TRANSPIRATION AND ROOT-PRESSURE. 



277 



root-Stock is connected with an exit tube, the water disposed of by the leafy top, 

 as well as the outflow from the root-stock, may be directly observed. This 

 proves now that the quantity of water required in suction or transpiration at the 

 top is far larger than that driven into the stem by the root-pressure ; the transpiring 

 leafy top of a Tobacco-plant, for instance, took up 200 cubic centimetres of water 

 in the same time that the root-stock discharged only fifteen cubic centimetres. 

 It follows even from this fact that the transpiration-current cannot be explained 

 by the root-pressure. Yet more definitely, however, is this conclusion to be drawn 

 from the fact that, as a rule, no root-pressure at all exists in a plant in which 

 vigorous transpiration is going on. If the leafy top of a Tobacco-plant, Sunflower, 

 Potato, or Gourd-plant, &c., is cut off in sunshine, that is while vigorous transpiration 



Fig. 214.—-^ a stein of Helianthus annuus (a) r^ 

 tube b,c,d\ the water forced out at c by the ro 

 same plant (/) placed in a graduated cylinder (£). 



:-pressure is measur 



er-pot and furnished with an 

 ed in e. B the top of 



and ascent of water are proceeding, and a glass tube is at once placed on the 

 stump projecting out of the earth, and if a quantity of water is then poured into 

 this, it is observed that the latter is sucked into the wood of the root-stock. 

 This proves that at the time of transpiration the cavities of the root-stock are 

 not filled with water, but that they must contain rarefied air, which enables water 

 to penetrate into the cavities. This fact, again, completely excludes the assumption 

 that the forcing up of the water by the root can in any way co-operate in the transpi- 

 ration-current ; it shows, on the contrary, that the phenomenon of the weeping of 

 the root-stock only comes into existence when the transpiration-current has been 



