Phloem in the Stems of Dicotyledons . II. 447 
of protoxylem and one or two secondary vessels on the outer side. These 
medullary strands are very variable in size, some being very small. 
D. Merckii, Lehm. 
Stem. 
The bundles of the ring have very V-shaped xylem, are very variable in 
size, and thus suggest their original derivation from a scattered disposition. 
D. sp. 
Leaf. 
In the petiole is an incomplete ring of separate bundles and a complete 
absence of medullary strands. 
Hemizonia corymbosa, Ton*, and Gray. 
Stem. 
Medullary bundles, about three in any transverse section, occur each 
close to a bundle of the ring ; they are amphivasal in structure, with xylem 
mostly enclosing the phloem. 
The bundles of the ring have clearly been set back into line from an 
original scattered arrangement, being of very varying sizes and distinctly 
individualized. 
Inuloideae. 
Inula Helenium, L. 
Stem. 
In two individuals medullary strands were observed near the base and 
in the upper part of the stem respectively, and in one case in the base of 
a branch and in the peduncle. They occur near the vascular ring, and some 
of them are united with the sclerenchyma surrounding the xylem. They 
die out above. Each consists of a central strand of soft bast surrounded by 
lignified tissue which is probably sclerenchyma, but may be fibrous xylem. 
These medullary strands are distinctly rudimentary and evidently on the 
verge of extinction (Fig. 26). 
The bundles of the ring were clearly once scattered in the ground- 
tissue. 
Leaf. 
The original vascular system of the stem of this plant is revealed, as in 
so many cases, by a glance at the present vascular system of the petiole. 
In this organ the scattered disposition of the widely-separated (tangentially 
and radially) bundles is obvious. They are distinctly two-ranked and of 
