42 CHRISTISON—OBSERVATIONS ON 
simply the increase in girth, and cannot discriminate between 
the amount due to deposit of wood on the one hand, or to 
changes in the bark or cambium on the other. In this respect 
borings have the advantage, but as to the bark I may state 
generally that except in very old trees there seems to be little 
loss in mass, or even by gradual attrition—so little as to be 
inappreciable in a single year. Thus, even in old rough-barked 
trees, my painted distinguishing numbers often show little trace 
of wearing in ten or twelve years, although distorted and 
rendered illegible from the gradual widening and splitting of 
the bark. Notable exceptions among the species are the true 
Plane tree and the Yew, whose tendency to shed their bark is so 
manifest. 
_ Sir Robert Christison at first aimed at no finer division of 
his tape than tenths of an inch, and confined himself to annual 
observations, but very soon, with practice and improved tapes, 
he measured to the twentieth of an inch and took monthly 
observations. Since his death in 1882, I have continued to 
measure his original trees, but as many of them, even from the 
first, were old or prematurely old, it was evidently desirable to 
experiment on younger specimens, not only because the results 
would be probably more reliable, but in order to ascertain the 
increments of the species over a greater range of age. 
Accordingly, in 1887, I selected some thirty-five young 
deciduous trees, from six to twenty inches in girth, chiefly 
situated in the South and East shelter belts of the Arboretum, 
and a similar number of young Pinacee in the Botanic 
Garden, taking two of each species, when possible, so that in 
case of one failing the other might preserve the continuity of 
observations in the species. With the exception of a few that 
were cut down from death or degeneracy, all of this second set 
~ were observed annually till the end of the period, and monthly 
from 1887 to 1891. The deciduous specimens generally throve 
well, considering that they had been put in without preparation 
of the naturally poor sandy soil, before the Arboretum was 
handed over to the authorities of the Garden, but they were 
almost all transplanted after 1891, and thus the continuity of 
observations was interrupted. The Pinaceze, again, throve so 
badly as greatly ta mar the value of the results in them. 
