7l6 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS 



there is no distinct epiderm, so there are no stomates. In the erect 

 leaves of grasses, the Iris tribe, ivc., they arc 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 lias been estimated that no fewer than 160.000 are con- 

 tained in every square inch of the under surface of the leaves of 

 /ft/drangea and of several other plants, the greatest number seem- 

 ing always to be present where the upper surface of the leaves is 

 entirely destitute of these organs. In Iris y?.niifinic each surface 

 has nearly 12,000 stomates in every square inch ; and in Yucca each 

 surface has 40,000. In the oleander, Jltwksia, and some other plants, 

 the stomates 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 surface of the epiderm with a sharp 

 knife ; but the form of the cavities and the position of the stomates 

 can only be well made out in vertical sections of the leaves. 



The internal structure of Leaves is best brought into view by 

 making vertical sections, traversing the two layers of epiderm and 

 the intermediate cellular parenchyme ; portions of such sections are 

 shown in figs. 560, 562, and 563. In close apposition with the cells 



of the upper epiderm (fig. 

 562, a, ), which may or may 

 not be perforated with the 

 stomates (c, c, d, d"), we find a 

 layer of soft, thin- walled cells, 

 with their longest diameter 

 at right angles to the surface 

 of the leaf, and containing 

 a large quantity of chloro- 

 phyll ; these generally pi-ess 

 so closely one against an- 

 other that their sides be- 

 come mutually flattened; and 

 no spaces are left, save where 

 there is a definite air-chamber 

 into which the stomate opens 

 (fig. 562, e) ; a,nd the com- 

 pactness of this superficial layer is well seen when, as often happens, it 

 adheres so closely to the epiderm as to be carried away with this when 

 it is torn off (fig. 561 , c, c). This layer, usually peculiar to the upper 

 surface of lea\cs. is known as the palisade-'parenchyme. Beneath 

 this first layer of leaf-cells there are usually several others rather 

 less compactly arranged ; and the tissue gradually becomes more 

 and more lax. its cells not being in close apposition, and large inter- 

 cellular passages being left amongst them, until we reach the lower 

 cpiderm. which the parenchyme only touches at certain points, its 

 lowest layer forming a sort of network, the so-called spoixjy paren- 



FIG. 562. Vertical section of epiderm and of 

 portion of subjacent parenchyme of leaf of 

 Iris germanica taken in a transverse direc- 

 tion : H, a, rellsof epiderm; b, b, cells at the 

 sides of the stomates; c, c, guard-cells; d, d, 

 openings of the stomates; c, e, cavities in the 

 parenchyme into which the stomates open ; 

 f,f, cells of the parenchyme. 



