177 
Ipomoect versicolor , Meissn . 
which they enclose, are produced centrifugally on the inner 
side of the normal cambium (see Fig. 6). The same cambium 
subsequently adds lignified tissue outside them, and they 
thus become enclosed in the wood (cf. Figs. 6 and 7). The 
phloem-groups themselves may either be formed directly from 
the cambium or by the subsequent division of the parenchy- 
matous cells. Both processes occur side by side, as is also 
the case in Asclepias and Thladiantha , to be described 
elsewhere. 
The parenchymatous islands with their strands of phloem 
extend up into the first internodes above the cotyledons and 
gradually disappear. They seem never to end blindly in 
the wood, but ultimately to abut on the cambium. Above 
the cotyledons they can only communicate with the medullary 
phloem at the nodes. 
The main root in its lower portion is normal. Here, as in 
the stem, the interxylary phloem ends in contact with the 
cambium. 
Ipomoea versicolor then presents the curious case of a 
plant in which the greater part of both stem and root is 
normal, while the region for some distance on both sides of 
the transition possesses a complicated system of interxylary 
phloem. The anomalous transitional region, with its abundant 
conducting tissue, serves no doubt as a temporary store-house 
of food-material, reserved perhaps in part for the period of 
flowering. 
A word must be said on the relation of the medullary phloem 
to the phloem-systems of the mature root. As already ex- 
plained, the former is continuous, at the lower end of the 
hypocotyl, with the normal external phloem. Then secondary 
growth begins ; at the points where the medullary phloem 
passes out the cambium necessarily cuts through it. Its con- 
tinuity, however, is not altogether interrupted, for opposite 
each bundle-pair, through which the out-going phloem passes, 
the cambium forms a parenchymatous xylem-ray containing, 
as already stated, strands of interxylary phloem (see Fig. 5 ; 
the diagram, Fig. 2, is taken above the exit of the medullary 
