292 Scott and Brebner . — On Internal Phloem 
In the mean time other anomalies have made their appear- 
ance. At a certain point in the development, in some cases, 
when the normal wood has only twelve cells on the radius, in 
others, when it has formed from thirty to forty layers, inter- 
xylary phloem appears. In one specially instructive case the 
thickening was very eccentric : on one side the wood and 
bast were normal, on the other side the wood was much 
thicker and traversed by two tangential bands of interxylary 
phloem, each of which had a cambium on its inner side (cf. 
Fig. 19). In this stem, the first interxylary cambium of the 
anomalous part exactly fitted on to the external cambium of 
the normal portion. We call attention to this exceptional 
case, because it throws light on the origin of the concentric 
zones. As a rule, the thickening is approximately uniform 
round the whole circumference. The bands of interxylary 
cambium and phloem sometimes run completely round the 
stem interrupted only by the parenchymatous rays. But 
more often their arrangement is less regular, the number of 
concentric bands varying not only in the different woody 
wedges, but also within the same wedge. The general 
arrangement of the tissues is such, that the inner layer of 
each concentric zone of xylem contains the larger vessels. 
Its outer portion is formed of the densely packed smaller 
vessels with thin-walled parenchyma interspersed (Fig. 19). 
The wider phloem-zones have precisely the same structure as 
the normal external phloem (Fig. 20). The sieve-plates in 
autumn are often callous. The narrower zones of thin-walled 
tissue are occasionally destitute of sieve-tubes, consisting only 
of cambium on the inner and thin-walled phloem-parenchyma 
on the outer side. But this is exceptional and, as a rule, the 
interxylary zones consist of typical phloem. The interxylary 
cambium which always lies on the inner side of the phloem 
band, does not appear to be very active, for no crushing to 
speak of takes place. The radial rows of the interxylary 
phloem are continuous inwards (through the cambium), but 
not outwards, with those of the wood. There can be no 
doubt that the development of these phloem-bands is centri- 
