Page:Proceedings of the Royal Society of London Vol 1.djvu/140

Rh 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 attraction and peculiar conformation of the vessels, though the alburnum, consisting of such capillary tubes and vessels, appears manifestly to expand and contract under the various changes of temperature 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 experiments, our author is inclined to consider gravitation as the most extensive 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;