TEE EVOLUTION OF SEX IN PLANTS. 37 



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 physio- 

 logically that of heterogamy, for the female cell is essentially a quies- 

 cent 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, &), 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 Eng- 

 land) 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 sup- 

 posed 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 

 (Popular Science Monthly, November, 1901, pp. 70 and 71). The 

 Chaetophoraceae are likewise isogamous, but sex is more firmly estab- 

 lished in this group, and structurally its members {Drapamal&ia, Chae- 

 tophora, Stigeoclonium, etc.) are much more elaborate than the forms 

 of the Ulothricaceae. The Coleochaetaceae end the series with an 

 heterogamous type of considerable complexity. 



