Page:Popular Science Monthly Volume 62.djvu/313

Rh condition and furnishes the last link in the chain of stages through which gametes pass in their sexual evolution. The gametes of Ectocarpus siliculosus are morphologically isogamous, i. e., both are biciliate cells similar in form when first set free from their respective gametangia. At the time of fusion, however, the condition is physiologically that of heterogamy, for the female cell is essentially a quiescent egg sought by motile sperms. There are species of Ectocarpus whose gametes have habits similar to siliculosus and which are also of two sizes (see Fig. 3, b), so that the resemblance to egg and sperm at the time of fertilization is very marked.

Another brown alga should be considered in connection with the forms noted above. Cutleria represents a family quite removed from the Ectocarpaceae and much more highly organized. The gametes differ greatly in size as do also the respective gametangia in appearance (see Fig. 3, c). The female gamete is exceptionally large and deeply colored, the male small and almost colorless. At certain seasons of the year (late summer and autumn on the coast of France and England) the female gametes germinate parthenogenetically. If sexual it moves about for a short time and then comes to rest, when it is fertilized as a quiescent cell. The conditions are then the same as in the species of Ectocarpus previously described.

Before leaving the brown algae, I am tempted to call attention to a very interesting condition in the group of the kelps (Laminariales). These forms are structurally very complex and are unequaled among the algae in size and luxuriance. But the zoospores of the kelps are never sexual, as far as is known, and the group is conspicuous as a wonderfully successful assemblage that has established itself in nature without the advantage of sexuality, which some biologists have supposed to be absolutely necessary for the high development of any group of organisms. The genus Caulerpa among the green algae presents a similar illustration.

We are now ready to examine certain groups of the green algae (Chlorophyceae). There are three families above the unicellular algae that are certainly nearer the main line of ascent than any other groups. They are related to one another and present an ascending scale in vegetative and sexual complexity. The Ulothricaceae, typified by Ulothrix, is the lowest group. They are isogamous and illustrate especially well the origin of sex as described in my former paper ( November, 1901, pp. 70 and 71). The Chaetophoraceae are likewise isogamous, but sex is more firmly established in this group, and structurally its members (Drapanaldia, Chaetophora, Stigeoclonium, etc.) are much more elaborate than the forms of the Ulothricaceae. The Coleochaetaceae end the series with an heterogamous type of considerable complexity.