The science of life : fully illustrated in tone and line and including many diagrams

BOOK 2

CHAPTER 5

VEGETABLE LIFE

9 1. Stems, Leaves, and Roots. Seeds.

§ I Stems, Leaves, and Roots

ND now we come to a vast group of

living things laid out for the most part upon a common plan—the plant world ; its importance in our daily consciousness is second only to that of our own phylum of vertebrated animals. Its structural conception is still an aggregation of cells; it displays the phenomena of sexual reproduction, but in nearly everything else it is so different from the lay-out of any phylum hitherto considered that it is difficult even to put types side by side for comparison.

A plant—our crude description will apply to any plant that bears flowers—consists of three essential parts; the stem, more or less rigid and holding the plant in shape, and at the same time guiding sap from place to place, a system of green leaves spread like a net to catch as much sunlight as possible, and a system of roots branching eagerly in the soil, anchoring the plant, and sucking up moisture and other substances upon which it thrives. We may leave out for the present the reproductive parts, the flowers and seeds, and discuss very briefly the physiological processes underlying the ordinary day-today life of an individual plant, just as in the first Book we began by discussing the dayto-day processes, the digestion and breathing and excretion, that underlie animal life. The life of a plant is so different from that of an animal, different even in its chemical basis, that we shall have to start again nearly from the beginning before we can understand the vegetable world ; but the effort is well worth while, for we shall find in a simple physical fact a clue that explains and clarifies all the mysteries of vegetable life.

An animal, we noted, consumes food and uses it in various ways ; with parts of it he repairs and adds to his own living fabric, while part of it is burnt in order to give the energy upon which his life depends. The food that he consumes consists of complicated substances—substances with large and intricately constructed molecules—which can be broken up and decomposed, and therefore used as fuel. Now, a plant never takes in such substances. ‘The things upon which it subsists—water, carbon dioxide, and

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§ 2. Individuality in Plants. § 4. The Flowering Plants.

§ 3. Flowers and § 5. Ferns and Mosses.

various mineral salts—have extremely simple and elementary molecules, and can under no circumstances be burnt. They contain no bottled energy, as the food of an animal does. Whence then does a plant get its energy, the energy for its growth and breeding, and for the hardly perceptible movements that it performs ?

If we place a plant in a compartment so contrived that we can, from time to time, analyse the gas around it, and if we keep the plant in darkness—if, for example, we experiment at night—we discover a profoundly important fact; a plant consumes oxygen and gives out carbon dioxide. lit breathes just as an animal does. Tis leaves are covered with minute, barely visible pores, thousands of tiny nostrils, as it were, and through these pores the gases diffuse in and out. The plant, then, is breathing, and therefore it must be burning something as fuel—but what is it burning? ‘There is nothing in the substances absorbed by its roots that could be burnt.

Supposing now that we repeat our experiment by daylight, or even in the presence of an electric light ; we get a very different result. We find that things are just the other way round, for now the plant is absorbing carbon dioxide and giving off oxygen. And in this fact lies the whole secret of the apparently foodless plant existence, for the plant makes its food in its own tissues—makes it out of water, carbon dioxide from the air, and a few simple salts from the soil giving off oxygen in the process, a thing beyond the power of any animal.

The leaves of a plant are green. ‘This is because the tissues of the plant contain a characteristic green pigment called chlorophyll. Chlorophyll is a unique substance as far as the chemistry of animals and plants is concerned, for when it 1s present in a living cell that cell can absorb and make use of the radiant energy contained in light itself. That, in a word, is the secret of the plant. It can drink the pure energy of sunlight, and it can use that energy to build out of the elementary substances that it absorbs the higher complex molecules of which its tissues consist.

The plant breathes, as we have already noted, and it breathes continually, both by