168 STRUCTURAL BOTANY 



TYPE XI. SPIROGYRA 



1. STRUCTURE 



Our next type is another of the filamentous fresh-water 

 Algae, but very distinct from those already dealt with. 

 The genus Spirogyra contains about seventy species, 

 differing much among themselves both in size and in 

 details of structure. We will give a general account of 

 the genus, noting when necessary the points in which 

 the specific differences are of interest. 



Spirogyra occurs chiefly in ponds and lakes, that is to 

 say in still, rather than in running water. It is often 

 present in immense quantities, the filaments forming 

 floating masses, sometimes several acres in extent, buoyed 

 up by the bubbles of oxygen which their assimilation 

 has produced. It is characteristic of Spirogyra to float, 

 for its filaments are perfectly free, without attachment 

 to any kind of substratum. There is no distinction of 

 apex and base, and all the cells are alike throughout the 

 thread. 



In the larger species of Spirogyra the cells are, 

 microscopically speaking, of great diameter (reaching 

 25 mm. = one-hundredth of an inch, in extreme cases). 

 They are therefore very favourable for study. Within 

 the stratified cell-wall is the primordial utricle, in 

 which protoplasmic circulation can be well observed in 

 vigorous plants. Imbedded in this are the very con- 

 spicuous chloroplasts, which here take the form of green 

 spiral bands with toothed edges. They form the most 

 striking feature of the plant under the microscope, and 

 from them the genus derives its name. The chloroplasts 

 vary in number from one to ten in each cell (see Figs. 



