160 VEGETABLE ORGANOGRAPHY. 



ill the Champs-Ely sees, presented from -ninety-four to a 

 hundred layers. 



Notwithstanding these slight anomalies, which result 

 perhaps from this — that all the trees of a plantation may 

 not be exactly of the same age, it appears certain that the 

 number of layers affords the means of knowing how many 

 years old a branch or tree may be, — the number of con- 

 centric zones in a transverse section indicates the number 

 of years which have elapsed since the formation of that 

 part. In order to know the age of the whole tree, it 

 must be sawed off exactly at the neck ; but this part is 

 not always easily recognised with precision in very old 

 trunks ; and this is a slight source of practical error, but 

 which does not affect the principle upon which we judge 

 of the age of trees. 



All the layers of a tree are not of equal thickness, 

 neither with regard to each other, nor in their different 

 parts ; and this is easily known, since each is the pro- 

 duct of the growth of a year. The woody layer will 

 necessarily be more or less thick, according as the year 

 may have been favourable or unfavourable — according 

 as the roots of different lengths may have found a good 

 or poor stratum of earth — according to the care taken 

 of the tree, &c. 



Besides these causes of accidental anomalies, the age 

 alone of trees influences the thickness of the layers in a 

 regular manner. I have observed, in this point of view, 

 very old Oaks cut down in the Forest of Fontainbleau ; 

 the thickness of their woody layers goes on augmenting 

 until the thirtieth or fortieth year ; from the thirtieth to 

 the fiftieth, or even sixtieth year, it slightly diminishes ; 

 but between the fiftieth and sixtieth the thickness of the 

 layers becomes very regular, and probably continues so 

 until the death of the tree ; at least, the trees upon 

 which 1 have made this observation, were, for the most 



