THE GREAT GROUPS OF ALG^E 



261 



fore, two sorts of asexual spores are produced : (1) the 

 • tetraspores, developed in ordinary sporangia ; and (2) the 

 carpospores, developed in the cystocarp, which has been 

 produced as the result of fertilization. 



OTHER CHLOROPHYLL-CONTAINING THALLOPHYTES 



175. Diatoms. — These are peculiar one-celled forms, which 

 occur in very great abundance in fresh and salt waters. 



Fig. 230. A group of Diatoms : c and d, top and side views of the same form; e, colony 

 of stalked forms attached to an alga;/ and g, top and side views of the form shown 

 at e ; h, a colony; i, a colony, the top and side view shown at k. — After Kernek. 



They are either free-swimming or attached by gelatinous 

 stalks ; solitary, or connected in bands or chains, or im- 

 bedded in gelatinous tubes or masses. In form they are 

 rod-shaped, boat-shaped, elliptical, wedge-shaped, straight 

 or curved (Fig. 230). 



