274 Scott and Brebner.—On Internal Phloem 
number of separate strands, as indicated in the diagrammatic 
Fig. 7. The periderm is pericyclic. 
When secondary thickening begins, the cambium, as so 
often happens, forms secondary parenchyma opposite the 
protoxylem and true wood only opposite the phloem. The 
parenchyma may either be continued indefinitely by the 
cambium as a ray, or woody elements may be formed closing 
It in. As growth in thickness goes on, this process is 
repeated. New rays originate at the middle of each prominent 
xylem-mass and these also may be discontinuous, the wood 
closing in over them. Fig. 7 shows a typical case with the 
secondary rays only just beginning. In Fig. 8, the secondary 
ray shown is partly closed in. Considering the position of the 
parenchyma, analogy has led us to speak of it as forming 
rays, but when it becomes enclosed in wood the expression 
xylem-parenchyma would be more appropriate. The normal 
phloem is well developed, the sieve-tubes attaining the large 
diameter of ’045 mm. But phloem is also formed on the 
inside of the cambium. In the earlier stages it is limited to 
the ray-parenchyma. Some of the interxylary sieve-tubes 
are always situated at the beginning of the principal rays. 
Some of these are perhaps of primary origin. Others are 
scattered about near the sides of the ray. As soon as the 
secondary rays are started, phloem is formed in them also (see 
Figs. 7 and 8). Sometimes a nascent ray becomes almost at 
once enclosed in wood and in these cases the whole of the 
little parenchymatous group may be used up to form a 
phloem-strand. Some of the interxylary sieve-tubes are 
formed directly from the cambium ; others are differentiated 
later on in tissue which is already remote from it, so that here 
again the two processes distinguished by Weiss occur in the 
same root. The interxylary groups are small, in fact a 
single sieve-tube with its companion-cells is often isolated. The 
cells surrounding the phloem show a few divisions, but no con- 
siderable growth takes place in this way in the non-tuberous 
part of the root. The interxylary sieve-tubes are not so large 
as the external ones, but reach *03 mm. in diameter. 
