PHENOMENA OF PLANT-LIFE. 61 



in different parts of the tree, as recorded at the times speci- 

 fied. A reference to figure 44 will convey a correct idea of 

 the manner in which the mercury fluctuates during every hour 

 of the day and night. 



The following are some of the most interesting results 

 obtained from the several gauges : 



The wood of the detached root absorbed the water from the 

 gauge, so as to exert a suction, like the roots of most other 

 species of trees in early spring, but the pressure exhibited at 

 any time was scarcely worthy of mention. So strange did 

 this appear, that, on the fourth of April, the gauge was re- 

 moved to a healthy root, detached from another tree, and, to 

 avoid any possibility of error, it was afterward connected with 

 a third root, but the results were always similar. It is cer- 

 tain, therefore, from these observations, as well as those con- 

 nected with the water-gauge, described on a precedingpage, 

 that the rise and flow of maple sap is not directly caused by 

 the activity of absorbent rootlets. 



Secondly ; it is seen that the movements of the sap in the 

 heart of a tree are much less rapid and vigorous than those 

 occurring in the sap-wood at the same level. This is doubt- 

 less owing to the fact that the old wood is more dense, and 

 therefore less permeable to fluids than the outer layers of 

 alburnum ; and also to the circumstance that the variations 

 of temperature, at the depth of ten inches from the bark, are 

 necessarily slow and limited. 



Finally ; it remains to consider the extraordinary fact, that 

 the greatest suction, as well as the highest pressure, was 

 exhibited by the gauge in the top of the tree. On the eigh- 

 teenth of April, the lower gauge in the sap-wood indicated a 



