174 CHARACTER OF THE ZYGNEMATALES 



mother plant (Fig. 105, D). The walls of the mother plant soon 

 break down and the new colony is set free. Under different 

 conditions, smaller but similar bodies are formed which escape 

 from the mother plant. These bodies are gametes and incapable 

 of growth unless a fusion is effected between two of them. 

 Especial interest attaches to the study of this plant because Klebs 

 was able to change the reproductive character of the plant by 

 subjecting it to certain definite conditions. By exposing the 

 plants to bright light, slight increase of temperature and in- 

 organic culture solutions of definite composition, zoospores are 

 produced. On the other hand, plants growing in solution of 

 cane sugar in subdued light or darkness formed gametes. 



These reactions are of great importance in helping us to under- 

 stand the nature of plant life. They demonstrate that definite 

 factors may change motile plants to a stationary condition and 

 in Hydrodictyon we see that measurable factors, as light, heat 

 and concentration of the solution may bring about the motile 

 condition of the plant and may also cause a variation in the mo- 

 tile condition which we call sexualtiy. 



68. Order b. Zygnematales or Conjugating Green Algae. — 

 This order includes the most common and attractive forms of 

 green algae (Figs. 106, 108). They are of very general occur- 



FiG. 106. Two common forms of the Zygnematales: A, Spirogyra- 

 n, nucleus; p, pyrenoid. B, Zygnema. 



rence in the still waters of ponds and streams where they often 

 form the large floating and frothy green masses popularly known 



