116 DERMAL SYSTEM 



component cells ; exceptionally tall epidermal cells occur in certain 

 Commelynaceae, Orchidaceae, Begoniaceae, etc. In xerophytes, 

 and especially among desert-plants, the volume, and hence the water- 

 storing capacity, of the epidermal cells is often increased by a papillose 

 protrusion of their outer walls. Occasionally individual cells become 

 enormously distended; the familiar Ice-Plant {Mesembryanthemum 

 crystallinmn), the stems and leaves of which appear as if studded with 

 beads of ice, owes its peculiar appearance to the presence of large 

 numbers of such epidermal water-vesicles (Fig. 28). According to 

 Volkens similar structures occur in quite a number of the plants of 



Fig. 2S. 



Enlarged water-storing epidermal cell (water-vesicle) of Mesembryanthemum 

 crystallinum. (L. 8.) 



the Egypto-Arabian desert-region (e.g. Reseda pruinom, II. ardbica, 

 Aizoon canariense, etc). 62 The water-storing capacity of the ordinary 

 tabular epidermal cells is of course small, as compared with that of 

 specialised structures like the above-mentioned water-vesicles ; it is, 

 nevertheless, by no means a negligible factor in the normal water 

 economy of the plant. In order to obtain at least an approximate 

 quantitative idea of the water-storing capacity of an ordinary epidermis, 

 the author has attempted to estimate what fraction of the daily foliar 

 transpiration can be made good at the expense of the two epidermal 

 layers of the leaf, in the case of certain woody plants ; the calculation 

 is based on the supposition that the epidermal cells contract to the 

 extent of one half of their volume owing to abstraction of water by 

 the photosynthetic tissue. (Only the bare figures are stated, the details 

 of the calculation being set forth in a note.) 63 It appears that the 

 foliar epidermis of Aesculus Hippoceistanum would be able to cope with 

 the average transpiration of the mesophyll for a period of two hours ; 



