Ch. X] 



CONJUGATING ALGiE 



425 



Zygnemacece: the Pond Scums. These are very slender 

 filamentous plants, which are sometimes attached on the 

 bottoms of shallow ponds, but ordinarily float felted to- 

 gether in great numbers on the surface, where the bright 

 green masses are buoyed up 

 by bubbles of gas, which is 

 mostly oxygen set free in their 

 photosynthesis. They are 

 characteristically slippery to 

 the touch because of a gelat- 

 inous secretion of the walls. 

 The commonest form is Spiro- 

 gyra (Fig. 294). Its filaments 

 consist of similar, cylindrical, 

 smooth-walled cells which ex- 

 hibit a remarkable specializa- 

 tion in their contents ; for the 

 bright green chromatophores 

 form elaborate crenated spiral 

 bands, beaded, as it were, here 

 and there with pyrenoids. 

 Zygnema, another genus, pos- 

 sesses not a spiral, but two 

 star-shaped chromatophores. 

 The cells multiply in the 

 filaments by cross division. 

 Especially distinctive is the 

 sexual reproduction, 

 typical case (Fig. 294), the 

 walls of cells in neighboring filaments grow out towards 

 one another in tubular projections; where they meet, 

 an opening is formed, through which the protoplasm 

 of one cell passes by amoeboid movement into the other; 

 then the two protoplasts fuse together, and develop a 

 thick cellulose wall, producing a resting zygospore which 

 is later set free by decay of the filament. In some 



Fig. 294. — Spirogyra species ; X 

 300. 



The cells of the filament on the 

 left show successive stages in conju- 

 gation with the filament on the right, 

 which has one cell in the vegetative 

 condition, and one with a fully 

 „ formed zygospore. (From a wall 

 chart by L. Kny.) 



