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Worsdell. — Origin and Meaning of Medullary 
three or four sizes, the largest being situated farthest to the outside, while 
the smallest, save for a few inverted ones on the immediate ventral side of 
one or two of the largest bundles, occur nearest the centre of the organ. 
Some of these smallest bundles are quite rudimentary (Fig. 27). 
Further, an index to the primitive stem-structure, as regards the 
distribution of the main tissues, is here found in the fact that there is 
absolutely no distinction between cortex and pith : the two tissues are 
one and indivisible ; the structure is astelic, a general endodermis being 
Fig. 26. Inula Helenium. Segment of 
vascular ring of stem, showing medullary phloem- 
strands {mps). Leaf- bundles at lb. x 9. 
Fig. 27. Inula Helenium. Petiole, 
showing well-marked scattered disposition 
of the bundles, and the small inverted 
bundles {pms) on the ventral side of the 
median bundle of the arc. x 3. 
absent, and a particular one around each bundle being present. The whole 
structure supports the view, with which the present writer agrees, that the 
pith is cortical in its origin. 
Summary. 
The following are the main facts concerning the vascular structure of 
the stem and leaf in Compositae : 
1. The vascular cylinder of the stem is loosely constituted, the com- 
ponent bundles being markedly individualized. 
2. Their alinement in the vascular ring is more or less irregular, being- 
situated at varying radial distances from the centre of the stem. 
3. The stems of the vast majority of Compositae are herbaceous. 
4. Where vascular strands occur in the medullary region of stem or 
leaf they, as a rule, exhibit great variety of development ; some being 
throughout well-developed vascular bundles, others consisting of phloem 
only ; some possessing both xylem and phloem during part of their longi- 
tudinal course, while in another part of their course the same strands possess 
phloem only, or have lost the lignified portion of their xylem. Many 
bundles dwindle in size when traced upwards or downwards, lose their 
xylem, then their functional phloem, and finally die out in situ , — in other 
words, end blindly in the parenchymatous tissue. 
5. Within the same genus, some species possess medullary strands in 
the stem, while other species are completely devoid of them, e. g. Rudbeckia , 
L actuca. 
