532 ECOLOGY 



acting as a lens, converges the light upon the plastids; the emission 

 of reflected rays causes this moss and the alga, Bolrydium, to glisten 

 in the sunlight. The leaves of water plants and of shade plants are 

 deep green, which is due partly to the thin, transparent epidermis and 

 partly to the peripheral position of the numerous deep-green chloroplasts. 

 Xerophytes. Xerophytes contrast sharply with hydrophytes, dis- 

 playing prominent palisade tissue,, often in many rows (as in the olean- 

 der and in most cacti), while the spongy tissue is small in amount and 

 poor in air spaces, and often is best developed toward the leaf center 

 (fig. 807). Frequently there are palisades in the lower half of the leaf, 

 though less prominently developed than above; in the cottonwood the 

 palisades are developed nearly equally on the two sides, and in many 

 fleshy vertical leaves there is a cylinder of palisade cells about the color- 

 less leaf interior (figs. 926, 927). Xerophytic leaves commonly are thick, 

 and often without chlorophyll at the center ; sometimes also the epidermis 

 is very thick, so that the peripheral regions are free from chlorophyll (figs. 

 766, 928). Some succulent xerophytes, notably the Crassulaceae (fig. 

 925) and many xerophytic grasses (fig. 835), are quite without palisade 

 tissue, but the compactness of the chjorgnchyma distinguishes them 

 from hydrophytes. Xerophytic leaves usually are pale green in color, 

 by reason of a relatively non- transparent epidermis (due to hairs, wax, or 

 cutin). or by reason of the deep position of the chlbrenchyma; or the 

 chloroplasts may be pale in color or relatively scattered in position. 

 Thick leaves with compact tissues and prominent palisade layers char- 

 acterize not only the plants of ordinary xerophytic situations, but also 

 those of peat bogs and salt marshes. Alpine 

 plants have more palisade tissue than do arctic 



plants, while the latter have a more lacunar 



']'''! A 



fA P sponge tissue. 



Taxonomic variations. In some leaves, as in 

 Sambucus and Lilium (fig. 765), there are arm pali- 

 FIG. 765. A cross sec- sa des, in which wall infoldings give an increased inner 

 tion of the upper part of a f ... ,. . f ,_,. 



surface to the cell; in nine leaves wal! imoldmes are 

 lily leaf (Lilium lonetfiorurn), 



showing the so-called arm remark ^ly developed, and the outer cells are divided 

 palisades (/>), also the upper into palisade-like compartments (fig. 1039). In some 

 epidermis (e) with its cuticle succulents, as in Portulaca and Begonia (fig. 766), 

 (c) ; considerably magnified. there are funnel-shaped palisade cells with large and 

 immotile chloroplasts crowded at the narrow base 



near the conductive tract ; sometimes there is a festoon of such cells about the con- 

 ductive bundle (fig. 767), and occasionally palisade cells curve toward the bundle. 



