524 Boodle .— Comparative A natomy of the 
(Fig. 26, Z?), it leaves a horseshoe-shaped mass of xylem 
in the stele. The xylem becomes nearly closed again and 
the fourth leaf-trace passes off ; after which the stele pos- 
sesses a fair-sized group of central parenchyma surrounded 
by a ring of tracheides (Fig. 26, C), thus agreeing with the 
structure of the mature stem in other species of the genus. 
The xylem is now not quite mature, lignifkation not being 
complete in the two tracheides with dark walls in the diagram. 
One of the central parenchymatous elements in the same 
diagram is shown with dotted contents to represent mucilage. 
This cell differs from the other cells of the central parenchyma 
in possessing mucilage, and in this resembles the cells of the 
endodermis. It may quite possibly be a rudimentary vestige 
of a nodal endodermal pocket. Or probably a more correct 
statement of this supposition would be to say that this cell 
may represent part of an endodermal pocket, but has been 
differentiated (owing to correlation) in connexion with a node 
in a region of the stem, which probably never possessed 
complete endodermal pockets. 
The central parenchyma in the part of the seedling we 
have been considering, gives no indication of being of the 
nature of phloem, so that from the young plant we obtain no 
suggestion of reduction from solenostely. 
The petiolar bundles of the first and second leaf of S.pusilla 
are shown in Fig. 26, E and D respectively. The bundle of 
the first petiole, in the basal region of the latter, has two 
or three sieve-tubes and is collateral, but higher up (Fig. 26, E) 
sieve-tubes are not present. Fig. 26, D } is the collateral 
petiolar bundle of the second leaf. Thus there is no indi- 
cation in the early leaves of the collateral bundles of the leaf 
having been derived from concentric ones by reduction in 
this phylum. 
Britton and Taylor ( J 0I) give three figures of leaf-bundles. 
The structure of the largest of these appears to be of the 
same type as that found in other species of the genus, and 
fibres appear to be present in the usual position. 
