STRUCTURE OF LEAVES. 



69 



and arrangement. This may be seen on making a vertical section 

 of a leaf, as in fig. 127. Below the epidermis of the upper side of 

 the leaf, there are one or two layers of oblong blunt cells, placed 

 perpendicularly to the surface (fig. 127 p s), and applied so closely 

 to each other as to leave only small intercellular spaces (fig. 127 TO), 

 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. 127 p i), leaving cavities between them, 

 I , which often communicate with stomata (fig. 128 s s). On this ac- 

 count 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 colour- 

 less. In some cases the cellular tissue is deficient at certain points, 

 giving rise to distinct holes in the leaf, as in Dracontium pertusum. 



138. Submerged Leaves. Leaves which are developed under 

 water differ in many points of structure from aerial leaves. They 

 have no fibro- 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. 130 j>), but no 

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

 cells, disposed irregularly, and sometimes leaving spaces which are filled 

 with air for the purpose of floating the leaf (fig. 130 /). When ex- 

 posed to the air, these leaves easily part with their moisture, and be- 

 come shrivelled and dry. In some instances there is only a net-work 



of filamentous-like cells formed, the spaces between which are not 

 filled with parenchyma, giving a peculiar skeleton appearance to the 

 leaf, as in Hydrogeton or Ouvirandra fenestralis (fig. 131). Such a 

 leaf has been called fenestrate (fenestra, a window). 



Fig. 130. Perpendicular section through a small portion of the submerged leaf of Potamogeton 

 perfoliatus. p. Parenchyma. J, Lacunae. 



Fig. 13L Fenestrate leaf of Ouvirandra fenestralis, composed of vascular tissue, without inter- 

 vening cellular tissue or diachyma. 



