LEAVES 



631 



the centrally placed chlorenchyma occupies less space than does the epi- 

 dermal water mantle (figs. 928, 766, 767). Perhaps the commonest sort 

 of water mantle is represented in Tradescantia, where there is a single 

 or double layer of large turgescent colorless cells. The water tissue com- 

 monly is best developed on the upper side of the leaf, sometimes being 

 confined to that side. In most cases the cell sap is highly acid. 



In the " ice plant " (M esenibryanthemum crystallinum) a few of the epidermal cells 

 are much distended and project considerably beyond the epidermal level, causing 

 the leaves to glisten in the sun- 

 light. Not infrequently plants 

 possess isolated colorless tur- 

 gescent cells in the midst of a tis- 

 sue made up of cells of wholly 

 different character; when such 

 cells have walls with tracheid- 

 like thickenings, they have 

 been called storage tracheids 

 (fig. 772). In many succu- 

 lent monocotyls (as Aloe) the 

 cell sap is very mucilaginous, 

 and in some plants of similar 

 character mucilaginous mate- 

 rial is deposited in the form 

 of wall thickenings. 



FIG. 927. A sector from a cross section of a 

 succulent equilateral xerophytic leaf (Senecio sp.), 

 illustrating peripheral palisade chlorenchyma and 

 central water tissue, but showing a gradual transition 

 from the former to the latter, both in cell size and in 

 chlorophyll abundance; lettering as in Fig. 926; con- 

 siderably magnified. 



The causes of water 

 accumulation in succu- 

 lent plants. Experi- 

 mental data. Some suc- 

 culent plants (e.g. Sem- 

 pervivum assimile, figs. 

 1043-1045) when placed for a few weeks in a moist chamber develop 

 slender elongated shoots with thin expanded leaves, having little or 

 no suggestion of succulence, while subsequent removal to dry air 

 results once more in the development of short and stout shoots that 

 bear thick succulent leaves of small size. Such reactions are quite 

 like those previously noted as characterizing habitats that differ in 

 atmospheric humidity and hence in transpiration (see p. 598). But 

 while the xerophytic leaf is in all cases small and thick and the meso- 

 phytic leaf large and thin, the thickness in the succulent xerophyte is due 

 to an increased dorsiventral development of watery tissue, while that in 



