218 



PHYSIOLOGY OF NUTRITION. 



cemented together with a mixture of wax, mutton fat, and 

 olive oil, which melts at a comparatively low temperature. Into 

 the tubulus towards the bottom of the cylinder C is fitted the 

 graduated glass tube R, about 15 mm. in diameter, and contain- 

 ing water. The water in JB is covered with a thin layer of 

 oil. It is well to arrange for determining the temperature of the 

 water in the apparatus. In many cases it is advisable to cover 

 the water in the cylinder C with a layer of oil, the absorbing and 

 transpiring parts of the object being thus simply separated from 

 one another. The loss from transpiration can be estimated by 

 weighing the whole apparatus on a sufficiently sensitive pair of 

 scales. The amount of water absorbed is indicated by the tube 

 E, 1 attention of course being paid to the effect of changes of 

 temperature on the volume of the water. If a rooted plant be 

 exposed in the apparatus for a long time to constant external 



influences, equilib- 

 rium is set up be- 

 tween transpiration 

 and absorption of 

 water. But if, 

 owing to increased 

 temperature of the 

 air, the transpiration 

 is much intensified, 

 the plant loses more 

 water than it ab- 

 sorbs ; if, however, 

 it is now covered 

 with a bell-glass, 

 the absorption of 

 water preponder- 

 ates. 2 Good material 



FIG. 83. Scales for transpiration experiments. 



for the experiments is afforded by bean or maize plants developed 

 by the water-culture method, and cut shoots of woody plants, e.g. 

 Salix. In many cases, e.g. for purposes of demonstration, it is 

 sufficient to place cut shoots in a vessel of water, and cover the 

 water with a layer of olive oil. The loss from transpiration is 

 ascertained by weighing the apparatus. If very great accuracy is 

 not required, we may use scales (see Fig. 83). We may work 

 with twigs of Salix fragilis, Helianthus, Eucalyptus globulus (a 

 young shoot of this plant provided with twenty leaves transpired 



