136 PHYSIOLOGY OF NUTRITION 



apparatus, shown in Fig. 79, being well suited to such studies. The roots of 

 the plant, together with a thermometer, are placed in a tube of water (r), which 

 communicates below with a long capillary glass tube and also with a rubber 

 tube closed with a glass plug (gl). As transpiration proceeds, the water menis- 

 cus advances along the capillary tube. To refill the latter, the glass plug is 

 simply inserted somewhat farther into the rubber tube. By placing a bell-jar 

 over the plant the atmosphere surrounding the latter may be kept either moist 

 or dry. To keep it moist a sponge saturated with water may be placed under the 

 bell-jar, the walls of which may also be moistened. To keep the atmosphere 

 dry, air may be drawn by an aspirator through a series of wash bottles filled 

 with concentrated sulphuric acid or with pieces of pumice saturated with this 

 acid. The plant may be kept in darkness by covering the bell-jar with an 

 opaque paper cylinder. 



3. Finally, the amount of liquid water absorbed and the amount of water 

 vapor lost at the same time may be determined. In this connection, Vesque's 1 

 apparatus may be used, which consists of a U-shaped tube, one arm of which is 

 broad and the other narrow. This is filled with water and the roots of the plant 

 are placed in the broad arm with a tightly fitting stopper about the stem. Loss 

 in weight of the entire apparatus gives the quantity of water evaporated, 

 while the depression of the water in the narrow arm indicates the amount of 

 water absorbed by the plant." 



In addition to the apparatus already described, cobalt paper was employed 

 by Stahl 2 to study transpiration. Swedish filter paper is dipped in a 5-per cent. 

 solution 3 of cobalt chloride, and is then dried in the sun or in an oven. It 

 should be stored in a dry place. This paper is intensely blue when dry but the 

 color changes to a bright pink as water is absorbed. The paper is placed upon 

 the leaf surface that is to be studied, and is covered with a small glass or mica 

 plate. For example, a slip of dry cobalt paper, placed against the lower sur- 

 face of a leaf with stomata on this side only, turns pink in a few seconds on a 

 sunny day, but may remain blue for several hours when placed against the upper 

 leaf surface, where stomata are lacking. This experiment shows clearly the 

 influence of stomata upon transpiration.* 



1 Vesque, Julien, L'Absorption comparee directement a. la transpiration. Ann. sci. nat. Bot. VI, 6 : 

 201-222. 1877. 



2 Stahl, 1894. [See note 1, p. 36.] 



3 Weaker solutions (1- or 2-per cent.) are more suitable in delicate tests, where the differences in trans- 

 piration are small. 



It is not strictly true that loss of weight in these experiments is to be interpreted 

 solely as loss of water, though other losses are generally negligible. Perhaps the only case 

 where significant errors may be involved on account of this assumption is that in which 

 leaves, etc., fall from the plant during an experiment. For a complete picture of the 

 meaning of loss of weight, however, aside from such obvious accidents as the fall of leaves, 

 it should be remembered that carbon dioxide and oxygen leave the plant in the same way 

 as does water vapor, that absorption of these two gases also occurs, and that many vola- 

 tile oils, etc., also evaporate into the air to some extent. — Ed. 



h The cobalt-chloride method really furnishes a means for measuring only the power of the 

 leaf to retard water loss by transpiration, the transpiration rate itself depending upon the 

 evaporating power of the air and upon the intensity of absorbed radiant energy as well as 

 upon this power. On various improvements upon Stahl 's method and upon the transpiring 



