LEAVES 



531 



of many mesophytic monocotyls, especially among the grasses and the 

 sedges, have a somewhat compact and uniform chlorenchyma without 

 palisade cells (fig. 762), and some monocotyls have cells elongated par- 



FiG. 762. — A cross section of a meso- 

 phytic grass leaf, that of the blue grass 

 {Poa pralensis), showing relatively uni- 

 form chlorenchyma (c), an upper epi- 

 dermis (e) differing considerably from the 

 lower, especially in the large cells (&) 

 which are concerned in leaf movement; 

 note the vascular tracts (v) and observe 

 that the stomata {s) are confined to the 

 upper surface; considerably magnified. 



Fig. 763. — A cross section of a 

 submersed hydrophytic leaf, that of a 

 pondweed {Potamogelon lucens), show- 

 ing the few cell layers (here three) 

 characteristic of a water leaf; note the 

 abundance of chloroplasts (c) in the 

 epidermis (e) and the absence of cutin- 

 ized epidermal walls; highly magni- 

 fied. 



allel to the leaf surface. In submersed hydrophytes epidermal chloro- 

 phyll usually is abundant (fig. 1018), often exceeding in amount that 

 contained in the mesophyll, which may be loose and spongy by reason 

 of the large air spaces, or which may be reduced to a single layer (fig. 

 763). Hydrophytes with emersed leaves are 

 much like mesophytes, except for their large 

 lacunae (fig. 805). Shade plants are weak in 

 palisade tissue, but, except in ferns (fig. 754), 

 they rarely contain epidermal chlorophyll. In 

 some leaves, especially in shade plants (such as 

 Maranta and Asarum), the convex outer walls 

 of the epidermal cells converge the rays of inci- wild ginger (Asarum cam- 

 dent light; the reflection of some of these rays f"*^^', showing three epi- 



*^ dermal cells with convex 



gives a velvety aspect to the leaf (fig. 764). outer walls which converge 

 Occasionally (as in Fittonia) isolated epidermal the rays of incident light; 

 cells, known as ocellce, are more papillate than considerably magnified. 

 are their neighbors. It has been supposed on somewhat uncertain 

 evidence that epidermal cells with convex outer walls are organs of 

 light perception. A more probable role of such cells is the facilita- 

 tion of synthesis through light convergence. In the luminous moss, 

 Schistostega, the chloroplasts are near the base of a globose cell, which, 



?IG. 764. — A cross 

 section of the upper part 

 of a shade leaf, that of the 



