GREY GUM AND ITS ESSENTIAL OIL. 261 
has its leaves so “ thoroughly honeycombed ” (if one may use the 
expression) with oil cavities or glands as this one—a feature that 
can be better noted in dried specimens than fresh ones. 
The cavities are plainly visible by holding up a leaf to a medium 
light, but of course are more plainly seen under an one-inch or 
half-inch objective. 
We have almost invariably found that the oil glands are always 
attached to the under side of the upper cuticle of the leaf, so that 
when the two surfaces of a leaf are drawn apart the lower one is 
quite destitute of these organs. To this fact we are inclined to 
account for the singular character that eucalyptus leaves have of 
twisting on their petiole—a fact of which we have long been 
cognisant, but of which we could not hitherto advance any 
explanation. It seems to us very probable that in order to 
prevent the upper surface of the leaves being always exposed to 
the sun’s rays it is turned from them by a mechanical contriv- 
ance, the lower surface thus being brought round to bear the 
brunt of the heat, and so protect the oil glands. 
There is, perhaps, little to add to the explanation of Figs. 1 — 5 
The cuticle shown so distinctly over the oil glands, with Fig. 2 
and 3, is very possibly stretched or extended over that body, and 
thus the cellular walls are more emphasised than in the more 
Opaque parts of the leaf, although of course the same structure 
pertains over the whole leaf, but not so easily detected owing to 
_ the presence of the chlorophyll. In a dried leaf (the drawings 
are from a fresh one) the walls of the chlorophyll bodies are Nit 
distinct. 
In £. globulus the texture of the leaf is much thicker, the 
palisade layers being much more numerous, and consequently the 
oil glands are not too prominent, nor are they so numerous. The 
cuticle is apparently structureless as compared with that of 
£. punctata, the cell walls of which, as delineated, form ir r 
polygons (mostly hexagons and pentagons). 
Sections of the leaf of EZ. globulus are given by Baron von 7 
Mueller in his Eucalyptographia, so that we have not — nega 
