ABSORPTION OF GASES. 73 



A gas indispensable to the work of the leaf or other green 

 parts is carbon dioxide. The question arises as to whether this 

 is absorbed mainly or entirely through the stomata, or whether 

 it may pass through the epidermis itself. It has been settled, 

 as will later be referred to (page 90), that it passes through 

 the stomata into the leaf. The study of the living tissues of 

 the leaf shows that the walls of the individual cells are imper- 

 forate, and hence gases cannot pass through them directly. 

 As the living cell-walls are always moist, the question arises: 



30. Can a gas essential to the work of the living green cell, 

 i.e., carbon dioxide, pass through a moist membrane ? 



Answer by Experiment 15. 



EXPERIMENT 15. This may be tested by placing the gas in con- 

 tact with a wet membrane having on the other side a liquid which 

 gives a visible reaction with it. Fill a wide-mouthed vial with filtered 

 lime-water, and cover with soaked parchment paper tied tightly on 

 the mouth. Fill a large test-tube with a mixture of 90^ air and 

 about \Q% COa (by the usual method over water by aid of the CO 2 

 generator, see page 42), slip the vial into it, and cork up tighily. 

 If the CO 2 can pass through the membrane, the lime-water will be- 

 come milky in a short time. 



( The membrane must be kept moist by the lime-water ; if the vial is 

 not entirely filled by tJie liquid, tJie wJiole apparatus may be kept tipped 

 somewhat ; a control experiment in wJiicJi tJie CO- 2 is removed from the 

 air of a test-tube by potash solution is valuable but not necessary.') 



31. What is the nature of the absorption of substances other 

 than the water, minerals, and gases already considered, i.e., 



By Insectivorous Plants? 

 By haustoria of Parasites? 

 By Mycorhiza? 

 By tubercles of Leguminosae ? 



Experiment upon this subject is hardly practicable here. 

 The subject must be studied theoretically from reading, lec- 

 tures, etc. 



32. What ecological effects can you trace from the interaction 

 of the physiological conditions of water, mineral, and gas absorp- 

 tion, and of osmotic phenomena in plants, with the conditions of 

 the external world ? 



