96 BOTANICAL GAZETTE [February 



lacunae. Such leaves need fewer and are provided with a smaller 

 number of stomata. Stomata are not needed for transpiration, since 

 transpiration is believed to be a necessary evil. It seems strange, 

 therefore, that in xerophytic leaves, where there is effected the 

 greatest protection against the loss of water by the development of 

 a thick cuticle and a thick outer epidermal wall, there should be 

 the development of a large number of stomata, thus facilitating the 

 loss of water from the plant through stoma tal transpiration. 



In leaves whose outer epidermal wall and cuticle are thin, there 

 is less need of stomata to facilitate exchange of gases in photo- 

 synthesis and respiration, since under such circumstances consider- 

 able interchange of gases can take place through the epidermis. 

 There is no doubt that mesophytic rosette leaves with a reduced 

 number of stomata have an ample supply of stomata to meet their 

 needs. Moreover, rosette leaves are close to the soil and are there- 

 fore more advantageously situated for the intake of carbon dioxide 

 than are stem leaves. In stem leaves the pressure of C0 2 cannot 

 accumulate beyond 0.0003 A, or about o. 22 mm. Hg, since above 

 this pressure it diffuses outward. But in rosette leaves close to the 

 ground, where the exhalation of C0 2 from the soil often increases 

 the C0 2 to 10 or more times the normal amount, a much higher 

 pressure of C0 2 may accumulate. This increased amount of C0 2 

 in rosette leaves is available for carbohydrate synthesis in all cases 

 where the leaves are not too much shaded. But since plants under 

 normal conditions receive much more energy of sunlight (about 

 4 or 5 times as much) than is necessary to synthesize the small 

 amount of available C0 2 , rosette leaves are most advantageously 

 situated for photosynthesis in spite of the reduced number of sto- 

 mata and the diminished amount of light. These facts have an 

 important bearing upon the development of chlorenchyma and 

 air spaces in rosette leaves. 





Epidermal cells 



In monocotyledonous plants the epidermal cells are usually 

 elongated. In dicotyledonous plants they are generally elongated 

 along the ribs and larger veins, but elsewhere they may be polygo- 

 nal and nearly isodiametric in outline, or entirely irregular. The 



