WATER RELATIONS OF PLANTS 299 



Experiments with cut shoots and leaves are more convenient, 

 as there is no necessity of weighing heavy pots of soil, which 

 must inevitably affect the sensitivity of a balance. Besides, 

 no special containers are required for the growing plants. Hence, 

 this method of studying transpiration is widely used. The 

 shoot is placed with its cut end in water, so that the water lost 

 will be continually replaced. Precautions must be taken to 

 prevent the evaporation of water otherwise than through the 

 shoot. This is accomphshed by inserting the twig through a 

 hole in the stopper of the container (Fig. 94). Air is permitted 

 to enter the container through a glass tube inserted in the same 

 stopper. Experunents of long duration with cut plants are not 

 very reliable; for the conditions of water absorption are altered, 

 and when placed in direct sunlight, cut portions of a plant may 

 wilt rapidly. Frequently, cut leaves and shoots transpire more 

 rapidly, when placed in water, than do those remaining on the 

 plant, since they may have a better water supply. That is why 

 more reliable results are obtained if the cut parts of a plant are 

 immediately weighed upon cutting and the weighing is repeated 

 at short time intervals of 1 to 2 min. The cutting is best carried 

 out under melted paraffin in order to preclude the penetration 

 of air into the vessels from without, as well as the desiccation 

 of the surface of the cut. This method may give fairly exact 

 data on the transpiration of plants in the natural conditions of 

 their habitat. 



The determination of transpiration by measuring the volume 

 of water absorbed is carried out by means of various potometers, 

 which have already been discussed in Art. 67. Potometers are 

 useful for purposes of demonstration. They also have the 

 advantage that the readings may be taken uninterruptedly. 

 But the amount of water absorbed does not always coincide 

 with the amount of water transpired. Even during a limited 

 period, the error may be as high as 50 per cent. In exact experi- 

 ments, therefore, the gravimetric method of determining trans- 

 piration is to be preferred to the volumetric. 



In order to obtain a continuous record of the progress of 

 transpiration, self-recording mechanisms called 'Hranspiro- 

 graphs" are used. In the majority of cases, these are trans- 

 piration balances in which the movements of the hands or the 

 scales are transmitted to a revolving drum with a smoked surface. 



