252 



PRACTICAL BOTANY. 



visions of nature to protect from too great radiation of 

 heat during the cool hours of the night. Darwin has 



shown that leaves 

 which are prevent- 

 ed from taking the 

 positions natural to 

 them are killed or 

 seriously injured on 

 chilly nights, while 

 other leaves on the 

 same plant do not 

 suffer. This is but 

 a continuation of 

 the provision al- 

 ready noted under 



FIG. 179. Leaf of the Scarlet Ruiiner, Phassolus vr -L. 09 i 

 multiflora, in the sleep position. (After Sachs.) 'Dumber 6A, Wiier 



by nature provides 



that plants shall receive or retain an amount of heat suited 

 to their individual needs. 



ISO 



FIG. 180. Upper portion of the leaf stalk of the Scarlet Runner, showing the 

 motile organs of the leaflet. A, day position ; B, night position. (After 

 Sachs.) 



FIG. 181. C, cross section through the rigid portion of the stem of Scarlet 

 Kunner; D, cross section through a motile organ. (After Sachs.) 



