8 



THE PHYSIOLOGY OF STOMATA. 



much more frequently to an increase of water in the soil. As many as five 

 leaf coverings have been formed during the period from February to August, 

 three of which were induced by irrigation (Cannon, 1905). From these facts 

 it will be appreciated that this plant is a marked desert type, and on this 

 ground it was believed that the study of the stomata and their relation to 

 the loss of water by the plant would be of profit. 



Aside from other features which may be at present disregarded the one per- 

 tinent to the matter in hand is the anatomical structure of the leaves (fig. 1). 

 They are 0.3 mm. thick, small, obovate in form, 2 to 3 (or 4) cm. long, rather 

 light green, and slightly glaucous. The cuticle is not thick, and, save for the 



isolateral structure, the leaves do 

 not at all strikingly suggest the des- 

 ert type of foliage (plate 2, fig. 3). 

 The chlorenchyma has above and 

 below a relatively thick palisade 

 of, in each case, a single row of 

 deep, columnar cells. Between 

 them lies a spongy chlorenchyma, 

 one-seventh of the total leaf thick- 

 ness, of rounded cells, with small 

 intercellular spaces. 



The epidermis is of a single 

 layer of cells, 30 to 45 micra deep, 

 and of simple tangential outline, 

 the cuticle being more or less read- 

 ily corrugated from the stomata 

 as a center.* The cuticle is not 

 at all remarkable, since its thick- 

 ness is no greater than that on 

 the leaf of many mesophytes. A 

 small amount of "bloom" is present, giving a slightly glaucous appearance 

 to the leaf. 



The stomata are of very simple character, as seen in fig. 3. The only 

 structural feature which marks it as a supposedly desert type is the some- 

 what marked outer ridge. There is no inner ridge. When young the dorsal 

 and ventral walls are not markedly thickened, but become very much so with 

 age, and as the secondary thickening proceeds the mobility of the organ is 

 reduced. The lumen, as a result, becomes reduced to a thin space for the 

 inner half of the guard-cells, but remains approximately cylindrical in the 

 outer half.f The outer wall remains thin, and by its stretching, when the 



*A not uncommon character; e. g., it appears in Ampelopsis. 

 firmer, i. e., adjacent to the pore; outer, adjacent to the epidermis. 



Fig. 1 . — Transverse section of portion of leaf of 



Fouquieria splendens. 



