IRRITABILITY 221 



Francisco, in California, for there the respective minimum 

 and maximum temperatures of these forms are never at- 

 tained. The optimum temperatures for most plants in this 

 region are maintained for only very short times, owing to 

 the great daily range in temperature, but are frequently 

 repeated, owing to the slight seasonal ranges. The frequent 

 occurrence of optimum temperatures may more than com- 

 pensate for their brief duration — a matter, however, which 

 requires proof. 



Movements and locomotion, as well as the rate of growth, 

 are more or less controlled by heat. Thermotropism and 

 thermotaxis are the irritable responses to the stimulus 

 of heat. A certain degree of warmth will induce positive 

 thermotropism or thermotaxis — respectively, growth and 

 locomotion towards the source of heat — while a higher or 

 lower degree of heat may induce the opposite, negative 

 thermotropism or thermotaxis. Roots, for instance, grow 

 toward but not into contact with steam or hot-water 

 pipes passing through the soil between buildings. Roots of 

 Indian Corn* will grow toward the source of heat till they 

 reach a point where the temperature is 37.5° C. They will 

 bend and grow away from any warmer part of the soil. 

 Ordinarily in nature, however, thermotropic sensitiveness is 

 of little advantage to the plant. There is no differentiation 

 into sensory and motor zones, the whole growing region 

 being equally sensitive. A certain degree of warmth is re- 

 quired in order that the organism may be sensitive to other 

 influences. Thus heat must arouse the parasitic dodder 

 ( Cuscuta) to a certain degree of sensitiveness else it will not 

 be able to respond to the contact stimulus (see pp. 244-5) 

 upon which its attachment to the host absolutely depends, f 



The sensitiveness and the response of motile organisms to 

 heat is manifestly important. The sensitiveness to heat 



The organs especially active in absorbing nutrient aqueous 



* Wortmann, J. Uber den Einfluss der strahlenden Warme auf wachsende 

 Pflanzentheile. Bot. Zeltung, 1883. Uber den Thermotropismus der Wurzel. 

 Bot. Zeitung, 1885. 



fPeirce, G. J. A contribution to the physiology of the Genus Cuscuta. 

 Annals of Bot., Vol. VIII., 1894. 



