144 PLANT PHYSIOLOOY 



change to a pink hue when exposed to dampness. This fact 

 has been employed by Stahl in demonstrating that transpira- 

 tion through the walls of epidermal cells is much slower than 

 through open stomata. The iodine test for starch demon- 

 strates whether carbon-dioxide enters the leaf through cell- 

 walls in suflBcient quantities for starch manufacture. In 

 most plants the epidermal walls are too impermeable for this. 



Through the relatively impermeable superficial tissues — 

 epidermis and cork — there must evidently be openings of the 

 intercellular aerating passages. The most important and 

 the most perfect of these openings are the stomata, found 

 on leaves and other young parts. On older and persisting 

 parts the enclosing cork layer may be interrupted by lenti- 

 cels — masses of rounded cells, unchanged as to their walls, 

 and enclosing intercellular spaces continuous with those 

 deeper in the body of the plant. Cork may be replaced on 

 submersed organs, or on those growing in the mud of 

 swamps and marshes, by a homologous tissue, like that 

 composing the lenticels, and called aerenchyma. * Lenticels 

 can be closed, and the passage of gases through aerenchyma 

 can be stopped, only by the growth of new tissue, of cork, 

 which seals the openings. The stomata, on the contrary, are 

 intercellular spaces bounded by a pair of delicately balanced, 

 and in most cases freely movable, epidermal cells. Lenticels 

 are found especially in the bark of stems and branches, 

 though also on older bark-covered roots. Aerenchyma is 

 found almost exclusively on roots. These organs secure 

 mainly the entrance of oxygen in sufficient quantities into 

 the older and not necessarily very active tissues, and at the 

 same time the exit of carbon-dioxide. When they occur on 

 aerial organs they of course facilitate transpiration. Sto- 

 mata, on the contrary, form most abundantly in the epi- 

 dermis immediately covering chlorophyll-containing cells. 



Oxygen forms about twenty per cent, of the atmosphere 

 and carbon-dioxide one-twentieth of one per cent. The chloro- 

 phyll-containing cells must be very active during the hours 

 when they are supplied by sunlight with tlie energy neces- 



* Schenck, H. tJber das Aerenchym : ein dem Kork homologes Gebijde. 

 Jahrb. f. wiss. Botanik, Bd. XX., 1889. 



