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

VEGETABLE LIFE

of the fern-plant; it is an exalted prothallus. Now let us turn back to the flowering plant. It has vascular tissue like the ferns, and through a great number of intermediates, we can trace its homology to the fern-plant. Tt is a still more exalted sporogonium. Where is its spore ?

Well, it happens that among some of the so-called club-mosses, which are related to ferns, there are two sorts of spores and two sorts of prothalli. There are female prothalli producing egg-cells only, waiting in their tiny flasks, and male prothalli producing the active spermatozoa. Moreover, the spores that grow into female prothalli are bigger than those that give rise to the male prothalli—indeed, the latter are just about the size of a pollengrain. And the lives of the prothalli are short, they are even smaller and more insignificant than the prothalli of ferns. The plant-tissue is, so to speak, going all out for the sexless generation ; it bustles perfunctorily through that ancient and venerable formality the prothallus and gets it over as soon as possible, like a worldly chaplain saying a rudimentary grace before he gets down to the business of eating.

And now let us turn to the more primitive seed-plants—to the conifers and cycads and the like. Here there is an even more swiftly gabbled prothallus. A pollen-grain, we noted, does not fertilize an ovum simply and _straightforwardly aS a spermatozoon does; it settles near the ovum and germinates and sprouts into a little mass of tissue, growing like a thirsty root towards the ovum, and only a bit of it, becoming detached from the rest, actually accomplishes fertilization. This looks suspiciously like the growth of a male prothallus. And the comparison is clinched by a curious fact: whereas in most seed-plants the fertilizing elements are very different in appearance from the spermatozoa of ferns there are one or two exceptions—the ginkgo or maidenhair tree and the cycads—in which the end of the growing pollen-tube gives rise to spermatozoa for all the world like the spermatozoa of ferns or mosses, which swim up to the ova and fertilize them. The pollen-grain of all these plants, then, is not a gamete but a spore, and it gives rise to a rudimentary male prothallus. And on the female side there is a similar story. In the young female catkin of a coniferous plant certain special cells, which we may call

“spore-cells,” are set aside. Each of these spore-cells divides and proliferates and produces by its own unaided efforts a special mass of tissues, and in this mass a little cluster of egg-cells is developed. The spore-cells are indeed spores, but they are not allowed to escape from the parent plant ; the masses of tissue that they give rise to are female prothalli, reduced and insignificant, which

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Fig. 111. A Moss-plant, showing the slender sporebearing generation rising parasitically from the leafy

sexual stems.

stay imbedded in the substance of their parent. The supremacy of the sexless

generation is almost complete. . In the higher flowering plants, the angiosperms, we can trace the same story, but here the sexual generation is even more reduced. (We would make it clear that our account Is a very much abridged one ; there is a beautiful series of stages leading from fern to ae 109