CHAPTER V. THE THALLOPHYTA. 



289 



the softening of the gelatinous membrane and move about indi- 

 vidually by means of their cilia, finally conjugating and forming 

 zygospores which, when mature, acquire a brownish-red color. 

 These after a period of rest germinate, produce a few swarm- 

 pores, and these in turn divide to form colonies of sixteen. See 



Fig. 484- 



Volvox globator'xs a more highly developed but similar plant also 

 common in our fresh waters. When mature it has a diameter of 

 about half that of an ordinary pin-head, and the cells composing 

 it, which are quite numerous, are so arranged as to form a hollow, 

 spherical colony. The cells are chlorophyll-bearing and im- 

 bedded in a tough, gelatinous, transparent membrane, which 

 forms the wall of the sphere ; they are ordinarily connected with 

 each other by threads of protoplasm, which form a delicate net- 

 work over the surface, and each cell is provided with a pair of 

 cilia which project beyond the enveloping membrane, and by 

 their rhythmic vibrations communicate a rolling motion to the 

 coenobium. 



Asexual reproduction takes place as follows : Some of the 

 vegetative cells, which are of larger size than the rest, escape from 



the envelope into the interior 

 of the sphere, lose their cilia, 

 and by cell-division form 

 miniature colonies similar to 

 the parent one. Often two or 

 three of these colonies can 

 be seen within the parent 

 coenobium. These continue 

 to grow until the walls of the 

 colony can no longer con- 

 tain them, when by its rup- 

 turing, they escape and be- 

 come independent. 



In the sexual reproduc- 

 tion, which occurs toward 

 the close of the season, some of the cells develop into oftspheres, 

 and others into antheridia, containing numerous antherozoids. 

 The former become larger than the vegetative cells, and escape 



into the interior of the colony. The antheridia also increase 

 20 



Fig. 485. — Volvox globator, magnified about 

 75 diameters, a, one of the oogonia ; 6. one of 

 the antheridia from which most of the anthero- 

 zoids have escaped. 



