144 GENERAL BOTANY 



very little transpiration during rain or fog, when the humidity of 

 the air approaches 100 per cent, while the transpiration is less on 

 a moderate day, with the humidity at 70 per cent, than it is on 

 a dry day, with the humidity at 50 per cent. Air currents, by 

 removing the water as fast as it evaporates from the stomata, are 

 also important factors in causing excessive evaporation, espe- 

 cially when vegetation is parched by a combination of low humid- 

 ity and high winds. In this case the external coatings of hairs, 

 already mentioned, is an important factor in preventing loss of 

 water, since it helps to maintain a cushion of moist air over the 

 entire leaf surface, protected from evaporation by the hair layer. 



It is evident also that the amount of water available from 

 the soil may modify transpiration from the leaves through their 

 tendency to wilt as soon as evaporating overbalances absorp- 

 tion, and so to cause the closure of the stomata, with a conse- 

 quent check on transpiration. It is quite probable also that the 

 leaf cells have some control over their own loss of water in a 

 vital way, although the nature of this control is only indicated 

 by recent experiments, which need elaboration and confirmation. 



Water ascent. The path of water ascent has already been ex- 

 plained as occurring in the great water ducts which form a part 

 of the conducting and supporting vascular system of the plant. 

 The forces necessary to accomplish the task of lifting water from 

 the roots of tall trees to the crown can best be appreciated after 

 a brief statement of the volume of water transpired and the rate 

 at which it moves up the ducts in the wood of plant stems. The 

 volume of water exhaled from the leaves of ordinary plants is 

 indicated by the rate of transpiration from the leaves. Ganong 

 estimates that the average daily transpiration from a square meter 

 (10J square feet) of leaf surface is 50 grams per hour in daylight 

 and 10 grams per hour in darkness. A birch tree with 200,000 

 leaves is supposed to give off from 300 to 400 kilograms (from 

 660 to 880 pounds) of water on a hot day in summer. Sachs 

 estimates that a sunflower plant the height of a man would evap- 

 orate from 800 to 1000 cubic centimeters (about 1 quart) of 

 water from its leaves on an average July day. The rate at which 

 this water moves up the ducts varies in different plants, as the 



