222 PHYSIOLOGY OF NUTRITION. 



necessary to introduce into the apparatus the small dish 8 con- 

 taining' hot sand. It may be supported on a small tripod. In 

 observations on the influence of the quantity of aqueous vapour 

 present in the air on the consumption of water by the plants, P is 

 provided in parallel experiments with Calcium chloride and water 

 respectively. In experiments which I made in diffuse light with 

 Syringa shoots, the meniscus retreated 10 mm. every ten minutes 

 when the air surrounding the leaves was poor in aqueous vapour. 

 In a parallel experiment with air rich in vapour under Cr, the men- 

 iscus only moved back 3 mm. in ten minutes. For many detailed 

 researches on transpiration the apparatus of Kohl is also suitable. 

 In using this, and also with the arrangement depicted in Fig. 85, 

 it is to be observed that, while the absorption of water by plants 

 in general rises and falls with the transpiration, still its amount is 

 by no means under all circumstances determined by the transpi- 

 ration. The arrangements referred to may, however, be strongly 

 recommended for comparative investigations of short duration. 



We may get an arrangement very similar to that of Kohl by 

 replacing the dish P in Fig. 85 by a ground glass plate perforated 

 in the middle, taking great care to make the junction air-tight, 

 and introducing through the cork of the bell G tubes for leading 

 in and leading off gases. By means of an aspirator, we suck 

 through the apparatus air which, e.g. in experiments on the influ- 

 ence of temperature on transpiration, has first been thoroughly 

 dried by passage through vessels containing concentrated Sulphuric 

 acid and Calcium chloride. We readily warm the air by heating 

 the in-lead gas tube, in the neighbourhood of the bell-glass G, by 

 means of a spirit flame. 



It is instructive to prove that light accelerates transpiration. I 

 used in my investigations the apparatus represented in Fig. 79, 

 and experimented chiefly with Cucurbita. We make the observa- 

 tions in a room which during the experiments only receives diffused 

 light, and can without difficulty be quickly darkened, absolute ex- 

 clusion of light, however, not being at all necessary. It is best to 

 place the apparatus in front of a window on a balance. We illu- 

 minate it for one or two hours, then darken, e.g. by closing the 

 window shutters, and again illuminate at the end of one or two 

 hours, and so on. During each period the temperature of the soil 

 and air must be kept constant, and, further, the hygrometric con- 

 ditions of the air must not vary. As a hygrometer, it is convenient 

 to use a simple stand supporting two thermometers, of which one 



