SEAWEEDS AND THEIK ALLIES. 97 



SPIROGYRA. S. longata, qulnina, etc. 



General Characters. 



The soft, green material called " pond scum," growing 

 on the surface of still water, is usually made up largely 

 of Spirogyra, not infrequently several species together. 

 Notice 



I. The color, varying according to conditions, so that 

 specimens from different places, or gathered at different 

 times of year, may present a wide range of shades. 



II. The delicate and slippery feeling, reminding one of 

 silk when taken between the fingers. 



III. The remarkable difference in size of the filaments 

 when examined with a hand lens, or even with the naked 

 eye, if specimens of extreme sizes are compared. 



Microscopic Structure. 



Mount in water and examine with the compound micro- 

 scope. 



I. Observe that each filament is composed of a single 

 row of cells. Follow one of the filaments to the end. 

 Are the cells composing it of uniform diameter? Of 

 uniform length ? How does the terminal cell differ from 

 the others? 



II. Study critically the cell structure. 



1. Focus slowly and compare one cell with another 



until you are satisfied as to their geometrical form. 

 Are they " rectangular " or cylindrical ? 



2. Separate the cell-contents from the cell-membrane 



by applying a plasmolyzing agent. Two per cent 



