96 PHENOMENA OF PLANT-LIFE. 



FIG. 44 represents the variations of pressure, as indicated by the mercurial 

 gauges, on the twenty-first of April, 1873, observations having 

 been taken every hour, from twelve A. M., to twelve p. M. 

 Every vertical line marks an hour, and every horizontal line 

 an inch on the column of mercury. Zero represents the point 

 where there is neither pressure outward from the tree, nor 

 suction inward. 



The line A shows the record of the sugar maplo, which at 

 midnight exhibited a suction equal to -6 inches, and at 7 A M. 

 had increased this to -22.9 inches. As soon as the sun 

 warmed the tree, the mercury began to rise and at 9.15 A. M., 

 had reached 16.3 inches. Then it declined very gradually till 

 at 12 P. M. it was at -3 inches. The temperature at 7 A. M. was 

 37 F. ; at 2 P. M. was 50.1 F. ; and at 9 P. M. it was 39.5 F. 



The line C marks the fluctuations of the mercury in the 

 lower gauge of the black birch, which was at the level of the 

 ground, and the line B shows the pressure in the upper gauge, 

 which was placed 30.2 feet above the lower one. The remark- 

 able fall, indicated as occurring at 12.45 P. M., was caused by 

 boring into the tree near the ground for the purpose of deter- 

 mining whether the tree was acting simply as a cylinder of 

 water filled by a force from beneath, as seemed evident from 

 the correspondence between the two gauges. The reduction 

 and restoration of pressure from simply opening and closing 

 the orifice were so rapid and extraordinary as to lead to the 

 conclusion that the force operating to produce the pressure 

 was simply the absorbent power of the roots, and this led to 

 the application of a gauge directly to a root, with the sur- 

 prising result described on page 253. 



The proportion borne by the cuts to the natural size of the object 

 represented is expressed by a fraction under the figure. Thus, in figure 

 one, the fraction indicates one-sixth the natural size, while in figure 

 three, the fraction * indicates that the object is magnified fifty times. 



