FILAMENTOUS ALG^E. 



19 



24. Base and apex. — But other filamentous algse show a 

 distinction between base and apex. In Uloihrix (fig. 19) 

 the basal cell is elongated and pointed, and 

 is colorless, because it is not fiirnished with 

 chloroplasts like the others. By this pointed 

 cell the plant is loosely attached, at least when 

 young, to the substratum, while the green por- 

 tion waves freely in the water. Thus arises a 

 distinction into two parts, viz. , the rhizoid and 

 the thallus. 



In Cladophora, Vaucheria, and their allies, 

 the plants are generally attached by a well- 

 developed rhizoid region, which is often 

 branched {w, fig. 20), as is also the thallus. 

 In contrast with the preceding, therefore, 

 localization of growth, producing branching, 

 may be observed. 



25. Branching. — A branch begins by the 

 growth in area of a limited portion of the cell- 

 wall. Since growing cells are usually stretched 

 by the water they absorb, the pressure upon 

 the enlarged region causes the wall to bulge out- 

 ward there. The convexity gradually increases Fig. 19. — zz/o- 



. . •! 1 IT T tkrix .zonata, 



as the region grows, until the swelling becomes a young fiia- 



, , /-111. • ment with rhi- 



an outgrowth whose further lengthening consti- zoid ceii, r, at 



, , . ., , , -, base. Magnified 



tutes a branch similar to the mam filament. 482 diam. — Af- 



1-1 1 T1..1 1. .-ter Dodel-Port. 



Growth in length may be limited to the tip of 



a filament, or to a narrow zone including one or more cells, 



or it may occur indifferently in any cell, or in all cells. 



EXERCISE V. 



Cladophora. — If fresh material is at hand* observe in a white dish ; if 

 preserved specimens are used stain for a few minutes in eosin. 

 ■ I. How is the plant attached ? 



