Page:Popular Science Monthly Volume 60.djvu/79

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71
THE ORIGIN OF SEX IN PLANTS.

4 or 8 in a cell. They are relatively large structures with 4 cilia and have the appearance shown in Figure 2, c. These 4-ciliate zoospores are never sexual and they develop new Ulothrix filaments like their parent. This simple method of reproduction may be continued for

PSM V60 D079 Ulothrix.png

Fig. 2. Ulothrix. a, Vegetative Filament. b, Development of Asexual Zoospores. c, Zoospore, d, Sporelings. e, Cell containing Gametes. f, Gametes, g, Conjugation of Gametes, h, Sexually formed Spore.

many months, but at times the conditions are such that another form of swarm-spore appears. These elements are much smaller than the usual zoospores, are developed more numerously in the mother cell and have 2 cilia as is shown in Figure 2, e, f. They are gametes and as a rule fuse readily with one another in pairs. The free-swimming gametes are shown in Figure 2, f, and two stages in the conjugation appear in g and h. If conjugation does not take place, the gametes settle down and in certain instances have been observed slowly germinating; but they develop feeble plants.

Now what are the causes that make the plant produce asexual zoospores on the one hand and gametes on the other? Are they deeply seated in the protoplasm of Ulothrix? In the first place there is no rule or rhythm in the appearance of zoospores or gametes, no time when conditions within the plant demand their development. And again, structurally, there is no hard and fast line between the zoospore and gamete; on the contrary, there are gradual transitions between these two forms of swarm spores. The problem thus resolves itself into an inquiry as to the precise environmental influences, the chemical and physical factors affecting the Ulothrix filament, whether they are actually able to make the plant form zoospores or not according to certain conditions. The habits of Ulothrix show us clearly that there are such factors, but the adjustments are so delicate that, apart from a very clear relation to temperature and the character of the salts in solution, it has not been possible to formulate them with exactness.

But other studies of Klebs, on forms that lend themselves more readily to cultivation than Ulothrix, have given some very definite results. Hydrodictyon, the water-net, is an alga^ that may be cultivated