WATER-TISSUES 399 



contact between the two systems cannot fail to facilitate the emptying 

 and filling of the water-reservoirs. A similar physiological significance 

 doubtless attaches to the wedge-shaped masses of water-tissue observed 

 by Lippisch in the leaves of Ravenala madagascariensis and of certain 

 species of Strelitzia (S. farinosa, S. Beginae, S. alba); the masses in 

 question consist of colourless palisade-shaped cells, and represent 

 local intrusions of the adaxial water-tissue into the photosynthetic 

 system. 



The quantitative development of the water-tissue varies within wide 

 limits. One end of this extensive and finely graduated scale is exem- 

 plified by certain epidermal layers, which reveal the accentuation of 

 their water-storing function by an increase in height, or by a 

 tendency to undergo tangential division ; the opposite extreme is 

 illustrated by those succulent photosynthetic organs in which the 

 massive water-tissue enormously exceeds the photosynthetic tissue in 

 thickness. It is an interesting feature of various leaves that the 

 margin, which is the region most exposed to damage by excessive loss 

 of water, is provided with a local water-tissue, composed of a varying 

 number of layers, although water-storing arrangements are altogether 

 absent, or, at any rate, far less conspicuously developed in the rest 

 of the leaf. Such marginal water-tissues occur, according to Hintz, 

 in various species of Acacia (A. leprosa, A. salicifolia, A. longifolia, 

 etc.) and Quercvs (Q. pedunculata, Q. rnacrocarpa, Q. Ilex, Q. cocci/era, 

 and Q. Suber), in Ilex Aquifolium, Hakea eucalyptoides, etc. Lippisch states 

 that in Musa the margin of the young leaf is wholly composed of 

 water-storing cells, but that this marginal tissue shrivels up later on, 

 when the leaf-blade becomes broken up into numerous parallel strips. 

 Schimper has made a remarkable observation regarding the quantitative 

 development of the water-tissue in the epiphytic species of Peperomia and 

 in certain Gesneraceae. In these plants the water-storing cells elongate, 

 and the whole water-tissue consequently increases greatly in thickness, 

 as the leaves grow older. In Codonanthc Devosii (Gesneraceae), for 

 instance, Schimper found that, while the average thickness of a middle- 

 aged leaf is 2' 5 mm., old leaves, which are just beginning to turn yellow, 

 may be as much as 5 mm. thick. This very appreciable difference 

 depends entirely upon the greater thickness of the water-tissue in the 

 older leaves. Schimper has further succeeded in proving experimentally 

 that the older leaves actually serve as water-reservoirs, which are 

 drawn upon by the younger leaves. The author has observed a similar 

 relation in Rhizophora mucronata ; on shoots of this Mangrove gathered 

 by him on the shores of the coral island of Edam, near Batavia, the 

 old, yellow leaves were exactly twice as thick as leaves which had just 

 reached their full size. The following table of measurements shows 



