80 THE WOOD. 



leaves more or less surround the whole stem, the new layer at length completes a zone, 

 and perfectly encloses the -wood of all former years. This is the explanation of the 

 term exogenous, which is derived from two words signifying to grow outwardly, for 

 the stem increases in thickness by successive layers on the outer side of the previously- 

 formed wood. That this is the mode of growth has been abundantly proved by experi- 

 ment, and demonstrated by accidental discoveries. Thus, if a plate of metal be inserted 

 between the bark and wood, it will in progress of time become inclosed by the new wood 

 which has overlaid them. So in like manner, if letters be cut deeply through the bark 

 and into the wood, the spaces will not be filled up from the bottom, but may be seen 

 in subsequent years overlaid by new wood. A statement appeared in a daily paper, 

 during the past year, to the effect that in cutting down a tree a cat had been discovered 

 inclosed in the wood of the trunk. These facts prove that the wood is applied from 

 without. Again, if a branch be stripped of its leaves down to a certain point, it will 

 not grow above that point ; and so, in like manner, if branches be stripped from one side of 

 a tree, the tree will not grow on that side. If a circle of bark be removed from a 

 branch above and also below a leaf, it will be found that increase of size will occur 

 below, but not above that bud ; and so, likewise, whenever a ring of bark is removed 

 from a tree, the new woody fibre will not proceed from the lower but from the upper free 

 edge. Further, if a scion be engrafted upon a stock having wood of a different colour 

 from that of the scion, it will be found that the wood produced from the scion overlays 

 that of the stock. This may actually be seen in operation in the spring season, if a 

 leaf be exposed immediately below its base ; for then bundles will be seen to shoot 

 below the ring of bark or cuticle, and to divide into two sets, one of which proceeds to 

 the liber, and the other to the wood of the trunk. 



These facts are undoubted, and the inferences seem to be indisputable ; but yet 

 various men of eminence have held contrary opinions. Thus, Linnaeus believed it to be 

 the produce of the pith, and Malpighi, that of the last year's wood ; whilst Du Hamel 

 affirmed that it was produced by neither, but solely from the cambium, which, according 

 to him, was secreted by the bark. It cannot be denied that the bark exercises an 

 influence in the formation of wood ; for if a zone of red bark be made to grow upon a 

 tree having white bark, all the wood appearing below this new bark will be red. But 

 this is not the result of any power in the bark to form wood, but simply that the wood, 

 as a part of the horizontal cellular system of a plant, has a controlling influence over 

 its secretions. These experiments, and others of a similar character, may be most 

 readily performed by any one of ordinary ingenuity. And what amusement could be 

 more instructive to our young friends of both sexes, and possibly through them to the 

 world at at large ? 



If our readers will cursorily glance at the cut surface of any stem, they will at once 

 perceive another fact in relation to the zones of wood, viz., that whatever may be the 

 thickness of the zone for the year, it is rarely equal around the whole circumference of 

 the stem. This is no matter for wonder ; but, on the contrary, it is surprising that 

 there is any approach to regularity, seeing that the position of leaves upin the branches 

 seems to be an accidental rather than a circular or spiral one. The occurrence is 

 readily accounted for on the theory above propounded, and is due to the lesser abun- 

 dance of leaves on the branches of one side than on the other, or to the prevalence of 

 winds, or some other physical cause, acting in that direction in opposition to the 

 growing process. 



