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

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VIDENCE FROM PLANT AND ANIMAL STRUCTURE

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Fig. 137. Ancestral reminiscence in the Feather-star’s life-history. The adult Feather-star (Antedon) can swim from place to place by waving its arms, or anchor itself temporarily by

a set of jointed claws, as the specimen in the centre is doing. Jrom a depth of about a hundred fathoms in the North Sea. On the left, a young Feather-star in the stalked stage.

ends in a root-like tuft.

On the right is an adult Sea-lily (Rhizocrinus), This animal is permanently fixed by a stalk which After growing for some time in

this form, the animal breaks off and swims away, leaving its stalk behind. Thus the Feather-star passes through a fixed stage which is like the final form of its more primitive relatives, the sea-lilies.

to die) and become a free-creeping featherstar (Fig. 197).

An interesting case among plants of the preservation of phases of the ancestral lifecycle which are now unnecessary is that of the maidenhair tree or ginkgo. This lovely tree, with its leaves like little fans, is extinct in the wild state, but has been preserved to us by being cultivated as a sacred tree in the gardens of Chinese temples. It is a naked-seeded plant, related to the pines and firs. As in all other seed-plants, its fertilization is effected by means of pollen. The _pollen-grain, as we have already explained in Book 2, if it alight on the pistil of a female flower, sends down a long tube towards the egg-cell embedded in the ovary. From its original single nucleus, three nuclei are produced by division ; two of these nuclei act as male gametes, either of them capable of fertilizing the egg. In almost all seed-plants these male gametes

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are merely more or less ordinary nuclei which pass down the tube to the ovum ; but in the ginkgo and the cycads the nuclei, associating themselves with some of the surrounding protoplasm, become transformed into two actively swimming, ciliated sperms, like the sperms of fern, moss, or seaweed, which swim on within the tube to fertilize the egg (Fig. 112). They would get there just as surely, we judge from the higher plants, if they had no cilia. These tiny sperms are as revealing as the gill-slits in our own embryonic neck. ‘They show that once upon a time, a very remote time, the ancestors of seed-plants lived in the water, where free-swimming sperms provide the natural method of achieving fertilization. And here, long after the pollen-tube had been evolved to provide a dry method of fertilization for dry-land plants, the motile character of these transitory sperms survives to recall the watery past. Even in some

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