45 8 W 'or s dell. — Medullary Phloem in Stems of Dicotyledons. II. 
easily explicable ; for these are the very features which always characterize 
vestigial, ancestral structures. 
The writer does not feel called upon, for the purposes of the present 
thesis, to determine the precise factors which govern the sporadic distribu- 
tion of the medullary strands : why, for example, they occur in the stem of 
Lactuca saligna and not in that of L. macrantha. The fact by itself is all- 
important and self-sufficient. To cite comparable cases : the presence of 
staminodes in the flower of some Primulaceae (e. g. Samolus ) and not in 
others, and the presence of inflorescence bracts in some genera of Cruciferae 
and their complete absence in others. It is enough that these facts indicate 
that an outer whorl of stamens and inflorescence bracts were once constant 
and normal features respectively in the ancestors of these plants. 
Having now weighed all the facts and hypotheses bearing on the 
question, the writer arrives at the final conclusion that the intraxylary 
phloem occurring in the stem of Compositae (a type of many other orders) 
is the vestige of a formerly well-developed system of medullary bundles 
which, along with the bundles of the vascular ring, constituted a scattered 
system of bundles, which was the normal feature of the stem in every 
member of the order, or its ancestry, at a time when the geophytic or semi- 
geophytic habit prevailed in these plants. The more conservative foliar 
organs very often retain, the ancestral structure, more or less well preserved, 
when it has become extinct, or almost so, in the stem. 
The writer is indebted to the authorities of the Royal Gardens, Kew, 
for the greater part of the material used in this investigation. He also 
desires to thank Mr. L. A. Boodle, F.L.S., for some critical suggestions, and 
for taking part in numerous discussions, affecting certain aspects of the 
theory embodied in the paper. 
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