268 PLANT PHYSIOLOGY 



This table shows that those conditions which contribute 

 to increase and to maintain those activities which supply 

 the plant with food, also favor reproduction by non-sexual 

 rneans. Light, enabling the plant to manufacture carbohy- 

 drates, and the salts needed in elaborating carbohydrates 

 into amides and proteids, stimulate zoospore formation. On 

 the other hand, the copious supply of already elaborated 

 carbohydrates, especially when no unusual amount of in- 

 organic salts stimulates to the further elaboration of carbo- 

 hydrates into amides and proteids, is most favorable to the 

 formation of gametes. Light is a specific stimulus to zoo- 

 spore formation, apart from its influence on nutrition. 

 Given a vegetating cell, healthy and of such size that it can 

 divide into zoospores or gametes, it will continue to vege- 

 tate until it is subjected to some influence which will cause 

 it to grow more slowly or to cease growing, and which will 

 stimulate it to reproduce itself in the one or the other of the 

 two ways in which it can reproduce. Hydrodictyon shows 

 plainly that the time and the manner of reproduction are 

 not fixed, but that both are determined by the influences to 

 which the plant is subjected and is sensitive. Additional 

 evidence of this is furnished by Klebs's observation that 

 Hydrodictyon nets with an already marked tendency to 

 form zoospores can be so influenced by external conditions 

 that they will cease to form zoospores and will thereupon 

 produce gametes instead. The same net, darkened at one 

 end and illuminated at the other, will form gametes in the 

 one end, zoospores in the other, respectively. 



Livingston* has studied one of the polymorphic algae, a 

 StigeoclonJum, particularly with a view to determining the 

 influence of varying concentrations of the medium upon the 

 form and reproduction of the plants. He finds that de- 

 creased osmotic pressure acts as a stimulus to zoospore- 

 formation as well as favoring vegetative activities, and that 

 high osmotic pressure checks zoSspore-formation and the 



* Livingston. B. E. Nature of the stimulus which causes change of 

 form in polymorphic algre. Bot. Gazette vol. 30. 1900. Further notes 

 on the physiology of polymorphism in green algae. Bot. Gazette, vol. 32, 

 1901. 



