May, 1919] Transpiration of Tobacco and Mullein 423 



When compared with another individual of the same species, 

 the removal of the hairs from mullein leaves produced the 

 following effects: 



4. The removal of the hairs from the mullein leaves does 

 not alter the resistance of the leaves to water loss in still air 

 and light. 



5. The removal of the hairs slightly decreases the resistance 

 of the leaves to 'water loss in wind and light when compared 

 with still air and light because the cuticular surface is more 

 exposed to the air. 



6. The removal of the hairs greatly decreases the resistance 

 of the water loss in still air and darkness as compared with still 

 air and light, due to the cuticular surface being more exposed, 

 because the water loss in darkness is entirely from the external 

 surface, the stomata being closed. 



7. Hairs as "protective" covering against ordinary intensi- 

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

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

 surface of the leaves and not from the external hairy surface. 

 The internal water loss is from twenty to forty times greater 

 than the external or cuticular water loss. 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. 



A record of the size of the pore openings of the stomata was 

 obtained by measuring camera lucida drawings of the pores. 

 The drawings were made from small pieces of epidermis from 

 a plant which was under the same conditions as those used in the 

 measurement of the water loss. Small pieces of the epidermis 

 were stripped from the under side of the leaf and immediately 

 put in absolute alcohol. Samples of the epidermis for different 

 hours of the day were treated in this way, and later mounted 

 in absolute alcohol and a representative number of the pore 

 openings quickly drawn on white paper under the high power 

 of a microscope with the aid of a camera lucida. These drawings 

 were measured and the figures reduced to their actual size in 

 microns. The results were expressed both in terms of the total 

 area, (square microns) and in terms of the length of the peri- 

 pheries (microns) of the pore openings. Curves of these values 

 were then plotted for the day on cross section paper. 



