272 Scott and Brebner.—On Internal Phloim 
conclusion that these interxylary phloem-strands end blindly 
above, and are not continuous in that direction with any of 
the phloem-systems of the stem. The lower part of the stem 
is in fact to be regarded as an extension of the storing-tissues 
of the root, and the phloem-strands may serve for the con- 
duction of proteid food -materials from one part to the 
other. 
Starting from one of the lowest internodes and tracing the 
tissues downwards the following changes are observed : The 
wood becomes more and more parenchymatous, i. e. nearly all 
its elements except the scattered vessels come to have 
cellulose walls and starchy contents. The interxylary phloem- 
strands become larger and more numerous. The medullary 
phloem-strands ramify and at a certain stage are found 
crowded all over the pith, which gradually becomes smaller, 
some of them reaching its centre. In the mean time the 
primary xylem-groups approach the middle of the axis 
ultimately fusing to form the diarch xylem-plate of the root. 
Thus some of the medullary phloem-groups, and ultimately 
all of them, pass to the outside of the primary xylem. There 
is no doubt, that the more central phloem-groups of the main 
root are the direct downward continuation of the medullary 
phloem of the stem 1 . The process is so far essentially the 
same as in Browallia . But in Asclepias the medullary phloem 
does not pass entirely to the outside of the cambium. It 
passes, in part at least, straight down the root, adjacent to the 
primary xylem, and thus forms the first ring of phloem-islands. 
The rest of the interxylary phloem is of secondary or tertiary 
origin as described above. Some light is thus thrown on the 
frequency of phloem-islands in the roots of plants with bicolla- 
teral bundles in the stem. The medullary phloem of the stem 
becomes the interxylary phloem of the root. The formation of 
interxylary phloem by the cambium is not a new departure, 
but an extension of the primary structure. The phloem- 
strands, whatever their origin, are connected by anastomoses. 
Our results agree in many points with those obtained by 
1 Just as Weiss found in Oenothera , loc. cit. ; Bot. Centralblatt, XV. p. 409. 
