WATER-TISSUES. MUCILAGINOUS CELLS 403 



cally sensitive pulvini which form the distal termination of the petioles. 

 The tissue in question takes the form of a continuous layer of elongated 

 cells, with their long axes directed as a rule obliquely upwards [and 

 outwards] ; the lateral walls of these cells show no signs of folding 

 when water is removed, but always remain fully extended. Clearly, 

 the mechanism that permits of a diminution in volume, when water 

 escapes, cannot be the same here as in the case of ordinary water- 

 tissues. In the present instance it is found, that the angle which the 

 longitudinal axes of the water-tissue cells forms with the longitudinal 

 axis of the whole pulvinus, diminishes when water is removed. As a 

 result, the peripheral ends of these cells move a little nearer to the 

 centre of the petiole, and the radial diameter of the water-tissue is 

 correspondingly reduced. At the same time the individual cells undergo 

 a certain amount of radial contraction. 



Some Bryophytes possess water-tissues, which, however, often serve 

 for the storage of plastic substances as well. Among Liverworts, the 

 Marchantiaceae call for special mention in this connection ; here the 

 colourless large-celled tissue situated immediately beneath the photo- 

 synthetic air-chambers is mainly utilised for water-storage. In Mosses 

 the occurrence of water-tissues is restricted to the sporogonia. In this 

 group of plants, the leafy shoots of the gametophyte generation have 

 adapted themselves to temporary failure of the water-supply in the 

 most straightforward manner ; for they are able to undergo complete 

 desiccation, without suffering any permanent ill effects. The sporogonial 

 water-tissue is sometimes sub-epidermal, in which case it may be located 

 either in the capsule-wall (Funaria hygrometrica), or in the apophysis 

 ( Webera nutans) ; in other instances there is an internal water-tissue, 

 situated in the columella, which, however, often also contains starch, 

 especially when the capsule is young. 



B. MUCILAGINOUS WATER-STORING CELLS. 186 



Vegetable mucilage subserves a great variety of physiological and 

 ecological purposes. In the present section, however, we shall confine 

 our attention to such mucilaginous cells and tissues as can act as 

 water-reservoirs, owing to the fact that the more or less viscid mucilage 

 which they contain has a stronger attraction for water than ordinary 

 cell-sap. Where the mucilage forms part of the cell-contents, as, for 

 example, in various succulents (Aloe, Agave, Cactaceae, etc.) in bulbs 

 and in the tuberous roots of certain Orchids, it is difficult to draw a 

 sharp line of distinction between mucilage-tissues and typical water- 

 tissues, for the simple reason that the latter also frequently contain 

 mucilaginous cell-sap. Separate consideration must, however, be 

 given to those instances in which the mucilage is derived from 





