Jan., 1920] Resistance of Leaves to Transpiration 57 



increased, hairy coverings increased in density also. Brenner (8) 

 found that in certain species of Quercus hairy coverings became 

 thicker when exposed to greater intensity of sunlight. " Wiegand 

 gives the following statements concerning his own work: 



"It is quite generally recognized that by far the most 

 important factor in transpiration is evaporation; indeed, we 

 may say that transpiration is really evaporation modified and 

 regulated by the plant. Therefore, it seems that a study of 

 the relation of hairy coverings to evaporation would throw 

 much light on the subject of transpiration. Good quality 

 blotting paper was used as the evaporating surface. The loss 

 of water was determined by weighing. Cotton made water- 

 proof by paraffin in gasoline, was used for the protective cover- 

 ing and an electric fan was used to produce wind." He draws 

 the following general conclusions from his experimentation. 

 "The evaporation experiments tend to show that porous 

 coverings like cotton, wool, or hair must be very thick to 

 produce any appreciable effect in retarding evaporation if 

 the surrounding atmosphere is quiet, but become very efficient 

 even in thin layers when the air is in motion. It seems probable 

 that those plants that live in situations where a moderate water 

 supply is available, but there transpiration must be reduced in 

 excessively dry times, but not interfered with when the sur- 

 rounding air is damp and transpiration therefore difficult 

 employ a hairy covering to retard transpiration." 



These conclusions were drawn from evaporation experiments 

 in which cotton, wool and hair were used for protective cover- 

 ings, and the water loss from a standard evaporating surface 

 was measured. It is quite definitely known that very little 

 water is lost from the cuticular surface of a leaf as compared 

 with the internal surface, and while it is obvious that cotton, 

 wool, or hair would produce those effects on a standard evapo- 

 rating surface, it does not mean that the water vapor from the 

 internal surfaces of the leaf would be retarded by these coverings. 



■ In conclusion it may be said that very little critical experi- 

 mentation has been done on the relation of hairy coverings to 

 transpiration. The general opinion is that leaves with hairy 

 coverings have a greater resistance to water loss in bright 

 sunshine and in wind than leaves without these coverings. 



