DESCENDING SAP. 71 



Upon this has been built the theory that wood grows downwards from 

 leaves — ^whieh is now known to be erroneous ; but although wood does 

 not descend from leaves, it is certain that the organised matter out of 

 which wood is formed does descend. If, indeed, the descent of elabo- 

 rated sap is denied, it becomes impossible to explain why, when a ring 

 of bark is removed from a branch, the new growth takes place princi- 

 pally on the upper edge of the wound and very slightly on the lower ; 

 and why gum, prepared by the leaves of a Potato, is afterwards found 

 in the tubers, in the final form of starch. It is impossible for those 

 who have any practical acquaintance with living plants not to agree 

 withProf. Mohl(2%eFe^e<a6fe cell. p. 71, English edition), that a denial 

 of a descending current of sap in bark is quite incomprehensible. 

 Certainly, as he says, it is no improvement upon the theory which men 

 attempt to cast aside to say that increased growth above an anntdar 

 wound is explained by artificial interruption of the upward current of 

 crude sap, in consequence of which the fluids contained in the upper part 

 of the plant must soon become greatly concentrated and potential for 

 development. "When we can succeed in fattening an animal by 

 depriving it of a portion of its accustomed food this explanation may be 

 received as satisfactory." 



This explains why there is usually more wood on the south 

 side of a tree than on the north ; and why depriving trees of 

 their branches (and leaves) is invariably attended by a 

 diminution of the quantity of timber. 



Upon this curious subject some of the best observations are those of 

 Van HaU, as recorded in one of the reports of the Ray Society. This 

 gentleman remarked that the growth of trees in thickness only com- 

 mences after the leaves are capable of fulfilling their functions ; this 

 was proved by aU hia measurements. The influence of the leaves upon 

 the increase of trunks in thickness exhibited itseK most distinctly in 

 the Italian Poplar. On one of these trees being deprived of almost all 

 its branches, in the month of March, the increase in thickness was pro- 

 portionably sUght during the months of Jxme and July. The growth 

 of a Lime-tree, on the other hand, in which the side branches, also 

 those lower down on the trunk, as weU above as beneath the point of 

 measurement, had, for the greater part, been purposely left, was consi- 

 derable, and increasing every year. An experiment was made with 

 two equal sized Oaks, situated under the same circumstances ; all the 

 lateral branches were taken from one and left on the other ; the result 

 was, that the iacrease of thickness, in the tree which had not been 

 pruned, was much more considerable than in the one which had been 

 pruned. But the fact is familiar to all intelligent woodmen. 



It may be regarded as an axiom in horticulture that the health of 



