47 RELATION TO ENVIRONMENT. 



tion, assimilation, and growth, so that growth is much more lux- 

 uriant than in the cooler climate, but they fail to flower or seed 

 because they are not able to raise the cardinal point for these 

 processes. Such plants become acclimatized with reference to 

 certain functions only. So some plants taken from warm cli- 

 mates to cooler ones may grow well, but fail to flower or fruit 

 because they cannot lower the cardinal temperature-point for 

 these functions to meet the new conditions. Different species 

 vary in this respect. The same species also may possess the 

 power of raising or lowering the cardinal points of all its func- 

 tions to a remarkable degree, and this probably accounts for the 

 very wide north-and-south distribution of certain species. 



912. Protection against cold. Intense cold prevents root 

 absorption, and has a drying effect on vegetation, so that vege- 

 tation protects itself in several ways through the severe winter 

 in temperate and arctic climates. The deciduous habit of 

 trees and shrubs, the underground stem of perennial herbs, the 

 rosette habit of perennials, the bud-scales on winter shoots, and 

 the low stature of arctic and alpine plants are modifications in 

 response to the harmful effects of extreme cold. 



913. Effects of freezing. In freezing weather plants are 

 injured in three ways: ist. The chilling effect of cold is suffi- 

 cient to kill some plants which are very sensitive to low tem- 

 peratures. 2d. Others are killed even by comparatively light 

 frosts, or freezes (examples : potatoes, tomatoes, corn, many herbs, 

 you g leaves and shoots of many plants). In these cases t.ie injury 

 is chiefly caused by the loss cf water from the protoplasm in the 

 cells. As freezing takes place in the tissues the ice crystals are 

 usually not formed within the cells. But some of the water, 

 under the influence of the extreme cold, is gradually withdrawn 

 from the cells into the intercellular spaces where the ice crystals 

 are gradually formed. This is in reality a protection to the 

 protoplasm, since it becomes "drier" and thus more resistant to 

 cold. When the plants "thaw" out, if the protoplasm has not 

 been killed by the cold, this water may be absorbed by the proto- 

 plasm again, and the plant is not injured. Freezing, then, to a 



