214 PICTURESQUE EXPERIMENTS 



partial closure, and is seen to occur in the periods 

 of darkness (black), and to rise when the plant 

 is re-illumined. These changes are necessarily 

 accompanied by rise and fall in the evaporation 

 of the leaf, but into the question of the accuracy 

 of this correlation I shall not enter. 



There are other methods of demonstrating the 

 movements of the stomata. Stahl had the happy 

 inspiration of making use of the colour-changes of 

 cobalt chloride. A piece of filter paper soaked 

 in a 5 p.c. solution of this salt is blue when dried, 

 and turns pink in damp air. A dry piece of this 

 material, applied with proper precaution to the 

 stomata-bearing surface of a leaf, rapidly changes 

 to pink if the stomata are open. When, however, 

 the same trial is made on the upper surface of a 

 leaf, where stomata do not occur, no such change 

 occurs. If two leaves are treated at the same time, 

 one in the normal position and the other upside 

 down, it is delightful to watch the appearance of a 

 pink picture of that leaf whose stomatic surface is 

 in contact with the paper, while no such change 

 takes place over that which exposes no stomata to 

 the tell-tale material. Another method was dis- 

 covered by the accident of finding in an old house 

 in Wales a Chinese figure of a man, cut out of a 

 thin shaving of horn, which writhed and twisted 

 when placed on the hand. It was clearly very 

 sensitive to moisture, and it seemed possible that 

 horn-shavings might be used to test the condition 

 of the stomata. The first difficulty was to obtain 

 a supply of this material. Having discovered from 

 the P.O. Directory that there Were two horn- 



