InfJuence of Moisture on Growth mid Distribution 121 



water and its loss of water will be reduced. In the tropics many 

 trees shed their leaves during the dry season. Other plants pass 

 periods of drought as seeds or as spores. Many lower animals form 

 cysts of one sort or another. Terrestrial mollusks close their shells 

 with a thin epiphragm of secreted mucus, and more highly organized 

 animals, such as the ground squirrels, go into a state of torpor in 

 their burrows. Dormancy of this sort occurring under conditions of 

 heat and drought is referred to as aestivation, and is found character- 

 istically in desert and tropical communities. Using various combina- 

 tions of the methods for procuring and conserving water discussed 

 above, plants and animals in arid habitats may live for months or 

 years without rain or access to free water. 



Influence of Moisture on Growth and Distribution 



The growth of plants is often more directly and obviously affected 

 by the water factor on land than is that of animals. The availability of 

 moisture influences not only the rate of plant growth but also the 

 growth form. Since the roots of most land species will not grow into 

 saturated soil nor into soil devoid of available water, plants will be 

 shallow rooted either if the water table is high or, in the contrasting 

 situation, if the soil water is limited to the uppermost layer. In the 

 latter case growth will be confined to the rainy season. The growth 

 of plants whose roots extend to the permanently moist subsoil are 

 largely independent of rain periodicity. 



Competition for water and the horizontal spread of roots often con- 

 trol the spacing of plants in regions where water shortage is critical. 

 The extent of the root system in a typical cactus is shown in Fig. 4.12. 

 The roots are seen to be spread widely just beneath the surface of the 

 soil where they can absorb whatever rain falls before it evaporates. 

 This cactus will capture all the available moisture within the area 

 shown and prevent any other perennial plant from gaining foothold 

 in close proximity. Desert vegetation is often spaced out in a strik- 

 ingly regular pattern as a result of this intense root warfare for water. 



The water factor also controls the geographical distribution of 

 plants on both small and large scales. Such trees as willows and cot- 

 tonwoods are characteristically limited to the moist banks of water 

 courses because their seeds will not survive more than a few days 

 unless the soil upon which they fall is wet. The seeds of other species 

 can remain dormant in a completely dry condition for months or years 

 but will germinate only when sufficient water is available in the soil 

 immediately surrounding them. In other instances the critical period 



