BOTANICAL GEOGRAPHY 



331 



trees on ascending a mountain is well shown in Fig. 234. 

 The treeless character of the mountain summit is also 

 plain. 1 



Recent experiments have shown that many ordinary 

 plants promptly take on alpine characteristics when they 

 are transferred to moderate heights on mountains. For 

 instance, a rather 

 commonly culti- 

 vated sunflower, 2 

 when planted at a 

 height of about six 

 thousand five hun- 

 dred feet, instead 

 of having a tall 

 leafy stem pro- 

 duces a rosette of 

 very hairy leaves 

 lying close to the 

 ground, thus be- 

 coming almost un- 

 recognizable as a 

 sunflower. The 

 change was even 

 greater than that 

 shown in the rock 

 rose (Fig. 235) cultivated by the same experimenter. The 

 peculiarities of alpine plants appear to be due mainly to 

 the intense light which they receive during the daytime, 



1 Part of the diminution is only apparent, the effect of distance, but the 

 growth at the highest levels is often less than waist high. 



2 Helianthus tuberosus, the so-called Jerusalem artichoke. 



FIG. 235. Two Plants of Rock Rose (Helianthemum). 



(Both drawn to the same scale.) 

 A, low ground form ; B, alpine form. 



