Plant life

Front matter

Rradrury Agnew s Co 9. BouverieSt.

LONDON

I'miiiliwaixiiM

Preface

— * —

Rightly to understand wliat work is done byliving plants, and how it is effected, not only requires a student to be a botanist in the ordinary sense of the word, but necessitates that he should also have a comprehensive knowledge of physics and of chemistry.

In few individuals can such an extensive knowledge now-a-days be expected. The practical cultivator especially, harassed by the daily cares of his occupation, is not able to master the endless details of these sciences ; and yet experience shows the increasing necessity for furnishing him with new tools and new weapons to enable him to utilise the resources of Nature, and to contend against adverse circumstances. Such tools, such weapons are furnished by the armoury of science. It is the object of this Handbook to point out the nature of these resources, and suggest the methods of utilising them. Something will be gained if only a right appre-

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PREFACE.

ciation of what cannot be done is obtained, as thereby labour on a sterile soil will be avoided, to be applied with more reasonable hope of success elsewhere.

In the following pages an attempt has, therefore, been made to supply a sketch, necessarily in faintest outline, of the physiology or life-history of plants ; of the way in which they are affected by the circumstances under which they exist, and of the manner in which they in their turn react upon other living beings and upon natural forces. Of necessity, there has been a little overlapping in the case of some of the subjects treated of in the companion volume, “ The Chemistry of the Farm/’ by Mr. Warington ; but as the matters are looked at from a different stand-point, and as no pretence is here made to impart special chemical knowledge, it is hoped that Mr. Warington and the reader also will forgive any slight incursions into a territory which the writer has no claim to enter except upon sufferance.

Structural botany, whether dealing with the outer conformation or the internal organisation of plants, is only incidentally treated of in these pages ; the classification of plants is also passed over without notice, as not coming within the scope of this Handbook. Information of an elementary character on

PREFACE.

Vll these points is given in the author’s “ Botany for Beginners,” * to which the present volume is in some sense complementary.

Detailed text-books of Botany, or of Vegetable Physiology, expressly adapted to the requirements of agriculturists do not exist, but there are many works from which a comprehensive general idea of the present state of knowledge on these subjects may be obtained.

Foremost among them is the second English Edition of Sachs’ “ Text Book of Botany,” edited by Dr. Vines.

The “Elementary Text Book of Botany,” by Prantl, of which an English edition has been published, also under the supervision of Dr. Vines — is in some sense an abridgment of Sachs’ larger volume. Of similar nature is “ The Text Book of Structural and Physiological Botany,” by Thomd, edited by Mr. Alfred Bennett. The third edition of Henfrey’s “ Elementary Course of Botany,” edited by the present writer, is, like Bentley’s “ Manual of Botany,” more comprehensive in scope than the works last named.

In the compilation of the following pages the writer has availed himself of the foregoing works, as

* Published by Messrs. Bradbury, Agnow, & Co.

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PREFACE.

well as of Van Tiegliem’s “ Traitd de Botanique,” the French translation of Sachs’ “ Physiologie Vegetale” by Micheli, and Deherain’s “ Conrs de Chimie Agricole.” The works and memoirs of Darwin have, of course, been laid under contribution, as well as numerous scattered papers by various authors. More especially the writer has to acknowledge his obligations to the voluminous records of the noble series of cultural experiments carried out at Rothamsted for so many years by Sir John Lawes and Dr. Gilbert. So far as at present published, these records and reports are chiefly to be found in successive volumes of the Royal Agricultural Society’s Journal. The very elaborate papers on the mixed herbage of Permanent Meadow from which most of the facts alluded to in the chapter headed “The Battle of Life” have been taken, are published in the “Philosophical Transactions” of the Royal Society. The “Memorandum Sheet” published by these experimenters supplies annually a condensed summary of the results of their experiments, and is a document that should be carefully studied with due reference to its professed object, by all who have the advancement of agricultural knowledge at heart.

M. T. M.

Contents

CHAP. PACK

I. — Plant Nutrition: The Work and the Materials. 1 — 16 II. — Nutrition ( continued ). — The Machinery . . . 17 — 41

IY. — Sensitiveness 55 — 71

Chapter I. Plant-nutrition : The Work And The Materials

Introductory remarks. — What plants do and how they do it. — Receipts. — ■ Expenditure. — Accumulation. — Transformation. — How plants feed. — Influence of temperature.— Water and the machinery by which it is supplied and distributed. — Protoplasm. — Cells and their contents. — Ingress and movements of water. — The first stage of nutrition. — Diffusion.— -Osmosis and the requisite conditions forit. — Saturation. — Varying degrees of, according to the nature of the liquid.— Amount absorbed. — Supply and demand. — Differences of composition of plants grown in the same soil, how explained. — Continuous change.— Nutritive value of water. — Nitrates; agency of Bacteria. — Potash. — Sulphur. — Phosphorus. — Iron. — Lime. — Principles of manuring. — Power of selection.

He who can make two blades of grass grow where only one grew before is universally looked on as a benefactor to bis kind. No one will dispute his title to our gratitude ; but at the same time it must not be overlooked that the claims of him wffio can make one grow where none at all existed before are even greater, because the difficulties to be overcome are more formidable, for where one exists already it is relatively easy to bring about its increase.

In any case, it is clear that, before either problem can o

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he satisfactorily solved, as full a knowledge as possible of all the conditions requisite for the process must he in some way or another obtained. Success in a practical pursuit like agriculture depends largely on the extent of our knowledge, and still more upon our power of applying it under various circumstances. In the following notes an attempt , will be made to supply indications of some among the phenomena of the life of plants of which it seems most desirable that the cultivator should take heed. The word “ indications ” is used because no attempt at completeness of detail or exhaustive treatment will be made, for that would be out of place here; and, moreover, the subject has already been treated from its chemical aspect in a Handbook by Mr. Warington. Some slight knowledge of the general conformation of plants on the part of the reader is assumed, but explanations of the more important points will be given ; ■while for further illustration of the construction of plants the reader may be referred to the writer’s “Botany for Beginners ” (Bradbury, Agnew, & Co.).

A living plant feeds, breathes, grows, developes, multiplies, decays, and ultimately dies. In so doing it receives, it spends, it accumulates, it changes. Some of these processes are always in operation, very generally more than one is going on at the same time, and the action of one is modified by and controlled by that of another. Some circumstances and conditions favour these operations, others hinder them. The practical cultivator has his concern in all these matters, so that it is of no slight moment to him to realise what is the work which a plant does, and how it does it.

How Plants Feed. — The nutritive process has to be

NUTRITION: THE WORK AND THE MATERIALS.

entered on the creditor side as a receipt. The plant will indeed feed upon itself for a time, or rather it will feed upon what its predecessor left it as an inheritance for this very purpose, or upon the stores accumulated in the plant itself during the preceding season ; thus, when a seed, or rather the young plant within the seed, begins to grow, it is at first unable to forage for itself, hut it depends for its sustenance on the materials laid up for its use during the preceding season by the parent plant. So the bud of a tree awakening into life, and beginning its career as a shoot which is to hear leaves and flowers, derives its first meals from the reserves accumulated the autumn previously in the parent branch. Very generally a little water, supplied from without, is required before the plant can avail itself of these stored-up provisions, but this is not always indispensable. Potatoes begin to sprout in their cellars or pits, as growers know, to their cost, before they can have obtained a drop of water from without. In this latter case there is water enough already in the tuber to allow of food being utilised.

Effect of Temperature. — A certain degree of useful heat is, of course, quite indispensable. Practically, no plant will feed when its temperature is reduced as low as the freezing point, and in most cases the heat requires to he considerably greater. Each kind of plant, each individual plant, and indeed each part of a plant, feeds, and performs each item of its life-work, best at a certain temperature, and , ceases to work at all when the temperature falls below or rises above a certain point. The particular degree, whether most or least favourable, varies according to the plant, its age, stage of growth and various external cir-

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cumstances, "which we need only mention, as their effects will he readily understood without the necessity of explanation.

It is clear then that a suitable temperature and access of water, either liquid or in the form of vapour, are the first essentials in the feeding process in plants. Practically, and from force of circumstances, the gardener has more control over both temperature and the supply of water than the farmer ; nevertheless by drainage, by choice of aspect, site, by shelter, and other means, even the farmer has some power to regulate the temperature and the amount or influence of water to which his crops are subjected.

Water. — Leaving, however, on one side, the temperature, we have to consider the water which is so essential, not only in the feeding processes with which we are now concerned, but with every other action of plant life. Fortunately there is, in general, no lack of it ; the earth and the air contain their shares of this elementary compound in varying proportions and varying modifications as liquid or gaseous. Besides, the plant itself has so much of it that even at the driest condition compatible with life it still constitutes a very large proportion of the entire weight. Now, it is as a rule when the plant, the seedling, or the bud is at its driest that growth begins, the necessity for food first manifests itself, and the demand for a further supply of water becomes imperative. How is the demand supplied ? We have seen that there is no lack of that fluid. How is it to get into the plant? The answer to this question brings us at once to the consideration of the raw material and of the fabric of plants by whose agency alone it is that the water gains entrance to the plant.

Ingress and Movements of Water; Diffusion, Osmosis.

• — Our first inquiry, then, must be to ascertain how the water, whose presence in sufficient quantity we have assumed, gets from without through the cell-membrane into the protoplasm — how, in fact, the first stage in independent nutrition is accomplished. When one liquid, say spirit, is poured into another, say water, the two gradually mix. If we suppose these liquids to consist of a number of molecules,* then, mixture may he taken to be the result of the displacement say of one molecule of water by one molecule of spirit, and so, throughout the whole quantity of liquid, there is displacement and replacement till at length equilibrium is restored and a thorough diffusion results. This power of diffusion does not always exist. The molecules of water and of oil will not mix or diffuse freely through each other. Water containing carbonic acid gas will not mix, in this sense of the term, with water containing acetate of lead; and when the attempt is made, chemical change is set up, the heretofore clear solutions — that containing the gas and that containing the lead — become when combined opaque and milky, owing to a chemical change, resulting in the formation of a white insoluble lead carbonate.

* It may be well, once for all, to explain the sense in which the term “molecule ” is here used. It is now generally assumed by physicists that every substance in nature is made up of excessively minute particles called atoms, which are indestructible. An atom cannot exist by itself, but in association with others. Such a group of atoms is called a “molecule.” A molecule, therefore, is the smallest group of atoms capable of existing separately and independently. These molecules may be of different sizes in different cases, and they are believed to be so arranged as just not to touch, but to leave spaces between them ; smaller in the case of a hard solid, wider in that of a liquid, still wider in that of a gas. The extent, moreover, of these interspaces may be increased or diminished bv varvinn degrees of heat. &

G

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It may be a truism to say, that for the process of diffusion the liquids must be diffusible, but the fact must be carefully borne in mind in all questions relating to the feeding of plants. In the case of plants, the phenomenon of diffusion, or the gradual admixture of two liquids of different natures, is complicated by the presence of a membrane in the shape of the cell-wall. The water from the outside has to pass through the membrane to reach the protoplasm on the other side. Speaking broadly, there are no holes in the membrane through which the water can pass. Ingress is secured by that process of diffusion to which reference has just been made, and by virtue of which the molecules of the membrane and the molecules of the water shift and change places ; the space that was occupied by a molecule of membrane is now occupied by a molecule of water, and vice versa. The access, therefore, of water into the interior of a closed cell is the result of the process of diffusion. Where two liquids mix without any intervening membrane, the mixture is called diffusion simply ; where there is an intervening membrane, the diffusion process is known as “ osmosis.”

Protoplasm— Cells.— The raw material (the term is not quite accurate, but for illustration sake it may pass) is that very marvellous substance now called “protoplasm.” We must leave it to chemists and microscopists to explain its composition and indicate its appearance. Suffice it here to call it, as Huxley did, “the physical basis of life.” Without it or when it is dead the plant is dead too ; with it the plant lives, without it it dies. It is a viscid, colourless, jelly-like substance, endowed with all those varied properties which constitute in the aggregate all that we can tangibly realise of the manifestations of life.

With few exceptions, which it is not necessary here to particularise, the protoplasm does not exist in one unbroken mass, but is cpntained in little membranous bags called “ cells.” These cells are of various shapes and sizes, and may undergo various modifications during the growth of the plant. They are large enough to be seen by the naked eye in the pulp of an orange, but usually they require the aid of the microscope to discern them. They are lengthened into tubes, placed end to end to form conduits, or thickened into fibres. The cells, then, variously combined and modified, constitute what we have termed the fabric of the plant. Each living cell consists essentially of a certain proportion of protoplasm contained within a membranous bag or bladder, called technically the “ cell-wall.” There may be, and generally are, other things besides the protoplasm within the cell-wall, such, for instance, as a small ovoid body known as the “nucleus,” and green colouring matter or “ chlorophyll ; ” but these other things, important as they are, we may leave out of consideration for the present.

Every plant and every part of every plant is made up of cells such as have been mentioned. As a cell a plant begins its independent life; with and by cells it lives, grows, multiplies ; by their decay it dies. It is, as has been said, the protoplasm, which is the essential agent in all these processes ; but, subject to a few exceptions, which need not now be specified, this protoplasm is always shut up within a cell-wall. Nor is it absolutely necessary that there should be more than one cell. Most plants with which the cultivator has to do consist of aggregations of

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such, but there are myriads of other plants which consist of but one cell ; in such a case the cell is the plant, the plant is the cell. Now this is important, because it shows us that all the processes of life can be, and often are, carried on in one cell only, that is, by one fragment of protoplasm. Where the fabric becomes more complex, one cell is more or less dependent on another, but still there is always a measure of independence left to each individual cell. Were it not so, the scythe of the mower or the grazing of the sheep, by destroying a portion, would kill the entire plant.

It follows that the life-history of a plant is, in essence, the life-history of protoplasm and of its covering, the cell-wall ; and hence it is that the microscopist or the chemist in the laboratory studying what goes on in isolated cells, placed as far as possible under uniform conditions, is really adopting the best means of investigating what takes place in the entire plant ; a circumstance which the “practical man,” so called, compelled to work in the field under such very different, more complex and much less definite conditions, finds it difficult to realise.

Conditions of Diffusion. — Diffusion, it will readily be understood from -what has just been said, is not equal or alike in all cases ; it depends upon the extent to which the twro liquids are diffusible, upon their different densities, upon temperature, and a variety of other conditions. So, in the case of osmosis, we have not only the nature of the two fluids to consider, but their relation to the membrane that separates them. The membrane may be much more permeable to one of the two fluids than to the other. Thus, in the case of a living cell, the membrane or wall is much more permeable to water than it is to protoplasm ; and so it happens that, while water readily penetrates the membrane and diffuses itself in the protoplasm, protoplasm does not nearly so readily permeate the membrane as the water. Ingress of water is easy and of constant occurrence, egress of protoplasm is rare and exceptional.

Pure water or weak saline solutions, such as are generated in the soil under certain circumstances, pass readily through membrane — that is, the molecules of the one shift and change places with those of the other — while those of gummy or albuminoid substances like protoplasm do not. After a time, if there is no outlet for the water absorbed, or if it is not utilised within the plant in some way, absorption and diffusion cease, the cell becomes saturated with water, and until something happens to disarrange the balance, no more is absorbed. But, even in the case where the cell is saturated with water, it may still take up other liquids, because the diffusive power of those other liquids, in relation to the cell-wall and to the protoplasm, is different from that of water, and this absorption may go on in its way till saturation point is reached for each one of them, just as in the case of water. On the other hand, it may happen that the plant may be saturated with other substances, and incapable of taking up more of them, while at the same time pure water may be freely taken up.

Quantity absorbed. — Just so much and no more of each particular substance is absorbed, the exact quantity of each being regulated in all cases by the condition and requirements of the cells, their membranous walls, and their contents. Thus it happens that some particular substances may he found by the chemist to exist in large relative pro-

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portions in tlie plant, wliile the quantity in any given sample of the soil from which it must be derived is sometimes so small as to elude detection. The plant in this case, or some part of it, is so greedy, if we may so say, for this particular substance, that it absorbs all within its reach, and stores it up in its tissues or uses it in someway, the demand ensuring supply. On the other hand, the soil may contain a large quantity of some particular ingredient which is incapable of being absorbed, or which the plant does not or cannot make use of, and, in consequence, none is found within the plant. The supply is present, but there is no demand.

The different physical requirements of the plant supply also the explanation of the fact that different plants, grown in the same soil, supplied with the same food, yet vary so greatly in chemical composition. Thus, when wheat and clover are grown together, and afterwards analysed, it is found that while lime is abundant in the clover, it is relatively in small quantity in the wheat ; and silica, which is abundant in the wheat, is absent from the clover. Poisonous substances even may be absorbed, if they are of such a nature as to be capable of absorption ; and so the plant may be killed by its own action — by suicide, as it were.

The entrance of water into the plant and the entrance of those soluble materials which a plant derives from the soil are therefore illustrations of the process of osmosis, and are subjected to all the conditions under which osmosis becomes possible, or under which it ceases to act. The study of these conditions is a question for the physicist, and the full explanation of them must be sought in works relating to physics. So the investigation of the substances which are absorbed with, the water, of the food materials, and their transformations within the plant, is the work of the chemist, and their history must he sought in chemical hooks.

Continuous Changes in Plants. — In this place we must confine ourselves to the few passing references already made, but one thing we must strive to impress forcibly on the reader, because, if the notion is well grasped, it will enable him to understand plant life so much more vividly. We allude to the continual changes that are going on throughout the whole living fabric of the plant while in its active condition. Cell membrane, the protoplasm, the entire mass of liquid and solid constituents of which the plant consists, are, as we have seen, made up of molecules, each, as it were, with a life of its own, undergoing continual changes according to different circumstances, acting and reacting one upon another so long as any active life remains. Active life, indeed, is ceaseless change ; dormant life is a condition of equilibrium, more often talked about than realised — in fact, it is merely relative — it implies merely a lessened degree of activity. From this physical point of view the death of a cell is only a change, a rearrangement of particles, never, however, to be recombined into a new growing cell, as happens in the case of a still living cell in the full tide of growth and activity.

Nutritive Value of the Substances absorbed by Plants.

— The importance of water may he judged from the fact that while succulent vegetables contain more than 90 per cent, of water, timber felled in the driest time seldom contains less than 40 per cent. (Warington).

As to the nature of the saline substances, reference

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must, as lias been said, be made to the “ Chemistry of the Farm ” and other works for full details. Suffice it here to say that certain of them, though always in relatively small proportions, are essential to the life of the plant ; certain others generally met with, though useful, are not indispensable. The former comprise salts of potash, magnesia, lime, iron, and in addition phosphorus and sulphur. The latter comprise salts of soda, silica, manganese, together with chlorine and occasionally other ingredients.

Of the salts just mentioned, the nitrates are of extreme importance, inasmuch as nitrogen is an essential constituent of protoplasm — without nitrogen there can be no protoplasm, without protoplasm there can be no plant. The nitrogen is supplied to the plants from the soil in the form either of nitrates (potassic nitrate, sodic nitrate), or of ammonia salts in which the nitrogen is in combination with hydrogen. The ammonia in the soil is made to combine with oxygen, and thus to form nitric acid, through the agency of minute organisms called “ Bacteria,” which, like the yeast fungus, act as ferments ; and by their agency it is, as Mr. Warington has pointed out, in confirmation of the researches of Schloesing and Muntz, that the ammonia salts, which themselves are inert, or it may be harmful, get converted into useful nitrates. Ammonia salts applied to some soils do no good, because the needful germs or ferment bodies are not present in the soil ; but where they do exist, they convert the useless into the useful, as before said. These bacteria occur in all fermenting material, such as farmyard dung, whose value as manure is in part accounted for by their presence and agency. It is probable in the future that just as the brewer uses his yeast to secure the conversion of starch into sugar, and the chemist “ seeds ” his solutions to effect the changes he wishes to bring about, and just as the gardener sows the spawn or germs of mushrooms in his mushroom bed, and obtains thereby a crop of succulent fungi, so the farmer may be able to apply to the soil the ferment-producing germs needed to change its quality, and render it available for plant food. When we have arrived at that point, manuring will be reduced to a science, and a pinch of the right material will be as efficient as a ton of our present compounds, the larger part of which are undoubtedly wasted under existing circumstances.

Potash salts are also essential, more so in some cases than in others. At Kothamsted, potash, after having been employed for a number of years as a manure-constituent on a certain grass plot, was discontinued ; the produce, in consequence, rapidly declined, and the quantity of carbon fixed in the tissues of the plants proportionately diminished, although the amount of nitrogen absorbed was the same in the two cases. The presence, therefore, of an adequate supply of potash, in the soil, seems essential to the full assimilation of the carbon which is derived, as we shall presently see, from the air, in the form of carbonic acid gas. It is believed from recent experiments that without potash no starch can be formed ; and starch, as we shall see hereafter, is of primary importance in the nutrition of the plant. In any case the value of potash manures for increasing the yield of certain crops, particularly potatoes, is a fact beyond dispute.

Sulphur and phosphorus are also derived from the soil as sulphates and phosphates. Both occur in association with the albuminoid contents of the protoplasm; and phosphorus seems specially needful in the formation of the pollen — the fertilising powder in the flowers — and in the

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ripening of seeds, while its effect on the growth of turnips is familiar to all practical men.

Iron is essential to the formation of leaf-green — “chlorophyll”— and chlorophyll is essential to the production of starch ; hence iron in some shape is essential to plants, and it also is supplied from the soil in the form of saline solutions.

What precise function lime plays in the plant’s economy is not known ; hut indirectly it is of importance as a means of introducing phosphorus and other essential ingredients. At Rothamsted, in two of the plots upon which barley has been grown for thirty years in succession, a mineral manure with nitrogen has been applied ; but in the one case lime has been added, in the other (otherwise treated exactly in the same way) no lime has been added. The plants on the plot without the lime are always of a darker green colour, but they are relatively deficient in carbon. Under equal conditions, it is seen that the amount of carbon assimilated from the atmosphere in the manner to be hereafter mentioned is directly dependent on the amount of available nitrogen, which latter is derived from the soil — (Lawes and Gilbert).