WATER RELATIONS OF PLANTS 



297 



The more rapidly this change takes place, the higher the rate 

 of transpiration. By this simple method, it can readily be 

 demonstrated that there is a considerable difference in the rate 

 of water loss from the upper and lower surfaces of leaves that 

 have their stomata mainly or wholly on one surface. The 

 cobalt chloride method is only a qualitative one, though during 

 recent years repeated endeavors have been made to raise it to 



Fig. 91. — Transpiration clips. Spring wire is bent as shown and cemented to 

 small glass slips about 1)^ by 2 cm. in size. Two-millimeter squares of cobalt 

 chloride paper are then clipped to the leaves as shown, to measure their relative 

 transpiration rates (after Loomis and Shull) . 



the level of a quantitative method by measuring with a stop 

 watch the time required for the blue paper to turn pink. 



The most reliable methods of studying transpiration are those 

 based on changes in weight of the plant resulting from loss of 

 water. It may be supposed that such methods might introduce 

 a considerable error; for changes of weight depend not only on 

 the loss or gain of water, but also on the gain or loss of dry 

 substances, due to assimilation and respiration. Numerous 

 experiments have shown, however, that the weight of water lost 



