THE MOVEMENTS AND SENSITIVITY OF PLANTS 337 



"sleep of plants." This expression is inappropriate, as such 

 nyctinastic movements have nothing in common with the sleep of 

 animals, which is a peculiar condition of rest. 



Changes in temperature or the intensity of light, or both, are 

 the actuating causes of these movements. The first instance is 

 concerned with thermonasty; the second, with photonasty. The 

 opening of the tulip, or saffron, Crocus sativus, flowers, which may 

 be easily observed when these plants are carried from a cold into a 

 warm room, are typical instances of thermonasty. Under these 

 conditions the flowers will open in 3 to 5 min. (Fig. 138). The 



Fig. 138. — Thermonastic opening of saffron flowers (after Kerner). 



mechanism of this movement lies in the fact that with an increase 

 of temperature the growth of the inner surface, morphologically 

 the upper surface, overtakes the growth of the lower surface of the 

 petals, which then curve externally and the flower opens. When 

 the plant is left for a considerable period in a warm temperature, 

 however, then the growth of the lower side also will be accelerated, 

 and the flower will again close. 



The movements of petals of the water lily and of flowers of 

 the dandelion are good examples of photonasty. Changes in the 

 intensity of light make these plants close not only in the evenings, 

 but in dull weather as well (Fig. 139). It may be produced also by 



