380 



THE MICROSCOPE AND ITS REVELATIONS. 



In the epiderm of Yucca, however, the opening is bounded by two pairs* 

 of cells, and is somewhat quadrangular (Fig. 268); and a like doubling 

 of the boundary-cells, with a narrow slit between them, is seen in the 

 epiderm of the Indian corn (Fig. 269). In the stomata of no Phanero- 

 gam, however, do we meet with any conformation at all to be compared 

 in complexity with that which has been described in the humble Mar- 

 chantia ( 332). Stomata are usually found most abundantly (and 

 sometimes exclusively) in the epiderm of the lower surfaces of leaves, 

 where they open into the air-chambers that are left in the parenchyma, 

 which lies next the inferior epiderm; in leaves which float on the surface 

 of water, however, they are found in the epiderm of the upper surface- 

 only; whilst in leaves that habitually live entirely submerged, as there is> 



FIG. 273. 



Fie. 271.* 



Portion of Epidermis of Leaf of Iris ger- 

 manica. torn from its surface, and carrying 

 away with it a portion of the parenchymat- 

 ous layer in immediate contact with it: a, 

 a, elongated cells of the cuticle; 6, 6, cells 

 of the stomata c, c, cells of the parenchy- 

 ma ;d, d. impressions on the epidermic cells 

 formed by their contact; e, e, cavities in the 

 parenchyma, corresponding to the stomata. 



Vertical section of Epidermis and of portion of 

 subjacent parenchyma of leaf of Iris germanica. 

 taken in a transverse direction: a, a, cells of 

 epiderm; 6, 6, cells at the sides of the stomata; r, 

 c, small green cells placed within these; d, d, 

 openings of the stomata; e, e, cavities in the- 

 parenchyma into which the stomata open; /, / 

 cells of the parenchyma. 



no distinct epiderm, so there are no stomata. In the erect leaves of 

 Grasses, the Iris tribe, etc., they are found equally (or nearly so) on both, 

 surfaces. As a general fact, they are least numerous in succulent plants, 

 whose moisture, obtained in a scanty supply, is destined to be retained 

 in the system; whilst they abound most in those which exhale fluid most 

 readily, and therefore absorb it most quickly. It has been estimated that 

 no fewer than 160,000 are contained in every square inch of the under 

 surface of the leaves of Hydrangea and of several other plants; the great- 

 est number seeming always to be present where the upper surface of the 

 leaves is entirely destitute of these organs. In Iris germanica, each sur- 

 face has nearly 12,000 stomata in every square inch, and in Yucca, each 

 surface has 40,000. In Oleander, Banksia, and some other plants, the 

 stomata do not open directly upon the lower surface of the epiderm, but 

 lie in the deepest part of little pits or depressions, which are excavated 

 in it and lined with hairs; the mouths of these pits, with the hairs that 

 line them, are well brought into view by taking a thin slice from the sur- 

 face of the epiderm with a sharp knife; but the form of the cavities and 

 the position of the stomata can only be well made-out in vertical sections 

 of the leaves. 



383. The internal structure of leaves is best brought into view by 

 making vertical sections, that shall traverse the two layers of epiderm 



