72 JOHN W. SHIVE AND WILLIAM H. MARTIN 



square was then folded back and over one of the openings was 

 placed a slip of cobalt paper, over the second and third openings 

 were placed slips of the dark blue and pale blue standards, respec- 

 tively. After this the squares of tape were cemented together 

 with the paper slips between, thus allowing each of the three paper 

 surfaces to be exposed only through the circular openings in the 

 tape. Each finished composite slip was permanently mounted 

 (by means of the free gummed surface of the tape) upon a small 

 cover glass cut to a suitable size (about 1.2 cm. by 1.5 cm.), 

 and the margins of the slip were trimmed to the edges of the 

 cover glass. Thus each composite slip was exposed to the leaf, 

 always with the same surface, the observations being made 

 through the cover glass. This feature of mounting the com- 

 posite slips on cover glasses does not interfere with the stand- 

 ardization nor with the drying of the slip, and it reduces the 

 possibility of the influence of the lateral leakage of moisture 

 upon the time required for the color change of the cobalt chlo- 

 ride paper, when the slip is in contact with the leaf surface. 

 Repeated tests with single mounted slips, over the standard 

 water surface, showed less variation in the time required for 

 the color change of the hygrometric paper than did similar tests 

 with unmounted slips. 



The Livingston method of applying the slips of hygrometric 

 paper to the plant surface makes use of a spring wire cUp, the 

 movable ends of which are extended by two small glass plates 

 which lie face to face when the clip is closed. The clip used 

 in the present work differed from the usual form by having a 

 small metal plate soldered to one of its free ends, the other free 

 end resting against the face of the metal plate at right angles 

 to its surface when the clip is closed. In making a test, the 

 leaf is held between the metal plate and the mounted slip, the 

 free end of the wire clip resting upon the cover glass of the slip 

 and near its center. The pressure exerted by the spring of 

 the clip is, therefore, imparted to the composite slip in such a 

 way as to secure uniform contact between the slip and the 

 leaf. 



The composite shps were dried over an electric hot-plate and 



