276 Scott and Brebner . — On Internal Phloem 
crowded around the middle of the root in the region where 
dilatation has chiefly taken place. They are also massed 
along the margins of the isolated strands of secondary 
lignified tissue, but are by no means limited to the neighbour- 
hood of woody elements. 
As the dilatation of parenchyma goes on chiefly in the 
inner part of the root, the outer region of the xylem is the 
more woody. Here we find, immediately within the cambium, 
a series of woody wedges (containing the tracheae), separated 
from each other by broad parenchymatous rays. They are 
arranged usually in four groups, corresponding in position to the 
primary phloem-bundles. As the root-tubers increase in thick- 
ness the course of both xylem- and phloem-strands becomes 
very oblique, with frequent anastomoses. The development 
has been traced in detail up to a diameter of about half-an-inch 
(1*25 cm.). The further growth of the root-tuber appears to 
be chiefly due to dilatation. Almost the whole tissue in the 
old specimens is parenchymatous, the wood being almost 
limited to a narrow zone just inside the cambium. 
This root shows with remarkable clearness the character- 
istic changes involved in the assumption of this type of fleshy 
structure. Complicated as the final condition is, it is easy to 
refer it to its origin from a comparatively normal dicotyle- 
donous root-structure. 
We have not, in this plant, specially investigated the 
transition from root to stem, but we have no doubt that here, 
as in Ascleptas, the innermost interxylary phloem of the root 
is continuous with the medullary phloem of the stem. 
Altogether this root has much in common with that of 
Asclepias , the most essential distinction lying in the different 
relation of the interxylary phloem to the ‘medullary’ rays. 
Sieve-tubes in the xylem have been observed in the root 
of Cucurbita by Van Tieghem 1 and Fischer 2 . From the 
description of the latter they seem to be limited to the 
transitional region. He regards them as forming the down- 
ward continuation of the sieve-tubes found in the medullary 
1 Ann. des Sc. Nat. Bot. Ser. 5, vol. XIII. p. 215. 2 Loc, cit. , p. 53. 
