The life of plants.

Front matter

LIFE OF PLANTS.

OREERUR

Le.

Contents

The use of the food-materials for construction and power-production. Growth and differentiation of cells and tissues. Respiration. Aerobic and anaerobic life. Fermentation and decay. Self-supporting (Autotrophic) and non-self-supporting (Heterotrophic) flowering plants: Saprophytes, Parasites, Semi-parasites. Symbiosis. Insectivorous plants

The environment of the land plant: water-supply and sunlight. Stomata as regulators of gaseous exchange. The mechanism of stomatal movement. The role of transpiration in the economy of the plant. The transpiration stream (ascent of sap)

Variation and Heredity: Evolution. Reproduction : asexual and sexual. Cell and nuclear division: the Chromosomes as the material bases of Heredity. The experimental breeding of plants. Mendelian inheritance.

Index

PAGE meoeOrPTTELUSTRATIONS

FIG. PAGE Acer platanoides, showing one year’s growth : Frontispiece I. Stages in the development of buds of horse-chestnut . : 7 2. Scale leaves from bud of horse chestnut . : 9 A species of Chlamydomonas, a unicellular green alae mad a species of Euglena, a unicellular animal . : : II 4. Fucus tai pus: a brown seaweed of benthic habit : Ls 5. Yeast: budding and spore-formation : : : : 2 6. Agaricus campestris, the common mushroom : ; 26 7. Spore-formation in the mushroom . : : ; : Z 8. The structure of the wheat grain ; ; 22 g. Effect of potassium nitrate on the growth lee seedlings ; 34 10. Starch grains from endosperm of barley . 37 It. Seedlings, illustrating the behaviour of cotyledons in ie mination . : ; : : : : ; : 39 12. Water-cultures of maize . : : : : : : 49 13. Root nodules of the pea . : ; ; : ; 53 14. Root-hair, enlarged and dbewewanienite : : 57 15. Apparatus for demonstrating continuous aieory tion of carbon dioxide by illuminated green leaves : 60 16. Surface view of epidermis of cherry laurel, showing erorats 64 17. ‘Transverse section of the leaf of the beech : a 18. The dependence of starch formation on Dirarestion aca supply of carbon dioxide. . : : 78 19. Record of the rate of diffusion of asec in Sluition : : 99 20. Osmotic apparatus : : : . Kon: 21. Stages in development of a Diane Bil : F 103 22. Diagram of plasmolysed cells of the lower apletinite of a leaf of Tradescantia discolor . : 105 23.) Lransverse section oi the ee stem of a Miieoe ders the sunflower . 5 117 24A. Transverse section of ihe youn Gece of a ice tenon (buttercup) 118 248. Part of Fig. Bre more ieniy Mae rnece shai sae cyunder . ne MURS) 25 A. Transverse section of a Monecou leon mee) 120

25 B. Transverse section of one of the vascular bundles diaers in Tio? 57 Aces ; : : cay Ni ae ; , SNES

List of Illustrations

Transverse section of em of Cucurbita 7 ee showing sieve-plates

Sieve-tubes of Cucurbita eae in tomer ioe eta. Transverse section of part of the stem of the sunflower Transverse section of the stem of the lime

Transverse section of part of the stem of the lime! in its fourth year

Longitudinal section of weed ay sina heres

Cells of the endosperm of the castor-oil plant . Longitudinal section through root-tip of maize . Nepenthes Ratclifiana. Photograph by Meee ; Broom-rape, parasitic on clover

Dodder, parasitic on the common tansy Guard-cells of a stoma

Porometer

Potometer :

Xerophytic plants nailed in the Raven Sexual reproduction of Chlamydomonas

Flower of Cypripedium Leeanum

Magnolia Watsonit. Photograph by Malby

44A and B. Diagrams illustrating (A) the structure of the peal and (Bs) the process of fertilization

45and 46. Semi-diagrammatic representation of nucle Aiton:

47: 48.

ay

Ve

Mitosis (below) and meiosis (above) in Addium sp. Diagram of mitosis and meiosis

Inheritance of palm- and fern-leaf Ares in ih: Giineee primula

Sensitive plant (Mimosa pudica), Hennes the ipeRes ni oe normal position and after stimulation

Seedlings of the grass Phalaris canariensis subred to phototropic stimulation

Response of root of broad bean to Resta quran

PAGE

522 122 [23 126

205 209

I

INTRODUCTORY

The part which plants play in the world. Plants of high and low degree ; unicellular and multicellular. The flowering plant. The origin of land vegetation. Plankton and benthos. The garden of the world.

THE study of plants is an adventure into another world the inhabitants of which are strangely different from those of the animal world. Members of the world of plants feed without mouths, move without limbs, ‘see’ without eyes, and find their way in and about the world without the aid of brain or nervous system.

In the exploration of the earth and the settlement of the land plants are the pioneers and animals but camp-followers. The plant world is a world of pioneers. Its members prepare the way, and only after plants have occupied a land may animals possess it. Plants not only colonize tracts of new land but also make them fertile. Even on the barren face of a limestone cliff the grey and yellow lichens gain a hold, Pf aerate tne Suriace of the tock, and help the forces of Nature, rain and running water, frost and snow and wind, to disintegrate it and create from the rock the mineral debris which is the beginning of soil. Not until many generations of some of the simpler and less exacting plants have colonized the soil and have lived and died thereon is it rendered fertile enough to support higher kinds of vegetation. Soil fertility is a memorial of plant activity. The fertility of the earth, created by plants, is also maintained by them. A soil from which all vegetation was banished would in course of time pass back into the sterile state in which it was before its advent.

Plants are the universal food-providers ; they—or rather some of them, and, in particular, green plants—possess the power of manufacturing the food-materials on which their own nourishment and that of the animal world depend. The process of manufacture, of which man has discovered so many important details that he is now able to some extent to imitate it, involves, like all manufacture, the use of raw materials and a source of power. The food-materials which the green plant manufactures are chiefly of two kinds, sugars and proteins: the former composed of the three elements carbon, oxygen, and hydrogen, and the latter composed of these, together with nitrogen and other elements. The raw materials used by the green plant in the manufacture of sugar are water and carbon dioxide—simple substances abounding on the earth. The energy utilized is the radiant energy of the sun. In the process of manufacture the raw materials disappear, and so does the energy. But neither the elements of which the raw materials are composed nor the energy are lost or destroyed. They abide in the manufactured food substances until, in course of time, one or other of these substances, for example sugar, uniting with oxygen, undergoes combustion, when the original raw materials from which it was formed are reproduced and the energy locked up or potential in the sugar is liberated. In this manner the manufactured food-materials provide the energy used by living beings. Not only, as George Stephenson said, does the sun drive the steam-engine ; it likewise drives the living engines —plant and animal.

As for the steam-engine: the story of the source of its energy is briefly this :—The sun shone in far-off times on the leaves of prehistoric green plants which grew, it may be, by estuarine waters.. Of the radiant energy which fell on a leaf, some was absorbed by the green tissues and served for the manufacture of sugars. Much of the sugar was consumed by

The Importance for the Animal World 3 the plant in the course of its life, but some, undergoing relatively little chemical change, became part of the woody skeleton. Presently the tree died and fell into muddy ooze. Before decay could complete its work of destruction the plant was embalmed and carbon and hydrogen and oxygen, united originally by the sun’s energy and the plant’s activity, remained united still, though as time went on some of the hydrogen and oxygen was lost and the plant-remains became more and more carbonized. Therein the energy derived from the sun remained dormant until in the furnace of the steam-engine the coal unites with oxygen, re-forms water and carbon dioxide, and liberates the long-stored energy: and so in the singing of the escaping steam there is an echo of the incidence of a ray of sunlight on a green leaf ages before the beginning of recorded time.

The study of plant nutrition discloses also the facts, surprising to any one who is used to contemplate the differences between plants and.animals, that the materials used by plants in the construction of their tissues are of the same kinds as those used by animals, and that the power employed by both animal and plant worlds in the work of living is derived from the same ultimate source. The green plant is an accumulator of energy. Each molecule of sugar or other food-material which it makes may be likened to a storage battery of such efficiency that only when the molecule is decomposed is the store of energy liberated. So lavish is the manufacture by green plants exposed to sunlight that the products suffice not only to form the fabric of their own bodies and those of animals but also to supply energy for both worlds.

It might seem impossible to add to this recital of the obligations of the animal to the plant world; but recent discoveries have shown that it is by no means complete. Beside the food-materials supplied by plants which serve the animal for body-building and as sources of energy, there are others of more obscure nature which play the part of natural preventive medicines. If they are present in the food of

B 2

4 Lifer Oje lasts animals bodily health is maintained ; but if they are absent from it malnutrition and disordered growth result. These accessory food bodies or vitamines are, at all events in the main, plant products, though curiously enough it is not yet known what, if any, is the part they play in regulating the growth of plants themselves.

The fact that fresh vegetables are conducive to health has long been known. It is recorded that Nelson, when he kept the sea with the Grand Fleet watching the French ports year after year, maintained the health of his seamen by exercise and onions ; and it is a matter of history that Captain Cook discovered during his voyages the value of fresh vegetables in preventing scurvy. From hints such as these, science, by isolating and testing the materials contained in plant tissues, has discovered a new class of food substances which behave as regulators of bodily growth and health; and in so doing has shown that the dietary debt of animals to plants is greater than was previously supposed.

Plants constitute the Ministry of Health of the world. They are also its Sanitary Authority. By the ordered activities of successions of plants of simple organization dead remains of things which once lived are decomposed stage by stage until they are resolved into the simple inorganic substances wherefrom green plants constructed them.

The burden of universal food production borne by the world of green plants has left its mark indelibly on the bodies of every individual belonging to that world. For, as mechanism made by man reveals in plan and detail of construction the purpose for which it is designed and the manner whereby it fulfils its purpose, so does living mechanism. Each organ of the plant and animal body has, as it were, engraved in its tissues the function that it has been called into existence to discharge; and the more specialized the organ, that is, the more it is dedicated exclusively to work of one kind, the more its plan and details of construction are indicative of its function.

Diversity of Form 5

The division of labour which is the mark of all higher organisms is accompanied by and based on an adaptive differentiation of the body. For example, the thin green sheets of tissue which are called leaves (see Frontispiece) show by their disposition and delicacy and by their structure that to them is confided as a prime function the task of absorbing and utilizing the radiant energy of the sun. The scales which enclose the buds of flowering plants are also leaves of like origin with, and of construction essentially similar to, that of foliage leaves; but, as indicated by their position, shape, and texture, their function is the more mechanical though none the less important one of affording protection to the frail tissues of the bud (Fig. 1). In the hard dryness or the varnish-like covering of their surfaces, their diminished size, and in the curiously opportune habit of dying and falling off just when the foliage of the bud begins to unfold may be seen evidence of the specialized function of bud-scales.

Comparative study of buds and bud-scales of common trees and shrubs as they appear at different times of the year is worth making, not only as an exercise in observation, but also because it throws light on the nature of the almost endless diversity presented by plants. All flowering plants are built on a common plan. Each is composed of a very limited number of members—root or descending axis with lateral roots, shoot or ascending axis with lateral branches and leaves, and reproductive organs borne by the flower.

‘ As is illustrated by comparison of the buds and bud-scales of different kinds of plants, not a little of the diversity among plants is brought about by old members undertaking new functions. - So far-reaching may be the change of appearance that a member charged with this or that special function may cease to look like what it is. In such cases the member in question may only be shown to belong to this or that category —root, stem, leaf, &c.—by tracing its origin and course of development, or by comparing the plant, member by member, with more normal and closely allied plants.

In the case of some shrubs and trees, e.g. dogwood (Cornus sanguinea) and wayfaring tree (Viburnum Lantana), the buds have no specialized scale-leaves. They are naked and the progressively smaller leaves which arise in due order from the rudimentary axis of the bud and cover its growing-point are all of one kind. When spring comes and the bud unfolds, none of the small leaves falls off. All persist and grow into foliage leaves. Each develops a leaf-stalk and becomes large and green, and as the axis of the bud elongates at the same time to form a branch each pair of leaves, borne on somewhat swollen nodes of the stem, becomes separated from the succeeding pair by a longer or shorter joint or internode of the stem.

In the majority of shrubs and trees, however, the buds are enclosed by bud-scales and in all of them the bud-scales, no matter how rudimentary and unleaf-like they are, may be shown to be of leaf-lke nature, that is, morphologically equivalent to leaves. Bud-scale and foliage leaf alike arise each as a superficial lateral outgrowth of the axis or young stem of a bud; each is of limited growth and not like a stem which may exhibit unlimited growth, increasing in length and thickness year after year. The order in which successive bud-scales are borne is identical with that of the foliage leaves which precede and succeed them. For example, in the horse-chestnut (Aesculus lippocastanum) the foliage leaves are in opposite pairs with successive pairs alternating with one another, and so also are the brown varnished scale-leaves of the buds (Fig. 1) ; in the plum and many other plants a line joining the point of attachment of the successive foliage leaves to the stem forms a spiral, and so also are the bud-scales spirally disposed on the miniature axis of the bud.

Although the morphological equivalent of leaves, bud-scales are rarely complete leaves. A fully developed foliage leaf may be seen to consist of blade (lamina), stalk (petiole), and leaf-base (see Frontispiece) which sometimes, as in roses and violets, &c., bears leaf-like outgrowths termed stipules.

——=

AN r i en , WM Mi (Re

(ET Gael

(Aesculus hippocastanum). 3, fully opened.

SS SS

= oes SSS ~ ~ NS SO ds DEBE SS

1, buds in resting stage ;

Stages in the development of the buds of horse-chestnut

2, unfolding ;

In some few plants, lilac (Syringa vulgaris), and privet (Ligustrum vulgare), each bud-scale consists of a leaf of which only the blade is developed. Yet, although the petiole is suppressed, it may be caused to appear. If, in spring, when the buds begin to break, the young inner leaves of a privet or lilac bud are removed, growth of the scale-leaves is resumed, and in the absence of their younger rivals they develop into normal, stalked, green foliage leaves. More frequently, as in the horse-chestnut, maple (Acer campestre), sycamore (Acer pseudoplatanus), and flowering currant (Ribes sanguineum), the bud-scale is the morphological equivalent of the leaf-base—that part of the normal foliage leaf which attaches the leaf-stalk to the stem. In these cases suppression of blade and stalk is attended by excessive growth of the leaf-base. In any of these plants, but particularly in the flowering currant, sycamore, and horse-chestnut, transitions may be seen as the bud opens (Fig. 2). They show bud-scales, some consisting of leaf-base only, others of leaf-base with a rudimentary blade, and yet others half-way between scale- and foliage leaves in which all three parts of the leaf may be distinguished. In the alder (Alnus glutinosa), beech (Fagus sylvatica), and oak (Quercus vobur), it is the stipules only which are represented in the scale-leaves. In some of the bud-scales of the alder a lamina may develop and thereby reveal the morphological nature of the bud-scale, but in the beech and oak the bud-scales develop no farther, and it requires the application of comparative methods to determine that they are nothing but leaves reduced to stipules.

Inasmuch as organized mechanism exhibits adaptation to its function, it is to be expected that the green plant will show throughout its plan and details of construction no less than in its mode of life, signs of adaptation to its work of food production ; and if that be so, its general structure must be very different from that of animals which are discharged by Nature from the obligation of sharing in that work. Of these signs the most impressive, after greenness, is the

Immobility of the Plant World 9 immobility of the plant world. Animals must seek their food and tend therefore to be rovers. Plants—at all events those that form the dominant vegetation of the land—must win the ingredients of theirs from earth and air and sun, and are therefore stay-at-homes. Although the immobility is more apparent than real, nevertheless the general run of green plants is fixed and stationary, each rooted in its narrow plot.

Fic. 2. Scale-leaves from a bud of the horse-chestnut, showing transitions from leaf-base only to leaf-base with lamina—petiole suppressed.

Thus firmly established, the plant may explore the earth and atmosphere and reach up towards the sun. In the earth the roots seek and find not too precarious stores of water and mineral substances. In the atmosphere the least movement of the air brings to the leaves new supplies of carbon dioxide ; and by reaching upwards towards heaven the stem may outdistance competitors for light. Because some raw materials come to it from below and others, as well as radiant energy, from above, the fixed green plant is two-ended. It extends by growth in both directions and makes the best of both worlds. It is perhaps conceivable that green plants might have secured supplies of raw materials by entirely different means, without surrendering power of moving about and without becoming half-subterranean and half-aerial beings. It is not impossible that distant ages may see a plant world composed of individuals of appearance and habit altogether different from those of the present day, a world which has succeeded in solving the problem of food-manufacture by novel methods better than those which are now in vogue among plants. If so, the descendants of our trees and shrubs and herbs, instead of dominating the world, may be ousted from their predominant position, and either pass away altogether or survive here and there, limited in number and restricted in range, occupying only the poorer regions which the new types of vegetation—at present only evoked in the imagination—may leave uncultivated.

Though not inconceivable, the advent of this new world of plants is not likely; for, since the remotest times in which there are fossil records of vegetable life, all the many different kinds of land plant which have lived upon the earth and have passed beyond the lowest stages of development have been characterized by the same essentials of habit and organization as those manifest to-day in the green plants of forest and field. They have all been fixed organisms and have all developed the two-endedness which men are so accustomed to see that they do not marvelatit. So constant is the adherence of land plants of all epochs and of very different stages of development to this plan of construction that the suggestion has been made that two-endedness was already a fixed character of the plant world before members of that world existed on the land at all.

According to this suggestion plant life began in the sea. There it passed through the earliest stages of its evolution, acquired green pigment, and developed forms of many, different kinds, all minute and of relatively simple organization, and all alike in the enjoyment of the freedom of the seas. They were free to swim or float wherever light drew them or

Origin of Land Vegetation seit the current bore them. In this marine, free or plankton stage, green plants, of microscopic size, possessed no roots to anchor them permanently to the ground, but floated passively buoyed up by the waters, or propelled themselves by motile extensions of their bodies. Plants which may be survivors of this stage still live in the sea and also in fresh water. One of them (see Fig. 3 A) illustrates the relative simplicity of this type of organism and the difference between plankton andland plants. Invisible to the naked eye and so small that a thousand might lie easily side by side on a line one-eighth of an’ inch long, each individual shows nevertheless, when examined by means of the microscope, a definite and elabor- Fic. 3. A, a species of Chlamydomonas—a unicellular green plant; B, a species of Euglena—a unicellular animal. ate structure. The ch, chloroplast; mn, nucleus; ¢@, eye-spot; f, flagellum;

+ v, vacuole; py (in A), pyrenoid (a mass of protein oz oval body which uncertain function). d constitutes the cell

or protoplast is composed of living substance, cytoplasm, in part of which (the chloroplast) green pigment, chlorophyll, identical with that of higher green plants, is contained. A denser, specialized mass of living substance, the nucleus, is to be seen in the clear, upper, flask-shaped part of the cytoplasm. The body terminates in a minute cytoplasmic knob from which project two slender, living threads—the flagella—by the rhythmic movement of which it is propelled through the water. On one side of the body is a small spot of pigment—the eye-spot—which helps the organism to register the direction in which light falls upon it and thereby to move toward the light.

At the moment of its birth this organism may have no visible covering, but its nakedness is almost immediately covered by the formation from the surface layer of cytoplasm of a membrane of non-living substance—a cell-wall which, though thin, is continuous and hence serves as a barrier across which no solid particles may pass. Through it only water and materials dissolved therein may be absorbed.

Although the plants of the primitive plankton were doubtless, like those now found in the surface waters of the sea, of varied structure, this organism, a species of the genus Chlamydomonas (coated monads), serves to illustrate the kind of beings they were. Active, freely moving, possessed of a rudimentary ‘sense organ’, this single-celled organism seems strangely different from the higher members of the world of land plants. Yet it may have been not distantly related to their remote ancestors. Its green pigment and continuous solid coat or cell-wall are characters which Chlamydomonas shares with other and even the most highly organized plants.

It is probable that in the earliest phases of the primeval plankton-period the world of life showed no distinction into plant and animal kingdoms, and that it was only in later phases of the period that organisms arose with distinctive animal characteristics. Even Chlamydomonas, so plant-like in the possession of chlorophyll and in the complete investiture of its body by a coat of cell-wall substance, may pass through phases in which it assumes animal-like characteristics. It may settle on organic debris, withdraw its flagella, round itself off into a sphere, and lose its chlorophyll. In this stage it may increase by growth and division of its body, obtaining, after the fashion of an animal, the material for growth from organic substances. In a similar way, simply organized aquatic forms of hfe of animal nature may show upon occasion plant-like characters. For example, one of the common, microscopic, unicellular organisms which exists to-day (Euglena sp., Fig. 3 B) possesses the distinctly animal characteristic that the investiture of its body

Plankton and Benthos T3 stops at and does not cover a short tract or gullet, which leads from the surface into the body. The existence of the open passage permits of the entry of solid particles which, if of food-yielding kind, undergo digestion and serve for the nourishment of the animal. This mode of nutrition is characteristic of the animal world ; to sweep solid substances into: a digestive tract and there to extract from them and to absorb into the body such nourishment as these solids may yield. In sharp contrast with this mode of nutrition is that of the plant world. The continuous solid coat or wall which invests. the plant-body admits of the passage of no solid particles. Only substances in solution may pass across the wall and. reach the protoplasm, that is, the cytoplasm, nucleus, and all other living parts of the cell.

The animal obtains its ready-made food-materials from without in solid or liquid form. The green plant, though it uses essentially the same food-materials, makes them within. its body from solutions of raw materials which it obtains from. soil and air. When these opposed characteristics became fixed, the one in one set of organisms, the other in another set, the dividing line between animal and plant world was estab-. lished and henceforth the members of the two kingdoms pursued their several and divergent ways.

Yet, in spite of their animal attributes, species of Euglena. contain pigment identical with the chlorophyll of green plants, and like green plants make use of it in the manufacture of food-materials.

There is no reason to doubt but that during the later plankton-phase the seas contained both plant and animal organisms. If it did the wastage of life must have been great indeed. The physical conditions of plankton life are severe enough for frail, minute, and ephemeral things. Rough seas and tides and untoward currents take a heavy toll: for to be borne below the depth to which the sun’s rays penetrate: spells darkness and death. To these must be added the no less formidable risks of incorporation and digestion within the:

bodies of the animals of the plankton. Yet in spite of everrecurring wastage the plant plankton maintained itself and assumed diverse forms of which some have left descendants which survive in the seas to-day.

For how long the primitive world of life consisted only of plankton cannot even be guessed ; but in view of the multiplicity of forms which were evolved, and of the slowness as measured by years with which evolution seems to progress, it was probably very long. Nevertheless in course of time the plant world developed a new kind of organism. Some of the plankton plants acquired a new habit of life. True to their role of pioneers, primitive plankton adventurers gave up their freedom and became fixed organisms or benthos. Settling on the shelving shores of shallow seas, the forerunners of the higher plants threw out anchor-like projections from their bodies and by these holdfasts grappled themselves to the rocks or anchored in the ooze. The world of plants was now divided into two sub-kingdoms: plankton, which remained free, and benthos, fixed and immobile. With the latter lay the future. They developed their two-endedness. The lower root-end became a strong holdfast, the free end grew out to form a thallus consisting of threads, or plates, or masses of tissue composed of many cells. It even in some cases became differentiated into an axis bearing lateral extensions—first crude foreshadowings of stem and leaf, which later on were to become fixed and standardized in the higher forms of vegetation. So the bodies of plants of the benthic phase, fixed by their holdfast root-ends, floated in the sea after the fashion of seaweeds (Fig. 4), which indeed may perhaps be their much-changed descendants.

The phase during which plankton and benthos divided the marine plant world between them, like the plankton-phase which preceded it, may have endured for ages, lasting until the land surfaces of the earth became hospitable to plants. When that happened the casual but sure hand of chance could scarcely fail to lead some benthic plants to land. For

Fic. 4. Fucus platycarpus, a brown seaweed of benthic habit.