134 



PHYSIOLOGY OF NUTRITION 



ascends. If a ring of bark is removed from a drooping branch of this willow, 

 the swelling develops not above but below the wound. 



§4. The Transpiration Stream. — The upward movement of the soil solution 

 in the plant depends upon a large number of conditions. Water can enter the 

 plant only if a part of the water already present be lost.'* Water is removed 

 from the plant by evaporation from the leaves, the process being called trans- 

 piration, and this is the main condition determining the movement of water. 



(a) Transpiration. 6 — Transpiration may be studied in a number of ways, 

 some of which will now receive attention. 



Fig. 77. — Apparatus for showing negative gas 

 pressure in wood. (After Pfeffer.) 



Fig. 78. — Malpighi's girdling experi- 

 ment; the twig is immersed in water to the 

 line h. 



i . The quantity of water transpired may be found by determining the loss in 

 weight of the plant and its container. The pot in which the plant is rooted is 

 hermetically sealed in a sheet-metal container. The seal (which may be of 

 plastiline or of a mixture of paraffine and petrolatum, etc.) should have three 



d While this is the main consideration, it may be remembered that enlargement alone, with- 

 out any loss of water, must necessitate water entrance into the enlarging cells. Also, water may 

 be removed from a cell and still not pass out of it, as when it becomes chemically combined 

 within (formation of carbohydrate from water and carbon dioxide, formation of glucose from 

 starch, etc.).— Ed. 



* For an excellent review of the literature of transpiration, see: Burgerstein, A., Die 

 Transpiration der Pflanzen. Jena, 1904. Also: Zweiter Teil (Ergänzungsband). Jena, 

 1920. — Ed. 



