LEAVES 



S8i 



Infolded and •withered leaves. — Many grass leaves (known as involute) roll in- 

 ward when exposed to desiccation, owing to the loss of water from large, turgescent, 

 thin-walled cells, wliich are arranged in longitudinal rows, and often are below 

 the level of the other epidermal cells (figs. 835-837; also fig. 762). The surface ex- 

 posure of a grass leaf may, by such a process, be decreased to a fraction of its 

 original area, doubtless resulting in a much-reduced transpiration, particularly be- 

 cause the stomata are on the infolded surface. Often the leaf ridges meet, making 



Figs. 835-837. — Cross sections of the involute leaf of a xerophytic grass, the sea 

 sz.nd rt^A {Ammophila- arenaria): 835, a cross section (highly magnified), showing chlo- 

 renchyma (c) composed of cells that differ from palisades in form and from sponge cells 

 in compactness; the stomata (s) are confined to the upper (here the more protected) 

 epidermis (e); note in the sinuses large water-containing cells (b), whose changes in 

 water content account largely for leaf closing and opening; v^ conductive tracts; m, 

 bundle sheath; m, mechanical tissue; 836, a diagrammatic cross section of a turgid 

 open leaf; 837, a diagrammatic cross section of a desiccated inrolled leaf; 836 and 837 

 slightly magnified. 



closed chambers of the furrows. The wilting of leaves, also due to reduced turges- 

 cence, may involve some reduction of transpiring surface, but its significance proba- 

 bly is small. Surface reduction is illustrated in several lichens (as in species of 

 Cladonia), whose thallus edges turn up when desiccated, exposing the white under 

 surface. Various mosses (as Ortholrichum), the rose of Jericho {Anastatica) , and 

 some ferns (as Selaginella lepidophylla and Polypodium polypodioides) curl up when 

 desiccated, exposing a reduced transpiring surface ; their sudden uncurling when 

 moistened has given rise to the term resurrection plant. Such movements as those 

 exhibited by the leaves of Polytrichum, due to changing water content (figs. 901, 902), 



