THE MOLECULAR FORCES IN PLANTS. 



219 



under by no means very favourable conditions 7 gr. of water per 

 hour), etc. In winter we employ for this and other transpiration 

 experiments shoots of Fuchsia, which, especially in direct sun- 

 light, transpire very strongly. 



To make sure that plants 

 transpire far more feebly in 

 a moist atmosphere than in 

 one poor in aqueous vapour, 

 we first place our apparatus 

 for half an hour under a, 

 bell-glass whose inner surf ace 

 has been moistened with 

 water. We then expose it 

 for half an hour to the air. 

 We make the experiment, 

 however, not in the open air, 

 but in a room, taking care to 

 keep the plant exposed to the 

 same conditions of tem- 

 perature and illumination 

 throughout the experiment. 



At a high temperature a 

 plant gives off much more 

 vapour than at a low one. 

 The temperature in itself 

 and the hygrometric con- 

 ditions usually co-operate 

 in bringing this about. 



To bring out very clearly 

 the dependence of the 

 amount of transpiration on 

 the quantity of water in the 

 air and on the temperature, 

 I made experiments with 

 the apparatus illustrated in 

 Fig. 84. On a tripod stands 

 the zinc cylinder Z, 40 cm. 

 high and 24 cm. in dia- 

 meter. In the cylinder is a 



porcelain dish, P restino- FlG> 84 --Apparatus for investigating transpira- 

 tion. The zunc cylinder Z is supposed ta be trans- 



on a small tripod, while the pareat 



