HOW PLAINTS ABE INFLUENCED 357 



the "lay of the land" is very important for plants. 

 It often happens that one side of a valley yields good 

 crops, while the opposite side is valueless for cultiva- 

 tion. 



It is interesting to notice that during the day the 

 hillside (especially the southern and southwesterly 

 exposure) i^ warmer than the valley (except when a 

 cold wind blows) ; the same is true at night, for the 

 cold air, being heavier, sinks down into the valleys, 

 so that they often have frost at night when the hill- 

 sides above are exempt from it. 



Plants which grow in high altitudes have stunted 

 stems with limbs densely branched or hugging the 

 ground in the form of rosettes. These features have 

 been produced experimentally to a certain extent by 

 growing ordinary plants under favorable conditions 

 by day and keeping them in an ice-house during the 

 night. This imitates natural conditions, since the 

 plants in question are comparatively warm during the 

 day when the sun is shining, but cool off very rapidly 

 after sunset and remain cold during the night. 

 Normally the greatest growth takes place at night, 

 hence chilling them at this time explains, in part at 

 least, their stunted growth. 



Alpine plants have very commonly larger and 

 brighter colored flowers, which are also richer in 

 honey, so that the mountains are particularly good for 

 bee-keepers. The foliage of these plants shows many 



