THE GREEN ALG^ (CHLOROPHYCE^) 191 



Spores that have special structures by means of which they 

 swim are called zoospores, meaning "animal spores," or "mov- 

 ing spores." They were thus named when it was supposed that 

 self -caused movement 

 is a distinguishing fea- 

 ture of animals. 



Plants which have 

 swimming spores have 

 means of more ready 

 distribution than do 

 those that reproduce 

 entirely by means of 

 fission. 



177. Spirogyra: its 

 habitat and structure. 

 The algse commonly 

 spoken of as "pond 

 scums" are found in 

 standing water, often 

 floating upon its sur- 

 face. One of the most 

 abundant of these is 

 Spirogyra. Within the 

 water, it appears as 

 long threads of a clear 

 green color, at times 



-n 



FIG. 156. Spirogyra 



ch, the spirally arranged chloroplast; n, the nu- 

 cleus which is supported by the bands of cyto- 

 plasm (c?/) ; cytoplasm also appears just within the 

 cell wall. The central cavity of the cell is usually 

 occupied by one or more large vacuoles. At the 

 right are two cells of one plant. At the left are 

 parts of two plants whose cells have conjugated 

 to form zygospores (Z). Greatly enlarged 



attached by one end to 

 some support; when 

 floating upon the sur- 

 face of the water, it 

 commonly appears in 

 yellowish-green mats 

 in which are many 

 bubbles of gas or air. It is soft like silk, and may thus be 

 distinguished from most other algae likely to be found in 

 similar places. 



