120 



from the following experiment. Several young standard apple-trees 

 were, by means of stakes and bandages, prevented from yielding to 

 the impulse of the wind up to about the middle of their stems, the 

 upper parts of the stems and the branches being left in their free na- 

 tural state. In the course of one summer it was found that much 

 new wood had accumulated in the parts which were kept in motion 

 by the wind ; whereas the lower parts of the stems and roots had 

 increased very little in size. One of these trees was afterwards con- 

 fined in such a manner that it could only move in one direction, viz. 

 north and south : thus circumstanced, the diameter of the tree from 

 north to south, in that part of the stem which was most exercised by 

 the wind, exceeded that in the opposite direction, in the following 

 autumn, in the proportion of 13 to 11. Several curious inferences 

 may be hence deduced as to the growth of trees in different situations. 



In those which are exposed on high grounds, and are kept in al- 

 most continual motion, the sap circulates with great rapidity, and 

 will be accumulated chiefly in the roots and lower parts of the trunk; 

 and hence the diameter of the trunk will diminish rapidly as it re- 

 cedes from the root : the progress of the ascending sap will of course 

 be impeded, and it will thence cause lateral branches to be produced, 

 the forms of which will be similar to that of the trunk ; and thus the 

 growth of an insulated tree on a mountain will be, as we always find 

 it, low and sturdy, and well calculated to resist the heavy gales to 

 which, from its situation, it is constantly exposed. Trees, on the 

 other hand, which grow in clumps or sheltered situations, where, for 

 want of motion, the sap is retarded both in its ascent and descent, 

 will acquire a very different habit, and even their wood a different 

 texture, insomuch that a great deal of the timber found in old build- 

 ings in and about London, which has always been considered as 

 Spanish chestnut, appear on close examination to be most evidently 

 forest oak. When a tree is wholly deprived of motion, it often be- 

 comes unhealthy, and not unfrequently perishes, apparently owing 

 to the stagnation of the descending sap under the rigid confinement 

 of the lifeless external bark. Stripping off this bark has been found 

 singularly beneficial towards the increase both of the trunk and 

 branches. 



As to the third cause of the descent of sap, viz. the capillary at- 

 traction and peculiar conformation of the vessels, though the al- 

 burnum, consisting of such capillary tubes and vessels, appears mani- 

 festly to expand and contract under the various changes of tempera- 

 ture and moisture in the atmosphere ; and though the motion thus 

 produced must be in some degree communicated to the bark and 

 other contiguous parts, yet combining the results of all his experi- 

 ments, our author is inclined to consider gravitation as the most ex- 

 tensive and active cause of motion in the descending fluids of trees. 

 An observation which corroborates his assertion is, that if the sap 

 impelled by causes more powerful than gravitation were to pass and 

 return as freely in the horizontal and pendent as in the perpendicular 

 branches, the growth of each would be equally rapid, or nearly so ; 



