SEAWEEDS AND THEIR ALLIES. 99 



III. Draw one of the cells with great care large enough 

 to show its complete structure. This will require close 

 attention to details. Repeat, if necessary, until you are 

 satisfied that your drawing represents truthfully a Spiro- 

 gyra cell. 



Describe fully what you have seen so far. 



NOTE. Possibly some things have escaped notice. The septa between 

 adjacent cells differ widely in different species. There are still other 

 points not likely to be observed except by comparing different forms. 



Reproduction. 



Spirogyra is reproduced sexually by zygospores and non- 

 sexually by cell-division. 



I. By zygospores. These may be found in the summer 

 time in specimens that look faded or discolored. They 

 are not to be looked for in bright green material. 



1. Observe the marked contrast presented by the conju- 



gating filaments to those in the vegetative condition. 

 The filaments occur in pairs, one with empty cells, 

 the other containing in each of its cells a large, 

 commonly oval zygospore. 



2. Notice the structure of the zygospore, with its heavy 



wall and dense contents. 



3. Compare different specimens, and try to make out the 



way in which the zygospores have been produced. 

 Notice the connecting-tube by which the cells of 

 the empty filament are connected with those of the 

 one containing zygospores. See if there are any 

 cases in which it contains protoplasm. Look for 

 specimens in which instead of a complete tube there 

 are protuberances from the opposite cells of the 



