WATER AS FACTOR IN DISTRIBUTION OF PLANTS 165 



less effective than a much smaller total precipitation which falls in 

 such a way as to be largely absorbed by the soil. Tlie frequency of 

 effecti^•e rains and the seasonal distribution of the rainfall are, there- 

 fore, usually more important than the amount of the annual pre- 

 cipitation. 



In general, forests occur in regions where there is adequate rainfall 

 during all seasons of the year. There must also be a warm vegetative 

 season and, if there is a winter the atmosphere must be reasonably 

 moist in order that transpiration may be reduced, since the trees are 

 always exposed to transpiration and they cannot absorb water from a 

 frozen soil. In regions of high winter rainfall and low summer rain- 

 fall, such as occurs in parts of southern California, the scleroph\llous 

 type of forest, composed of low trees or shrubs with broad, sclero- 

 phyllous, evergreen leaves, is found (Fig. 72). 



Grasslands occur chiefly in regions of high summer rainfall and 

 low winter rainfall, while deserts occur where there is low rainfall 

 both winter and summer. If we represent the summer season by the 

 letter S and the winter by W, using a large letter for a high rainfall 

 and a small letter for a low rainfall in each case, the amounts of 

 rainfall needed by the different types of vegetation may be repre- 

 sented symbolically as follows : 



SW-ordinary forest. Sw-grassland. 

 sW-sclerophyllous forest, sw-desert. 



It may be seen by the following diagram that there may be transi- 

 tion from one to another of the types in all cases except between 

 sclerophyllous forests and grasslands. 



*sW 



The influence of water on the distribution of the vegetation types 

 is well brought out by what we may call Schimper's Third Law. 

 This states that the type of vegetation in the tropical and temperate 

 zones is determined by the amount and seasonal distribution of the 

 rainfall and bv the humiditv of the air. 



