54 
Rev. T. A. Jefferies. 
and varied outline. The groups of xylem have their protoxylem 
adjacent to the pericycle and large vessels to the inside. The 
protoxylem (*) is composed of narrow annular and spiral vessels. 
The xylem consists of tracheides, large vessels, and xylem 
parenchyma. Both annular and pitted tracheides are found, their 
ends tend to be blunter than usual, and they are very thick walled ; 
this tissue either in transverse or longitudinal section is easily 
confused with the sclerenchymatous ground tissue of the pith which 
in general appearance it closely resembles. The vessels (Fig. 3, V) 
are exceedingly large, their size being all the more striking because 
they are distributed singly, not more than one to a xylem group ; 
they are almost circular in outline, with abundant conspicuous pits, 
and each is surrounded by a layer of flat parenchymatous cells. 
Usually these vessels are developed only on the alternate xylem 
groups, the other groups of xylem remaining inconspicuous through 
the absence of the large vessel and the resemblance between the 
smaller wood elements and the ground tissue : but occasionally two 
neighbouring xylem groups converge on one vessel, forming a V 
section with a phloem group between the arms ; and sometimes two 
neighbouring groups have each a vessel and these two vessels 
become laterally apposed, when the circular outline of the single 
vessel is replaced by one roughly elliptical, including the two vessels, 
and these are separated by a flat partition of parenchyma, one cell 
thick, along the minor axis. These large vessels form a well marked 
zone around the pith. The xylem parenchyma, surrounding the 
vessels and scattered among the tracheides, has its walls slightly 
thickened, although in comparison with the other elements of the 
stele, the phloem excepted, it is thin walled ; in transverse section 
some of these cells around the large vessels are of considerable size. 
The whole of the central portion of the stele within the ring of 
large vessels is occupied by a sclerenchymatous pith (Fig. 3, P). 
This tissue is composed of exceedingly thick walled, long, scleren- 
chyma fibres, with fine pits and with ends rather bluntly pointed 
for such a tissue and sometimes flat; towards the centre these 
elements increase in sectional area but there is no thinning of their 
walls : towards the margin the same tissue spreads out between the 
large vessels and occupies the regions between the vascular bundles; 
the middle lamellae are very prominent and give rise to strongly 
marked polygonal outlines as show in the figure. From the above 
description it will be realised that very thick lignified walls are 
a pronounced feature of the stele; indeed it is chiefly to this 
