MOVEMENT (»К MATERIALS IN THE PLANT 



135 



openings, through one of which the stem of the plant projects. The second 

 opening is usually closed and bears a tube through which water may be added 

 to the pot, and the third bears a small glass tube drawn to a fine, open point 

 above. Through the capillary opening of this tube the air in the apparatus 

 remains in equilibrium with that of the external atmosphere. The loss in weight 

 of the apparatus is due almost entirely to the loss of water from the plant by 

 evaporation. 1 A tall cylindrical vessel of water may be used for small plants in 

 experiments of short duration. The plants are fastened, by means of silk- 

 wrapped wire, with their roots in the water and their green parts projecting into 

 the air, a thin layer of oil being placed over the water surface to prevent evapora- 

 tion. 7 The loss in weight of the apparatus, in this case also, is due almost 



Fig. 79. — Kohl's apparatus for the study of plant transpiration. 



wholly to evaporation of water from the plant. 2 



2. The amount of water absorbed by the plant may be measured, Kohl's 3 



1 Hales, Stephen, Vegetable Staticks. London, 1727. 



- Wiesner, Julius, Untersuchungen über den Einfluss des Lichtes und der strahlenden Wärme auf die 



Transspiration der Pflanze. Sitzungsber. (math.-naturw. Kl.) K. Akad. Wiss. Wien 74' : 477-531 



187 7. 



3 Kohl, F. G., Die Transpiration der Pflanzen and ihre Einwirkung auf die Ausbildung pflanzlicher 

 Gewebe. Braunschweig, 1886. 



f Oil is apt to penetrate into the stem, and the wax seal is much to be preferred. For a 

 short distance above and below the water surface, the stem may be covered with some material 

 (as plastiline, chicle — the base of the common chewing-gum of the American market — etc.) 

 that does not absorb water and prevents the oil from coming into contact with the plant, in 

 which case the oil-seal method may be satisfactory. Some of the plastiline on the American 

 market is unsuitable, however, for it injures some plants.— Ed. 



