THE MOLECULAR FORCES IN PLANTS. 



215 



use, we see, even after a very short time (a few seconds ta a 

 minute), that the paper 

 in contact with the lower 

 surface of the leaf as- 

 sumes a red tint, while 

 the other retains its blue 

 colour. This result is 

 always obtained when 

 we experiment with 

 leaves (leaves of Phaseo- 

 lus, Salix Caprea, Populus 

 nigra, Liriodendron, 

 Syringa vulgaris, Cycla- 

 men leaves of the last 

 plant are available even 

 in winter) whose upper- 

 surface contains few or 

 no stomata. The red- 

 dening of the paper is 

 due to the action on the 

 Cobaltous chloride of the 

 aqueous vapour escaping 

 from the stomata ; only 

 traces at most traverse 

 the cuticle, so that the 

 paper in contact with the 

 upper leaf surface suffers 

 little or no change of 

 colour. 



In investigating the 

 evaporation of a leaf on 

 the intact plant, it is 

 generally advisable to 

 employ large thin sheets 

 of mica instead of the 

 heavy glass plates. They 

 may be fixed by means of 

 small clamps. 



FIG. 80. Apparatus for investigating transpira- 

 tion. 



1 See Garreau, Ann. d. Sc. Nat., 1850. 



2 See Stahl, Botan. Zeitung., 1894. 



