1.2 2 



PHYSIOLOGY OF NUTRITION 



ascends. If a ring of bark is removed from a drooping branch 6f this willow, 

 the swelling develops not above but below the wound. 

 • §4. Th^ Transpiration Stream.^ — The upward movemerit 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, thp process being called trans- 

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



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

 some of which will now receive attention. 



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Fig, 77-— Apparatus for showing negative gas Pig. 78.— Malpighi's girdUng experi- 



pressure in wood. {After Pfeffer.) meitt; 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 4of 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 



■i WhUe 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 stiU not pass out of it, as when it becomes chemicaHy combined 

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

 starch, etc.). — Ed. 



• For an excellent review of the earlier literature of transpiration, see: Burgerstein A Die 

 Transpijation der Pflanzen. Jena, 1904.— £(i. ' "' 



