452 Dr David Christison on ascertaining 



number of concentric waves, as in the last, distinct enough, 

 yet too vague to be counted accurately. Groups of three or 

 four, more distinctly marked, occur, but they do not in the 

 least correspond with the age of the tree. Judged by a 

 British standard, the tree might be considered to have badly- 

 marked rings, occasionally arranged in more distinct groups 

 of two or three, indicating, from the more distinct lines alone, 

 an age of about twenty years, but it is only six and a half. 

 In the illustration, Plate IV,, which I owe to the skilful 

 pencil of Mr Gustav Mann, the natural appearance is shown 

 in the finished part of fig. 1 ; but the lines of demarcation of 

 the first five years are accentuated to show their position 

 better. The sixth line has not been so treated, as it is 

 scarcely possible to make it out. 



Acacia lopliantha. — The smooth surface shows about forty 

 concentric waves, very distinct at a glance, but with no sharp 

 definition, and difficult to count. They do not differ much 

 in distinctness, except a well-marked dark one near the 

 centre. 



The general results, so far as naked-eye surface character- 

 istics of the blocks are concerned, are, that the zones are 

 quite distinct, reliable, and annual in Robinia ; that in Melia 

 zones are clearly marked out, but are considerably more 

 numerous than the years of age of the tree ; that in 

 Acacia melanoxylon the annual zones are vaguely marked by 

 an accentuation of colour in certain groups of numerous 

 rings which are not annual ; that in A. moUissima and 

 lophantha the annual zones are either unrecognisable, or are 

 only to be made out with difficulty and doubt, amidst the 

 numerous wave-like rings. 



Microscopical Appearances. 



It remained to ascertain whether, in the instances in which 

 the rings were indistinct or unrecognisable to the naked eye, 

 the true annual rings could be made out by aid of the micro- 

 scope. Fine sections were accordingly made of Acacia 

 moUissima and lophantha by Dr Macfarlane, and I had the 

 advantage of his assistance in examining them. As the 

 results in the two species were not materially different, I 

 shall confine my observations to the Acacia moUissima, as 



