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TEXTBOOK OF PLANT PHYSIOLOGY 



stopper of the container (Fig. 68). Air is permitted to enter the 

 container through a glass tube inserted in the same stopper. In 

 experiments of a short duration with slowly transpiring plants, 

 cut parts from a plant are weighed without the water. In either 

 case, however, cutting is not without an influence on transpiration. 

 Even when placed in water immediately, cut portions of a plant 

 may wilt rapidly in direct sunlight. 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. If 

 one tries to calculate the total amount of water 

 given off by the tree from the transpiration of 

 a single leaf, and the number of leaves on a 

 tree, he runs the risk of obtaining abnormal 

 values. Experiments with cut organs of the 

 plant, therefore, give no reliable data as to the 

 magnitude of their transpiration under normal 

 conditions. 



Still less reliable results are obtained by 

 methods that are based on the determination 

 of the volume of water absorbed by the plant, 

 as when different kinds of potometers are used, 

 to which attention already has been called in 

 Art. 46. Potometers are useful for purposes 

 of demonstration. They have one advantage: 

 the readings may be taken uninterruptedly. 

 But the amount of water absorbed does not 

 Fig. 68. — Method of a } wa ys coincide with the amount of water 



determining the water , _ . .. , . , ,, 



used by a shoot. transpired. Even during a limited period the 

 error may be as high as 50 per cent. In exact 

 experiments the gravimetric method of determining transpiration 

 is to be preferred to the volumetric. 



In order to obtain a continuous record of the march of trans- 

 piration, self-recording mechanisms called "transpirographs," are 

 used. In the majority of cases, these are transpiration balances 

 in which the movements of the hands or the scales is in some way 

 transmitted to a revolving drum with a smoked surface. Experi- 

 ments with transpirographs are usually successful only in a closed 

 room, such as a greenhouse. When used out-of-doors, the accu- 

 racy of the records is interfered with by wind. 



