THE BEGINNINGS OF PLANT LIFE 15 



atoms. Without any visible effort or means of 

 propulsion they slowly glide in a straight line 

 through the water, and should anything obstruct 



l-'i,2;. 7- Diatoms common in [)Onds and lakes 



their passage they make no effort to round it, but 

 without turning reverse their motion and glide 

 backwards, apparently on the same straight track 

 on which they came, often repeating the move- 

 ment several times as if unconvinced that the path 

 was impassable. 



Other common forms do not lead such a free 



