LEAVES 



S6i 



surface is without stomata, while they are abundant on the emersed 

 upper surface; the submersed leaves of water lilies are quite 

 without stomata. Subterranean organs have no stomata, as a rule, 

 though the latter are present in some instances. Most xerophytes 

 have few or no stomata on the upper leaf surface, though various 

 conifers (as Jtiniperus) and most Crassu- 

 laceae have numerous stomata on that 

 side. 



The influence of external factors upon 

 the development and arrangement of 

 stomata. — The variations. — Xerophytic 

 leaves, though smaller than the meso- 

 phytic leaves of the same species, usually 

 contain about the same number of epi- 

 dermal cells, their size being considerably 

 less. Hence the stomata are often more 

 numerous per unit surface in xerophytes 

 than in mesophytes, though in the aggre- 

 gate no more numerous, and perhaps 

 even less numerous. Occasionally an 

 increase of atmospheric moisture results 

 in an increase of stomata in proportion 

 to the other epidermal cells. In some 

 plants (as Asperula tinctoria) the orienta- 

 tion of stomata varies with the habitat, 

 mesophytic individuals exhibiting irregular 

 orientation, while xerophytic individuals 

 exhibit longitudinal orientation as do 

 monocotyls. The most pronounced in- 

 fluence of external factors is found in 

 amphibious plants, where stomata ordinarily can be induced or 

 inhibited at will by growing the plants respectively in air or in 

 water. Stomata occur on both surfaces in the air leaves, but on 

 the upper surface only in the floating leaves of Polygonum amphibium 

 and Ranunculus sceleratus, the air leaves of the latter having more 

 stomata on the upper surface in moist air and on the under surface 

 in dry air. Whatever the conditions of germination, the first leaves 

 of Proserpinaca have stomata, but they originate within the seed in 

 contact with air instead of with w^ter; subsequent submersed leaves 



Fig. 805. — A cross section 

 through a leaf of the yellow water 

 lily {Nymphaea advena), showing 

 a strong development of palisade 

 tissue {p) in the upper half which 

 is above the water level {w), and 

 an unusually loose spongy tissue 

 with large lacunae (/) in the lower 

 half which is below the water 

 level; the stomata (j) are con- 

 fined to the upper surface; a, 

 slime gland ; m, a stellate stereid ; 

 considerably magnified. 



