1899.] NATURAL SCIENCES OF PHILADELPHIA. 113 



XIII. Eccentricity of the annual wood circles in 

 Rhus Toxicodendron l. 



In an abstract of an address' made by myself on excrescences 

 and eccentric wood growth in trees, I pointed out that the thicken- 

 ing of the annual layer of wood, more in some portion of the circle 

 than in others, was due to a greater generative power of the cells 

 in the thickened portions owing to more abundant nutrition having 

 reached them. " Where the whole mass of tissue seems equally 

 and regularly vitalized, the generative tissue forms a new layer of 

 wood of about equal thickness all round." "In some cases 

 masses of cells seem to easily draw from others more than their 

 share (of nutrition), and the latter are corresj^ondingly weakened. " 

 ■" All the cases of peculiar eccentricities, Hedera, Toxicodendron, 

 Ampelopsis, and the peculiar cases in ordinary timber trees, could be 

 explained by this, so far as to note that the immediate law was a 

 loss of generative power in the cells of the annual layer. Of course 

 the indirect causes leading to this would be numerous, and left 

 room for much more investigation." At the conclusion of that 

 address the report published says Mr. Meehan was asked to explain 

 the square section of trunk of Picea amabills on exhibition at the 

 American Centennial, and the reply was that in all probability the 

 thickened portions of the annual layers at the corners forming the 

 square were due to strong roots near these four corners supplying 

 abundant nutrition in those directions. 



Though twenty-two years have elapsed since that communication 

 was made, the matter does not seem to have been further investi- 

 gated, and our biological text-books still endeavor to explain these 

 variations in the width of the concentric rings to various causes 

 other than the one to which I have alluded. Though the bulk of 

 nutrition must be by way of active roots, the carbon necessary for 

 the structure must be contributed through the leaves. Healthy 

 leaves and vigorous roots must be reciprocal. On the other hand, 

 the power exercised by any force is in inverse ratio to distance ; and 

 it is probable that a weak series of foliage, near the active roots, 

 juight aid nutrition to better advantage than stronger ones further 

 away. 



Some interesting observations were recently made on the ordinary 



^ Proc. Acad. Nat. Sci., December 19, 1876. 



