572 Worsdell .— The Origin and Meaning of Medullary 
Lagenaria vulgaris. 
Seedling. 
In the hypocotyl occur six bundles, having the usual bicollateral 
structure, surrounding a central lacuna. The transition from stem to 
root structure takes place below the ‘ heel ’, approximately in the region 
where the first lateral roots are given off. In tracing the structure down¬ 
wards by means of a series of transverse sections, the internal 1 phloem- 
groups pass outwards and unite with the outer phloem-groups immediately 
before the transition to root-structure occurs, and therefore just before 
the junction of the lateral roots. As each phloem-strand passes out it 
revolves on its axis through i8o°. The phloem-strands do not all pass out 
exactly at the same level. When two strands occur opposite the proto- 
xylem of a single bundle, they pass out on opposite sides of the latter. 
A fairly wide pith is present even after root-structure is formed. 
The fate of the internal phloem-groups is thus very different from that 
of those in the hypocotyl of Cucarbita Pepo , according to Fischer’s data, 
and in Cucurbita maxima , according to Lamounette. 
From such developmental facts as have just been given Gerard arrived 
at (what will be seen to be later) his erroneous conclusions. 
There is, however, one developmental feature of some importance. 
Various observers, besides the present writer, have noted that the internal 
phloem arises at a later period than does the external phloem. It is com¬ 
monly found that vestigial structures arise later in the ontogeny than is the 
case with other parts of the tissue-plexus. This fact would tend to indicate, 
therefore, that the internal phloem is a vestigial structure. As far as this 
goes some little clue has been gained from a study of the ontogeny. How¬ 
ever, Faber’s observations (see above) point in exactly the opposite direction. 
L. clavata. 
Stem. 
The zone of sclerotic fibres, which is present in all Cucurbitaceous 
stems, always marks the limit of the central cylinder. This is an important 
character. Exactly the same is true for most, if not all, Monocotyledonous 
stems. 
In this plant, immediately within the sclerotic ring, occur great numbers 
of rudimentary phloem-strands. 
The central cylinder consists, as in so many Cucurbitaceae, of two 
circles of bundles. At the sides of the bundles of the inner ring, or, in 
1 The term ‘ internal phloem ’ will, in the following pages, always be applied to the phloem- 
strand which is attached to the ventral or inner side of the ‘ bicollateral 5 bundle of the cylinder. 
To all other phloem-strands occurring in the pith or the embouchement of the rays the. term 
‘ medullary phloem ’ will be given. These distinctive terms are for purposes of clear description. 
