DEVELOPMENT OF PLANTS 



185 



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 

 motile condition which we call sexuality. ( 



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- 





-Jit-:- 



j*t. 



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 

 as pond scum or frog spittle. They can usually be recognized 

 by their slimy nature which is due to the copious exudation of a 

 gelatinous substance from the cell walls. In one group of this 

 order the plants are multi-cellular (Fig. 106), the cells being 

 attached end to end and forming a filament — though it should be 

 stated that each cell is practically independent in its life process 

 13 



