THE STEM OF VASCUl.AR PLANTS. IGl 



part, between two and three hundred years old; the 

 oldest of all was three hundred and thirty-three. After 

 sixty years an oak increases from about eight to ten 

 lines in diameter in ten years ; and from two to three 

 inches when it is between twenty and thirty years old. 

 Finally, these observations are necessarily subordinate to 

 the difference of species, soils, seasons, and cultivation. 

 They would seem to indicate that it would in general be 

 profitable in regular cuttings to cut down every thirty 

 years, rather than every twenty years, since it is from 

 the twentieth to the thirtieth that the trunk of oaks 

 increases the most. 



Not only are the layers unequal with regard to each 

 other, but their thickness is not often the same through- 

 out their circumference. Malpighi was the first who care- 

 fully observed that the pith seldom occupied the exact 

 centre of the trunk ; or, what is the same thing, that the 

 concentric layers are often larger or more numerous on 

 one side than on the other ; this phenomenon has been 

 designated by the name of Excentricity (excentriciti) 

 of the woody layers. Among the ancients, some asserted 

 that the pith was nearer the bark on the south side ; 

 others said that this was the case on the north. 



Neither the one nor the other have omitted hypotheses 

 to explain this fact ; several said that it was a means of 

 knowing the situation of the tree in the forest, &c. &c. ; 

 but all the marvellousness has vanished with an exact 

 observation of the fact. Duhamel and Bufibn proved 

 that the excentricity had no connexion with the position 

 of the tree relative to the points of the horizon, but with 

 its purely local situation. When on one side of it a good 

 stratum of earth, or a place free from all other roots, 

 is found, those roots which are directed there receive 

 more nourishment, furnishing more to the corresponding 

 part of the trunk, which increases most on this side. In 



VOL. I. M 



