CHAPTER V. THE THALLOPHYTA. 



287 



far from the natural size, and b, one of the cells greatly enlarged, 

 showing numerous small cells in the interior, in the act of arrang- 

 ing themselves into a net. 



Fig. 482. — The Hydrodictyon or Water-net. 

 a, portion of colony nearly natural size ; b, one 

 of the cells of the colony greatly enlarged, 

 showing in the interior minute cells arranging 

 themselves to form a reticulum. 



The Volvocineae consist 

 of cells either occurring 

 singly or grouped in coeno- 

 bia, and endowed in the veg- 

 etative, as well as in the 

 spore stage, with the power 

 of locomotion. The plants 

 are mostly fresh-water forms, 

 and even the colonies are of 

 small size, most of them 

 microscopic. They multi- 

 ply asexually by cell-divi- 

 sion. The mode of sexual 

 reproduction in some is by the conjugation of swarm-spores, 

 while in the higher forms of the group, the process is that of 

 fertilization, the germ-cell being of larger size and quiescent, 

 while the fertilizing cell is small and provided with cilia. 



Among the unicellular forms are Chlamydomonas and Eu- 

 glena. Euglena viridis may be taken to illustrate this section of 

 the group. The cells are somewhat fusiform in shape, but capa- 

 ble of spontaneously changing their form. At the anterior end 

 is a long flagellum, by which the cell is propelled, and near the 

 base of the flagellum a red spot, commonly called the " eye-spot." 

 Increase, during the vegetative period, takes place by fission in 

 a longitudinal direction. After a period of activity of longer or 

 shorter duration, the cells come to rest, shrink into a round form 

 and acquire a firm integument. On the recurrence of conditions 

 suitable for growth, the cells divide repeatedly, and finally resume 

 the active, flagellate form. See Fig. 483. Sexual reproduction 

 has not yet been observed in this genus, but in Chlamydomonas, a 



