No. 463.] STUDIES ON PLANT CELL. VI. 457 



I refer to many lower algae such as Ulothrix, forms of the 

 Volvocaceas, CEdogonium, Coleochaete, and many others. How- 

 ever, the homologies of primitive gametes and their origin from 

 types of asexual zoospores is very clear in a number of groups. 

 We can see nothing in the morphology and mode of develop- 

 ment of these reproductive cells to suggest reduction phenomena 

 when gametes are produced. The primitive gamete is generally 

 somewhat smaller than its homologue the zoospore, often because 

 the protoplasm of the gamete mother-cell becomes distributed in 

 a greater number of daughter elements. It is well known that 

 the conditions that lead to conjugation are exceedingly variable, 

 depending upon environmental factors and one often cannot tell 

 at the time whether a swarm spore will show sexual habits or 

 germinate without conjugation. The most satisfactory theory 

 of the origin of sex in plants regards primitive gametes as 

 weaker or lacking in certain potentialities of vegetative growth 

 and the conjugation as a mutually cooperative process resulting 

 in a rejuvenescence of the protoplasm. The fact that many 

 simple types of gametes will germinate without fertilization and 

 produce small and weak sporelings shows that vegetative possi- 

 bilities are not entirely lost. Investigations on the chromosome 

 history among these forms, difficult though they be, are some of 

 the most interesting subjects of botanical research. We know 

 some general principles of the origin and evolution of sex in 

 plants (Davis, :oib, : O3a) but of the chromosome history in the 

 simplest types of gametogenesis nothing is known. 



With respect to the history of the chromosomes in the sim- 

 plest sporophytes we are also as ignorant as in the simplest 

 types of gametogenesis. We have excellent reasons for believ- 

 ing that the sporophyte generation is represented among the 

 thallophytes in a number of very simple conditions. Numbers 

 of zygospores and oospores (c. g., Ulothrix, CEdogonium, forms 

 of the Conjugales and Volvocaceas, etc.) give rise on germination 

 to several daughter cells. In higher forms this growth period is 

 lengthened to the formation of a reproductive tissue (Coleochaete) 

 and in the great groups of the Rhodophyceae, Ascomycetes, and 

 Basidiomycetes there is present an extensive development from 

 the fertilized female cell (or its equivalent when apogamy obtains) 



