FARMERS' REGISTER— VEGETABLE PHYSIOLOGY 



225 



Extracts from the Foreign Quarterly Review. 



DE candolle's vegetable physiology. 



The process used by nature to heal the wounds of 

 trees — theory of pruning. 



As these details are somewhat foreign to the spe- 

 cific object of his work, we shall not dwell upon 

 them, but pass on to a subject more strictly phy- 

 siological in some of its details, and of nearly equal 

 importance in the practical results which it in- 

 volves — we mean the process employed by nature 

 in healing whatever wounds trees may receive ex- 

 ternally. It is upon this that the whole success of 

 pruning rests, and we shall the more willingly 

 dwell upon the subject, as it is one which has late- 

 ly given rise to considerable discussion, it having 

 become a matter of doubt among some extensive 

 landed proprietors of this country, whether the 

 old system of " close pruning" has not been pro- 

 ductive of immense damage to our forests, and 

 whether it ought not to be entirely laid aside in fa- 

 vor of another system, which has been termed "fore- 

 shortening." These inquiries appear to have ori- 

 ginated from an examination of the effects which 

 are always produced in timber whenever a branch 

 is cut otF close to the trunk of the tree ; and it is 

 rightly asserted, that wherever this has been done, 

 a complete solution of continuity exists between 

 the old wood and that which had been formed over 

 the wounded surface ; and that in many cases, 

 though by no means in all, decay and rottenness 

 has been introduced to a greater or less extent into 

 the very heart of the tree. Where the pruned 

 branch is large, the blemish thus introduced is 

 proportionably great, and must always diminish 

 the value of the timber, even though it should not 

 have caused the introduction of any rottenness. — 

 But although such, it must be allowed, are unde- 

 niably the bad effects of close pruning, it does not 

 follow that the proposed alternative of fore-short- 

 ening is any way preferable to it; and we shall 

 first consider the mode in which all wounds are 

 healed over, and then examine the question whe- 

 ther the damage produced by fore-shortening is 

 more likely to be counteracted by the subsequent 

 efforts of vegetation than those which are intro- 

 duced by close pruning. 



Whenever the stem is wounded by the removal 

 of a portion of the bark, and the wood becomes 

 exposed to the action of the atmosphere, a double 

 influence is exerted in gradually decomposing its 

 texture : one arises from the union of its carbon 

 with oxygen, as in the ordinary process of decom- 

 position ; and the other from the humidity by 

 which it is penetrated, dissolving some parts of the 

 tissue, and in reducing it generally to a soft and 

 disorganised state. These results will of course be 

 very different according to the nature of the wood 

 attacked, and to the length of time that the wound 

 requires to be healed over. As soon, however, as 

 the wood is secured Irom further exposure to the 

 atmosphere, the damage ceases to increase, though 

 the blemish which has been introduced admits of 

 no remedy. The new wood and bark which form 

 over the wound, are derived Irom the growth of the 

 alburnum and liber, which gradually extend them- 

 selves from its upper edge, and from along each 

 side, till they meet in the middle and then unite 

 and blend together as in the case of grafting. By 

 judiciously splitting a rock vertically at the zone 

 which corresponds to that year's growth in which 

 Vol 1,-29 



the surface of a pruned branch was covered oyer 

 by the fresh wood, every mark of the pruning 

 knife will be found on the discolored surface of the 

 old wound, as fresh as when it was first impressed 

 upon it, and the new wood will have received a re- 

 verse impression of this surface as accurately as a 

 counter receives the stamp of a die. Vertical 

 wounds on the surface of the trunk are those which 

 heal the most readily, because their direction tal- 

 lies with the course of the cambium, which soon 

 forms a tumor at the upper extremity of the wound 

 and down each side, in the manner just described ; 

 and this is more readily extended over the wound 

 in proportion as its surface is smoother. V^arious 

 composts are useful for protecting the exposed sur- 

 face from the atmosphere, whilst the healing pro- 

 cess is in progress : but nothing of an oily or pois- 

 onous description should be employed. When- 

 ever, therefore, pruning is absolutely necessary, 

 it is advisable to prune close, in order to reduce 

 the exposed surface to the condition of a vertical 

 wound ; unless, indeed, the limb be very large, 

 when it may be more adviseable to prune at some 

 little distance from the trunk, lest the blemish 

 which would be introduced into the timber should 

 be so considerable as to more than counterbalance 

 any advantage that wou'd be obtained. There is 

 no direct means by which a transverse section 

 through the wood may be healed over, and if a 

 branch be lopped at a distance from its point of 

 union with a main branch, or with the trunk itself, 

 the exposed surface never heals over, but causes 

 the decay and death of the branch for some dis- 

 tance back, until this is stopped at some spot where 

 the returning sap is in sufficient quantity to pro- 

 duce fresh wood and bark. The system of " fore- 

 shortening" rests upon the gradual decay of the 

 pruned branch, until it be ultimately killed by the 

 increasing shade of the superior branches, Avhen its 

 fall will take place in the natural way ; as in all 

 branches which grow low upon the stem, and are 

 early stifled by the shade of the upper branches, 

 and which slough off, without producing any very 

 marked blemish in tlie heart of the tree. Here, 

 however, we must observe, that there is no pro- 

 cess for " sloughing off" the decayed parts of ve- 

 getables which at all resembles that which takes 

 place in animals ; but when the branch has be- 

 come so completely rotten, as to fall off upon the ap- 

 plication of the slightest force, it will be found that 

 the new bark and alburnum which are formed 

 round the base of its stump, always envelopes more 

 or less of the rotten wood, which forms a rough 

 and jagged surfixce to the wound. It is erroneous 

 to suppose that those branches which fall off by a 

 sort of natural pruning, resulting from their being 

 killed by an obstruction of the light, leave compara- 

 tively little or no trace of their decay in the heart 

 of the tree ; but since it happens that those branch- 

 es which perish early are always proportionably 

 small, when compared with the bulk to which the 

 trunk attains, the blemishes which they leave may 

 easily be underrated, and this we believe to have 

 been the origin of the error which supposes that 

 the blemish introduced upon the natural decay and 

 fall of a branch is, ceteris paribus, of less conse- 

 quence than that which results upon closely pru- 

 ning it. The danger which attends all pruning 

 may be diminished by paying attention to a few 

 rules, such as cutting the surface quite smooth, 

 cutting it obliquely so as to prevent the wet from 



