398 STORAGE SYSTEM 



sodium, chloride in the leaves ; whether this is the correct explanation 

 or not, is still an open question. 



Water-tissues always consist of living parenchymatous cells, which 

 may reach a very considerable size. Apart from a thin peripheral 

 layer of protoplasm, in which are embedded the nucleus and a few 

 chloroplasts, the cell-cavities are entirely tilled with a clear, watery 

 or, in the case of deep-seated water-tissues, often somewhat slimy 

 liquid. The cell-walls are typically thin, and devoid of pits, but in 

 some cases develop a slight collenchymatous thickening along the 

 edges of the cells. In the leaf of Peperomia incana, the layer of water- 

 tissue immediately above the photosynthetic tissue has the edges of its 

 cells thickened to a quite unusual extent. These thickened strips of 

 cell-wall form an elegant, resistant framework, which completely roofs 

 in the thin layer of photosynthetic tissue. The latter is thus pre- 

 served from deformation, even when a large amount of water escapes 

 from the leaf. 



As regards their shape, the cells of water-tissues are in general 

 approximately isodiametric. If, as sometimes happens, they are dis- 

 tinctly elongated (e.g. in Hoheiibergia strobilacea), the orientation of their 

 longitudinal axes is such as to diminish the distance which the water 

 has to traverse on its way to the photosynthetic tissue. In Hat leaves 

 the cells of the water-tissue often assume a palisade-like form for 

 a similar reason (Carapa molncccnsis, Wiizophora mucronata) . 



According to their situation, water-tissues may be described as 

 peripheral or internal. Peripheral water-tissues, which may be regarded 

 as elaborations of the epidermal water-storing system (cf. p. 114), are 

 most frequently found in ordinary flattened leaves, where they are 

 situated close to one or other surface. In dorsi-ventral leaves the 

 water-tissue is mainly or exclusively developed on the adaxial side. 

 Internal water-tissues, on the other hand, are characteristic of those 

 photosynthetic organs which are specialised, even as regards their 

 external form, for life under xerophytic conditions. The Cactaceae, 

 and the genera Agave, Aloe and Mesembryanthcmwm, provide excel- 

 lent illustrations of this type of structure. Both forms of water- 

 tissue are generally located in close proximity to the photosynthetic 

 system ; the surface of contact between the two sets of tissues 

 also invariably tends to be as large as possible. In the leaf 

 of Maranta arundinacea, for instance, the water-tissue consists of 

 a layer of vertically elongated cells situated immediately beneath 

 the adaxial epidermis with obtusely conical lower ends, which project 

 into the underlying palisade-tissue. The photosynthetic system and 

 the water-tissue are still more closely dovetailed together in the leaf 

 of Hohenbergia strobilacea ; the resulting enlargement of the surface of 



