66 



GENERAL BIOLOGY 



FIG. 48. Chara. 



small branch ; b, 



of practically independent cells that merely hang togeth- 

 er. Then there are those that show some differentiation 



of parts, and some mu- 

 tual relations between 

 them.Scenodesmus cau- 

 datus (fig. 46 c) shows a 

 very moderate begin- 

 ning of differentiation 

 in the modified form of 

 the two end cells. Then 

 we have a differentia- 

 tion between base and 

 apex, the one end tak- 

 ing up the duty of se- 

 curing attachment, the 

 other providing for 

 growth as in Cladophora 

 (fig. 47). Finally, we 

 have in Chara, a solid 

 aggregate of greatly 

 Chara, like Nitella, is made up 

 of a succession of nodes and internodes, but in each of the 

 latter there is one central cell completely surrounded later- 

 ally by a layer of slenderer cells, (fig. 48.) Thus the central 

 cell is completely inclosed and removed from the source of 

 supply of food and air ; and it is rendered dependent on its 

 neighbors for its living. And in Chara and in many other 

 algae there is a high degree of division of labor, 

 certain cells of the plant body being set apart to serve the 

 reproductive process, while others perform the nutritive 

 functions. 



The purpose of the following study is to observe in a 

 variety of representative algae the phenomena of cell aggre- 

 gation and of cell differentiation. Incidentally there should 



. . . , 



piece of the stem containing a node and 

 part of two internodes, the lower one hav- 

 ing the cortical cells spread apart from 

 the central cell; o, ovary (archegonium) ; 

 5, spermary (antheridium) ; c, the mature 

 ovary more enlarged, showing the egg cell 

 within; d, e, and /, successive stages in the 

 development of the ovary ; g, the mature 

 spermary in section; fe, a pair of sper- 

 matic filaments; i, a bit of one of the 

 filaments more magnified to show the 

 sperms developing within the cells; i, a 

 single sperm, set free. 



differentiated cells 



