270 Scott and Brebner . — On Internal Phloem 
The external phloem is considerable in amount, but is 
largely parenchymatous; the sieve-tubes are similar in size 
to those of the interior, and in a stem 1 cm. in diameter are 
scarcely more numerous. 
As regards the root, the early stages are normal, with a 
diarch or triarch vascular cylinder. But all roots, which attain 
a diameter of much more than 1 mm. have groups of phloem 
in the wood. In a root about 3 mm. in thickness, such as 
that from which Fig. 6 is drawn, much the greater part of the 
phloem is situated in the wood. In roots of this size periderm 
has not yet formed, though the external cortex is withering 
away, and the vascular cylinder forms the bulk of the root. 
The secondary tissues are extremely parenchymatous and the 
cells crowded with starch ; xylem- and phloem-parenchyma 
are quite similar, and the isolated phloem-groups in each arise 
in the same way by the sub-division of cells cut off respectively 
on the inner and outer side of the cambium. On the xylem- 
side the parenchyma consists, partly of the medullary rays, 
partly of xylem-parenchyma in the narrower sense. Except 
in the form of the cells, there is no difference between the two, 
but the phloem-islands appear to be limited to the latter. As 
regards the distribution of the interxylary phloem, some of 
the groups are in contact with the protoxylem-elements. These 
groups have arisen, at any rate in part, from primary paren- 
chyma. Otherwise the phloem-islands are all secondary. Some 
of them lie at the sides of the groups of tracheae, which are 
scattered about among the parenchyma. In this case cambial 
divisions take place on the side towards the tracheae, and we 
have the same structure which Weiss figured in Bryonia 1 . Other 
phloem-groups are isolated in the secondary parenchyma re- 
mote from the tracheae. No considerable cambial growth of 
the phloem-islands takes place. The neighbouring cells divide 
and add a few more sieve-tubes to the strand, but there is no 
obliteration of the older elements. The long-continuing divi- 
sions show that the development of the phloem-island is a 
1 Loc- cit., Flora, 1880, Fig. 6. 
