132 THE PLANT WORI.D 



Our Teachers' Department. 



Edited by Professor Francis E. IvLoyd, 



Teachers College^ Columbia University, New York City. 



COBALT CHLORID IN TRANSPIRATION EXPERIMENTS. 



I DESIRE to bring to the attention of teachers of elementary botany a 

 well-known but little-used method of demonstrating quickly the loss of 

 water by plants. This is done by the use of cobalt chlorid. All that is 

 needed is a small amount of a 5-per-cent solution of this substance into 

 which some filter paper is dipped. When dry this paper will be brilliant 

 blue in color, but upon the slightest access of moisture the brilliancy of 

 color is lost and gradually the paper turns to pink. Cobalt chlorid is 

 thus seen to be a sensitive indicator of the presence of moisture. 



One method of using this cobalt paper is to suspend a bit of it, which 

 has been previously dried, inside a bell-jar, under which is placed a 

 growing plant, after taking the precaution however of covering the soil 

 and flower-pot in which it grows. After a short time and long before 

 any condensation of water takes place on the sides of the bell-jar, the 

 increase of humidit}^ of the enclosed air is indicated by the cobalt paper. 

 In order to make this experiment logical a similar bell-jar should be set 

 up with the cobalt paper inside, but without the plant. 



This experiment as here outlined was first introduced into elementary 

 work by Professor Atkinson of Cornell University,* and is especially 

 useful for demonstrating transpiration to a class. Individual students 

 may however use cobalt paper in a still more instructive way. 



If an uninjured leaf is laid on the table and a bit of dry cobalt paper 

 is laid upon it and covered with a small piece of glass, with merely press- 

 ure enough to apply the paper close to the surface of the leaf without 

 injury, the water escaping by transpiration will gradually change its 

 color and the rapidity of the change will depend of course upon the kind 

 of leaf and upon the number of stomata present. By this method, the 

 upper and lower surfaces of the same kind of leaf may be compared and 

 also leaves of different texture with different surface characters may be 

 studied comparatively. 



A very simple and effective little device for applying cobalt paper to 

 the upper and lower surfaces of the same leaf at the same time mav be 

 made as follows : 



Take two pieces of transparent celluloid about 1 Vz inches square and 

 a strip of cobalt paper about 2 inches long and % inch wide. Fold this 

 strip once across the middle. Place it between the pieces of celluloid, 

 which are to be held together by a wire paper clip. The spring of the 



*See " First Lessons in Plant lyife," p. 97. 



