66 



GENERAL BIOLOGY 



of practically independent cells that merely hang together. 

 Then there are those that show some differentiation of 



parts, and some rnutu, 

 relations between them. 

 Scenodesmus caudalus 

 (fig. 460) shows a very 

 moderate beginning 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 

 differentiated cells. Chara, like Nitella, is made up 

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

 there is one central cell completely surrounded laterally 

 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 



FIG. 48. Chara. a, a small branch; b, a 

 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 (archegomum) ; 



x the mature 



;g cell 

 in the 



development of the ovary; g, the mature 

 spermary in section; h, a pair of sper- 

 matic filaments; i, a bit of one ot the 

 filaments more magnified to show the 

 sperms developing within the cells; j, a 

 single sperm set free. 



