THE TIIALLOPHYTA 



275 



The Pond Scums, of which Spirogyra (Fig. 153) is the common 

 example, are all filamentous algae. In this genus the chloroplast 

 is a broad, strap-shaped structure running spirally around the cell 

 and on it appear a series of small, rounded areas, the pyrenoids. 

 The nucleus is suspended in the middle of the sap cavity by 

 threads of cytoplasm extending to the walls. In sexual reproduc- 

 tion, adjacent cells of two filaments which are lying side by side 



m 



^M 



Fig. 154. — Desmids of various tyj 



send out projections or "conjugating tubes" toward one another. 

 The tips of these touch, the wall between them breaks down, and 

 through the channel thus formed the whole protoplasmic con- 

 tents of one cell enters the other and the living portions of the 

 two cells fuse into a thick-walled zygospore. Occasionally the 

 two cells conjugate in the tube itself, and sometimes two adjacent 

 cells of the same filament may unite. 



The Desmids (Fig. 154) are unicellular plants of the utmost 

 variety and beauty of form. The cell is composed of two per- 

 fectly symmetrical halves separated by a zone, the istJmius, which 

 is often constricted and under which lies the nucleus. Aside 



