condition 



RELATION OF PLANTS TO ENVIRONMENT 



453 



* 



condition of the soil is very marked in alkaline areas where the 

 concentration of salt in the soil permits a very limited range of 

 species. So the physical and 

 mechanical conditions of the 

 soil influence plants because 

 the moisture content of the 

 ground is so closely depend- 

 ent on its physical condition. 

 Rocky and gravelly soil, other 

 things being equal, is dry. 

 Clay is more retentive of 

 moisture than sand, and mois- 

 ture also varies according to 

 the per cent of humus mixed 

 with sand, the humus increas- 

 ing the percentage of moisture 

 retained. 



616. Effects of cold and 

 frost. Intense cold prevents 



root absorption, and has a drying effect on vegetation, so that 

 vegetation 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. 



617. Effects of freezing. In freezing weather plants are 

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

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

 atures. Second. Others are killed even by comparatively light 

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

 herbs, young leaves and shoots of many plants). In these cases 

 the injury is chiefly caused by the loss of water from the pro- 

 toplasm 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 



Fig. 412. 



Main trunk straight, branches all bent and 

 fixed to one side by wind from one direction 

 (Rocky Mountains). 



