SPIROGYRA 



315 



abundant fresh water plants, and in the examination of other 

 forms of fresh water Algae with the microscope one usually finds 

 some Desmids present. The cell is pecuhar in being organized 

 into symmetrical halves, which are separated by a constriction 

 that forms an isthmus. The nucleus is in the isthmus, and in 

 each half there is a chloroplast and a number of pyrenoids. 



They reproduce in two ways, by cell division and by zygo- 

 spores. In multiplying by cell 

 division, the cell divides at the 

 isthmus, the halves separate, and 

 the portion of the isthmus re- 

 maining to each half develops a 

 new half and thus a new individ- 

 ual is formed. In sexual repro- 

 duction the cells pair and the 

 protoplasts, which escape through 

 ruptures at the isthmus, fuse and 

 form a zygospore. Sometimes 

 the cells after pairing become con- 

 nected by a tube through which 

 the protoplasts reach each other. 

 In either case the entire proto- 

 plasts of cells conjugate. 



Spirogyra. — Spirogyra (Fig. 

 271), very common in ponds, 

 sluggish streams, and watering 

 troughs, is the most familiar fila- 

 mentous form of the Conjugales 

 and the one most commonly 

 studied in elementary classes. It 

 gets its name from its large and 

 beautiful spiral chloroplasts. Its 

 cells are all alike and it pro- 

 duces no zoospores. Its sexual reproduction, in which the gam- 

 etes reach each other through tubes, is its important feature. 

 Under certain conditions, filaments pair and line up side by side. 

 In this position, the cells of the filaments grow toward each 

 other in tubular projections which unite and form open passage 

 ways between the cells of the paired filaments. The protoplasts 

 of one filament pass through these tubes and fuse with the pro- 



FiG. 271. — A species of Spiro- 

 gyra. A, a portion of a filament 

 showing a vegetative cell with its 

 spiral chloroplasts (c) and nucleus 

 (n) (X 100); B, filaments conju- 

 gating and two zygospores (z) 

 fully formed; C, a zygospore 

 germinating and producing a new 

 filament (X 150). A and B from 

 nature, and C from West. 



