Chap. I] 



WATER 



ii 



occurring singly or united into tissues ; sometimes, as in Philodendron 

 cannaefolium, intercellular spaces assume the same function. A rich 

 development of parenchymatous living aqueous tissue occasions the 

 succulence of leaves and axes which has been already described. This 

 aqueous tissue is either external, between the epidermis and the chloren- 

 chyma {pcricJiylous), as in many Bromeliaceae, Rhizophora (Fig. 17), and 

 other plants ; or internal, and then within the chlorenchyma [endocJiylous)^ 

 as in Cactaceae, succulent Euphorbiaceae, and most other stem-succulents 

 (Figs. 13, 14). In perichylous construction the aqueous cells have a watery 

 sap, in endochylous construction they are usually filled with mucilaginous 

 contents. 



Solitary living aqueous cells are less frequent than 

 aqueous tissue. They are very conspicuous, for instance, 

 in Mesembryanthemum crystallinum, where certain epi- 

 dermal cells expand into large water - bladders ; in 

 Tillandsia usneoides and others they are scattered in the 

 chlorenchyma. 



Living aqueous cells always remain filled with proto- 

 plasm and cell-sap ; they never contain air. The volume 

 of water that they contain, however, varies between wide 

 limits. When transpiration is slack, they may be gorged 

 with water, for instance at night or in dull weather, but 

 during strong transpiration they supply the assimilating 

 cells with water, and then they collapse strongly. 



Water-storing tracheids, as opposed to living water-cells, 

 contain air or water according to the amount — greater or 

 less— of transpiration of the green tissues. They are most 

 frequently present at the ends of vascular bundles in 

 leaves ; and only in the leaves of certain xerophilous 

 distributed through chlorenchyma (Figs. 15, 16). 



Fig. 12. Xerophilous 

 structure. Cold soil of 

 Greenland. Cassiope tetra- 

 gona with small leathery 

 leaves folded inwards. 

 g Young shoots. Magni- 

 fied 2. After Warming. 



orchids are they found 



The water-reservoirs in many xerophytes are not uniformly distributed 

 in the leaves or axes, but are confined to certain members, whose chief 

 function is the storage of water. Leaves that are ageing and have become 

 abnormally thick owing to the subsequent great enlargement of their 

 aqueous tissues in many cases serve as water-reservoirs of this kind, as 

 we see in epiphytic Gesneraceae and species of Peperomia, Rhizophora, 

 Sonneratia and other mangrove-trees ; and these older leaves supply the 

 younger ones — which are at the height of their assimilating activity — with 

 water until their store is completely exhausted 1 (Figs. 16 a and 17). 

 Amongst such water-reservoirs are numbered the well-known pseudo-bulbs 

 of epiphytic orchids, the spindle-shaped petioles of Philodendron cannae- 

 folium, and other like structures. 



1 Schimper, III, p. 42 ; Haberlandt, Physiol. Pflanzenanat., p. 349. 



