CAUSE OF GUM IN STONE-FRUIT TREES. 



227 



inclies below the surface. I have taken as 

 much as one pound of gum from a single tree, 

 and those trees which have the largest exuda- 

 tions, I generally find the naost vigorous, and 

 have made the largest growths. ■ In fact, I have 

 never seen trees, either in the cold climate of 

 Britain, or in the northern New England 

 States, exude gum so excessively as in the 

 warm and temperate climate of Maryland, un« 

 der conditions the reverse of those which 

 Knight, Loudon, and other eminent horti- 

 culturists, attributed its production. 



It has likewise been supposed, that the de- 

 positions of gum Avere made in tlie spring, from 

 the ascending juices, a kind of spontaneous 

 extravasation, resulting from a superabundance 

 of ascending sap, which the leaves are unable 

 to assimilate, or to throw off by perspiration ; I 

 am fully satisfied that such is not the case, for 

 I have made incisions in trees for the purpose 

 of extravasation, when the vital forces of the 

 trees were most active, in the spring, but in 

 every case the wound thus made became ex- 

 siccated after tlie bleeding had ceased; in no 

 case has the sap thus exuded been transform- 

 ed into gum. As the season advances, small 

 portions of the gum are formed, and towards 

 the end of summer and autumn, it accumu- 

 lates in large quantities, and the accumulation 

 is always greatest, if not wholly from the up- 

 per side of the incision, showing that it has 

 been poured down from the leaves with the 

 descending current, and taking the first oppor- 

 tunity of exuding itself, instead of finding 

 egress as it would otherwise do at the points 

 of the roots. 



This gum, then, is no loss to the tree, and 

 I am convinced the trees would be better if 

 they threw off more gum than they generally 

 do, for it affords neither materials for wood 

 nor fruit, any more than other organic sub- 

 stances. Its chemical constitution is different 

 from that of woody fibre, the latter containing 

 a larger proportion of carbon, showing that the 



leaves have already appropriated as much car- 

 bon as the exercise of their functions enables 

 them to fix, and convert along with the sap 

 there accumulated, into woody fibre. Thig 

 fixation of carbon, is analogous to the func- 

 tions of digestion in animals. In the solid 

 food of all animals, carbon is one of the prin- 

 cipal ingredients ; so is it the principal ingre- 

 dient in the formation of every fibre, out of 

 the crude sap drawn from the roots by the 

 leaves. Of the whole amount of sap absorbed 

 by the roots, a very small portion is appropri- 

 ated to the tree, and when we consider, that 

 in a short time, a tree will absorb by its roots 

 three or four times its own weight of water, 

 we will have some faint idea of the large quan- 

 tity that it must get rid of by exhalation and 

 other means, before the fluid can be concen- 

 trated to that degree in which it is converted 

 to solid matter by the addition of carbon, un- 

 der the influence of light, 



"Woody fibi'e contains about 50 per cent, of 

 carbon. The gum is chiefly composed of hy- 

 drogen and oxygen, in the proportions which 

 constitute water, and doubtless the small quan- 

 tity of carbon contained in it, though insuffi- 

 cient for its conversion into wood, is neverthe- 

 less sufficient to preserve it in a gelatinous 

 state. It is evident that this gum must un- 

 dergo a great change before it could be con- 

 verted into woody fibre, and, though I have 

 minutely examined it in this respect, I have 

 never seen the slightest appearance or approach 

 to the cellular formation of organized tissue, 

 whereas if the viscid substance termed cambiiun 

 which is prepared by the leaves, to form the 

 new layer of wood, be drawn off from the stem, 

 it not only shows a tendency among its particles 

 to arrange themselves in the form of cells and 

 vessels, but frequently does so, forming by 

 this kind of coagulation as perfect tissue as that 

 formed in the interior of the stem, as may 

 frequently be seen in nodules and hard con- 

 cretions on the stems and branches of various 



