Jan., 1920] Resistance of Leaves to Transpiration 75 



5. The removal of the hairs sHghtly decreased the resistance 

 of the leaves to water loss in wind and light as compared to still 

 air and light, because the cuticular surface is more exposed to 

 the air. 



6. The removal of the hairs greatly decreased the resistance 

 of the leaves to water loss in still air and darkness as compared 

 with still air and light. This is due to the greater exposure of 

 the cuticular surface. In darkness the stomata are closed and 

 transpiration is almost entirely cuticular. 



7. Hairs as a protective covering against ordinary intensities 

 of wind and light on mullein leaves may be disregarded. The 

 water loss from the leaves is mostly from the internal (mesophyll) 

 surface of the leaves. The internal water loss is from twenty to 

 forty times greater than the external or cuticular water loss 

 (on the basis of increased transpiration due to stomatal openings 

 minus increased evaporation caused by increased environmental 

 factors) . The removal of the hairs increases total transpiration 

 only to the extent that the cuticular surface is more exposed 

 and has apparently no effect on stomatal transpiration. 



LITERATURE CITED. 



1. Warming: Ecology of Plants. 1909. 



2. Jost: Plant Physiology. 1917. 



3. Gager: Fundamentals of Botany. 1916. 



4. Coulter, Barnes, and Cowles: Text Book of Botany. 1910. 



5. Wiegand, K. M. The Relation of Hairy and Cutinized Coverings to Transpira- 



tion. Bot. Gaz. 49 : 430-444. f. 1. 1910. 

 G. Kerner: The Natural History of Plants. 1893. 



7. Vesque, M. J. et Viet, M. C. Die L' influence du milieu sur la structure 



anatomique des Vegetaux. Ann. Sci. Nat. Bot. VI. 12 : 176. 1881. 



8. Brenner, M. Die Transpiration de Pfianzen. 1904. 



9. Transeau, E. N. x\pparatus for the study of Comparative Transpiration. 



Bot. Gaz. 52 : 54-60. f. 1-5. 1911. 

 10. Livingston, B. E. A Rotating Table for Standardizing Porous Cups Atmom- 

 eters. Plant World 15 : 157-162. 1912. 



