188 PLANT PHYSIOLOGY 



and suitable wax, while the latter also makes tight the junction of the 

 two halves of the plate and of the bell jar with the plate. If the halves of 

 the plate tend to spread apart, they may be kept together by a small wooden 

 wedge forced down between one of them and the iron ring. 



In the bell jar, which should contain a thermometer and hygrometer, 

 the conditions can be varied one at a time, — light by various forms of shad- 

 ing, humidity by drawing air through calcium-chloride tubes, or a wet 

 sponge, into the chamber, temperature by drawing the air through a glass 

 tube heated by a spirit-lamp or cooled by cold water or ice. 



Demonstration Methods. These have been indicated above. Potom- 

 eters of all degrees of accuracy and sensitivity can be adapted from sim- 

 ple materials, and show very striking results when the conditions are varied 

 before a class. 



The foregoing methods yield satisfactory data as to the effects 

 of external conditions upon transpiration, but do not admit of 

 a comparison between the different plants. For this it is neces- 

 sary to expose them to precisely the same conditions, which 

 for potted plants requires some form of meteorostat, but for 

 shoots can be effected by potometers in bell jars. Of course 

 the comparison must be made between equal leaf areas, prefer- 

 ably expressed in the gm 2 h system. The same end may, how- 

 ever, be attained in another way by taking the means of the 

 results given by the foregoing experiments, the various fluctua- 

 tions tending to balance one another through considerable inter- 

 vals of time. 



T he external conditions which m a y p<W» t r fi n ipirnt hn...nrr, 



nhvinnsly j not confined to thny ciirr^rrnrl ;„„ ^ Q transpiring 



leav es, but must include those which bear upon the absorbing 

 roots. Accordingly we now face this problem: 



What effect is produced upon transpiration by the principal 

 conditions affecting the water-supply? 



These conditions, in order of importance, are (a) quantity of 

 water available; (6) the temperature of the soil; (c) the soluble sub- 

 stances present. 



Experiment. Prepare a plant as for the preceding experiments, 

 and, keeping all other conditions as constant as possible, withhold 

 all water and determine the transpiration. 



Experiment. Prepare a plant as for the preceding experiment, 

 but with a good thermometer in the soil, and for a day determine its 

 transpiration under ordinary conditions. Then immerse the pot 



