164 PLANT PHYSIOLOGY 



textured leaf, and over the cut end slip a tightly fitting rubber tube; through 

 this, by aid of a blast or air-pump (e.g., a bicycle pump), force air into the 

 leaf; then plunge the latter under clear water in a glass vessel and observe 

 closely the surface of the leaf. Also by a similar method force air through 

 a long internode of some soft-textured stem, a considerable length of root, 

 or other selected part, holding the free end under water. 



The best material for this experiment is afforded by the long-petioled 

 floating leaves of water-plants, some of which are so porous that air can 

 almost be blown through them by the mouth. Limnocharis Humboldti, 

 commonly grown in greenhouses, is good. Next to water-plants come some 

 of the Begonias, then Rubber Plant and Orange, while the tropical house 

 plants Jacobinia magnified, Codi&um aucubfffolium, and Clerodendron Thomp- 

 sons are also good. 



Since this experiment is liable to be marred by closing of the stomata, 

 or by clogging of them through water entering by capillarity, it is best to 

 ensure their open condition by first keeping the plant for a short time in a 

 bright light, well watered, and in a somewhat humid atmosphere; while it 

 may also help to keep the stomata open if the leaf is cut off under water, 

 thus preventing entrance of air into the ducts. Also the clogging of the 

 openings may be prevented by having the air-current in action before the 

 leaf is plunged under water. It is these difficulties which have led to the 

 recommendation of a method, e.g., DETMER, 172, whereby the air is drawn 

 through the leaf and escapes under water from the petiole; and other modi- 

 fications have been described. 



Turning from the gas-absorbing system to the gases absorbed, 

 the student should now work out, and should tabulate for car- 

 bon dioxide and oxygen (and preferably also for nitrogen), their 

 chemical composition and molecular weights, their properties 

 and affinities, their relative diffusibilities and solubilities, and 

 their proportions in the atmosphere. He should then turn par- 

 ticularly to their remarkable power of diffusibility, so impor- 

 tant in this connection, and his understanding of this matter 

 will be much aided by some practical study of the phenomena, 

 which he may well bring before him as a problem thus: 



What are the principal phenomena, and the physical explana- 

 tion, of the diffusion of gases? 



EXPERIMENT. Prepare four tubes of rather thick glass, of some 

 10-12 mm. bore and 8 cm. length, with one end ground flat across. 

 Cut two discs of parchment paper, one of thin rubber sheeting, and 

 one of a leaf having stomata on only one surface, such as Hedera 

 Helix (English Ivy), Codiaum, or Ficus elastica (Rubber Plant); seal 

 these air-tight by sealing-wax to the ground ends of the tubes (Fig. 



