54 



THE SMALLEST LIVING THINGS 



the palmella condition is the dominant form and non-motile 

 colonies are very characteristic, although the individual cells 

 readily revert to the motile phase. The smaller motile forms 

 are included for the most part in the group represented by the 

 globular colony of volvox (Fig. 26), but equally small non- 



Fig. 25— A PLANT FLAGELLATE, 

 SPHAERELLA LACUSTRIS 



It may be motile or quiescent, red, green, 

 or colorless. A, a red-colored resting form; 



B, a red-colored, biflagellated motile form; 



C, D, green-colored motile forms; E, F, 

 parent cell and flagellated zoospores from 

 it; G, H, I, a green cell goes into palmella 



stage and forms four red motile cells 



After T. E. Hazen 

 Magnification, 600 



motile forms, either alone or in the palmella phase, may develop 

 flagella and become motile for a short period, e.g., the chloro- 

 coccales. Still another group of small forms — the desmids — 

 are permanently quiescent and do not have a palmella phase. 

 In other groups of this class, the organisms are filamentous and 

 are usually found in aggregates which are visible to the eye. 

 In the present volume, therefore, we shall limit treatment of the 

 isokontae to these three non-filamentous forms, in all of which 

 grass-green chlorophyll is characteristic. 



The Volvox Group 



In the first of these, the volvox group, the central unicellular 

 type is well represented by small unicellular forms of the genus 

 Sphaerella (Fig. 25). Two equal flagella found at the anterior 

 end draw the cell briskly through the water while at the same 

 time it is rotating on its long axis. A definite membrane cover- 

 ing the cell gives rigidity to the body, so that change of shape 



