INFLUENCE OP WATER ON LAND-PLANTS 35 



Summary of Xerophytic Factors. 



I. Causes which reduce Absorption, and so set up a 



state of physical or physiological dryness : 



1. A physically dry soil. 



2. A cold soil. 



3. A salt soil. 



4. A sour soil. 



II. Causes which increase Transpiration, and so set up 



a condition in which the loss of water tends to outrun 

 the supply : 



(a) Physical factors : 



1. A dry atmosphere. 



2. A high temperature. 



3. Wind. 



4. Rarefaction of the atmosphere. 



(6) Physiological factor : 



1. Intense illumination. 



Natural Regions where Physical or Physiological Dryness 



prevails. 



1. Deserts (lack of water, dry air, intense illumina- 

 tion). 



2. Steppes and prairies (soil dry and hot in summer, 

 air dry, intense illumination, intense heat, especially at 

 noon). 



3. Rocks and stones (lack of water). 



4. Sandy and gravelly soils (lack of water). 



5. Chalk - downs (exposed to wind chalk is porous, 

 and therefore apt to become very dry). 



6. Bark of trees (lack of water). 



7. Salty seaside - soils, salt swamps and marshes 

 (presence of salt). 



8. Peat-bogs (presence of souring acids). 



9. Polar regions (cold). 



10. Alpine regions (cold soil, wind, intense illumination, 

 rarefied air). 



11. Wind-swept, exposed situations (drying winds, 

 intense illumination). 



