Page:Popular Science Monthly Volume 60.djvu/81

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produce zoospores if grown in solution of inorganic salts and in bright sunlight. Nets with strong inclinations to form zoospores can be made to produce gametes by cultivating in a sugar solution in subdued light or darkness. Plants that have no special inclination to form either zoospores or gametes may be decided one way or the other by the illumination, bright light producing zoospores and darkness gametes. It is also fair to say that sometimes the tendency to form zoospores is so strong that a plant will not yield for several generations to the conditions that generally bring about the immediate production of gametes.

Let these studies on Hydrodictyon and Ulothrix stand as illustrations of the kind of evidence presented in varying degrees by many algae and fungi and constantly increasing as investigations in physiology proceed. The general trend seems unmistakable. We may feel sure that sexual elements, gametes, have arisen from asexual reproductive cells with an immediate relation to and probably because of certain environmental factors. In a general way these factors are known to be light, temperature, osmotic pressure and, most important of all, the chemical nature of the environment with especial reference to the kinds of foods.

What was the change that came over the asexual reproductive cell when it took on the stamp of sex? The differences are best measured in the possibilities of the two elements. The asexual zoospores may quickly and readily produce a new individual. The gamete, generally speaking, must fuse with its kind or else die. We have seen that primitive gametes may germinate without conjugation but the resulting plants in the cases best known are weaker than normal individuals. We also know that the lower stretches of the plant kingdom furnish abundant illustrations of parthenogenesis, that is, the power of an egg cell to develop without fertilization. These exceptions, however, strengthen the evidence that the essential differences between gametes and asexual zoospores are qualities lacking in the former, and especially the ability to continue and sustain the mechanism demanded by vital processes.

With conjugation all is changed, and the sexually formed spore has the qualities lacking in the two gametes from which it arose. The protoplasm is in a sense rejuvenated and with the stimulus comes sooner or later an expression frequently more vigorous than that of the asexual spore.

The most striking conjecture on the significance and origin of sex has been presented under the name 'autophagy.' It is a very simple hypothesis. However, its simplicity is its greatest danger and will probably be its complete undoing, for enough is known to indicate that the factors and conditions that produce the sexual act are im-