156 VEGETABLE ORGANOGRAPHY. 



remarked as many as fifty layers of the alburnum in 

 Pldllyreas about 200 years old, which I have been 

 obliged to cut down in the part of the Garden of Mont- 

 pellier planted by Belleval. 



We easily understand that in trees of different species 

 there must exist numerous varieties in the number, 

 thickness, hardness, and colour of the layers of the 

 alburnum, compared with those of the wood ; but in 

 each species itself, some differences are also found in 

 different individuals. Ttius, in general, trees which 

 grow in damp places, or during wet seasons, have more 

 alburnum than those which grow in dry places, or dur- 

 ing dry years. Duhamel assures us that in different 

 stunted oaks he has counted from seven to twenty-five 

 layers of the alburnum. 



The relation of the thickness of the alburnum to the 

 wood varies in different species and different individuals, 

 not only from the preceding causes, but, moreover, from 

 the age of the tree. Thus, the alburnum is equal to the 

 wood in an oak six inches in diameter ; it is as two to 

 seven in a trunk of a foot ; as one to nine in one of two 

 feet, &c. ; still these proportions given by Duhamel are 

 very variable. Mustel has observed that different parts 

 of the same layer of the alburnum maybe transformed into 

 perfect wood at different periods ; thus he has seen some 

 Oaks which had, on one side, fourteen layers of the 

 alburnum, on the other, twenty ; or, on one side seven, 

 on the other twenty-two, &c. The layers of the albur- 

 num are almost always thicker on the side where they 

 are less numerous ; that is to say, in other terms, that 

 when a root meets a good stratum of earth, it nourishes 

 the corresponding part of the tree more abundantly. 

 Those parts which are most nourished have the woody 

 layers thicker, and they arrive more quickly to the state 

 of perfect wood, whilst the roots which fall in with poor 



