Actinostemma biglandulosa. 641 
adopts this conception of bicollaterality, distinguishing me¬ 
dullary phloem from independent strands of phloem by means 
of the following characters :— 
‘ 1. The medullary phloem-strands in every case accom¬ 
pany the leaf-traces within which they lie, on their exit into 
the leaf. 
2. They arise almost at the same time, or only slightly 
later than the parts of the phloem outside the xylem. 
3. Where a cambium forms in connexion with them, it 
never produces wood also 1 .’ On the other hand, Herail 2 
declares that in the majority of plants with bicollateral 
bundles, the medullary phloem does not appear at the same 
time as the rest of the bundle, neither does it proceed from 
the same meristem. According to him it is only as applied 
to the bundles of the Cucurbitaceae that the term bicollateral 
is strictly appropriate ; for in their case ‘ the internal phloem 
is as primary as the tracheae themselves.’ Lamounette 2 
agrees with Herail that the secondary origin of medullary 
phloem is the general rule, but denies that the Cucurbitaceae 
are exceptions to it. From observations of the apex of 
Bryonia dioica , this investigator concluded that the pro- 
cambial meristem and the meristem which gives rise to 
medullary phloem are distinct from the first, the medullary 
phloem being a later development from those conjunctive 
tissue-cells which abut internally on the procambium. 
In Bryonia dioica , according to Lamounette. the medullary 
phloem appears almost, but not quite at the same time as the 
rest of the bundle, and therefore at a point close beneath the 
growing point of the stem. The primary structure of the 
stems of species of Bryonia , Trichosanthes , Momordica , and 
Melothria was examined with reference to this particular. 
In all of them the vascular bundles at this early stage 
exhibited an abundant medullary as well as external phloem, 
of which the former was often greater in proportion. 
In Actinostemma biglandulosa , however, the medullary 
1 This distinction was shown by Scott and Brebner (op. cit.) to be invalid. 
2 Scott and Brebner, 4 On Internal Phloem/ Annals of Botany, vol. v. 
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