Practical Plant Physiology

CHAPTER I.

THE problems of plant-physiology and the method by which they are to be solved. The scientific method. Classification of physiological problems.

A SEED sown under suitable conditions germinates, giving rise to a seedling. The seedling grows, puts forth leaves and branches, drives its roots further and further into the soil, and becomes a mature plant. Presently and in due season the flowers appear, endure in their delicate beauty for a while, and, their work being done, fade away. The stalks which bore the flowers now support the swelling fruits within which the seeds are ripening. When the seeds are set, the fruit bursts open, scattering them far and wide, or, falling to the ground and rotting, sows the seeds near the parent plant. Such are the more striking episodes in the life of a flowering plant.

This regular sequence of events—the germination of the sown seed, the formation of roots and branches, leaves and flowers, the setting of the seeds and the ripening of the fruits—seems so natural that we are apt to accept it as needing no explanation. But when we begin to observe the several processes more closely, to reflect upon them, or to compare one plant with another with respect to them, we find ourselves asking all sorts of questions. Why is it that the chickweed of the hedgerow runs its course from seedling to fruiting stage in less than one brief season, whilst a foxglove does not reach the flowering stage till its second year? By what gymnastic exercises does the seedling extricate itself so neatly from its seed coat? How is it that the plantain on the lawn hugs the ground so closely as to escape almost uninjured the knives of the cutting machine? Whence come the power and the material by means of which the giant oak with gnarled trunk and spreading branches forms itself from an acorn? How is it that, no matter which way up we plant a bean seed in the soil, the root of the seedling turns downward and burrows in the earth, whilst the stem twists itself so that it comes to grow upward into the air? From what source does the apple obtain the sugar to which it owes its sweetness or the rose the perfume with which it scents the air?

When once we begin to interest ourselves in plants, we find that the problems which they suggest are as varied as they are numerous, and we realise that in every plant of hedgerow, field, or garden, all sorts of strange events are happening.

Could we but find answers to the questions which living plants suggest to us, we should be in possession of a great body of knowledge concerning their life and work; in other words, we should have taught ourselves not a little of the science of plant physiology.

Hence the most pressing of our problems is, how are we to set about obtaining answers to any of the questions which arise in our minds when we observe living plants? Curiosity suggests the problems, how does science seek to solve them?

In order to find out the method of scientific discovery, let us fix our attention on some particular phenomenon exhibited by a plant, and consider how we may ascertain its significance.

The phenomenon which we will choose for investigation is the origin of the drops of water which appear on the leaves of certain plants, such, for example, as the oat.

1.†We sow oats in ordinary soil in two pots, and, when the leaves of the seedlings are four or five inches in height, we may find, on examining them in the early morning, that near the apex of each leaf is a shining drop of water looking like a dew-drop (Fig. 1).

We want to discover whence the water-drops come!

Though we were to sit up all night watching the plants, we should obtain no solution of the problem: all we should see is that the drops, when first formed, are small, and that they may increase in size very rapidly. Observation, therefore, though it provides us with scientific puzzles, does not necessarily help us to solve them. Observation, careful and continued observation of the living plant, is essential for the study of plant physiology, but something besides observation is wanted now.

FIG. 1.—OAT SEEDLINGS.

Drops of water (w), excreted from water pores (hydathodes) in the tips of the leaves. From a Photograph.

Evidently all we can do is to make a guess as to the origin of the water-drops. Confronted with the problem, we call imagination to our assistance, and by its aid guess at the answer. As the result of guessing, we suggest that the water may be dew formed from water-vapour in the air. Next we ask ourselves; suppose the guess is wrong, what then? At once common-sense makes answer; if the water does not come from the air, it must come from the plant itself. Having exhausted our guessing powers, we proceed to look coldly at the alternative suggestions, and to review them in the light of common-sense. In this case, commonsense admits that either guess may be right. But our guesses cannot both be right. Therefore we must discover some way of deciding between them. If the problem were one of a kind with which we are more familiar, for example, as to the height of a friend; and if two people guessed differently with respect to this, we should not hesitate as to our method of verification. We should stand the friend against the wall and measure him. That is, we should put the guesses to the test of experiment. Whereas no amount of discussion would determine the correctness of the guesses, a yard measure properly used would settle the matter in a minute.

In like manner, to solve the problem of the origin of the water-drops, we submit it to the test of experiment. But how is this to be done? Once again we must appeal to imagination and common-sense. We must use these faculties conjointly in order to invent an experimental test. In our particular problem, it is easy enough to devise a method. We know that plants take up water from the soil, and so we argue thus: if the drops of water on the leaves come from the air, they may make their appearance as readily on the unwatered as on the watered plants; but if the drops come from the plants themselves, it will probably matter fundamentally whether the plants contain much or little water. Thus we arrive at our method of experiment. Water one pot thoroughly, withhold water from the other, and examine the plants on successive mornings. When we do this, we find that the water-drops are plentiful on the leaves of the watered seedlings, and are absent from, or, at all events, fewer on the others.

To complete the proof we devise a further experiment.

2.For example, give water to the previously unwatered pot, stand it under a bell jar, and observe that in five or ten minutes drops of water appear on the tips of the leaves. Therefore we conclude that the water which appears on the leaves comes from the plant and not from the air.

But in solving the particular problem, we have discovered also the answer to the general problem—how to set about getting replies to special questions? The answer is—by using exactly the same method as that which we have just employed. There is indeed no other way. It is called the scientific method, and involves, as we have learned, processes of guessing, reasoning, and trying.

Thus the processes involved in the use of the method are as follows:

1. The guessing process in which the imagination is invited to suggest possible answers to the problem under investigation. Our guesses may be as wild as we like to make them. The more the imagination is allowed to run riot, the more likely are we to open up new paths for investigation. Indeed, it is no exaggeration to say that the greatest discoveries are the outcome of the wildest guesses.

2. The judging process, in which common-sense assumes the part of advisor, recommending this or that guess as more likely to prove true, and rejecting any guess which runs counter to established truth.

3. The testing process, which consists in the devising and execution of experiments calculated to demonstrate the truth or falsity of the guess, or, as it may be called, the hypothesis, which gains the approval of common-sense.

4. The summing-up process, by which we decide, whether the evidence provided by the results of the experiments is convincing or not.

If the evidence is absolutely conclusive in favour of our hypothesis, we speak of that hypothesis as a fact; if the evidence is inconclusive, we may yet continue, for want of a better, to hold the hypothesis and to use it in our arguments; though in doing this we have to be extremely cautious, and to remember that our hypothesis is not proven.

Hence to study a science aright is not to become a narrow specialist, but to develop all the highest faculties of the mind. This scientific method is not peculiar to plant-physiology: it is the method employed in all the sciences, and by its use all the knowledge of nature which we possess has been obtained.

All that remains to be done in this introductory chapter is to classify our problems, that is, to arrange those of like nature in groups, and the groups in convenient order. In doing this we will make an assumption, which may not, at first sight, seem very probable; but which will be of great service to us. Whether the assumption is true or false we shall discover as we proceed with our investigations. We assume that the life of a plant is not different in essentials from that of an animal or from that of man himself. Unless this assumption is wholly false, and, in that case, we shall soon discover our mistake, it will be of great assistance to us in the otherwise puzzling problem of the arrangement of our questions. For we know, without the aid of science, and from our common experience, a good deal about our own life-processes. We know, for example, that we feed, and that without food of certain kinds we cannot live. We know that we breathe, and that we cannot exist for more than a few minutes without air. We know also that we, like animals in general, move, and that some movements, as, for example, getting up in the morning, depend on an effort of will, whilst other movements, for example, the beating of the heart, are independent of consciousness. We know also that animals and plants grow, give birth to young, and ultimately die.

Hence we arrive at the following classification of the problems of plant physiology:

(1)Feeding processes (nutrition).

(2)Breathing processes (respiration).

(3)Growth processes.

(4)Phenomena of movement and of sensitiveness (or irritability).

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† The numerals, in heavy type, which occur throughout the book refer to the experiments which are to be performed, see Preface, p. xi.

CHAPTER II.

THE mode of germination of seeds: the parts of the seed and seedling: the resting and active states of seeds: the resisting powers of resting seeds: germination capacity: the visible order of events in germination. The nature and function of cotyledons and of endosperm: adaptation in plants: large seeds and small seeds.

A FULLY grown plant is by no means a convenient subject for experimental purposes. Not only is it bulky, but its roots are hidden in the ground and cannot be disturbed without damage to the plant. On the other hand, a handful of pea or bean seeds may be obtained for a penny, and, when planted, the seeds produce seedlings in the course of a week or two. Moreover, inasmuch as the seedlings grow rapidly, we may assume, from analogy with young children, that they are likely to feed hungrily. Hence seeds and seedlings should prove very useful to us in our studies in plant-nutrition. We will therefore commence our work by an examination of seeds and seedlings.

Since we shall require seeds for all sorts of experiments, we must take every opportunity of getting together a large and varied collection. At the proper times of the year, ripe seeds of garden plants, weeds, and common trees should be gathered, dried, and stored in corked or stoppered bottles. The bottles should be labelled, and on each label should be written the name of the seed (or fruit), the locality whence it was obtained, and the date of gathering. If it is not possible to collect a sufficiently varied assortment of seeds, some may be purchased from seedsmen and stored in labelled bottles. Samples of the seeds and fruits should be affixed to cards with the name, natural order, and other interesting details, such, for example, as locality and weight, and the cards placed in the physiological museum, in which records of experiments, specimens, and photographs, etc., should be kept. In case the beginner does not know how to distinguish seeds from fruits—and some fruits look exactly like seeds—he should refer to an elementary text-book (Bibliography, 3, 5), which deals more particularly with the morphology of plants, that is, with the characters and peculiarities of their form and structure. For, though we, in studying the work of plants—that is their functions—shall have to take notice of their form and structure, we have not space to deal fully with the morphological branch of botany.

Having become familiar with the shapes, sizes, and peculiarities of the seeds and fruits of the commoner plants, we proceed to germinate some peas. At once the question arises: since the seeds in our collection do not germinate whilst in the bottles, what is to be done in order to make them begin to grow? Now everyone who has access to a garden or to the country knows how quickly weeds and other plants spring up in showery weather, and hence we make the sure guess that a supply of water is necessary for germination. Even though we know this, we prove it; for, by so doing, we shall extend our knowledge and make it more precise.

3.To this end, prepare three pots of garden soil, dry one thoroughly in a kitchen oven, and, in order to prevent the soil from getting moist again, set it to cool under an inverted marmalade jar, or similar vessel. See that the soil in the other two pots is thoroughly moist. Determine the average weight of ordinary, dry pea seeds by weighing several samples of twelve each. Put a couple of dozen seeds to soak in tepid water, and, after twenty-four hours, dry their surfaces by means of a cloth, weigh and compare them with respect to weight, size, and shape with the dry seeds. Calculate the percentage of water taken up. Now plant four or six peas in each of the three pots: putting dry seeds in the pot with the dry soil, dry seeds in one of the pots with moist soil, and soaked seeds in the remaining pot. Label the pots 1, 2, 3, and note in a rough note-book the details of time of planting, and states of seed and soil. Cover each of the pots with a glass plate or piece of cardboard or stiff brown paper, and see that the soil in pots 2 and 3 does not get dry. Record the dates of appearance of the seedlings in each of the pots. Copy out the results neatly in a note-book kept for the purpose, and add any remarks that seem interesting. Records should be made of the results of every experiment that is performed, and, whenever it is useful, sketches should accompany the records, which should be arranged in brief, tabulated form.

4.Take the remaining soaked seeds, wipe them, put them in a dry place—for instance, on a shelf in a living room—and weigh them at daily intervals, and thus determine the rate at which they lose water. When they seem fairly dry, put them in a thin paper bag in a desiccator. (See Appendix A.) At the same time, weigh and place in a paper bag a dozen dry, unsoaked peas, and put this bag, properly labelled, in another desiccator. After an interval of about a week, weigh the two lots and determine how much each has lost in weight. Leave them exposed to the air of a room for some hours, weigh them again, and compare these weights with those of the same seeds when taken from the desiccator. From the experiments, it is evident that seeds are hygroscopic, that is, they take up water from moist air and give up a certain amount of water when the air to which they are exposed is dry. The bearing which these facts have on such matters as the following should be considered:—the importance of storing seeds out of contact with moist air: the difference that the weather at the time of harvesting is likely to make to the viability of the seed: the advantage and possible disadvantage of soaking bean or pea seeds before sowing in the garden: the fact that, in wet autumns, seeds of various plants may be found germinating whilst still attached to the parent plant.

In cases where students work in groups, some should use one kind of seed for the above experiments and others another, e.g. barley grains (which are strictly fruits), horse chestnuts, onion seeds, etc. The results obtained with these different seeds should be compared with one another.

We have now confirmed our knowledge that seeds, in order to germinate, require water; we have found that the amount of water which seeds, such as peas, can absorb is surprisingly large; and we have learnt also that seeds are hygroscopic. We recognise that a knowledge of these facts helps us to store our seeds properly, and shows us how we may hasten their germination. We will next determine whether seeds dried as thoroughly as possible in a desiccator are absolutely dry, or whether they still contain water.

5.To this end weigh a dozen peas which have been in the desiccator for a week, soak them till they are soft, wipe them with a cloth, and pound them in a mortar; transfer the whole of the mash to a weighed, dry porcelain dish, and dry it thoroughly in a drying oven at about 100° C. After two days, take the dish out of the oven, stand it in a desiccator to cool, and then weigh it. Replace the dish in the oven and continue the weighing at daily intervals till no further loss of weight is recorded. We thus obtain the dry weight of the substance of the seeds, and a comparison of this weight with that of the desiccator-dried seeds tells us how much water the latter, apparently dry seeds really contained. The result of the experiment proves that even the driest seed contains a considerable percentage of water. The above experiment will be the more instructive if, at the same time, other vegetable tissues, e.g. carrots, turnips, and also fresh leaves (grass or spinach, etc.) are weighed, dried in a desiccator, weighed again, then chopped up (there will be no need to soak them first, as they are not so flinty hard as the seeds), pounded in a mortar, dried in the drying oven, and their dry weights determined.

6.A ready way of proving that ordinary air-dry seeds contain a considerable amount of water is as follows: half fill a wide-mouthed glass bottle with peas. Stopper the bottle, and place it in the drying oven at about 90-100° C. After 1-2 hours remove the bottle and observe that, as it cools, water, given off by the peas, condenses to form drops on the sides. By using different kinds of vegetable tissue it will be discovered that they all contain a certain amount—some a very large amount—of water, and that, of vegetable structures, seeds contain far less water than any others. That it is to this fact that seeds owe their resistant powers we demonstrate in the following way.

7.Prepare a saucepan of boiling water, place a few ordinary dry peas and equal numbers of soaked and of desiccator-dry peas in small canvas or muslin bags, plunge them in the boiling water for a few seconds, plant the three lots (after soaking the dry seeds) in pots, and record their germination. Whereas the thoroughly dry seeds have not been injured by their short immersion in boiling water, the soaked seeds show by their failure to germinate that they have been killed. It is noteworthy that advantage is taken of the resistance of dry seeds to high temperatures in treating grains of oats, the surfaces of which are suspected to be contaminated with the spores of a disease-producing fungus called smut. The grains are plunged for five minutes in water at a temperature of 55° C., and subsequently sown. The effect of the hot water is to destroy the smut spores without injuring the oats.

8.Next, the effects of low temperatures on very dry and on soaked seeds should be determined. The most convenient way to do this is to pound up ice and salt and to put the freezing mixture into a small pail, in the middle of which a glass flask is placed. The several small lots of peas, each lot in a muslin bag, are put into the glass vessel and left there for some hours. The bags of seeds are then withdrawn, and the germination capacities of the three lots of seeds tested.

From the result of this and similar experiments it is learned that dry seeds are more resistant to adverse conditions than soaked seeds.

During the summer we make a comparison between unripe and ripe peas. Definite experiment is not necessary to convince us that the unripe seeds in their young pods contain far more water than the ripe seeds, and we may take it that, during ripening, one process which goes on is the gradual loss of water by the seeds. When, on the one hand, we call to mind the extremes of temperature to which the seeds of plants are exposed during their long winter’s sojourn in the ground, and when, on the other hand, we realise the great resistant power of dry seeds, we cannot doubt that this natural drying process, which takes place during the ripening of seeds, is of advantage to the plant, making undoubtedly in many cases the difference between destruction and survival. That the resting state is due, in large measure, to the natural drying during ripening may be inferred from the experiments we have made, and also from the fact already noted that, in wet autumns, various kinds of plants may be met with the seeds of which are already beginning to germinate on the parent plants. Specimens illustrating this phenomenon should be collected and added to the museum.

Experiments recently made have proved that certain seeds may retain their capacity for germination for a great number of years; in one instance, among seeds known to have been kept for 87 years, some were found to be capable of germination, and it is interesting to know that experiments are now in progress to determine for how long seeds, which have been dried as thoroughly as possible, will retain their vitality. Though, as we have just learned, dry seeds may survive for many years, there is no evidence to prove the truth of the statements which are often made that wheat grains and seeds of other plants deposited a thousand or more years ago with mummies in mummy cases in Egypt have retained till the present day their powers of germination. Indeed, there is good reason to believe that such mummy wheat has long ago lost its vitality.

Our experiments demonstrate that a seed is a structure which, by reason of its dryness, is capable of passing through a long resting stage. Whilst in the dry state, it is far more resistant than is the growing plant. By providing it with water, the seed may be caused to pass from its resting or latent state into one of activity. What is true of seeds is also true of the simpler reproductive bodies of many of the lower plants. For instance, it has been shown that the resting spores of some bacteria are not destroyed by exposure to such high temperatures as 100°-120° C., at which temperatures the bacteria in their active, growing state are killed. That this great power of heat-resistance is due to the dryness of the spores is proved by the fact that, if the spores are brought under such conditions of moisture and warmth that they begin to grow, they lose their resistant powers. Since the group of plants known as bacteria includes many disease-producing forms, and since some of the latter produce resting-spores, the bacteriologist and the doctor have to take the resistant powers of spores into account in their efforts to exterminate disease-producing germs.

Let us now return to our study of seeds, and set ourselves to find out what is the first visible sign of germination.

9.In order to do this sow samples of soaked seeds, some in earth, others in germinators. Germinators of various patterns may be obtained ready-made (Appendix B), but one of the simplest and most useful may be made from a couple of ordinary saucers. Several layers of thick white blotting-paper are moistened thoroughly and fitted neatly into one saucer. A few soaked seeds are distributed on the blotting-paper, and the other saucer, into which moist blotting-paper may also be fitted, is inverted over them. The only precautions necessary are that the blotting-paper should not be too wet to begin with