104 STOMATA. [CH. IV 



horn acquires a permanent curvature. In reading the 

 instrument it is best to note the position of the index 

 after a fixed interval, say 10", or fairly good readings may 

 be taken by waiting until the index comes to approximate 

 rest. To make an observation the leaf should be fixed, 

 stomatal side upwards, on a horizontal support by means 

 of small metal weights. 



With the instrument the experiments 118 B and 118 G 

 may be performed; in 118 B a preliminary opening of the 

 stomata may often be observed to occur at the moment 

 of severing the leaf from the plant. The closure of the 

 stomata in artificial darkness or as evening comes on may 

 be easily seen. 



(118 bis). The temperature of leaves in relation to trans- 

 piration 1 . 



Cat a Tropceolum leaf ( W) and wrap it round the bulb 

 of a thermometer graduated in T 1 Q C. and secure it with 

 a ligature ; then do the same with a Trop&olum leaf (F) 

 still attached to the plant. As soon as leaf W has had 

 time to wither, its thermometer will give a higher reading 

 than that of F. In sunshine the difference may be 4 C. 

 If the stomata of F are made to close, e.g. by means of 

 darkness, the temperatures of F and TF become practically 

 identical. 



(118 A) StahVs cobalt method 2 . 



It is well known that paper impregnated with a solution 

 of a cobalt salt, e.g. cobalt chloride, changes from blue to 

 red when it is placed in damp air and reassumes the blue 

 colour when dried. Stahl has been able by taking ad- 



1 Francis Darwin, Phil. Trans., 165 B 1898, p. 583. 



2 Stabl, Botan. Zeitung, 1894, p. 117. 



