zoospores. These lowly types of the Protococcales are certainly most nearly related to the parent forms of all the higher algae. The principal stages in the life history of such a type (Chlamydomonas) are illustrated in Figure 1. The free-swimming cell is shown in a and b. In c we have the conjugation of two individuals which gives a sexually formed spore such as appears in d. e and f 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 development 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 remote 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. Recent studies by Livingston have shown for one type (Stigeoclonium) that zoospores immediately follow the transfer of cells in a resting condition from a certain solution 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 zoospores 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
