THE EVOLUTION OF SEX IN PLANTS. 39 



There are several other groups of algae that confirm and illustrate 

 in various particulars the principles of sexual evolution that we have 

 traced in the three lines of development here described. The Sipho- 

 nales duplicate the history in most of the stages. Isogamy with 

 variation in the size of the gametes is found in several of the remaining 

 groups. Heterogamy in its most extreme form is presented in such 

 isolated types as the stoneworts (Charales), Oedogonium, Bulbochaete 

 and Sphaeroplea. All these types stand as representatives of lines 

 of extinct ancestry, whose sexual evolution must have passed through 

 the stages that we have described. 



The red algae (Rhodophyceae) do not enter into this discussion. 

 They started with sexuality at an advanced stage of heterogamy. 



Let us now briefly summarize with examples the steps in sexual 

 evolution which we have discussed, beginning with isogamy, at the 

 dawning of sex, and ending in heterogamy. 



First Stage. Isogamy with exactly similar gametes; the condi- 

 tion at the origin of sex. Exemplified by many of the lower algae, 

 Hydrodictyon, Ulotlirix, Viva, Cladophora, etc., certain species of the 

 lower brown algae and unicellular green. 



Second Stage. Isogamy with gametes similar in form but of dif- 

 ferent sizes, the female large and richly nourished, the male relatively 

 small. Illustrated by species of Chlamydomonas and Ectocarpus, 

 Bryopsis and the forms that also illustrate the third stage. An 

 index to this condition is the differentiation of the gametangia with 

 respect to the number of gametes developed. The female gametan- 

 gium tends to reduce the number until a single egg takes all the 

 material of the oogonium. The male gametangium increases the 

 number of sexual products, becoming an antheridium that may develop 

 numerous sperms. 



Third Stage. Isogamy in that peculiar form when the gametes are 

 similar in form at the time of their discharge from the gametangia, 

 but different at the time of fusion, because the female gamete becomes 

 a motionless cell. Examples: Ectocarpus siliculosus and secundus 

 and Cutleria multifida. This stage is the transition point between 

 isogamy and heterogamy. Morphologically these gametes are iso- 

 gamous; physiologically they are heterogamous. 



Fourth Stage. Heterogamy which has several grades in the degree 

 of differentiation and specialization of the egg and sperm. 



Fifth Stage. The retention of the egg in the oogonium (female 

 gametangium), a condition peculiar to but not at all universal among 

 heterogamous higher algae. Illustrated by Oedogonium, Bulbochaete, 

 Coleochaete, Sphaeroplea, Chara and Vaucheria. This stage would 

 be developed quickly when once started, and a tendency in this direc- 

 tion is probably shown in Aphanochaete. 



