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

BOOK e

sperm and showing the gradual reduction of the sexual generation with which we will deal later.)

Now what does all this signify ? The earliest and most primitive plants had an alternation between sexual and _ sexless forms ; gradually the former was compressed until in the higher plants it is an unimportant gesture, a little mass of tissue packed away in the Ovary or creeping over its top. Why should this be? Well, as we have seen, the spermatozoa, when they exist, swim towards the ova. And they must have something to swim in. In dry conditions the spermatozoa are absolutely helpless, as helpless as a fish on land, so that if there 1S a separate prothallus stage in the life - history it must grow in a damp place. And since the SECExalVeysss

s ta ge Fig. 110. Sprouts (lower left). directly out

166

Fig. 112.

At the lop a p the egg below. nuclei (shown black). contains an egg-cell (shown very dark). lilies the fertilizing nuclei or moss sperms.

THE SCIENCE OF LIFE

Fertilization in the higher plants.

ollen-tube is growing sinuously through the ovary tissue and nearing The tube is a vestigial generation; it contains two Sertilizing The grey mass below is a vestigial female generation ; it

This is the typical condition.

human embryo.

But in some in the pollen-tube are spiral like the nuclei of fern This is drawn (on a larger scale) on the lower right ; And in the primitive Cycads the pollen-tube produces active sperms These are evolutionary reminiscences, like the transient gill-clefis of a

CHAPTER 5

of the prothallus such a plant can never get very far away from moisture. So that the cutting down of this stage, first the tucking away of the whole process into the moist cracks of a cone on the parent plant and then the elimination of a free spermatozoon altogether, is a process of emancipation from water. It is quite parallel to the way in which, in all vertebrata above the amphibia, the larval gill-breathing phase of development is reduced and hurried = through in the eggcell vor: uterus as the case may be. In both instances, in higher plant just as in higher vertebrate, a progressive emancipation from water and an adaptation to dry land is traceable. And with the plant just as with the animal, it was the Waterinhabiting forms which preceded and produced the dry land ones.

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