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

BOOK 2

living, just as our protoplasm is. But there are general differences between animal and vegetable cells. In the first place, the cells of the green parts of a plant contain, in addition to the nucleus, mitochondria, and so forth, that we noticed in the animal cell, little round spherules within which the vital chlorophyll is confined. After what has been said there is no need to stress the importance of these plastids, as they are called. The second point is of minor importance ; plant cells almost invariably secrete little boxes round themselves, boxes of a complex substance related chemically to the sugars, and called cellulose. Because of these myriads of microscopic but indigestible boxes in which the protoplasm of a plant is enclosed mammals like cows or horses which lead an exclusively vegetarian life have to carry longer and more complicated digestive tubes than we do. Moreover their own digestive juices are incapable of attacking the cellulose and thus these herbivores are dependent, every time they digest a meal, on microbes that live in their bowelsbut that is another story, that we shall recount later.

‘The urgent need of an animal is food ; that of a green plant is sunlight—and herein lies the clue to above-ground vegetable architecture. Air is plentiful enough, and, once its roots have struck in a suitable soil, a supply of moisture and the necessary mineral salts is guaranteed ; but the third vital necessity, light, is a thing that has to be fought for. All over the world, wherever roots can get a grip and an adequate supply

Three plant cells from a moss leaf.

Note the tough boxes in which they are imprisoned and the oval green plastids ranged along their walls.

Fig. 105. typical

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THE SCIENCE OF LIFE

CHAPTER 5

of nourishing moisture, plants are wrestling silently with their neighbours for light. Leaves are a net, spread to catch the invigorating rays. The reader will have noticed the beautiful leafmosaics of elm or ivy, the leaves fitting into each other like the fingers of clasped hands so that as little as possible of their area will be shaded, as much as possible directly bathed in radiance. ‘The primrose, starting early in the year and sacrificing this neatness for a grimmer purpose, flops its rosette of broad leaves hastily around so that any other seedlings that may be sprouting near by are stifled ; thus there is no risk of the primroses being put in the shade, in a literal sense, by a taller and more stalwart neighbour. Other plants race each other upwards, to be the first to drink the raining rays. The trees, spreading their leaves high in the air, get their fill of light and the undergrowth below has to content itself with their leavings, with diffused light and an occasional bright fleck that has filtered through—the crumbs of light, so to speak, falling from the rich table above. Because of light-hunger, although plants cannot move about as we do, they can grow with astonishing rapidity; a sprouting seedling, in its eager upward rush, can increase in length very much more rapidly than any animal can do. Growth, the flinging out of an effective leaf-net to catch light, is the dominant, driving activity of the plant organism.

Below the leaf-net is the stem, supporting

Fig. 106. Regeneration in Bryophyllum calycinum, a plant from Bermuda.

A leaf and a piece of stem, cut off and dipped in water, are sprouting

out new roots and shoots. (After Loeb.)