386 Chandler . — On the Arrangement of the Vascular Strands 
might possibly be interpreted as sieve-tubes, but, on the whole, it is 
probable that sieve-tubes are not differentiated in the central £ pith ’ until 
after the first leaf-trace. The latter is quite simple, but of relatively large 
size, leaving a gap which remains open through a considerable distance, 
a character which is maintained throughout the plant. The corresponding 
root is quite normal, and, just before the second leaf-trace, definite sieve- 
tubes appear scattered in the enlarged pith. After the closing of the 
second gap, the stem presents a characteristic appearance in transverse 
section. The xylem ring is very narrow (consisting of only two or three 
rows of tracheides), and encloses a relatively large pith of phloem in which 
the sieve-tubes are very conspicuous. In these early stages it frequently 
happens that isolated tracheides, sometimes in small groups, are differen- 
tiated in the central phloem, a point of some interest which will be referred 
to in connexion with Nephrodium setigerum. Immediately outside the 
xylem is a very regular row of parenchyma cells, followed by an almost 
continuous single layer of sieve-tubes. The most striking feature, however, 
is the pericycle, which is composed of two or three layers of large squarish 
cells with conspicuous nuclei and readily-staining contents. The endo- 
dermis is well marked, but its cells are not always in exact sedation with 
those of the pericycle, as so commonly occurs. The parenchyma of the 
ground- tissue is thin- walled except at the periphery, where the walls are 
sclerosed and deep red in colour. 
The first four root- and leaf-strands are formed in a perfectly normal 
manner, the xylem gradually dilating and enclosing the enlarged central 
strand of phloem. In connexion with the exit of the fifth or sixth leaf- 
trace we have the formation of a typical ground-tissue pocket as described 
for other forms. The vascular ring becomes strongly oval in outline, and 
towards one end of the central phloem a group of two or three endodermal 
cells appears. At a slightly higher level the endodermal cells surround 
a strand of thick-walled parenchyma very similar in appearance to the 
sclerotic elements of the ground-tissue (Fig. 174). As in similar cases, the 
inner and outer endodermis become continuous at the exit of the trace, 
and the ground-tissue is differentiated to a considerable depth in the 
Central phloem (Figs. 174, 175). The apparent encroachment of the ground- 
parenchyma upon the vascular tissue increases until the phloem and peri- 
cycle merely line the concavity of the xylem, as indicated in Fig. 176. 
At a slightly higher level the leaf-gap is closed, and we have the very 
interesting appearance represented in Fig. 1 77. The resemblance of this 
isolated section to a similar section of Gwynne-Vaughan’s true solenostelic 
ferns is very striking. 
During these changes the xylem ring has become thicker, and exactly 
opposite the previous leaf-gap begins to bulge outwards in preparation 
for the next trace. The formation of the latter is indicated in Figs. 177 
