28 THE STORY OF PLANT LIFE 



temperature is lowered, heat being lost when evapo- 

 ration takes place. Not only is the soil rendered 

 drier, but the plant is exposed to a drying process. 

 The stunting effect of the wind is seen on the sea- 

 coast or on hills. 



The effect of wind may be seen on the opposite 

 sides of a range of hills lying in or across the path of 

 the prevalent south-west winds. On the south-west 

 side the plants are exposed to the full force of the 

 wind, and more or less xerophytic ; on the north- 

 east side the vegetation is more luxuriant as a rule. 

 Rain is deposited on the east side of the hills, and 

 whilst the sunny side is the south-west the plants 

 are exposed to drying action and more sun than on 

 the north-east, where the winds are drier. 



{d) and (e) Light and Heat. 



We get light and heat from the sun. Without the 

 solar energy life would be to-day impossible, what- 

 ever may have been the conditions in the past, e. g. 

 in the Carboniferous period, when the huge arbores- 

 cent Horsetails, Club-mosses, etc., lived in an atmo- 

 sphere charged with humidity and a far greater 

 proportion presumably of carbonic acid gas than 

 there is to-day. But such conditions have to do 

 with the secular history of the earth with which we 

 are not here concerned, and of which, moreover, 

 little reliable information is available, save hypo- 

 thetically. 



