gg MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



species of the same genus in dry sites, e. g., Populus nigra had 135 

 stomata per square millimeter, while P. alba had 315. 



Brenner (21) found that leaves of Quercus pedunculata on the drier 

 sites of southern Europe had considerably fewer stomata per unit 

 area than those on the moister sites of central Europe. 



Hirano (96), in studying the water relations of stomata in about 

 40 varieties of Citrus in California, found that the number of stomata 

 was more closely related to the rainfall during the spring months when 

 the leaves were developing than to the total annual rainfall or to light 

 and temperature. The hardier varieties, it should be noted, also were 

 characterized by a low stomatal density. 



Although Eberhardt (54) found in Salix and Populus that as the 

 relative humidity increased the stomata became fewer but larger, most 

 workers have found an increase in stomata in moist regions, all other 

 factors being equal. We have learned, however, that sun leaves fre- 

 quently have more stomata than shade leaves if there is an abundant 

 moisture supply. In considerations of stomatal numbers, as else- 

 where, simultaneous consideration of temperature, water, and light is 

 important. No one factor in silviculture can be considered without 

 taking into account the other two. 



Changes in number of stomata in dry regions are frequently associ- 

 ated with changes in structure. In conifers, the stomata are very 

 frequently clogged up with waxy granules similar to those on the 

 epidermis. Wulff (244) noted also in Betula alba that where this tree 

 grows as a shrub on the dry, wind-blown sands of Spitsbergen, the 

 stomata on the under side were filled with large, shiny, brown granules 

 of wax, while the leaves of similar species in the botanical garden at 

 Lund showed no wax deposits. The stomata may also be plugged up 

 by the adjoining parenchyma cells, which sometimes grow inward in 

 such a fashion as to close the stomata; this has been noted in old leaves 

 of Camellia japonica L. and Prunus laurocerasus L. by Schwendener 

 (187). 



CONTROL BY STOMATA 



The stomata are of prime importance in controlling transpiration, 

 however, through the movements of their guard cells. When condi- 

 tions are favorable for photosynthesis and there is plenty of light, 

 moisture, and C0 2 , the guard cells, which also contain chlorophyll, 

 manufacture carbohydrates; and the increased osmotic pressure re- 

 sults, owing to their peculiar morphology, in a wider opening between 

 them. This permits C0 2 to enter easily to the mesophyll cells below 

 and at the same time permits water vapor to pass outward. If any of 

 the conditions for optimum photosynthesis are lacking, such as insuffi- 

 cient light or water, the stomata then tend to close, serving somewhat 

 as automatic regulators. This was the accepted view for many years, 

 but in 1908 Lloyd (139) published a comprehensive monograph in 

 which he questioned the regulatory function of the stomata. He 

 worked chiefly on desert plants, and furthermore his methods have 

 been questioned by Loftfleld (140) and others, so that his attempt to 

 discredit stomatal regulation of transpiration in mesophytes has met 

 with a rather hostile reception; the conclusion at the present time 

 seems to be that while the stomata perhaps do not regulate the trans- 

 piration so accurately or completely as was once supposed, they are 

 able to regulate water losses to an appreciable and valuable extent. 



