WATER-STORAGE IN THE EPIDERMIS 115 



physiological peculiarities of epidermal cells are connected with this 

 partial function of water-storage. In the first place, every epidermis 

 possesses the power of alternately accumulating and delivering up 

 water. When an organ is transpiring too actively, the epidermis parts 

 with a large portion of its store of water to the green photosynthetic 

 cells, which exert a stronger " osmotic suction." Hence when an organ 

 begins to wilt, the first signs of collapse appear in the epidermis ; the 

 thin lateral walls of the epidermal cells are thrown into folds, and the 

 cells gradually contract in the radial direction, as water is withdrawn 

 from them. When the cells are once more supplied with abundance of 

 water, their lateral walls straighten themselves again. That this 

 " bellows-action " of the epidermal cells, which can be repeated when- 

 ever required, constitutes a very characteristic property of the water- 

 storing epidermis, may be inferred, as has already been hinted, from the 

 thin and flexible condition of the radial walls. The following circum- 

 stance, upon which Westermaier quite properly lays stress, points to 

 the same conclusion. Photosynthetic organs are often provided with 

 radial buttresses, consisting of specially thick cell-walls {Kingia australis), 

 or, more usually, of entire mechanical cells (Olra europea), which serve 

 to prevent the green tissues from collapsing when water is withdrawn 

 from them ; wherever these buttresses are present they never extend 

 outwards beyond the inner epidermal walls. Evidently it is not 

 intended that the partial collapse of the epidermal cells themselves 

 should be prevented. 



As it is improbable that the same amount of water will be simul- 

 taneously withdrawn from the epidermis in every part of an organ, it 

 would seem to be desirable that water should be able to circulate 

 freely through the epidermis, so that it can readily flow towards the 

 points at which it is most in demand at any given moment. The 

 requisite continuity of the epidermal water-storing system is in general 

 sufficiently ensured by the circumstance that the radial walls are thin, 

 or, if thickened, at any rate abundantly provided with pits. There are 

 in addition a number of special features, designed to produce the same 

 result, which occur especially in the cells overlying subepidermal 

 fibrous strands. In the Cyperaceae, for example, the epidermal cells 

 which are situated above fibrous strands, in stem and leaf, are often 

 very much flattened. The inner walls of these cells are provided 

 with peculiar, strongly silicified conical processes, which serve, according 

 to Westermaier, to arrest the downward movement of the outer wall, 

 and thus to prevent the interruption of the epidermal water-circulation 

 which would result from the total collapse of these cells. 



The water-storing capacity of the epidermis, and consequently its 

 importance as a water- tissue, varies according to the height of its 



