THALLUS OF GREEN ALG^ 



197 



are found in young stages of most filamentous Algae (Figs. 115, a ; 

 117, g), but in forms like Spirogyra, (Edogonitim, and Ulothrix, 

 the filaments usually break off and become free-floating as they 

 grow older, except in the case of species inhabiting flowing water. 

 The lobes of the attaching cells grow into all the irregularities 

 of the substratum 

 (Fig. 104, B), to which 

 they cling after the 

 manner of a sucker, 

 adhesion being often 

 increased by the secre- 

 tion of a cementing 

 substance. The great 

 efficiency of the at- 

 taching cells is very 

 evident in those 

 species of Cladophora 

 which grow on wave- 

 beaten rocks or in 

 rapidly flowing 

 streams. 



The restriction of 

 the power of division 

 to the terminal cells 

 of Cladophora marks 

 a considerable step in 

 the direction of divi- 

 sion of labour as com- 

 pared with Ulothrix, 

 and this is also seen 

 in the formation of 

 reproductive units, 

 which is almost con- 

 fined to the cells of the lateral branches (cf. p. 215). A more 

 extreme condition is seen in Draparnaldia (Fig. 105), which 

 is not uncommon in slowly flowing water. Here the main 

 axes, which serve almost solely for support, consist of large 

 cells with smaU chloroplasts, whUst assimilation and reproduction 

 are relegated to the densely branched laterals. 



Fig. 105. — Photograph of part of a plant o 

 the Green Alga Draparnaldia, seen under 

 the low power of the microscope. 

 [Photo : E. J. S.] 



