hut finally gathering in small groups and sorting themselves in pairs. The elements in such a pair begin to fuse together; the process is called conjugation and represents the simplest form of sexuality. The two sexual cells are called gametes, but they are nothing more than zoospores so constituted that they must fuse with one another in order to live.
The gametes show their relationship to zoospores in various ways and there is no doubt that they arose from the latter. In the first place they have the same general structure and are developed in the same sorts of cells on the mother plant. But the most important evidence of affinity is exhibited by certain gametes that are so much like zoospores that they will sometimes settle down and germinate without conjugation. This means that their sexual characters are not strongly enough developed to overcome the vegetative tendencies of their parents the asexual zoospores. However, the sporelings that come from these abortive or perhaps parthenogenetic gametes are weaker than the products of the ordinary or normal zoosopores and sometimes never reach full development. As may be guessed, this curious intermediate condition between the zoospore and gamete furnishes a most important clue to the fundamental distinctions that separate the one from the other. These differences are evidently physiological rather than morphological in character.
It is only recently that botanists have in part understood and attempted to define precisely the conditions that determine the development on the one hand of zoospores and on the other of gametes. In a general way it has been believed for a long time that the problem was a physiological one and that various environmental conditions of season, temperature or light were responsible for the results. But in the past ten years there have been numerous studies, on various types of the lower plants, attempting to establish as exactly as possible the chemical and physical factors at work. In this field of research the botanist, Klebs, has been especially active, and he, above all others, deserves the credit of developing certain experimental methods of attack. These have yielded important results and justify the belief that we may in the future obtain much precise knowledge.
Klebs treats the forms to as many well-defined conditions as he can devise, various as to the food, the osmotic properties of the water, the light and the temperature. The results have been very remarkable considering the difficulties of the problems. We can not do better than to follow his studies on one or two forms to illustrate the possibilities of investigations in this difficult field.
His studies on Ulothrix are interesting. This is a lowly type of unbranched filamentous alga common in both fresh and salt water. The zoospores (Figure 2, b) are formed in varying numbers, but usually