3 o2 POPULAR SCIENCE MONTHLY. 



Among the remaining groups of algae, and containing by far the 

 greatest portion of this class, are a number of orders and families 

 comprising isogamous and heterogamous forms clearly related in var- 

 ious ways to one another, by vegetative structure and similar life- 

 histories, but offering a wide range of variation in the form and 

 habits of the sexual cells. And it is among these algae that we may 

 find material upon which to base general conclusions on the sexual 

 evolution of plants. These processes are illustrated more or less 

 completely by stages in several groups, but especially so in three lines 

 of development which we shall use as the illustrative basis of this 

 paper. They are the Volvocaceae, certain groups of the brown algae 

 (Phaeophyceae) and the region of the green algae comprising the 

 Ulothricaceae, Chaetophoraceae and Coleochaetaceae. 



These three lines are very far removed from one another and 

 must have become separated at an exceedingly early period of develop- 

 ment, probably before the origin of sex and certainly before any 

 extended sexual evolution. The Volvocaceae is an extreme side line, 

 by which we mean that its higher members are very far removed from 

 the theoretical main line of accent that runs through the algae to the 

 next higher group of plants, the Bryophytes. The groups of the Phaeo- 

 phyceae form a system of side lines very much more highly organized 

 vegetatively than the Volvocaceae. The Ulothricaceae, Chaetophora- 

 ceae and Coleochaetaceae are the nearest of all algae to the theoretical 

 main line of ascent, and some of their representatives are very close 

 to this chain of extinct forms; it is, of course, too much to expect 

 that any living alga should be actually one of the links. Each of 

 these three series tells the same story of the general events of sexual 

 evolution as do all other lines of ascent amrng the algae, fragmentary 

 though some of them are. 



We shall take up the Volvocaceae first. This is a peculiar family 

 of plants, remarkable for the fact that the vegetative conditions are 

 motile. Its highest member and a well-defined climax type is Vol- 

 vox, one of the most highly specialized of the algae in its peculiar 

 way. The lowest representatives of the Volvocaceae are unicellular 

 (CMamydomonas, Sphaerella, etc.), and between these simple organ- 

 isms and the complex Volvox there is a series of forms, all cell colonies 

 {Gonium, Pandorina, Eudorina and Pleodorina), each more complex 

 than the other as to the number, arrangement and degree of special- 

 ization of the cells. I know of no family of plants that illustrates so 

 many important evolutionary principles as clearly as the Volvocaceae, 

 and it might be made the subject of an interesting paper. But we 

 are to consider now only the differentiation of the sexual cells. 



The lower members of the Volvocaceae are mostly isogamous. In 



