RESISTANCE BY LEAVES TO WATER LOSS 



11 



the same leaf at different stages of its development, in respect 

 to the resistance offered to transpiration. Also, we have here a 

 method which should be very valuable in comparing the tran- 

 spirational retardation of the foliage of different plant forms, 

 whether growing in the same or in remote localities, a line of 

 investigation which has not heretofore been attempted and which 

 should do much to advance both ecology and agriculture. As 

 far as resistance to transpiration is concerned, it would seem that 

 we might now be able to give something of a much needed quanti- 

 tative turn to the description and classification of ecological forms. 



The only cases so far met with in which the method just de- 

 scribed fails utterly are those of extremely xerophytic leaves, 

 upon which the hygrometric paper does not completely lose its 

 blue color. Such is true, for example, of the upper surface of the 

 older leaves of the tree tobacco, Nicotiana glauca Graham. Here 

 we have apparently to deal with a question, not of rate at all, but 

 of vapor pressure. It seems that the leaf surface in such cases 

 possesses a lower vapor pressure than does the hygrometric paper 

 at its critical point of color change. This whole matter still 

 awaits investigation. 



EXPERIMENTATION 



Although the method of standardized cobalt chloride paper may 

 be applied to other plant surfaces, the experimental data to be 

 presented in this section deal with foliage leaves only; foliar 

 transpiration is generally by far the most important form of water 

 loss, and "it has seemed best, in the beginning of studies of this 

 kind, to confine attention to foliar conditions. The following 

 experiments have been selected from a larger number, the aim of 

 this presentation being to bring out some of the uses of the stand- 

 ardized paper method, on the one hand, and to indicate, on the 

 other, some of the possible variations in foliar resistance to tran- 

 spiration. 



The potted plants for these studies were grown without shelter, 

 in sheet metal cylinders, and, while not as large as others of 

 similar age in the open soil, they showed no evidence of being 

 otherwise unusual. 



