114 THE PLANT 



compared with those obtained from absorption experiments made with 

 plants rooted in soil before they can be relied upon. This necessity practi- 

 cally puts the potometer out of commission for accurate work, tinless future 

 study may show a somewhat constant ratio between the absorption of a plant 

 in its own soil and that of a plant placed in a potometer. 



4. Measuring absolute humidity. The cog psychrometer makes it 

 possible to determine the increased relative humidity produced within a 

 glass cylinder or special tin chamber by a transpiring plant. From this 

 result the absolute humidity is readily obtained, and by means of the latter 

 the actual amount of water given off. The evident drawback to this 

 method is that the increasing humidity within the chamber gives results 

 entirely abnormal for the plant concerned. 



5. Self-registering instruments. There are various methods for regis- 

 tering the amount of transpiration, based upon weighing, or upon the poto- 

 meter. The Richard recording evaporimeter has all the advantages of 

 weighing, inasmuch as the water loss is measured in this way, and in addi- 

 tion the amount is recorded upon a revolving drum, obviating the necessity 

 of repeated attention in case it is desirable to know the exact course of 

 transpiration. On the other hand, methods which depend upon the poto- 

 meter, while graphic, are not sufficiently accurate to b'C of value. 



6. The use of hygroscopic materials. Hygroscopic substances change 

 their form or color in response to moisture. As they indicate comparative 

 water loss alone, they are of value chiefly in the study of the stomatic 

 surfaces of leaves. F. Darwin^ has used strips of horn, awns of Stipa, and 

 epidermis of Yucca to construct small hygroscopes for this purpose. In 

 these instruments the error is large, but as no endeavor is made to obtain 

 exact results, it is negligible. Filter paper impregnated with a 3-5 per 

 cent aqueous solution of cobalt chloride is deep blue when dry. If a strip 

 of cobalt paper is placed upon a leaf and covered with a glass slip it turns 

 bright rose color, the rapidity of the change affording a clue to the amount 

 of transpiration. 



158. Field methods. The conditions which a satisfactory field method 

 of measuring transpiration must fulfill have already been discussed; they 

 are accuracy, simplicity, and normality. These conditions are met only by 

 weighing the plant in its own soil and habitat. This has been accomplished 

 by means of the sheet-iron soil box, already described under the determina- 

 tion of the chresard. The method is merely the familiar one of pot and 

 balance, slightly modified for field use. The soil block, which contains the 



1 Observations on Stomata by a New Method. Proc. Camb. Phil. Soc, 9:303. 1897. 



