184 



THE 



Volvox thus presents a great advance over Sphoerella, 

 Chlamydomonas, and other one-celled members of the Volvo- 

 cacece, in the highly developed sexual process as well as in the 



complex cell colony. There is, how- 

 ever, a series of genera (G-onium, Pan- 

 dorina, Eudorina, Pleodorina, etc.) in 

 the family, illustrating intermediate 

 conditions between these extreme 

 forms, which makes it clear that Vol- 

 vox stands at the head of a remark- 

 able line of development in the algae. 

 It may be considered the climax type 

 of a side line of evolution, that is, 

 a branch which departs widely from 

 the main line of ascent. 



ORDER 2. THE CONFERVAS, OR 



CONFERVALES 



216. The confervas. The Confer- 

 vales comprise many very common 

 filamentous algae and also such mem- 

 branous forms as the sea lettuce. The 

 algae which seem to be nearest to the 

 main line of ascent to the liverworts 

 and mosses are found in this group. 

 Some of the types illustrate espe- 

 cially well the principal forms of sex- 

 ual reproduction in the algae and 

 various types of life histories. 

 217. Ulothrix. This confervoid alga (family Ulothricacece) 

 is abundant on stones and rocks along the shores of the great 

 lakes, in quieter waters at the seaside, and frequently grows in 

 stone fountains or on stonework around park ponds. The fila- 

 ments are unbranched, and each consists of a row of similar 



FIG. 179. The water net 

 (Hydrodictyon] 



This is a remarkable form of 

 the Protococcales, whose cells 

 form the meshes of a net-like 

 cell colony, A. New nets are 

 formed in the interior of large 

 cells, B, which develop an 

 immense number of zoospores 

 that never escape from the 

 mother cell, but join with one 

 another to form daughter 

 nets, which are set free by the 

 breaking down of the mother- 



' cell wall 



