DESCENT OF THE SAP IN TREES. 101 



the bark has been most reduced, the greatest quantity of wood has been 

 deposited. 



Other causes of the descent of the sap towards the root I have sup- 

 posed to be capillary attraction, and something in the conformation of 

 the vessels of the bark. The alburnum also appears, in my former 

 experiments, to expand and contract very freely under changes of 

 temperature and of moisture ; and the motion thus produced must be in 

 some degree communicated to the bark, should the latter mb$t%4co. jJ bp 

 in itself wholly inactive. I however consider gravitatioii &s th& most- 

 extensive and active cause of motion in the descending mrids or trees; 

 and I believe that, from this agent, vegetable bodies, like unorganized 

 matter, generally derive, in a greater or less degree, the forms they 

 assume : and probably it is necessary to the existence of trees that it 

 should be so. For if the sap passed and returned as freely in the 

 horizontal and pendent, as in the perpendicular branch, the growth of 

 each would be equally rapid, or nearly so : the horizontal branch would 

 then soon extend too far from its point of suspension at the trunk of the 

 tree, and would inevitably perish, by the increase in a compound ratio of 

 the powers of destruction, as compared with those of preservation. 



The principal office of the horizontal branch, in the greatest number 

 of trees, is to nourish and support the blossoms, and the fruit, or seed ; 

 and as these give back little or nothing to the parent tree, very feeble 

 powers alone are wanted in the returning system. No power at all had 

 been fatal; and power sufficiently strong wholly to counteract the 

 effects of gravitation had probably been in a high degree destructive. 

 And it appears to me by no means improbable, that the formation of 

 blossoms may, in many instances, arise from the diminished action of 

 the returning system in the horizontal or pendent branch. 



I have long been disposed to believe the ascending fluids in the albur- 

 num and central vessels, wherever found, to be everywhere the same ; 

 and that the leaf-stalk, the tendril of the vine, the fruit-stalk, and the 

 succulent point of the annual shoot, might in some measure be substituted 

 for each other ; and experiment has proved my conjecture, in many 

 instances, to be well founded. Leaves succeeded and continued to per- 

 form their office when grafted on the leaf-stalk; the tendril and the 

 fruit-stalk alike, supplied a branch grafted upon them with nourishment. 

 But I did not succeed in grafting a fruit -stalk of the vine on the leaf- 

 stalk, the tendril, or succulent shoot. My ill success, however, I here 

 attribute solely to want of proper management, and I have little doubt 

 of succeeding in future. 



