Chapter XII: The Physiology of Plants

In this chapter I will endeavour to present to my readers a concise view of the nature and method of the various processes that go on continually in the growing plant.

These processes were incidentally referred to in our examination of the character of the various organs of the plant. Thus, in dealing with the root, we spoke of its physiology so far as concerned the absorption of water by its root-hairs. In the leaf, we touched upon the correlation between the shape and arrangement of the leaf tissues and the part the leaf plays in the economy of the plant. The physiology of the reproductive organs, again, we briefly explained in connection with their natural history.

In order to arrange our studies systematically, we may divide the physiology or function of plants into groups, and, taking each group separately study their effect on the plant.

We may then divide the functions of plants into

Nutrition,
Assimilation, and
Reproduction.

The first teaches us how a plant feeds and what it feeds upon; the second, how the food is prepared by the plant so as to enable it to use this food for growth and to store some of it away for future use. The third group deals with the various means adopted by plants for multiplying and increasing the species.

Plants, like animals, must _feed_ and _breathe_ in order to live; the food of plants, however, differs from that of animals in being more simple and elementary.

Plant food is of two kinds, water and gas. Water is an actual necessity to the plant, both as a direct food and as a medium to convey inorganic food. If we burn some wood to a white ash and then analyse it, six inorganic elements will always be found—potassium, magnesium, calcium, iron, phosphorus, and sulphur. These substances have been proved by experimental water-culture[25] to be indispensable to plant-life; others are found in larger or smaller quantities, but they are not, judging by experimental tests, essential to plant life. These inorganic elements do not enter the plant as such, but in the form of salts dissolved in water; the phosphorus and sulphur as phosphates and sulphates. Exactly how these salts and other elements are absorbed will be best learnt from a simple experiment.

[25] Testing the effect of plant food by water-culture is carried out in the following manner. Six large jars are filled with distilled water. In No. 1 all the six elements above mentioned are placed in small quantities, so as to form a weak solution. In No. 2 only five of them are added to the water, and in each succeeding jar one element is left out. A seedling plant which has been germinated on damp sand is suspended in each jar in such a manner that the leaves are in the air and the roots in the water without the seed touching the liquid. The growth of the young plants is carefully observed, and the result is found to be that No. 1 will grow and flourish, finding all its needful food in the water, whilst the rest of the seedlings will show plainly by their feeble and starved condition that, the food elements being absent, they cannot build up their stems and leaves, and must eventually perish.

We must first provide a large glass jar three parts full of clear water. Then a lamp chimney, to the bottom of which a piece of membrane (which any butcher will supply) has been affixed, should be partly filled with water coloured by sulphate of copper, and then suspended in the glass jar. Through a cork fitted to the top of the lamp chimney a long tube should be inserted. The fluid in the lamp-glass will be seen to rise in the tube shortly after the experiment is made, and the clean water in the large jar will become slightly coloured.

This experiment teaches us that liquids have the power of passing through a membrane; this power is known as diffusion, or _osmosis_. Further, we notice that the clear fluid passes into the coloured water more rapidly than the heavy coloured water passes out.

Now the fine _root hairs_ of a growing plant are _membranes_, having the same property as the membrane we placed on the lamp shade; inside the root hairs there exists heavy dense cell sap, outside are the films of hygroscopic water containing (dissolved) inorganic salts, and this water passes in through the membrane of the root, whilst a very little of the cell sap passes out into the soil, the quantity passing in being greatly in excess of that which escapes.

When once the crude water of the soil is inside, it is soon passed along to the stem and leaves by the pressure of more water coming in, and by what is called _capillary power_, this power we may easily see if we dip a fine tube into water, when at once the water will rise up some distance into the tube. I have pointed out that plant food is gaseous as well as aqueous.

Oxygen is absorbed by the root very freely from the soil, and, therefore, farmers and gardeners frequently plough and stir the soil of fields and gardens so that the roots may obtain a supply of this needful gas.

Let us now endeavour to see how the gaseous food is taken into the plant. In order to do so we must remember that the gases necessary for plant food form part of the air we breathe; this air is made up of two-thirds nitrogen, one-third oxygen, with a small and varying, but always present, quantity of carbon-dioxide, and of these the latter is the most essential to the life of plants.

We have learnt in our study of the leaf how it, by the aid of the green chlorophyll granules, and under the influence of sunlight, absorbs this carbon-dioxide and effects certain changes in it. One of the most essential elements in the growth of plants is _nitrogen_; this we have just seen constitutes two-thirds of the air we breathe, but the plant is unable to make use of it in this free form; that is to say, although the leaf can freely absorb carbon-dioxide it cannot absorb nitrogen; it has to be taken in by the roots of ordinary plants in the form of nitrates, that is, in conjunction with some other element. There is, however, an important exception to this rule; for what are called the insectivorous plants have the power to absorb nitrogen under certain conditions. These will be explained in the succeeding chapter. We can now summarise the processes of nutrition. The roots absorb water containing earthy salts as well as oxygen gas. The leaves absorb gaseous food in the form of carbon-dioxide, and I may add sometimes water vapour. There are two simple experiments that my readers can make which will prove these statements, and will give them a greater interest in the somewhat dry details of vegetable physiology. Our first experiment to show the absorptive power of roots is taken from Sir Joseph Hooker’s Primer on Botany.

“Take up three plants of the buttercup carefully by the roots; leave one (No. 1) on the table; place another (No. 2) with its roots in water; hang the third (No. 3) upside down over a tumbler of water with a few of the leaves in the water, but the root exposed. In due time No. 1 will have faded; No. 2 will be quite fresh; No. 3 will have the parts not in the water faded. No. 1 shows that water contained in the plant has evaporated from its surface; No. 2 that the water has been absorbed by the root and conveyed to the leaves; No. 3 that the immersed leaves have not supplied the other portions of the plant with water.”

The second function, assimilation, depends upon several processes that together go to make up the work of digestion and preparing plant food. These processes are transpiration, respiration, and evolution of oxygen; the latter process is associated with the feeding of the leaf—that is, the absorption of carbon-dioxide. This compound gas is under the influence of sunlight, and by the agency of the green colouring granules, decomposed into carbon-monoxide and oxygen; the latter is eliminated, whilst the carbon and a part of the oxygen is retained, and with the absorbed water is converted into material that the plant can use for the purpose of increasing its structure.

By a very simple experiment we can prove the escape of oxygen from the foliage of plants. A few sprays of such leaves as laurustinus, bay, arbor vitæ, and maiden-hair fern should be tied firmly to a piece of stone. We should have ready a soup-plate, a glass shade, and a tub full of fresh spring water (one large enough to allow the shade to be held upright under the water). When all is ready, place the bunch of leaves and stone in the glass shade held horizontally, and gradually sink it under the water till the shade is quite full; place the soup-plate at the open end where the shade is, and slowly raise the glass until it is upright, and then it can be lifted out and placed on a table in a window where the sun or bright light can reach it. The bubbles of oxygen will soon begin to form along all the edges of the leaves and the jewelled effect of the bouquet will be very curious and beautiful. It is hardly needful to say the stone is simply required to keep the group in an upright position. By the following day there will be a large bubble of oxygen collected in the upper part of the shade, eliminated from the leaves by the aid of chlorophyll and sunlight.

These changes resulting in assimilation are always in correlation with the process known as _transpiration_. The root is continually taking in fluids charged with inorganic salts; these are by the water conveyed to the leaves by means of the network of veins, which we know by the term fibro-vascular bundles. These, as we may see in skeleton leaves, traverse the entire substance of the leaves where the salts are used up in the constructive work of the plant. The water is not all wanted; part of it passes off in the form of vapour. Transpiration, then, is the passing off of this water.

We can easily see this process going on if we place a few tropæolum leaves in a cool tumbler, and then expose the tumbler to sunlight. In a short time the sides of the glass will show a film of moisture due to the transpiration of the leaves. This process takes place more freely in a warm temperature than in cool conditions; consequently, in hot weather there is rapid transpiration, and as the water is parted with more cell sap passes into the leaves and stems, and so the plant is kept cool. We can now see the great use of the little pores known as _Stomates_; these are found mainly on the under surface, and it is principally through these pores that the leaf transpires.

We must now carefully note the fact that all growing parts of the plant take up oxygen and give off carbon-dioxide. This power which is common to all life is known as _respiration_. It is a process that cannot be observed in daylight in green plants because this respiration is feeble, and also because the opposite power of assimilation is so strong that the action of breathing is obscured. In the absence of sunlight, however, it can be observed, as also it may be traced in connection with parts of the plant other than the green leaves. Seeds, for example, during their earlier growth (germination) give off carbon-dioxide freely by respiration. This we can prove for ourselves by taking a large glass jar holding about two or three quarts; fill this about half full of beans that have been well soaked in water so as to swell them and induce them to commence germination. Close the jar with a tight fitting cork; after six or seven hours the presence of carbon-dioxide may be easily seen. Have ready a small phial of clear lime water, and with a piece of twine let this down into the jar without spilling its contents; allow it to remain there some minutes, keeping at the same time the top closed with a handkerchief. We shall see that the clear lime water will after a short time become cloudy or milky; this is due to the carbon-dioxide, liberated by the seeds, forming chalk with the calcium of the lime water, the chalk being insoluble and easily seen. Now take out the phial and let it stand, well covered, when the chalk in the form of a fine precipitate will be seen at the bottom of the phial. If desired, a second experiment can be made with the same jar by lowering into it a lighted taper; we shall find it will go out owing to the presence of the carbon-dioxide; as this gas does not support combustion our lighted taper is quickly extinguished.

We can see from these experiments that respiration goes on in the growing plant and that this process is independent of chlorophyll. It is an essential part of the life of all plants, and my readers who may perhaps wonder why it is that two such opposite processes as I have described are both carried on in the plant must remember that in the main the feeding process which depends on sunlight and the presence of chlorophyll is carried on in the _daytime_, whilst respiration is practically counteracted in the daytime by the vigorous intake of carbon-dioxide. At night when the rays of light cease and no longer enable the plant to feed, the respiration is evident. Briefly, we learn that _in light_ the plant gains in weight, whilst _in darkness_ (by respiration) it loses. The green plant can only construct growing material out of simple substances in light, having no power to do so in the dark.

Heat is just as needful to plant-life; it must be above freezing point, and a somewhat high temperature is necessary to set in motion all those chemical processes that I have briefly described.

At a low temperature the work of assimilation and other processes are arrested; on the other hand, a rise in temperature increases the activity of these processes.

We now come to the third function called reproduction. We have seen in connection with the food of plants how they convert inorganic material into organic. This one fact is significant of the great office of plant-life in nature; animal-life could not exist without its help. Plant-life may be said to prepare the food of animal-life, and retain that balance of gases in the atmosphere necessary to healthy respiration. How important then it is that all kinds of herbs, trees and plants should multiply and be fruitful, life of any sort is of limited duration, and subject to all the vicissitudes of accident, constitution, and climate, and so we find that plants have been endowed with wonderful powers of reproduction in order that the earth may be constantly clothed with vegetation, necessary for the life of man and all animal nature.

By reproduction I want my readers to clearly understand the power possessed by the individual plant to multiply its kind or species; and this power is carried into effect in a variety of ways in different species. These various methods of reproduction then will occupy the concluding pages of this chapter. The protoplasm (or life principle) of any individual plant is endowed with the power of giving rise to an entirely new individual. This is accomplished in one of two ways. In the first by cells forming a part of the plant, but yet not specially modified for the purpose of reproduction. This mode of increase is known as vegetative reproduction. We will illustrate it by two examples widely apart. Many lowly plants like protococcus (the bright green substance which so beautifully colours tree trunks in moist situations) and yeast, are formed of one cell only, and when such cells attain their full size they simply divide into two or more cells which grow, and finally attain maturity when the process is repeated.

The other example is that known as the strawberry “runner,” this, as we know, is only an elongated stem bearing at the end a bunch of leaves, and from the base of the leaves a few roots, the whole being a new plant which may be removed from the parent and grown in some other place.

These, then, are examples of vegetative reproduction, and my readers can discover for themselves many other instances in the garden.

The plan of propagation by “cuttings” is simply the gardener’s practical application of vegetative reproduction.

The second mode of increase is by special reproductive cells, which are set free by the parent plants and become new individuals. The second mode is common to all plant-life, and in it two distinct processes can be observed. We often see on a decayed pear or apple a patch of brown mould (mucor). If we examine it with a lens we see a little forest of tiny erect stalks, and upon the apex of each is a round ball containing reproductive cells, each of these, which are called spores (the spore-case being called the sporangium), contains protoplasm, which is endowed with the power of giving rise to a new individual mould.

This process is typical of what is common to ferns, and many other cryptogamic plants, and is called _asexual reproduction_.

The second form is that in which two such spore-like organs as we have noticed in the mould, fuse together and form a spore capable of giving rise to a new plant.

This is known as _sexual reproduction_, and is dependent upon the fact that the protoplasm of either of the two organs is incapable of giving rise to a new individual plant, and that they must come in contact and fuse organically before a new plant can be formed. This process of fusion I have in an earlier chapter described as fertilisation. The pollen grain, the fertilising agent, is one of the reproductive cells, and the other, the ovule, is the cell that has to be fertilised. After this there is the subsequent development of the ovule into the seed, and in this seed we may recognise a plant in embryo endowed with powers not possessed by its parent, that enables it to resist extremes of heat and cold which would result in many cases in death to the parent plant. By way of experiment some seeds have been subjected to 40 degrees of cold, and yet have not lost their germinating power, whilst, on the other hand, it is known that seeds of some plants growing in sandy deserts lie baking in the sun for many months in a temperature of over 70 degrees, and yet begin to grow as soon as moisture reaches them.[26]

[26] From “Nat. Hist. of Plants,” p. 554: “It has been proved experimentally that seeds which have been deprived by calcium chloride of as much water as possible are not killed even at the boiling point of water.” Careful experiment has shown that there are three stages of activity in the life and work of a plant—(1) A _minimum or zero_, at which the processes are just possible; (2) a _medium stage_ or _optimum point_ where the activity is the greatest; and (3) a _maximum stage of heat_ where _growth is arrested_. So that we learn that plant-life can suffer from too high a temperature as well as that which is too low.

Things to be observed or collected:—Experiments to be made in order to show diffusion, transpiration, and respiration, collection of oxygen from water bouquet. Carbon-dioxide from germinating beans. Observe—

Blue mould on fruit.
Strawberry runner.
Rooted cuttings.
Stamens and pistil of any flowering plant.

CHAPTER XIII

_INSECTIVOROUS PLANTS_

“Beyond, the moorland has its wealth
Of pink and purple, blue and gold;
Heather and gorse, whose breath gives health,
And ling, a hive of bees that hold:—
And when there’s moisture in the brake,
The clammy sundew’s glistening glands
’Mid carmine foliage boldly make
Slaves of invading insect bands.”