LESSON XIX 



SPIROGYRA 



AMONGST the numerous weeds which form a green scum in 

 stagnant ponds and slowly-flowing streams, one, called Spiro- 

 gyra, is perhaps the commonest. It is recognized at once 

 under a low power by the long delicate green filaments of 

 which it is composed being marked with a regular green 

 spiral band. 



Examined under the microscope the filaments are seen to 

 be, like the hyphae of Penicillium, linear aggregates, that is, 

 to be composed of a single row of cells arranged end to 

 end. But in Penicillium the hyphae are frequently branched, 

 and it is always possible in an entire hypha to distinguish 

 the slightly tapering distal end from the proximal end, which 

 springs either from another hypha or from a spore. In 

 Spirogyra the filaments do not branch, and there is no 

 distinction between their opposite ends. 



The cells of which the filaments are composed (Fig. 43, A) 

 are cylindrical, covered with a cellulose cell-wall (c. iv), and 

 separated from adjacent cells by septa (sep} of the same 

 substance. The protoplasmic cell-body presents certain 

 characteristic peculiarities. 



It has been noticed in more than one instance that in the 



