570 Worsdell .— The Origin and Meaning of Medullary 
to medullary collateral bundles were noted. In the node the xylem of the 
medullary bundles unites with that of the ring. 
He says: ‘ The phloem-bundles situated on the inner edges of the 
normal vascular bundles of these plants [Cucurbitaceae] are structures quite 
analogous to the internal vascular bundles in the stems of Rumex and 
Rheum . The internal phloem-bundles in Cucurbitaceae, when provided 
with their own wood, represent, doubtless, independent vascular bundles/ 
Their independence is also shown by their branching and by their passing 
from one normal bundle to another. He reaches this conclusion by the 
method of comparative histology. 
He found that the differentiation of the first sieve-tubes in the internal 
phloem of Bryonia alba occurs much later than in the external phloem ; 
but in the same plant their development may be much earlier. The 
development of the internal phloem-bundles of the Cucurbitaceae is just the 
same as^that of the medullary bundles of Polygonaceae, &c. 
‘ The appearance of internal bundles in Dicotyledons should be 
regarded not as an anomaly in this type, but rather as the ulterior develop¬ 
ment and perfectioning thereof/ He regards it as an evolutionary develop¬ 
ment and cites its occurrence in some Gamopetalae in support of the idea. 
Wallace studied the stem structure of Actinostemma biglandulosa , in 
which he found that the bundles are primarily collateral and remain so until 
after a considerable quantity of secondary tissue is formed. Two of the five 
inner bundles are very small, and at first possess phloem only ; later on they 
acquire xylem, and still later become bicollateral like all the larger bundles 
of the two rings. Medullary phloem does not arise simultaneously in rela¬ 
tion to all the ten bundles of the internode : the three large inner ones first 
acquire it, then the larger of the two inner bundles, next, those of the outer 
ring, and finally the remaining inner bundle. The wood of the normal 
bundle becomes surrounded by phloem. Medullary phloem does not 
accompany the leaf-traces into the leaf. The older petiole has collateral 
bundles. 
Pitard found in Cucurbita Pepo tertiary phloem-strands in the rays 
of the stem at the edge of the wood ; they are connected by branches with 
the internal-phloem groups of the bundles. 
Faber, after tracing the development of the stem-bundles of Cucurbita 
Pepo , concluded that the internal phloem arises very early at the growing 
point; he found that the inner and outer phloem already existed before any 
vessels were formed. The sieve-tubes of the inner phloem arise from the 
same procambial strand as the rest of the bundle. He could discover 
no difference, either in the development or structure, between the outer and 
the inner phloem. The development of both is centripetal, i. e. towards the 
protoxylem. In one bundle only did he see two small xylem-elements 
formed from the cambium attached to the inner phloem. He says : ‘It is 
