222 PLANT PHYSIOLOGY 



which causes a man who is cold to approach the fire but 

 not to draw too close, and when w r armed to move to a 

 slightly cooler spot, is not peculiar to him or to higher 

 animals. This example of thermotaxis expresses the sensi- 

 tiveness of all living protoplasm and the habit of all motile 

 organisms to seek positions which are most comfortable 

 because most favorable to the accomplishment of their vital 

 functions. Thus slime-moulds* and zoosporest are known 

 to move toward sources of heat. 



Whether, corresponding with the influence of light, the 

 direction of movement of the internal organs of the cell is 

 affected by the direction of the source of heat seems not 

 to have been studied. The rate of movement of the 

 organs of the cell in relation to the degree of warmth has 

 been carefully examined. For example, the chlorophyll 

 grains in Elodea, Valisnerhi, and Chara circulate with in- 

 creasing rapidity up to a temperature of 34 to 38 C., but at 

 44 all movement ceases, though it will be resumed when 

 the temperature is lowered.]: The temperature at which the 

 protoplasm circulates most rapidly in these plants is near 

 the optimum for most of the activities of the cell. 



INFLUENCE OF WATER 



A sufficient quantity of water is a necessary condition 

 for active life (p. 6 ) . Water is also an essential component 

 of the living structure, protoplasm (p. 7). Given the 

 minimum amount of water for the erection of the living 

 structure after a period of inaction, the organism will re- 

 sume its functions. The germination of seeds and spores 

 shows this. Increase in the amount of water from the 

 minimum to the optimum is followed by increase in all the 

 vital activities. This is consistent with chemical and physi- 

 cal experience in the laboratory, a minimum quantity of 

 water being indispensable to many reactions, a larger ( opti- 



* Stahl, E. Zur Biologic der Myxomyceten. Bot. Zeitung. 1884. 



f Strasburger, E. Wirkung des Lichtes und der Warme auf Schwarm- 

 sporen. Jenaischer Zeitschrift, 1878. 



% Velten. W. Einwirkung der Temperatur auf die Protoplasmabewegung. 

 Flora, 1876. 



