MOVEMENT OF MATERIALS IN THE PLANT 



123 



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 opiening of this tube the air in the apparatus re- 

 mains in equihbrium 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 evapor- 



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



ation.-'' The loss in weight of the apparatus, in this case also, is due almost 

 wholly to evaporation of water from the plant. ^ 



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



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



2 Wiesner, Julius, Untecsuohungen uber den Einfluss des Lichtes und der strahlenden Warme au£ die 

 Transspiration der Pflanze. Sitzungsber. (math.-naturw. Kl.) K, Akad. Wiss. Wien74^: 477-S31. 1877. 



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

 webe. Braunschweig, 1886. 



' 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. — Ei. 



