216 



PHYSIOLOGY OF NUTRITION. 



83. The Influence of External Conditions on the Transpiration 



of Plants. 



Very different forms of apparatus have been employed in the 

 numerous researches on the influence of external conditions on 

 transpiration. We will here make use of only a few which 

 I can recommend from my own experience. One has already been 



described in 82 and 

 represented in Fig. 79. 

 It is there indicated 

 that the plants may be 

 cultivated in the covered 

 glass vessel itself. We 

 may also grow them 

 in flower-pots, and place 

 these in a sufficiently 

 large glass vessel with 

 a ground rim and pro- 

 vided with a glass cover. 

 If we are working with 

 comparatively large 

 plants (e.g. Helianthus, 

 Nicotiana), and there- 

 fore have to use large 

 flower-pots in order to 

 ensure very vigorous 

 growth, we place the 

 pots in receivers made 

 of sheet zinc, and pro- 

 vided with divided 

 covers. In each cover 

 there must be a hole 

 to receive a thermo- 

 meter, and another for 

 the stem of the plant 

 to pass through. The junction of the halves of the cover may be 

 made air-tight by smearing with a cement (1 part of wax and f 

 part olive oil melted together). Experiments with comparatively 

 large plants (e.g. Helianthus, Nicotiana) are instructive because 

 they show us that even in a short time, e.g. twenty-four hours, 

 they give off to the air a very considerable quantity of water. 



FIG. 81. Apparatus for investigating transpiration. 



