the Genus Hottonia. 
259 
produces roots which grow horizontally (morphologically) into the pith 
(int. rt ., PI. XX, Figs. 5 and 6, and Text-fig. 3) and finally vertically 
upwards, passing out through one of the breaks in the vascular ring. 
As in the case of the floral pedicels, the traces of the subtending leaves 
fuse with the branch steles before the latter unite with the main vascular 
supply, but the traces of many of the crowded leaves just below this point, 
which have no branches in their axils, enter the stem as shown in Figs. 5 
and 6, PI. XX, i. e. a complete gap is formed in the vascular cylinder through 
which the cortex and pith are in unbroken continuity and where the internal 
endodermis unites with the external. A very little lower down, however, 
no complete leaf-gap of this type is formed, as the external endodermis is 
not broken, and the internal parenchyma is thus never continuous with the 
external (Text-fig. 4). These two forms of leaf-gap characterize the two 
types of development described by G wynne- Vaughan ( 5 ), p. 317, for the 
seedlings of Primula japonica , of which the latter, i. e. where there is 
a break in the vascular tissue only, is much the more prevalent. It is this 
type which is constant for the whole of the subterranean and aquatic stem 
with the exception of that just below the transitional region, and it is 
probably characteristic of at least aquatic Angiosperms, as I have found 
it in the stem of Hippuris vulgaris , Myriophyllum proserpinacoides , and 
Menyanthes trifoliata. It is interesting to note, as affording a transition 
between the two types, that for some distance below the point at which the 
internal endodermis ceases to unite with the external it is inclined to run 
out into the leaf-gaps as shown in Text-fig. 4 and PI. XX, Fig. 8. 
Just at the base of the inflorescence the vascular ring is relatively very 
large, the pith measuring about 3 mm. across, but as we pass down the 
stem it narrows rapidly till only a few cells are enclosed by the internal 
thickened ring (PI. XX, Fig. 9), and finally none at all, the centre of the stele 
being occupied by a core of lignified cells (Fig. 10). Passing further down 
the stem, this core itself contracts till in transverse section it consists of two 
or three cells or even a single one (Fig. 11). At this point, which is usually 
several centimetres from the transitional region, the central lignified tissue 
may die out altogether, or it may persist for a long distance down the stem, 
though in this case it is almost always interrupted. Indeed, this interrup- 
tion may take place at any level ; the internal ring is often more or less 
incomplete, as well as irregular, and may be absent for several millimetres to 
reappear smaller a little lower down. 
Thickened, lignified cells in the centre of the stele have been found as 
far as 18 cm. from the transitional region, and it seems a point worth 
noting that they tend to occur more often at the nodes, especially where 
these are crowded. They are never found in the subterranean stem. The 
whole internal lignified structure is thus comparable with a single 4 endo- 
dermal pocket ’ described by Tansley and Chick ( 12 ), and Boodle ( 2 ), for 
s 2 
