Grafts et al. 



186 



Water in Plants 



Quantitatively the amount of water transpired by a known leaf area 

 may be most accurately determined by drawing air of known moisture con- 

 tent over the leaf and subsequently through an absorbent such as anhydrous 

 calcium chloride or magnesium perchlorate. The increased gain in weight 

 of the absorbent over the gain found when an equal volume of air of the 

 same original humidity is drawn through the absorbent gives the weight of 

 water transpired. Such a method used by Freeman ( 1908) is a modifica- 

 tion of the methods of E. and J. Verschaffelt (1890) and Geneau de 

 Lamarliere (1892) and has been further modified by Minckler (1936) 

 and HiESEY (1940). The latter investigators used a vacuum pump to draw 

 air over the plant part and flow meters to determine the volume of air mov- 

 ing. The main objection to this method is that an artificial atmosphere 

 exists around the experimental part. 



Qualitative and semi-quantitative investigations have been carried out 

 using cobalt chloride paper, hygrometers (of horn, cellophane, and Yucca), 

 and calcium chloride-containing vessels attached directly to the transpir- 

 ing leaf. Of these only the cobalt 

 chloride method has been used exten- 

 sively and it has been standardized as 

 a quantitative method. Livingston 

 and his students have been especially 

 responsible for the development of this 

 method (Livingston, 1913; and Liv- 

 ingston and Shreve, 1916). 



Indirectly the amount of water ab- 

 sorbed can be used as an indicator of 

 water lost by the plant. Potometers 

 for cut and intact plants are especially 

 good for class demonstrations and, 

 when combined with the weight meth- 

 od, yield valuable data on the lag be- 

 tween water loss by transpiration and 

 water absorption. In this way the 

 fluctuation in the water content of a 

 plant can be studied. Such an appara- 

 tus is shown in Figure 49. The artifi- 

 cial environment surrounding the ab- 

 sorbing area is again a source of error. 

 In summary, it may be said that 

 extreme difficulty has been encountered 

 in devising a method of studying trans- 

 piration which is accurate but does not significantly affect the environment 

 of the experimental plant. The gravimetric and potometric methods on 

 whole plants are most likely to alter the action of absorbing organs and in- 

 directly the stomata while the volumetric method may alter the environ- 

 ment of the leaves. Any method utilizing cut parts must be employed 

 with extreme caution and only after adequately checking the various pos- 

 sible effects of cutting. Choice among the methods available must be made 

 after carefully considering and, if possible, measuring the effects that the 

 method will have on the plant itself. 



Expression of Results: — In order to express the amount of water 

 transpired on an area basis exact measurement of leaves is important. The 



\^ 



Fig. 49. — Potometer with grow- 

 ing plant for study of water absorption 

 and loss. Redrawn from Meyer and 

 Anderson (1939). 



