458 TIIANSACTIOXS AND PKOCEEDINGS OF THE [Sess. lvii. 



" as beinw clearer of the swellins; of the roots."' Failing 

 this, he took the narrowest point. He, however, took no 

 pains to make his views known, and, in ignorance of them, 

 Sir liobert Christison was the Jfirst to insist publicly 

 upon the necessity of establishing a common standard of 

 position, and, in a Paper communicated to this Society in 

 1878, he showed that this must be chosen where the 

 stem is unaffected by the swelling downwards towards the 

 buttresses and upwards towards the great branches — in fact, 

 at the narrowest part of the stem, a point generally to be 

 found about 5 fe6t from the ground, although in very 

 old trees it may be higher, and in unusually short stems 

 it may be lower. Of late years this principle has been 

 very generally adopted, although from the neglect of it 

 the vaguest statements as to the size of trees are still not 

 unfrequently met with. In the present Paper I have 

 adopted the girth of 5 feet from the ground, or, if 

 necessary, at the narrowest point, as the single criterion 

 of size, recording, however, other measurements in special 

 cases. 



Age of Tkees. — Considering that life is annually 

 renewed in our forest trees, there is no evident reason, 

 apart from disease, why their vitality should come to an 

 end ; yet it would seem that, just as in the animal world, 

 which lias no such annual rejuvenation, so in the vegetable 

 world, each species of tree has its natural limit of life. 

 The average limits of the species, however, have not been 

 determined, any more than the age which exceptionally 

 favoured individuals may reach, and it is only by the 

 gradual accumulation of facts that we can hope to form 

 some estimate on these points. The main obstacle, how- 

 ever, is the difficulty in ascertaining the age of very large 

 trees. 



As to the modes of ascertaining the age of trees in 

 general, the most reliable is when the date of planting is 

 known, Init it is surprising how seldom, in Scotland at 

 least, this kind of evidence is to be liad, even for trees of 

 moderate age, and it is quite unavailable for aged veterans, 

 except in a legendary and hilmlous form. 



Good evidence may also l)e obtained, but only in fallen 



