LEAVES 



567 



occasioned by water withdrawal. The abundance of wilted leaves in tropical forests 

 is due to the fact that the stomata remain open on account of the strong light and the 

 humid air ; once closed, the stomata reopen only after the leaves again become 

 turgescent. Old leaves sometimes lose water more rapidly than do young leaves, 

 because the stomatal mechanism becomes less perfect with increasing age. Thus, 

 while stomatal movements do not in any true sense regulate transpiration, through 

 closure they reduce its amount and thus contribute to the welfare of the plant. 



Protective stomatal structures. Doubtless transpiration is reduced by 

 various stomatal structures, such as guard-cell cutinization, hairs, wax 

 and resin deposits, and tyloses. The frequent restriction of stomata to 

 the under leaf surface probably has a similar effect. Of importance, 

 too, are pits, long or tortuous passageways, and alternations of cutin 

 ridges with vestibules, all of which 

 doubtless retard outgoing water. But 

 stomatal structures and activities can- 

 not stop transpiration; at best there 

 is only retardation. 



The advantages and disadvantages 

 of stomata. The great advantage of 

 stomata is the facilitation of synthesis, 

 and their great disadvantage the facili- 

 tation of transpiration. Stomata are 

 most necessary where they entail the 

 most harm, namely, in xerophytes, 

 where the heavy cutinization makes the 

 absorption of carbon dioxid through 

 the cuticle almost impossible. Where 

 stomata entail no danger, as in sub- FIG. 807. -A cross section through 



the leaf of the oleander (Nerium Olean- 



mersed hydrophytes, they are un- jer), showing a thick three-layered epi- 



necessary, since gases pass readily dermis (e) with a prominent cuticle 



through the non-cutinized epidermis. 



The epidermis. General features . 

 The leaf epidermis consists commonly 



(c), a striking development of palisade 

 tissue (/>) below as well as above, a 

 layer of sponge tissue (/) near the 

 center instead of near the lower epi- 



of a single layer of cells (figs. 760, dermis ' and a stomatal & (6 > wi * 



v stomata (s) that are slightly elevated 



761, 926), or sometimes of two or above the level of the pit epidermis 



more such layers (as in Nerium, fig. and with protective epidermal hairs 

 80 7 ; also figs. 7 66, 801), SO Com- <*>' considerably magnified. 



pactly placed that no spaces intervene, except where stomata occur. 

 On both leaf surfaces in submersed plants (figs. 763, 1018), and often 



