68 POPULAR SCIENCE MONTHLY. 



zoospores. These lowly types of the Protococcales are certainly most 

 nearly related to the parent forms of all the higher algae. The prin- 

 cipal stages in the life history of such a type (Chlamydomonas) 

 are illustrated in Figure 1. The free-swimming cell is shown in a and 

 &. In c we have the conjugation of two individuals which gives a sex- 

 ually formed spore such as ajDpears in d. e and / present a quiescent 

 condition when the cells multiply for a short time by fission. 



The evolution of the algae has led for the most part to the develop- 

 ment and long continuance of such phases of the life history as are 

 stationary, and from these the filamentous, membranous and otherwise 

 differentiated plant bodies have arisen. Finally the motile stage 

 became so shortened as to be only a method of reproduction on the 

 part of the plant and is passed over very quickly. 



The zoospore then takes on new interest when one contemplates 

 its relation to the past, realizing that it represents conditions of a re- 

 mote period when the algae were much simpler than they are now and 

 passed the greater part of their lives in a motile condition. It is not 

 likely that the first algal types were motile, for the lowest group of all, 

 the Cyanophyceae or blue-green algae, presents forms whose cells are 

 always stationary, reproducing by simple fission. 



But above the lower stretches of the algae, the zoospore appears with 

 great regularity and usually conspicuously in the life history. There 

 are certain types (unicellular Volvocaceae), whose life histories are 

 mostly or entirely alternations of motile conditions and quiescent 

 states when the cells come to rest, lose their cilia and remain motionless 

 for many days. Such resting cells are well known to students of the 

 lower algae, and it is an interesting fact that they may pass quickly 

 and readily back to the motile form. Indeed there is every reason 

 to believe that the one state or the other is largely determined by the 

 physical environment of the organism. Eecent studies by Livingston 

 have shown for one type (Stigeoclonium) that zoospores immediately 

 follow the transfer of cells in a resting condition from a certain solu- 

 tion of salts to a weaker solution, and this is an excellent illustration of 

 the sort of factors that influence the alga. 



In our discussion of the problem of the origin of sex we are to deal 

 chiefly with forms whose motile conditions are so shortened as to be 

 manifestly largely or wholly reproductive in their purposes. The 

 plants are stationary, but at times and under certain conditions zoo- 

 spores are produced in great numbers. These, after a brief existence 

 as free-swimming cells, settle down and give rise to a new stationary 

 plant body usually like the parent. 



Zoospores or swarm spores are wonderfully alike in structure in the 

 algae that are most closely related to one another. The prevailing type 

 among the green algae (Chlorophyceae) is a pear-shaped cell with 2 



