GROWTH BT THE STEM. 38 



central wood is of little consequence, and may be de- 

 stroyed, as it constantly is. in hollow trees ; and the 

 rind is of comparatively small importance, for it is 

 continually perishing under the influence of the at- 

 mosphere : but the liber and alburnum are naturally 

 in a state of constant renovation, and cannot -be per- 

 manently injured without injury to the plant. 



57. But although, under ordinary circumstances, 

 the sap of Exogens rises through the alburnum and 

 descends through the liber, yet the simplicity of 

 structure in plants is such, that, together with the per- 

 meability of their tissue, it enables them, in cases of 

 emergency, to alter their functions, and to propel 

 their fluids by lateral instead of longitudinal commu- 

 nications. 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 all longitudinal com- 

 munication 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 went on as before : 

 which could only have been effected by a lateral 

 transmission of this fluid through, or between, the 

 sides of the woody tissue. So when " ringing" is 

 practised, and the alburnum is partially destroyed, 

 the ascending fluid diverges into the stratum of wood 

 beneath the annulation ; and, when it has passed by, 

 it again returns into its accustomed channels ; at the 

 same time, it is probable, although not proved, that 



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