LEAF STRUCTURE AND TRANSPIRATION 203 



winds. In the former case it has been supposed that the whitish hairs reflect 

 such a large proportion of the incident light that the leaf temperature is not 

 as high as it otherwise would be, and that the rate of transpiration is corre- 

 spondingly lower. In the latter case it has been supposed that the hairs impose 

 a mechanical barrier to the effect of wind on transpiration. However, Sayre 

 (1920) found that removal of the hairs from the leaves of the mullein {Ver- 

 bascum thapsus) had little effect upon the rate of stomatal transpiration under 

 ordinary conditions of light intensity and wind velocity, although such treat- 

 ment did result in an increase in cuticular transpiration. 



3. Ratio of Internal to External Surface in Leaves. — Although trans- 

 piration rates are most frequently expressed in terms of the exposed area 

 of the leaf surface, most of the water-vapor lost from leaves evaporates from 

 the walls of the mesophyll cells which bound the intercellular spaces. The 

 area of this internal evaporating surface in proportion to the external surface 

 of a leaf varies greatly not only in leaves of one species as compared to those 

 of another, but in leaves of the same species if they have developed under 

 different environmental conditions. Computations of the ratio of internal 

 exposed surface to external exposed surface of leaves of a number of species 

 have yielded values ranging from 6.8 to 31.3 (Turrell, 1936). In general 

 the greater the proportion of palisade to sponge tissue the greater this ratio. 

 The thickness of the leaf will also obviously be a factor influencing this ratio. 

 The ratio is greater in sun leaves than in shade leaves of the same species; 

 for the lilac the values are 13.2 and 6.8, respectively. 



It has generally been supposed that the transpiration rates of leaves in 

 which a relatively large area of mesophyll cell walls is exposed to the inter- 

 cellular spaces as compared with the exposed epidermal area of the leaf are 

 greater than in leaves in which the contrary condition obtains. Theoretically 

 such an effect could be exerted only if the area of exposed mesophyll cell walls 

 exercised a controlling influence upon the steepness of the vapor pressure 

 gradient through the stomates. When transpiration is relatively high a steeper 

 vapor pressure gradient may be maintained by leaves possessing relatively ex- 

 tensive internal evaporating surfaces than by those which do not. The higher 

 transpiration rate of sun leaves as compared with shade leaves on the same 

 plant, which has been observed in some species, may be partly explainable on 

 this basis. 



4. Sunken Stomates. — The stomates of many species of plants are sunken 

 below the general level of the epidermis (Fig. 42, D) . Since the length of 

 a diffusion gradient is one of the factors governing its steepness, diffusion 

 through a small opening is slower if the gas must pass through a relatively 

 long tube before reaching the orifice, than if the diffusion is through a shorter 



