SPIROGYRA 35 



some seventy species. Spirogyra is found in floating 

 masses in lakes and ponds; it favours still water, but 

 I have found it in running burns in Scotland at an 

 altitude of 1,000 feet. The floating masses, regarded 

 from above, look like green scum. They are kept 

 floating by means of bubbles of oxygen given off by the 

 plants in the process of carbon-assimilation. The 

 masses are composed of innumerable filaments of cells 

 arranged end-to-end, the diameter of a single filament 

 of one of the larger species not being more than T i^ inch. 

 Yet, to the microscopist, this diameter is quite large, 

 and his lenses enable him to observe the structure of 

 the cells in much detail. The filaments are quite free 

 and unattached, and the cells show the same char- 

 acters throughout. In an examination of a single fila- 

 ment one cannot say which of the terminal cells is basal 

 or apical. We have but to tease out two or three 

 threads from a mass and examine them in water under 

 the microscope to realize how singularly beautiful these 

 components of despised pond scum really are. Each 

 cell is seen to be cylindrical, generally longer than broad. 

 The firm cell-wall, composed of cellulose, is lined on its 

 inside with a layer of protoplasm, distinguished as the 

 primordial utricle, and, if we are fortunate in our speci- 

 men, we shall see a process of circulation going on in 

 this lining of protoplasm. A large, clear central space 

 is apparent. This is the vacuole, not empty, but full 

 of cell-sap. The nucleus in some of the larger species 

 is suspended in the middle of the vacuole by strands of 

 protoplasm, which anchor it to the chloroplast; in 

 other species it is embedded in the primordial utricle. 

 It is the chloroplasts which give the name to Spirogyra. 



