THERMOTROPISM OF LEAVES 97 



of species living in alpine climates are very marked. The ex- 

 tremes of temperature are the tests for endurance of the proto- 

 plasts, but any species must receive a specific amount of heat or 

 a certain amount of radiant energy in the form of heat in order 

 to carry out the seasonal. activity. It is this that determines the 

 continued existence of a plant in any locality. These thermal 

 constants are found by adding the daily maxima during the vege- 

 tative season of the species. 1 



151. Thermotropism. The free moving organisms of the animal 

 kingdom show very marked movements in response to changes 

 of temperature both as to movements of organs, and locomotion 

 which will place their bodies in an optimum intensity of the radi- 

 ations. This capacity is shared by free swimming spores of plants 

 to some extent, while shoots, roots and secondary organs exhibit 

 curvatures in some instances toward the source of heat, or to 

 place their surfaces in such position as to decrease harmful radi- 

 ations during periods of low temperature. The effects of changes 

 of temperature are so intimately connected with the adaptive re- 

 actions to light, which are so regularly recurrent as to have 

 become rhythmic, that it is difficult to distinguish purely ther- 

 motropic movements. 



152. Thermotropism of leaves. The leaves of a number of 

 woody plants assume a drooping .position at temperatures under 

 the freezing point, and recover when the thermometer indicates 

 a point much above that. This movement is shown by the laurel 

 {Prunus laurocerasus), Portugal laurel {Prunus Lusitanica) and 

 may also be seen in branches of Tilia during the first frost of the 

 season. 2 



A very striking example of this action is shown by the great 

 laurel, or rosebay (Rhododendrum maximum), which is found 

 over the eastern United States. If specimens of this plant are ex- 

 amined after the temperature has fallen below the freezing point, 

 it may be seen that the leaves are of a deep green color with a 



1 Kerner. Natural History of Plants. I : 558. 1890. 



2 Darwin and Acton. Physiology of Plants, 163. 1894. 



