LEAVES 



559 



level, the stomata of conifers agree with those of many xerophytes, and 

 are, perhaps, to be regarded as xerophytic stomata. The stoma-bear- 

 ing under surface of xerophytic leaves is often hairy, contrasting w^ith 

 the smooth upper surface (as in Antennaria or in Populus alba, fig. 820), 

 while in other cases hairs may develop only in pits (as in Neriunt, fig. 

 807) or in furrows. Similarly, wax deposits often are observed on stoma- 

 bearing surfaces, and waxy or resinous excretions may even clog up the 

 stomata. Occasionally intrusive growths, known as tyloses, due to the 

 bulging of adjoining cells into 

 the stomatal air cavity, develop 

 to such an extent as almost to 

 block up the air passages (fig. 

 803); especially is this the case 

 in xerophytic leaves. The 

 stomata of vernal herbs (as 

 Medeola) are likely to be less 

 protected than are those of 

 trees or of estival herbs (as 

 Achillea), the latter being more 

 like those of xerophytes. 



In most hydrophytes and in 

 some mesophytes (as in ferns) 

 the inner cutin ridges of the 

 guard cells are lacking; in a 

 few cases, even the median 

 ridges are absent. Rarely, ex- 



FiG. 803. — A cross section through a stoma 

 from the upper leaf surface of Pilea elegahs, 

 showing the closure of the stomatal passageway 

 by the protrusion of a mesophyll cell (w) into 

 the stomatal cavity (c); such outgrowths are 

 known as tyloses ; note the great thickening of 

 the wall (w) just beneath the stoma; s, guard 

 cells; p, chlorenchyma cells with chloroplasts 

 (d) and nuclei (n) ; highly magnified. — From 

 Habeelandt. 



cept in some hydrophytes, are the stomata lifted above the epidermal 

 level. The stomata of cotyledons are relatively uniform in structure, 

 thus seeming to correspond with the uniform conditions under which 

 cotyledons are developed. 



The influence of external factors upon the structure of stomata. — A slight 

 decrease in the size of the guard cells and a slight increase in the cutinization of 

 their walls, when developing stomata are exposed to dry air or to other xerophytic 

 conditions, are about the only changes that have been experimentally induced in 

 the structure of stomata, a fact that seems remarkable in the light of the extreme 

 plasticity of other superficial cells and tissues. The extensive habitat variations 

 above noted make the structural rigidity of stomata all the more extraordinary. As 

 appears from the following paragraphs, stomata often can be induced or inhibited 

 at will, but their structure, when present, seems fixed; however, the paucity of ex- 

 perimental data makes any general conclusion hazardous. 



