18 PLANT SUCCESSION AND CROP PRODUCTION 



cient for growth so that as with temperature, light is not of su- 

 preme importance in plant distribution. There are few if any 

 places upon the surface of the earth where the light and tempera- 

 ture requirements of plants are not satisfied at least during some 

 season of the year. Moisture, therefore, remains the important 

 factor in detennining plant distribution. 



The importance of water as compared with light and tempera- 

 ture is easily shown. Our climatic zones, torrid, temperate, and arc- 

 tic, are east and west in their bearing, and one crosses them in a 

 journey from the equator to either pole. Desert and arid regions 

 are by no means restricted to any particular climatic zone. Desert 

 and arid regions are the extreme, but we may have dry, grassy 

 plains extending far northward beyond the tree formations. Look- 

 ing at it another way, we can see that water is irregularly distrib- 

 uted on the earth's surface, while temperature and especially light 

 tend to be somewhat regularly distributed. The water is the un- 

 balanced factor and is therefore the limiting factor. It is the unequal 

 distribution of water, both in amount and during a given period 

 of time, which is the cause of our distribution of various types of 

 vegetation. In order to fully understand this, the variations of cli- 

 mate should be examined in detail. 



The Relation of Climate to Plant Distribution 



Although the sun's rays strike the different parts of the earth's 

 surface in about equal amounts, yet the total energy derived from 

 them is quite unlike in different regions. What then happens to 

 prevent equal distribution of energy? 



In the first place, the land surfaces and the water surfaces 

 absorb heat at different rates. If we call the specific heat of the 

 water surface 1, then the specific heat of the land surface is ap- 

 proximately 4, for the land surface takes up and again gives oflf 

 heat about four times as fast as the water. Large land masses, 

 e.g., continents, have dry regions toward their centers where the 

 summer temperatures are extremely high and the winter tempera- 

 tures extremely low. The day and night temperatures also fluctu- 

 ate greatly. The loss of water from the centers of continents by 

 evaporation tends to intensify the variability of the climate. The 

 sea surface, on the other hand, or a small island in the center of 

 the ocean, has, at sea level, an equable climate in which the sum- 

 mer extremes and the winter extremes are much closer together 



