150 M. B. WELCH. 
the other and hence the relative number of oil glands per 
unit area of cross section is much greater in one case than 
the other. Again the size of the glands varies consider- 
ably, not only in the one leaf of a single species but also. 
between species. Thus in H. corymbosa the mean diameter 
of a number of oil glands measured was 0°075 mm., whereas 
in EK. phytocarpa, glands were found. measuring up to 0°825 
mm. in diameter. 
Since the volume or theoretical oil bearing capacity of 
each gland varies as the cube of the radial dimension it is 
apparent that the size of the gland is especially important 
in arriving at any comparison of the different species. In 
some Hucalypts the glands are remarkably regular in size, 
in Others, e.g., EH. hemiphloia, EH. ligustrina, etc., great 
variation was found and in one section may range from 0°25 
mm. —0°075 mm., taking into account medium sections only. 
To obtain the relative number of glands present, the 
average number in a transverse section 5 mm. in length, of 
the leaves was counted. The mean thickness of the leaves. 
examined was measured and thus it was possible to obtain 
the number of glands per sq. mm. of cross section. Although 
naturally the leaves vary in thickness, there is a certain 
amount of uniformity in mature leaves of any one species. 
Very often however, a single leaf is not uniform in this 
respect, even in section at right angles to the mid-rib, e.g., 
one leaf of H. robusta measured 0°480 mm. in thickness 
near the intramarginal vein, whereas nearer the mid-rib. 
the thickness was 0°375 mm. The thickest leaves are 
typically those of the Mallees, sometimes averaging in 
specimens examined 0°60 mm., although EH. coriacea 
measured 0°57 mm. and H. obtusiflora, a stunted Mallee- 
like coastal Hucalypt, averaged 0°60 mm. 
The figures taken for the yield from the various species. 
are those given in ‘* The Bucalypts and their Essential 

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