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Phloem in the Stems of Dicotyledons . II. 
first importance, for two chief reasons : in these species these phloem-strands 
represent the last vestigial remnant of medullary bundles in the stem, and 
it is of paramount interest that they occur in connexion with the main or 
median leaf-bundle ; as the phloem-strands are in all respects perfectly 
similar to those which occur on the ventral side of the leaf-bundles in the 
cortex (i. e. in the decurrent leaf-base), the two sets must be regarded as 
homologous ; it follows, further, from this that they must be regarded as 
homologous with the phloem-strands (or bundles) occurring on the ventral 
side of the bundles of the petiole of other species. The final obvious deduction 
is that the small ventral strands of the bundles of the petiole, not only of 
this genus but of all genera in which they occur, represent a peripheral 
medullary bundle-system of the petiole which is homologous with that of the 
stem. And the point of chief interest is that, where they occur in the leaves 
of those plants whose stems show no trace of me dullary strands , they indicate 
that medullary strands were once a constant feature of those stems. 
Even were the above deductions unconvincing, there is no other possible 
explanation of these small ventral strands of the leaf. 
The medullary phloem-strands or bundles, whether peripheral (‘ internal 
phloem ’) or central, of the free petiole arise, as regards their immediate 
origin, quite independently, and have no connexion with the internal-phloem 
strands of the ‘cortex’ of the stem above described. But this fact does 
not interfere with the conclusion that the two sets are completely homologous 
with each other. In a case like that of L. per ennis medullary strands are 
absent in the free petiole, but we see them beginning to be abstricted off 
from the external phloem of the larger bundles. In the decurrent base 
of the leaf (‘ stem-cortex ’) they have been completely abstricted off and 
occur on the ventral side of the bundles. 
Sonchus. 
This genus is closely allied to Lactuca. There is space only for a brief 
mention of the structure. 
S. asper, Hill. 
Stem. 
The medullary phloem-strands constitute a fairly independent system ; 
they form connexions at the node with the bundles of the vascular ring of 
both stem and branch. 
The most median of the internal-phloem strands of each leaf-bundle 
enter the pith and form there an independent strand, but the more lateral 
ones unite with the external phloem of the stem-cylinder. The former of 
these two facts is of great interest in connexion with what was observed in 
the case of Lactuca. 
