68 



STRUCTURE OF LEAVES. 



136. The Epidermis (fig. 127 e s, ei), composed of cells more or 

 less compressed, has usually a different structure and aspect on the two 

 surfaces of the leaf. It is chiefly on the epidermis of the lower sur- 

 face (fig. 128 ez), that stomata, s s, are produced, occupying spaces 



between the veins, and it is there also that hairs usually occur. In 

 these respects, the lower epidermis resembles the outer bark of 

 young stems, with which it may be said to correspond. The lower 

 epidermis is often of a dull or pale-green colour, soft, and easily de- 

 tached. The upper epidermis (figs. 127 and 128 e s) is frequently 

 smooth and shining, and sometimes becomes very hard and dense. In 

 leaves which float upon the surface of water, as those of the water- 

 lily, the upper epidermis alone possesses stomata (^[ 56). On removing 

 a strip of epidermis, part of the parietes of the cells below is often 

 detached in the form of a green net- work (fig. 129 p p), and on 



examination under the microscope, the 

 stomata, s s, are seen communicating 

 with colourless spaces, III, surrounded 

 by green matter. 



137. The Parenchyma of the leaf IS 



the cellular tissue surrounding the ves- 

 sels, and enclosed within the epidermis 

 (fig. 127 PS, pi). It has sometimes re- 

 ceived the names of Diachyma (S/, in 

 the midst, and xvpa., tissue), or Meso- 



phyllum (pwos, middle, and (pv^ov, a leaf), or Diploe (S/TrXo??, a cov- 

 ering). It is formed of two distinct series of cells, each containing 

 chlorophylle or green-coloured granules, but differing in their form 



Fig. 127. Thin vertical section of the leaf of a Lily, highly magnified, ef, Epidermis of upper 

 pagina or surface, e i, Epidermis of lower surface, p s. Parenchyma of upper portion of the leaf, 

 composed of close vertically-placed cells, p i, Parenchyma of lower portion, composed of loose 

 horizontal cells, in, Intercellular passages. 1 1, Lacuna;. 



Fig. 128. Similar section of the leaf of Balsam. The letters denote the same parts as in fig. 

 127. s s, Stomata. 



Fig. 129. Strip of the lower epidermis, e e, of the leaf of Balsam, showing a net-work formed 

 by a portion of the parenchyma below, p p, being detached. The spaces of the net are lacunae, 

 III, often corresponding to stomata, s s. 



