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

BOOK 2

grass, for example, has a tough, rapidly growing underground stem that sends up a bunch of leaves and flowers from time to time, much as the hollow main stalk of Obelia throws up polyps. The runners of a strawberry are similar, and the reader, if he or she has ever attempted to weed a garden, will be able to think of many troublesome examples of the same thing. Moreover, it is a well-known fact that many plants can be reproduced by means of cuttings ; in Begonias, for example, a single leaf or even a part of a leaf, detached from the plant, will sprout out other leaves and rootlets, and reorganize itself into a whole plant, just as an isolated Obelia polyp will sprout out a new axis-tube and found a new colony. Mosses are celebrated for their great power of regeneration from cut portions of all the organs, and they can reproduce themselves by means of gemmules like sponges, by separating off little bits of themselves which grow into new plants. The bulbils of some of the flowering plants are devices for doing the same thing.

To sum up, then, it is difficult and probably wrong to think of conscious individualized plants. One should think of plant tissue as organized in quite a different sort of way from our own, as an unconscious race, rather than a number of rigidly differentiated persons.

§ 3 Flowers and Seeds

Like animal tissue, vegetable tissue has to reproduce itself, to spread and colonize new localities, and although, as we have seen, it has all sorts of sexless ways of spreading and multiplying, the principal method employed, at least in the higher plants, is sexual ; as with ourselves, it involves the union of gametes of two kinds.

Now, there are great variations between the different kinds of plants in the method of reproduction and in the life cycle. We shall start by describing the reproduction of the flowering plants, and afterwards, in reviewing the vegetable kingdom, we shall note what arrangements occur.

The chief differences between the sexual process in animals and that seen in the higher plants are due simply to the fact that animals move about while plants are rooted. Among higher animals, male and female see or smell or hear each other, are smitten with desire, move towards each other, make love, and so their gametes are brought together. But plants have no eyes

158

THE SCIENCE OF LIFE

_over buttercups,

CHAPTER 5;

to see with, no brains to lust with, no muscles to move with, and they have to rely on outside agencies to bring about the union of their gametes. Plants woo each other “ by correspondence” ; they are provided with elaborate devices for utilizing the transport possibilities of winds, streams or insects, and inducing them to act as go-betweens in this necessary transfer.

Let us consider for a moment a buttercup flower, if possible in the presence of the real thing. There are five brilliantly yellow petals, the most obvious parts of the flower ; outside them are five little green sepals, the leaves that enclosed and protected the other parts of the flower when it was a tender, forming bud. If one of the petals be pulled off, a tiny yellow scale will be seen at its attached end; under the scale is a moist spot, moist with a droplet of the sugary fluid called nectar. ‘This nectar is a bribe for the black insects that are generally found crawling and the shiny golden petals are nothing more or less than advertisements, shouting to the insect world that there is nectar to be had. Inside the petals is a cluster of delicate structures, a ring of yellow stamens surrounding a bunch of green ovaries. The stamens are little stalked knobs, and when they are shaken each of them emits a few grains of the yellow dust called pollen; this dust is the male element of the plant. A pollen-grain is something much more than a spermatozoon, but that we will discuss later. The ovaries are green capsules, each containing a little round white body which is the female element. From the free end of the ovary a short neck projects, ending in a sticky yellow spot called the stigma. Just as in most cases the eggs of an animal cannot develop until they have been fertilized, so this waiting globule can do nothing unless a pollen-grain should fall on the end of the stigma. ‘This is called “ pollination.” |

When a grain of pollen gets on to a stigma it behaves like a minute plant in favourable circumstances ; it sprouts out a little stalk or filament, the pollen-tube, which burrows down towards the egg-cell like a rooilet growing down through the soil; the tip of the stalk ultimately touches the egg-cell and fertilizes it so that it may develop into an embryo.

The manner of operation of the whole affair is this. Insects are attracted by the petals and come to seek the nectar; the stamens shed their pollen outwards into the circular eroove at the bases of the petals, so that the insects, scrabbling for the nectar,