Anatomy of the Ophioglosseae . 379 
ment takes place in a centripetal direction (i. e. towards the 
phloem). The xylem is therefore monarch. 
For comparison with the transverse sections, a root-tip was 
prepared in the following way : — Some of the cortex was 
sliced off on two opposite sides ; the remaining part, which 
contained the stele intact, was then boiled in potash, washed, 
and mounted in strong glycerine. The stele was thus 
rendered transparent. It is represented in Fig. 2, where px is 
the first protoxylem element. Two of the tracheides are 
shown more highly magnified in Fig. 3, where it is seen that 
the thickenings in these early protoxylem-elements are 
annular, with an approach to reticulate thickening in places. 
The first protoxylem-element is marked px at about the 
same point in Figs. 2 and 3. 
To return to the transverse section ; the phloem is seen at 
the upper limit of the procambial strand. It consists of an 
arc of sieve-tubes (s.t. Fig. 1), (which are in contact with the 
endodermis, e), together with a certain amount of the 
parenchyma lying between the sieve-tubes and the xylem. 
The walls of the sieve-tubes are, at this stage, already 
thickened, and stain strongly with haematoxylin ; the paucity 
of contents also serves to distinguish these elements. A few 
sieve-tubes are still undifferentiated, viz. the inner ones 
opposite the middle region of the arc. ' Thus, when differentia- 
tion is complete, the row of sieve-tubes is usually one element 
thick at the ends, and often two elements thick in the middle. 
When mature the sieve-tubes show the characteristic granules 
in contact with the walls. 
A section at an earlier stage, before the differentiation of 
any xylem, showed the arc of sieve-tubes easily recognizable, 
and with one or two sieve-tubes near each end slightly thicker- 
walled than the rest. This suggests the presence of two 
protophloems. Their position, determined by comparison 
with younger stages, is shown at pp. in Fig. 1. 
It will be seen from Fig. 9 that in the mature root there 
is a considerable zone of parenchyma between the sieve-tubes 
(p/i) and the xylem. The outer part of this parenchyma, 
