Myosotis spp.; scurvy—Cochlearia spp.; sea- couch — Agrogyron junceum ; sheep’s fescue—Festuca ovina ; soft—Holcus mollis ; sweet vernal 337 —Anthoxanthum odoratum; Timothy—Phicum pratense ; tus- sock— Aira cespitosa; whitlow —Draba verna ; whorl|—Catabrosa aquatica ; wild oat—Avena fatua ; Yorkshire-fog—Holeus lanatus Grass-wrack—Zostera spp. Green man-orchid—Aceras anthro- pophora Gromwell—Lithospermum officinale Ground - ivy — Nepeta Gleshoma ; -pine— Ajuga Chamepitys Groundsel—Senecio vulgaris Guelder-rose—Viburnum Opulus Habenaria conopsea (fragrant or- chid), 261, 264 ; intacta, 213 Hair-bell—Campanula rotundifolia Halidrys siliquosa, 277 Hardhead—Centaurea nigra Hare’s-foot trefoil—Trifolium ar- Vense Hawk’s-beard—Crepis spp. ; -bit— Leontodon spp. Hawkweed—Hveracium spp. Hawthorn—Crategus Oxyacantha. Hazel—Corylus Avellana Heath—Frica spp. ; St. Dabeoc’s— Dabeecia polifolia Heather—Calluna vulgaris; bell— Erica Tetralix Hedera Heliz (ivy), 267, 293 Hedge-parsley—Caucalis Anthriscus Helianthemum Breweri, 209 ; Cham- ecistus (rock-rose), 259, 260, 290 ; guttatum, 213 Hellebore, green—Helleborus viride Helleborine (= E;ixactis) media, 272 ; purpurata, 272 Helleborine, white—Cephalanthera pallens Helleborus viride (green hellebore), 273 Helminthia 283, 294 Hemlock—Conium maculatum Hemp-agrimony—Hupatcrium can- nabinum ; -nettle—Galcopsis spp. Henbane—Hyoscyamus niger Heracleum Sphondylium (hogweed, cow-parsnip), 289, 294 Herb-paris — Paris quadrifolia ; -Robert—Geranium Robzrtianum Hieracium spp. (hawkweeds), 264, 271, 285; Iricum, 209; Pilocella (mouse-ear h.), 251, 258, 282, 290, 291, 296 echioides (oxtonguc), 22, 338 BRITISH Hippocrepis comosa (horso-shoe vetch), 260 Hippophaé rhamnoides (sea-buck- thorn), 281, 282 Hippuris vulgaris (mare’s-tail), 237, 239 Hogweed—Heracleum Sphondylium Holcus lanatus (Yorkshire fog), 263 ; mollis (soft grass), 271 Holly—Ilex Aquifolium Honeysuckle—Lonicera Periclyme- num Hop—Hamulus Lupulus Hordeum maritimum (sea-barley), 279 ; murinum (wall-b.), 295 Horehound, black—Ballota nigra Hornbeam—Carpinus Betula Horned pondweed—Zannichellia spp. ; poppy—Glaucium luteum Hornwort — Ceratophyllum demer- sum Horse-chestnut— 4isculus Hippocas- tanum ; -shoe vetch—Hippocrepis comosa ; -tail—Equisetum spp. Hottonia palustris (water-violet), 237, 239 Hound’s-tongue— Cynoglossuny offi- cinale Houseleek—Sempervivum tectorum Humulus Lupulus (hop), 214, 293 Hutchinsia petrea, 290 Hyacinth, wild—Scilla nutans Hydrocharis Morsus-rane (froghit), 237 Hydrocotyle vulgaris (marsh-penny- wort), 245, 247, 251 Hyoscyamus niger (henbane), 282 Lypericum Elodes (marsh St. John’s- wort), 242; perforatum (ccmmon St. J.), 294; pulchrum (upright St. J.), 296; quadrangulum, 245 ; tetrapterum (St. Peter’s-wort), 245 Hypocheris radicata (cat’s-ear), 264 Iceland-moss—Cetraria islandica Ilex Aquifolium (holly), 269, 270, 271 Impatiens fulva (orange-flowered balsam), 215, 241 Inulu Conyza (yloughman’s-spike- nard), 261, 272, 282 ; crithmoides (golden samphire), 284, 285; dysenterica (common flea-bane), 294 ; salicina, 214 Iris fetidissima (foetid iris), 272, 282; Pseudacorus (yellow flag), 242, 245 Isoetes lacustris (quillwort), 238 PLANTS Ivy, common—Hedera Helix; ground—Nepeta Glechoma Jack-by-the-hedge--Alliaria offici- nalis Jacob’s-ladder—Polemonium caru- leum Jasione montana (sheep’s-bit sca- bious), 291 Juncus spp. (rushes), 253, 254; articulatus, 246; biglumis, 257; bufonius, 241 ; conglomeratus, 246, 251; effusus, 251; Gerardi, 279; obtusiflorus, 245, 247 ; squarrosus, 251; supinus, 246 ; trifidus, 288, 289 Juniper—Juniperus communis Juniperus communis (juniper), 261, 271, 273, var. nana, 288 Kale, sea—Crambe maritima Kidney-vetch—Anthyllis Vulneraria Knapweed—Centaurea spp. Knautia arvensis (field-scabious), 298 Knot-grass—Polygonum aviculare Kobresia caricina, 259 Keleria cristata, 260 Lactuca alpina (alpine sowthistle), 287; muralis (wall-lettuce), 290, 291 Lady’s - bedstraw—Galium verum ; fingers — Anthyllis Vulneraria ; slipper-orchid—cypripedium Cal- ceolus ; smock—Cardamine pra- tensis ; tresses—Spiranthes spp. Lamb’s - lettuce —Valerianella olt- toria Laminaria, 277 Lamium album (white deadnettle), 216, 294 ; Galeobdolon (yellow d., archangel), 270, 294; purpurcum (red d.), 294 Lapsana communis 216, 263, 294 Larch—Larix Larix (larch), 273, 274 ae ia squamaria (toothwort), 269, 94. Lathyrus pratensis (meadow-pea), 263, 293 Laurel, cherry—Prunus Lauro-cera- sus ; spurge—Daphne Laureola Lavatera arborea (trec-mallow), 284 Lemna gibba (thick-leaved duck- weed), 237 ; minor (small d.), 233, 237 ; trisulca (ivy-leaved d.), 237 (nipplewort), INDEX TO NAMES OF PLANTS Leontodon spp. (hawk’s-bit), 264, 285 ; hirtus, 282, 283 Lepidium Smithii, 291 Lottuce—Lactuca spp. ; Valerianella olitoria Leucobryum, 272 Ligusticum scoticum (lovage), 284 Ligusirum vulgare (privet), 244, 269, 272, 293 lamb’s— Lily-of-the-valley—Convallaria ma-. jalis Lily, white water—Nymphea alba ; yellow water—Nuphar luteum. Lime-tree—Tilia spp. Linaria Cymbalaria (ivy-leaved toadflax), 216, 291; minor (small t.), 298 ; vulgaris (yellow t.), 294 Ling—Calluna vulgaris Linnea borealis, 274, 288 Linseed—Linum usitatissimum Linum catharticum (purging flax), 258, 259, 260: perenne (peronnial f.), 260; usitatissimum (common f., linseed), 260 Listera cordata (lesser twayblade), 274 ; ovata (common t.), 264 Lithospermum officinale (gromwell), 294 Littorella lacustris (shoreweed), 237 Live-long—Sedum Telephium Lizard-orchid—Orchis hircina Lloydia serotina, 288 Lobelia Dortmanna (water-lobelia), 238 ; urens, 213 Lolium perenne (perennial rye-grass), 263 London pride—Saxifraga umbrosa Lonicera Periclymenum (honey- suckle, woodbine), 267, 270, 271, 293 Loosestrife, common—Lysimachia vulgaris; purple—Lythrum Sali- caria Lotus corniculatus (bird’s-foot tre- foil), 259, 264, 282, 284, 285, 294, 296 Lousewort—Pedicularis spp. Lovage—Ligusticum scoticum Lungwort—Pulmonaria officinalis Luzula arcuata (alpine wood-rush), 287 ; campestris (field w. r.), 251, 58 5 Lychnis alba (=vespertina, white campion), 216, 294; dioica (=diurna, red c.), 270, 289, 294 ; diurna, see dioica; Flos-cuculi (ragged Robin), 294; Githago 339 (corn-cocklo), 216 ; vespertina, see alba Lycopodium spp. (clubmosses), 255: alpinum (alpine c.), 259 ; clavatum (common c.), 251, 259; Scelago (fir c.), 251, 255, 259. 289 Lycopsis arvensis (field-alkanet), 216, 282 Lycopus europeus (gipsy-wort), 243 Lysimackia nemorum (yellow pim- pernel), 270; vulgaris (common loosestrife), 243 Lythrum Salicaria (purple loose- strife), 243 Madder, field—Sherardia arvensis Mallow—Malva spp. ; tree- —Lava- tera arborea ; marsh—Althea offi- cinalis Malva rotundifolia (dwarf maliow), 296 ; sylvestris (common m.), 216, 294 Man-orchid—Aceras anthropophora Maple—Acer campestre Mare’s-tail—Hippuris vuigaris Marigold, bur—Bidens spp. ; com-- Chrysanthemum Leucanthemum ; marsh—Caltha palustris Marjoram—Origanum vulgare Marsh-cinquefoil-—_Comarum palus- tre ; figwort—Scrophularia aqua- tica; lousewort — Pedicularis palustris ; mallow—Althea offi- cinalis ; marigold—Caltha paius- tris; orchid—Orchis latifolia ; pennywort — Hydrocotyle vul- garis ; ragwort—Senecio aqua- tica ; St. John’s-wort—Hypericum Elodes ; speedwell— Veronica scu- tellata ; stitchwort—~. 8v0. 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