246 PHYSIOLOGICAL BOTANY. PART II. 



for instance, that the layers of wood at different depths, 

 in a hollow yew tree at Cholsey, Berkshire, varied con- 

 siderably in thickness ; and that some of those which 

 had been very recently deposited were 2 lines, whilst 

 others, which were more than a century older, were 

 only half a line in thickness. This tree is between 14 

 and 1 5 feet in circumference ; and there is another in 

 the churchyard of the neighbouring parish of Aldworth, 

 which is more than 19 feet in circumference, which, 

 estimated by De Candolle's rule, ought to be above 900 

 years old ; but may rather be considered as nearer ()00 

 years. 



3. The Baobab (Adansonia digitata.) The last ex- 

 ample which we shall select, is that of the enormous 

 baobabs, or monkey-bread trees of Senegal, whose great 

 ages Adanson has attempted to estimate from the fol- 

 lowing data. 



Thevet mentions, in his " Voyages aux Isles Ant- 

 arctikes," in 1555, some " beaux arbres," which Adan- 

 son found to be 6 feet in diameter in 1749- He 

 judged, from Thevet's expression, that these trees could 

 not have been less than 4 feet in diameter at the time 

 when he saw them ; and this opinion was strengthened 

 by observing the extent to which the letters of certain 

 inscriptions upon them had become deformed, and which 

 inscriptions were dated from the fourteenth and fifteenth 

 centuries. Allowing therefore that these trees had in- 

 creased 2 feet in diameter during two centuries, he 

 estimated their age at (>00 years. But there are trees 

 of this species which are 30 feet in diameter; and these, 

 at the above rate, would be 3000 years old. But 

 if the age of these trees be calculated upon mathematical 

 principles, it should seem that they must be much 

 older even than this. Thus, Adanson having ascer- 

 tained that a tree of 1 year old was 5 feet in height 

 and 1 inch in diameter, and a tree of 30 years was 

 22 feet high and 2 feet in diameter, he applied these 

 data to construct a table, which bhould give the heights 



