422 



STOMATA. STRUCTURE OF LEAVES. 



Fig. 227. 



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 greatest 

 number seeming always to present itself in species, the upper 

 surface of whose leaves is entirely destitute of these organs. In 

 Iris germanica, each surface has nearly 12,000 stomata in every 

 square inch ; and in Yucca, each surface has 40,000. — In Ole- 

 ander, Banksia, and some other plants, the Stomata do not open 

 directly upon the lower surface of the Cuticle, 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 Cuticle 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. 



314. The Internal Structure of Leaves is best brought into view 

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

 Cuticle and the intermediate Cellular Parenchyma ; portions of 

 such sections are shown in Figs. 225, 227, and 228. In close 



apposition with 

 the cells of the 

 upper Cuticle 

 (Fig. 227, a, a), 

 which may or 

 may not be per- 

 forated with Sto- 

 mata (c, c, d, d), 

 we find a layer of 

 soft thin-walled 

 Cells, containing 

 a large quantity 

 of Chlorophyll ; 

 these generally 

 press so closely 

 one against ano- 

 ther, that their 

 sides become mu- 

 tually flattened, 

 and no spaces are 

 left, save where 

 there is a definite 

 i • -i ,i a ,„. Air-chamber into 



which the Stoma opens (Fig. 227, e) ; and the compactness of this 

 superficial layer is well seen, when, as often happens, it adheres bo 

 closely to the Cuticle as to be carried away with this when it is 

 torn off (Fig. 226, c, c). Beneath this first layer of Leaf -Cells, 



Vertical section of the Cuticle, and of a portion of 

 the subjacent Parenchyma, of a Leaf of Iris germa- 

 nica, taken in a transverse direction :— a, a, Cells of 

 the Cuticle ; b, b, Cells at the sides of the Stomata ; 

 c, c, small green Cells placed within these ; <l, ,i 

 openings of the Stomata ; e, e, lacuna? of the Paren- 

 chyma into which the Stomata open ; /, /, Cells of 

 the Parenchyma. 



