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

VEGETABLE LIFE

Fig. 109.

On the right is a Fern-plant (Aspidium). Sexless fern begets.

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Below them is a highly magnified view of the tube, in which the egg-cell awaits fertilization.

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The Life-cycle of the Fern. 7

Above, on the left, are two Prothalli—the sexual generation which the

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ts at the bottom of the tube ; passing up the neck is a drop of an attractive fluid round which three sperms are hovering.

little swimmers. These spermatozoa make their way towards the female organs through the moisture that usually collects under the prothallus, and they pass down the necks of the flasks to fertilize the waiting egg-cells. All this is more like animal reproduction than anything we have hitherto related of plants. As the result of fertilization the eggcell begins to grow, and it develops into a fern-plant like the one we started from. Evidently, we have here in the fern an alternation of generations, like the alternation that we noticed in the liver-fluke. The fernplant produces spores ; the spore grows into a prothallus; the prothallus produces spermatozoa and ova; the fertilized ovum grows into a fern-plant, and so on. We may note that there is no very definite purpose in this arrangement, as far as can be ascertained ; it is not in any sense better than the arrangement seen in the higher plants. It is an arrangement that becomes intelligible, as we shall learn, from the evolutionary point of view; the ferns, like the amphibians, are transitional forms, caught in the act of passing from one style of existence to another. Like the amphibia, the ferns cannot get far away from wetness. They have not fully attained the dry land.

There are one or two other kinds of plant that show a similar life-history. The horsetails bear their spores on erect cones, and here again the spore develops into a sexual prothallus. As with the ferns, our temperate zone horse-tails are by no means the most impressive ; there are tropical species that reach a height of thirty feet, and at one time gigantic tree-like horse-tails were a conspicuous feature of the world’s vegetation. The curious “ club-mosses ”’ have the same cycle of reproduction. ‘These plants are classified together as the Pteridophyta, which means “‘ fern-like plants.”

Now it is very interesting to note that there are a number of plants that exhibit a similar alternation of generations, but with the emphasis on a different stage in the cycle. There is no need for us to describe in detail the appearance of the little tussocks of various species of moss that are found growing on walls and stones, on the boles of trees, in squelchy bogs and on the hard ungenerous faces of cliffs—indeed, almost anywhere we care to look. The structures that especially concern us are the delicate graceful rods with tiny globular or oval capsules perched on their upper ends that may often be found rising from moss-plants, for the capsules are

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