THE MOLECULAR FORCES IX PLANTS. 



21' 



The second arrangement which may be conveniently employed 

 in experiments on the evaporation from plants, especially in 

 lecture demonstrations, is depicted in Fig. 81. The U-shaped, 

 fairly wide glass tube G is filled with water. One limb is closed 

 by a cork, through which passes the lower end of the shoot, Sp, 

 whose transpiration is to 

 be measured. The other 

 linib is closed by a two- 

 holed cork. Through one 

 hole passes the thermometer 

 T, through the other one 

 limb of the narrow bent 

 glass tube G', which is 

 filled with water. We 

 place the arrangement in 

 a wide glass vessel, and 

 transfer the whole ap- 

 paratus to a balance. We 

 are now in a position to de- 

 termine the loss of weight 

 suffered by the shoot as the 

 result of transpiration. We 

 can, however, at the same 

 time observe the loss of 

 water directly, since the 

 evaporation causes a de- 

 pression of the column of 

 water in the tube G'. 



The apparatus indicated 

 in Fig. 82 is especially to 

 be recommended for in- 

 vestigating transpiration. 

 The object under examina- 

 tion is fixed water-tight Fl 82. Apparatus for determining the 

 , -i amount of water absorbed and given off by tran- 

 Ito the upper end of the 8piring pi an ts. (After Pfeffer.) 



vessel (7, which contains 



water, and is, say, 25 cm. in height and about 6 cm. in diameter. 

 Wi- may experiment either with shoots or with rooted plants. 

 In the former case we close the vessel with a perforated rubber 

 stopper, and pass the shoot through it; in the latter we support 

 the plant by means of a divided cork, the halves of this being 



