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INFLUENCE OF WATER ON LAND-PLANTS 35 
Summary of Xerophytie Factors. 
I. Causes which reduce Absorption, and so set up a 
state of physical or physiological dryness : 
1. A physically dry opt 
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). 
