80 



STEUCTTJEE OF LEAVES. 



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

 face (fig. 143 e i), 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 detached. The 

 upper epidermis (figs. 142 and 143 e s) is frequently smooth and 



Fig. 142. Pig. 143. 



shining, and sometimes becomes very hard and dense. Many tropical 

 plants present on the upper surface of their leaves several layers of 

 compressed epidermal cells. These appear to be essential for the pre- 

 servation of moisture in the leaf. In leaves which float upon the sur- 

 face of water, as those of the water-lily, the upper epidermis alone 

 possesses stomata (p. 30). 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. 144 pp), and on examina- 

 tion under the microscope, the stomata, 

 s s, are seen communicating with 

 colourless spaces, I 1 1, surrounded by 

 green matter. 



The Parenchyma of the leaf is 

 the cellular tissue surrounding the 

 vessels, and enclosed within the epi- 

 dermis (fig. 142 ps, pi.) It has 

 sometimes received the names of Diachyma {bia,, in the midst, and 

 ;^u^a, tissue), or Mesophyllum (/ietrog, middle, and (pvM.ov, a leaf), 

 or JDiploe {dmXon], a fold). It is formed of two distinct series 

 of ceUs, each containing chlorophyll or green-coloured granules, but 



Fig. 142. Thin vertical section o£ the leaf of a Lily, highly niagnifled. e s. Epidermis of 

 upper pagina or surface, e t, Epidermis of lower surface, p s, Parenchyma of upper por- 

 tion of the leaf, composed of close vertically-placed cells, p i, Parenchyma of lower portion, 

 composed of loose horizontal cells, m, Intercellular passages. 1 1, Laounaj. Fig. 143. 

 Similar section of the leaf of Balsam. The letters denote the same parts as in flg. 142. 

 s s, stomata. Fig. 144. Strip of the lower epidermis, e e, of the leaf of Balsam, showing a 

 network formed by a portion of the parenchyma below, p p, being detached. The spaces of 

 the net are lactmEC, III, often corresponding to stomata, s s. 



