134 COMPENDIUM OF GENERAL BOTANY. 



Communication between the intercellular spaces and air-cham- 

 bers and the atmosphere is brought about by breaks in the contin- 

 uity of the epidermal covering. Every breathing-pore (stoma, 

 compare Figs. 7479) is directly opposed to the function of the 

 epidermal system, because it increases the loss of water ; the pores 

 must remain open at least for a time in order to permit the ingress 

 of CO 2 and the egress of O. The closing of stomata is therefore a 

 physiological requirement. This most important requirement will 

 be considered a little later. The unavoidable loss of water is re- 

 duced very materially by the facts that the great majority of the 

 stomata of land-plants are on the lower surfaces of the leaves, and 

 in protected positions, as, for example, in depressions; also by 

 being covered with hair-cells, by the elongation of the ' ' entrance, ' ' 

 etc. Comparative anatomy reveals a series of instances in which it 

 is possible to know the habitat of a given plant from the position 

 and structure of its breathing-pores. (Concerning this consult 

 TSCHIRCH and other authors.) 



Let us add a few further physiological (also teleological in their 

 final results) observations concerning this important apparatus. 

 Submerged and subterranean organs are, in general, entirely free 

 from stomata; for example, they never occur on roots. They 

 occur mainly on green leaves and green stem-organs. It is also 

 worthy of notice that land-plants devoid of chlorophyll (saprophytes 

 and parasites) are almost uniformly free from stomata or contain 

 only a few. In bifacial aerial leaves the stomata are, as a rule, on the 

 lower surface, as has already been stated ; in floating leaves they 

 occur on the upper surface ; in centric leaves (not differentiated 

 into luminous sides and shade-sides) they are evenly distributed on 

 all sides. Their number varies greatly : from 40 to 300 per 

 square millimeter. In Brassica Rapa there are about 716 per 

 square millimeter. In leathery leaves they are smaller and more 

 numerous ; in succulent leaves they are larger and less numerous. 



Stomata are organs especially adapted for closing. Lenticels 

 take the place of stomata when the epidermis is displaced by cork- 

 tissue. Investigation in regard to lenticels shows that the relative 

 permeability to air, at least in some plants, is greater in the spring 

 than in winter. Lenticels are never entirely closed, while the 

 stomata may be. We shall now consider the stomata and lenticels. 

 more in detail. 



