iS COURSE OF SAP NOT PREVENTED BY WOUNDS. 



in hollow trees ; and the outer rind is of comparatively smaU 

 importance, for it is continually perishing under the influence 

 of the atmosphere : hut liber and alburnum are the seats of 

 vitality in trees and cannot he permanently injured without 

 destruction to the plant. 



If indeed this were absolutely the case it would be indis- 

 pensable that liber and alburnum should be most carefully 

 guarded ; and so they are in nature by the thick integument 

 of mere bark, which overlies them. But it continually hap- 

 pens that the usual vegetative processes are interrupted by 

 accidents, while the power of repairing injuries is so great 

 that many of the usual functions of a plant may be destroyed 

 without serious injury, such functions being performed ad 

 interim by other organs until the injury is repaired; so 

 that although, tinder ordinary circumstances, the sap of exogens 

 rises through the wood and descends through the liber, yet 

 the simpEcity of structure in plaiits is such, that, together 

 with the permeability of their tissue, it enables them to propel 

 their fluids by lateral instead of longitudinal communica- 

 tions. The trunk of a tree has been sawed through beyond 

 the pith in four opposite directions; namely, from north to 

 south, from west to east, from south to north, and from east 

 to west, at intervals of a foot, so as completely to cut off 

 aU longitudinal communication between the upper and lower 

 parts of the stem, as effectually as if those two parts had been 

 dissevered ; and yet the propulsion of the sap from the roots 

 into the head of the tree, and vice versd, went on as before : 

 which could only have been effected by a lateral transmission 

 of this fluid through the woody tissue. So when " ringing " 

 is practised, and the alburnum is partially destroyed, tixe 

 descending fluid diverges into the stratum of wood beneath the 

 annulation ; and when it has passed by, it again returns into 

 its accustomed channels. 



A striking example of this was given by Mr. Curtis in the Gardeners' 

 Chronicle for 1846 (p. 697). It was the case of a Pollard Ash-tree 

 (Fig. XI.) struck with lightning on the 7th or 8th of May, 1845 and 

 thus deprived of the bark all round the trunk for a space of eight feet at 

 fig. 3, and of three feet two inches at fig. 1. Nevertheless in July 1846 



