THE STEITCTUEE OF THE LEAP 



391 



on the lower face ; in submerged leaves there are none ; in 

 floating leaves they are confined to the upper surface. '~' 



In many leaves water stomata are present. These remain 

 permanently open ; in- some, again, there are passages or cracks 

 among the cells, which can hardly be described as stomata. 

 These irregular apertiu-es, like the true water stomata, serve for 

 the emission of drops of water. 



In some leaves, as in the Oleander (Nerium oleander) and 

 Banlisia, the stomata are seated in pit-like depressions on the 

 under surface {Jig- 776). 



The cells of the epidermis when the latter is many-layered 

 sometimes contain the eystoliths already described (fig. 775). 



Within the epidermis of the winged portions of the phyllo- 

 podium is the mesophyll of the leaf. This is variously arranged, 

 giving three main types of structure, known I'espectively as the 

 centric, the dorsi-ventral, and the iso-hilateral : of these the 

 second is the commonest form (fig. 111). It has derived its 

 name from the fact that the tissue is of different character 

 towards the upper and lower surfaces. Immediately under 

 the upper epidermis the cells are oblong, and are arranged with 

 their longest diameter at right 

 angles to the surface of the leaf, 

 constituting the so-called pali- 

 sade 'parenchyma. The cells 

 contain large numbers of chloro- 

 plastids or chlorophyll grams. 

 Sometime3_ there is only a single 

 layer of these cells, but often 

 there are several. The cells are 

 arranged close together, and 

 have relatively few intercellular 

 spaces among them. 



The lower half of the meso- 

 phyU is made up of the so-called 

 spongy parenchyma. The cells 

 are of irregular shape, and are 

 often arranged so as to be in con- 

 tact only at portions of their sur- 

 faces ; the tissue is consequently 



much less dense, and large intercellular spaces or lacunse are 

 present : there is always a conspicuous lacuna under each 

 stoma. 



The cells of the spongy parenchyma, like those of the palisade 



FiC!. 776. 



Fi(l. 776. Vertical section tliroagh 

 the leaf of a Banksia. a, a. Epi- 

 dermis, c. Spongiform parenchyma. 

 &. Hairs wiiich are contained in 

 little depressions on the under siu-- 

 face of the leaf, and at whose base 

 peculiar stomata are found. After 

 Schleiden. 



