478 SMITH 



said to be safranol. Sexual activity in sexually inactive female cells was in- 

 duced by pikrocrocin, and that of sexually inactive male cells was induced 

 by safranol. 



In an investigation of Monostroma Moewus (1940b) reports that zoospores 

 may be made to function as gametes by means of extracts obtained from 

 thalli. Monostroma is a close relative of Ulva, being generically distinguished 

 from it because the sheet-like thallus is but one cell in thickness. Monostroma 

 is heterothallic, all cells of a thallus producing gametes of the same sex. When 

 germinated in water the protoplast of the greatly enlarged zygote divides to 

 form 32 quadriflagellate zoospores. When zygotes are germinated in extracts 

 from thalli there is a formation of 64 swarmers that are biflagellate. A biflagel- 

 late swarmer formed when germination takes place in a thallus extract is 

 able to function as a gamete and one of the same sex as that of the thallus 

 from which the extract was obtained. 



The problem of the course of evolution among algae is of broad interest 

 because there is universal agreement that algae have given rise to all plants 

 standing higher in the evolutionary scale. For the Chlorophyceae and the 

 Xanthophyceae Blackmann (1900) proposed the widely adopted theory that 

 the most primitive members of these groups were of a unicellular flagellated 

 type and that there were divergent evolutionary lines from the primitive type. 

 One of these tendencies, the volvocine, was toward an aggregation of flagel- 

 lated vegetative cells into progressively larger and more specialized colonies. 

 This is a blind evolutionary end in which none of the colonies evolved a more 

 complex structure than is found in Volvox. Another tendency, the tetrasporine, 

 was toward an organization of non-flagellated immobile cells into colonies of 

 increasing complexity of structure. This was the line that gave rise to the 

 higher plants. Still a third tendency, the endosporine, was toward a unicellular 

 condition without cell division but with multinucleate cells of elaborate com- 

 plexity among advanced forms. Later, Pascher (1914) pointed out that sim- 

 ilar evolutionary tendencies are to be found among the Chrysophyceae. 



Evolution along the tetrasporine tendency has been accompanied by evolu- 

 tion of a variety of life cycles. The concept of a life cycle involving a rhythmic 

 alternation of sexual and asexual generations was first clearly enunciated by 

 Hofmeister (1851) as a result of his studies on the life cycle in certain 

 bryophytes and pteridophytes. However, it was not until forty years later 

 that the accompanying periodic doubling and halving of the number of 

 chromosomes was first pointed out by Strasburger (1894). The question of 

 an alternation of generations among algae was raised (Pringsheim, 1856) 

 shortly after discovery of alternation of generations in land plants. Even as 

 late as 1903 Davis, with considerable justification, stated that alternation of 

 generations with an accompanying alternation of chromosome numbers had 

 not been definitely established for algae. A year later Williams (1940a, 1940b) 

 showed that the brown alga Dictyota has such a life cycle, but one in which 



