. A manual of botany. Botany. 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- 77
. A manual of botany. Botany. 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 t
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