Exchange and Movements of Gases and Liquids. 73 



cells, and these finally become filled, and then some of the 

 liquid is pressed out through openings, collecting in drops. 

 The greater part of the dew formed on grass tips owes its 

 origin to this action. 

 ,1 67. Localization of transpiration. — If some method 

 of detection of watery vapors is applied to the surfaces of 

 a plant, the organs adapted for transpiration may be ascer- 

 tained. For this purpose two successful methods have been 

 found. In one, known as the " cobalt test," paper saturated 

 with salts of cobalt change color when coming in contact with 

 moisture. The second method consists in the employment of 

 a narrow strip of material which changes its form, on the addi- 

 tion and loss of moisture, in such a regular manner that it is 

 used as a hygrometer. 



To detect transpiration by the cobalt test, proceed as fol- 

 lows : procure a few small pieces of good filter paper, and sat- 

 urate in a 10 per cent, solution of cobalt nitrate in distilled 

 water, then dry thoroughly in sunlight. This should be done 

 by pinning the wet paper by one corner to a horizontal strip 

 of wood so that it may depend freely in the air. If the wet 

 paper is allowed to come into contact with other objects, it 

 may become stained and worthless. Secure a number of small 

 pieces of mica 2 cm. square. Cut a piece of the prepared 

 paper i cm. square, warm it over a flame, and lay it on the 

 upper surface of a leaf. Cover it with a square of mica, and 

 seal the edges of the mica to the leaf with a wax composed 

 of one part each of beeswax, vaseline, and resin. Attach a 

 similar preparation to the lower surface of another leaf. If 

 water vapor is given off by the surface underneath the mica, 

 the impregnated filter paper will be changed from a blue to a 

 reddish color. Compare reactions shown in ten minutes, an 

 hour, and two hours. Make tests of as many organs of a 

 species as possible, and describe the distribution of the sur- 

 faces from which watery vapor may be exhaled. 



