153 



to select from tlie soluble materials of the soil just those substances 

 which every species needs for its peculiar constitution. 



Herbert Spencer has attempted to demonstrate that the compression 

 and dilatation of cells and ducts, caused by the swaying of stems and 

 branches in the wind, is an important aid in promoting the flow of sap. 

 When we consider that many trees grow where the wind scarcely affects 

 them, and that plants flourish in glass-houses, where they are never 

 disturbed in this way, we shall see that this hypothesis is of small ac- 

 count. There is also here, as in the hypothesis of Malpighi, a need of 

 valves to prevent regurgitation, and we have during the present season 

 demonstrated that detached living roots, entirely underground, exert 

 an enormous force merely by their power of absorption. 



In 1720 Kev. Stephen Hales, an English clergyman, made some exper- 

 iments on the absorption and exhalation of water by plants, and on the 

 force with which absorption was exerted. He applied mercurial gauges 

 to several different grape-vines, and obtained, as the maximum pressure 

 exerted by rising sap, a force sufficient to sustain a column of mercury 

 38 inches in height, which is equal to a column of water 43 feet high. 

 To learn how far this might be true, and what were the facts concerning 

 the spring flow of sap in our forest-trees, and especially in the sugar- 

 maple, in regard to which scarcely any accurate observations had been 

 made, we began some investigations at the agricultural college last 

 March, the results of which may be summarily stated as follows : 



A gauge was attached to a sugar-maple March 31, which was three 

 days after the maximum flows of sap for this species, so that further ob- 

 servations are required earlier in the season to complete the record, and 

 determine with certainty the maximum pressure which it exhibits in 

 the spring. Of the record made the following facts are specially inter- 

 esting : 



First, the mercury was subject to constant and singular oscillations, 

 standing usually in the morning below zero, so that there was indicated 

 a powerful suction into the tree, and rising rapidly with the sun until 

 the force indicated was sufficient to sustain a column of w^ater many feet 

 in height. Thus at 6 a. m., April 21, there was a suction into the tree 

 sufficient to raise a column of water 25.95 feet. As soon as the morning sun 

 shone upon the tree the mercury suddenly began to rise, so that at 8.15 

 a. m. the pressure outward was enough to sustain a column of water 18.47 

 feet in height, a change represented by more than 44 feet of water. On 

 the morning of April 22 the change was still greater, requiring for its 

 representation 47.42 feet of water. These extraordinary fluctuations w^ere 

 not attended by any peculiar state of the weather, and happened twelve 

 days before there were any indication of growth to be detected in the 

 buds. These observations are quite new, and as yet wholly inexplicable, 

 but will receive further attention at a future time. 



On the 20th of April two gauges were attached to a large black birch, 

 one at the ground and the other 30 feet higher. The next morning at C 

 o'clock the lower gauge indicated the astonishing pressure of 50.65 feet 

 of water, and the upper one of 20.54 feet. The difference between, the 

 indications of the two gauges was thus 29.92 feet, while the actual dis- 

 tance between them is 30.20 feet, so that they corresponded almost 

 precisely as if connected by a tube. In order to learn whether the same 

 principle would prevail if the other gauge was moved, it was raised 12 

 feet higher. The same correspondence continued through nearly all the 

 observations of the season, notwithstanding the gauges were separated 

 by 42.20 feet of close-grained birch-wood. The sap-pressure continued 

 to increase until, on the 4th of May, it represented a column of water 



