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cases, it is not possible to ascribe the death of the tree to 

 any one single factor. It is known that, as a general rule, the 

 water-current from the roots passes chiefly through the younger 

 external layers of wood, but it must be remembered that all 

 living tissues are more or less capable of water conduction, and 

 that the ascending water-current under ordinary circumstances 

 only makes use of the young wood layers because it can pass 

 through them more rapidly than it could through any other 

 tissue. If a ring- wound through the young wood now interrupts 

 these rapidly conducting channels the water- current is compelled 

 to make use of some other and less satisfactory channels. 

 Girdling may thus result in the water-current being carried 

 through the inner and older wood-rings which, under ordinary 

 circumstances, would not have been utilized, and although 

 this interferes with the rapidity of the current and is thus 

 more or less injurious, the death of the tree may not result from 

 this cause for a considerable period. It should also be noted 

 that the longer the distance which the water-current has to tra- 

 verse in slowly conducting channels, the greater will be the 

 interference with the rapidity of the current and, consequently, 

 the wider the ring, the more likely it is to be effective in causing 

 the death of the tree. We must, however, consider not only 

 the effect of the wound on the upward water-current, but also its 

 effect on the supply of food materials descending the stem from 

 the leaves. These food substances are mainly carried through 

 the sieve-tubes of the cortex. That the downward passage of 

 these materials is interrupted by the ring- wound is indicated 

 by the fact that a callus often begins to form only along the 

 upper edge of the ring, the severed tissues along the lower 

 edge, which now receive no food supplies from the crown, 

 being unable to produce a callus. Now if the passage of the food 

 materials from the crown to the base of the stem and the roots 

 is thus prevented, it is obvious that although the reserve mate- 

 rials in the tissues of the roots and the base of the stem may 

 for a time suffice for the nourishment of the cambium, for the 

 extended growth of the roots, and for the production of new root 

 hairs and water conducting layers of wood ; this cannot continue 

 indefinitely, and if no other source of food materials is made 

 available, the supply of water obtained from the soil by the 

 roots will gradually decrease, and the death of the tree eventu- 

 ally result from this cause alone. That the reserve materials 

 in the roots are thus sometimes exhausted is indicated by 

 the fact that, in some cases, no coppice shoots arise from the 

 stumps of trees which have been killed by girdling, there 



