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

VEGETABLE LIFE

night and by day. (By day-time, however, its breathing is not noticeable because it is masked and overweighted by this process that works in the opposite direction, involving the absorption of carbon dioxide and the storage of energy inside the molecules of chemical compounds instead of its liberation by their disruption.) Nevertheless, it breathes, and underlying the whole physiology of its tissues is a scheme like the chemical scheme of an animal, an oxidation of complex substances in order to yield energy. The difference between animal and plant is that the latter has something in addition. Over and above this scheme it has another scheme founded upon sunlight and chlorophyll, by means of which it can synthesize its food out of the most elementary ingredients instead of having to hunt about and look for it. Take the most famous chef in the world ; give him water, a handful of nitrate of lime, a teaspoonful each of nitrate of potash, phosphate of potash and sulphate of magnesia, and a trace of iron (such as the rust from an old nail); what sort of dinner could he turn out for you from that? Give them to a plant, let it absorb them into the kitchens of its own cells, and if there be light it will turn them into delicious tissues, into asparagus or lettuce or beetroot, into spinach or turnip or artichokes or potato or wheat or peas, according to its particular bent. :

Naturally, the organization of a plant is very different from that of an animal—a mouse, let us say. A mouse has nerves and sense-organs and muscles and a busy little brain, but what should a plant want with these things? It has no food to seek, and that is the chief thing that an animal does with its brain. Indeed, if a plant could walk about, it would have to tear up its roots at every step! It sits still and spreads its leaves towards the light, and its roots into the soil. It lacks the complicated digestive apparatus of a mouse, for the mouse consumes energy-yielding fuels that have to be adjusted before they can be fitted into his tiny body. But the plant has no need to consume fuels; it contents itself with the elementary molecules that it finds in the air and soil. It does not rush about from place to place and has no muscles that toil strenuously, therefore it has no need of a heart-

pump and violently pulsing blood; the slow, steady ooze of sap, upwards and downwards along the stem, suffices to keep its parts in communication with each other, and is quick enough for its slow, dignified chemical interchanges. Nevertheless, it should not be thought that the organization of a plant is simple. It has a very great variety of specialized tissues, struts and girders, pipes, living laboratories, storehouses of various kinds, but they are planned and specialized altogether differently from the tissues of a mouse.

The plant is built up of tiny organized protoplasmic specks just as we are—indeed,

Aim

Fig. 104. Part of the surface of a leaf, magnified, showing the outlines of the cells and the pores or stomata, the microscopic nostrils through which the plant breathes.

A single oak-leaf has several million of these pores.

if we look into these intimate details of their organization the gulf between animal and vegetable, between man and cabbage, let us say, seems to narrow. In many plant-cells active movements can be seen in progress; the impassive stillness that distinguishes cabbage so markedly from man breaks down. ‘These movements can naturally be best seen in transparent plant tissue—in the leaves of the Canadian waterweed, in the stinging hairs of a nettle, or in the thin fronds of mosses—and in many of these the protoplasm can be seen to stream regularly round the little box in which it is enclosed, almost as if it were trying to find a way out. It is obviously and actively