98 



PLANT PHYSIOLOGY AND ECOLOGY 



may be produced either by temperatures that are more or less 

 constantly too low, or by shortness of season, which is equally 

 effective in reducing the total amount of heat available for the 

 uses of the plant. As a rule, these two factors act in unison, 

 producing marked reduction in size. Such reduction is character- 

 istic of the vegetation of alpine regions, although the dwarf habit 

 of alpine plants is due chiefly to adjustment to water. The gen- 



FiG. 26. Alpine mats of Sileae acaulia growing on the north side of a rock. 

 The effect of different tenipeiaLure sums is shown by the number of 

 flowers. 



eral effect of low temperatures may often be seen in field crops 

 during seasons in which the temperatvires are largely below the 

 normal. This effect may readily be demonstrated by growing 

 seedlings of the same species in warm and cold compartments of a 

 plant-house. The resistance of plants to low temperatures, espe- 

 cially those of winter, depends upon the relation between the 

 available water, which is reduced by freezing, and transpiration^ 

 rather than upon the actual cold. 



120. Influence upon vegetation. Temperature has no effect 

 upon the movement of plants, i.e., migration into new habitats^ 

 but it has a profound influence upon the establishment, or ecesis, 



