STRUCTURE OF I,EAVBS. 



81 



differing in form and arrangement. This may be seen on making a ver- 

 tical section of a leaf, as in figs. 142 and 143. Below the epidermis of 

 the upper side of the leaf there are one or two layers of oblong blmit 

 cells, placed perpendicularly to the surface (fig. 142 p s), and applied 

 so closely to each other as to leave only small intercellular' spaces (fig. 

 142 m), except when stomata happen to be present. On the under 

 side of the leaf the cells are irregular, often branched, and are arranged 

 more or less horizontally (fig. 142 p i), leaving cavities between them, 

 I I, which often communicate with stomata (fig. 143 s s). On this 

 account the tissue has received the name of cavernous. The form and 

 arrangement of the cells, however, depend much on the nature of the 

 plant, and its exposure to light and air. Sometimes the arrangement 

 of the cells on both sides of the leaf is similar, as occurs in leaves 

 which have their edges presented to the sky. In very succulent plants 

 the cells form a compact mass, and those in the centre are often 

 colourless. In some cases the cellular tissue is deficient at certain 

 points, giving rise to distinct holes in the leaf, as in Monstera Adan- 

 sonii ; such a leaf has been called pertuse (pertusus, bored through). 

 In Victoria regia perforations in the leaf seem to be subservient to the 

 purposes of nutrition, in permitting the gases collected beneath the 

 large expanded leaf to escape, and thus allowing its under surface to be 

 brought into immediate contact with the water. > 



SuBMEEGtED Leaves. — Leaves which are developed under water 

 differ in structure from aerial leaves. They have usually no fibro- 



Fig. 145. 



Pig. 146. 



vascular system, but consist of a congeries of cells, which sometimes 

 become elongated and compressed so as to resemble veins. They 

 have a layer of compact cells on their surface (fig. 145 p), but no 

 true epidermis, and no stomata. Their internal structure consists of 

 cells, disposed irregularly, and sometimes leaving spaces which are 

 filled with air for the purpose of floating the leaf (fig. 145 I). When 

 exposed to the air these leaves easily part with their moisture, and 

 become shrivelled and dry. In the submerged leaves of Trapa and 



Fig. 145. Perpendicular section through a small portion of the submerged leaf of Pota- 

 mogeton perfoliatua. p, Parenchyma. I, Lacuuse. Fig. 146. Fenestrate leaf composed 



of filamentous cells, with intervening spaces. 



G 



