DESMIDS, DIATOMS, AND FRESH-WATER ALG.E. 101 



2. PEDIASTRUM (Fig. 80). 



The green cells are usually so arranged as to leave 

 narrow colorless bands between them, and occasionally, 

 in those species formed of a great number of adherent 



Fig. 79. Scenedesmus quadricauda. Fig. 80. Pediastrnm granulatum. 



cells, several apparently empty colorless spaces are scat- 

 tered about the disk. In the latter cases the colorless 

 marginal teeth are often very numerous, but they are 

 usually more or less conspicuously arranged in twos. 

 In the species here figured the marginal teeth are gener- 

 ally twelve in number. P. granulatum^ Fig. 80. 



3. VOLVOX. 



A small sphere continually in movement, rolling 

 through the water in a very graceful manner, its sur- 

 face studded with green points. Under a low power it 

 seems like a hollow globe, and the cause of the mo- 

 tion is a mystery ; but the \ inch objective, when the 

 Volvox moves slowly or rests, shows that each green 

 point bears two fine cilia or little hairs continually vi- 

 brating and lashing the water. It is from their vibra- 

 tions that the Yolvox receives its rolling motion. The 

 deep green balls often seen within the globe are young 

 plants in different stages of development. When ma- 

 tured the mother-globe is broken and the young plants 



