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

VEGETABLE LIFE

inherently devitalizing in inbreeding or even in self-fertilization itself. The chief reason for its unpopularity, so to speak, lies in the fact that selffertilized populations are more homogeneous, less variable than those in which cross-fertilization is the rule. As we shall learn when we treat of Evolution, variability of the species is a profoundly important thing, for it gives a basis for natural selection to work upon; in the long run variable, cross-fertilized species will tend to supplant homogeneous self-fertilized species merely because they are variable. There will come to be more of the former. It is not that inbreeding is inherently bad and outbreeding inherently good ; it is merely that the latter method affords more opportunity for Evolution, for the adaptation of the race to the ceaselessly changing conditions of the organic world, that determines its predominance.

But we have dwelt long enough on flowers ; let us turn to fruits and seeds, to the embryos that result from successful pollination.

We have described the process of pollination in the buttercup ; let us proceed with our description, and trace the subsequent events. The litle bunch of green ovaries, each with a waiting egg-cell inside it, has been dusted with pollen ; the sepals, petals, and stamens, their purpose discharged, have withered and dropped off. The ovaries swell up and ultimately become dry and hard ; inside them the egg-cells are developing into tiny embryos, and being fitted out with little stores of food, like the yolk of a bird’s egg. The embryo, together with its food store and the tough membrane enwrapping both, is called a seed, and the enlarged ovary that contains it is called a fruit. So in the buttercup each fruit contains one seed.

At this stage the plant is faced with another problem, a problem analogous to that of pollen-transfer already discussed: Supposing that the fruits were simply to drop off and that the seeds were to germinate in the ground beside the parent plant, what would happen? There would result a_ bitter struggle between parent and offspring, their leaves wrestling and fencing for light, their roots for moisture. It is better for the seeds to be sown in distant places, so that they do not interfere with their parent or with cach other, and so that the species may extend its range. They have, to use the technical term, to be dispersed.

The fruits of plants are often variously and ingeniously contrived so that their contained seeds may be dispersed. The reader will be familiar with a number of examples.

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The plumed fruits of dandelions and thistles and the winged fruits of sycamore and ash sail in the wind for considerable distances. The dry poppy-head rocks in the wind, and as it sways it scatters little puffs of seed, like a sower scattering handfuls of grain. It is indeed a better and more even distributor of fine grains than an ordinary salt-shaker or pepper-pot ; and a household shaker deliberately designed from the model provided by the poppy has recently been patented. The fruits of violet and wood sorrel and the pods of gorse and the sweet-pea burst with surprising violence and eject their tiny offsprings into the world. The seeds and fruits of water-lilies have spongy coverings filled with air, by means of which they float for a time on the surface of the pond, later to become water-logged and sink. The fruits of goose-grass and agrimony and burdock bristle with tiny hooks that catch in the fur, feathers or plus-fours of passing creatures, and so are carried for great distances before they are dropped, brushed off or noticed, pulled off and tossed aside. And there are many other ingenious devices to reward the observant naturalist.

Finally, there are the fleshy and succulent fruits—blackberries, | yew-berries, elderberries, currants, apples, cherries, plums, oranges, bananas, grapes, peaches, figs. Here, again, we find plants overcoming their own immobility by bribing animals to act as their postmen. The flesh of an apple, a strawberry, or an orange is not a store of food for the growing embryo, as the flesh of a bean is ; it is a free gift, deliberately set out for birds or mammals to consume. In return, the consumers unconsciously distribute the plant. They often carry the fruit a little way before attacking it ; if the hard part within is large, like the stone of a cherry, it is dropped uneaten to the ground, and has thus achieved distribution. If it is small and inconspicuous, as in a blackberry, it is swallowed with the rest of the fruit, and being indigestible emerges unchanged from the other end of the alimentary canal; in the latter case it is not only distributed but manured. The conspicuous appearance of most fleshy fruits is again an advertisement, drawing attention to the gift. ‘The advertisements of fruit are mostly directed at vertebrates, those of flowers mostly at insects.

We may close by referring to the seeds of gorse, which employ humbler carriers ; they have a small, fleshy appendage that is appetizing to ants, and in addition to being exploded out of their pods they are therefore dragged about by the ants, often for some distance.

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