ii6 l^OT ALTOGETHER ABOUT PLANTS 



Evaporation is a cooling process. The more it is 

 checked at the surface of our bodies, the more we feel the 

 heat. Why is it that evaporation cools the surface at 

 which it occurs? Remember the molecules. Molecules 

 are always in motion. Though the mass which they com- 

 pose may be stationary, the individual molecules are always 

 dancing about as much as their surroundings will let them. 

 Now these dancing molecules at the surface of a Hquid 

 tend to dance off into the air, especially if it is dry air. 

 This is the cause of evaporation. The warmest molecules 

 are the most vigorous dancers. They are the most likely 

 to fly off into the air, and, as they go, they take their heat 

 with them. This lowers the temperature of the whole 

 surface. If, on the contrary, the warmest particles do not 

 fly off the surface on account of the abundance of them 

 already in the air, and if the surface concerned is a human 

 skin, the person inside of it is sure to be uncomfortable. 

 This fact that evaporation causes loss of heat has been 

 proved to be of advantage to plants that are exposed to 

 high temperatures. It appears in some cases to prevent 

 the internal temperature from rising to a point which 

 seriously impairs the life processes. 



With plants it is rapid evaporation rather than slow 

 evaporation which is likely to cause trouble. In fact, 

 plants thrive in greatest abundance in tropical forests 

 where the humidity is always great and where evaporation 

 is much less than in drier climates. On bright dry days 

 of summer, especially during a drought, when the soil 

 as well as the air has become dry, many plants are likely 

 to suffer. Water evaporates from them more rapidly than 

 the roots are able to supply it. Cells lose their turgidity, 

 and this causes wilting. (See page 75.) The grass turns 



