a 
Sykes. — The Anatomy and Morphology of Tmesipteris. 69 
no internal endodermis or phloem is present. At this stage there is a great 
increase in the number of tracheides, and sometimes the arrangement of 
a row of tracheides suggests secondary thickening, but is more probably 
due to long continued primary growth. Very soon the pith and phloem 
cells become collenchymatous ; it is interesting to note how simultaneously 
this change takes place in the two tissues. 
Figure IV. A-F, illustrating the changes which take place in the stele during transition. 
A — typical rhizome structure ; F = typical stem structure, below the first leaf ; B-E = transition 
region, (e = endodermis, p = phloem, x = xylem, pxy — protoxylem, pp — pith, i and 2 = the 
protoxylem groups of the rhizome stele ; 1 gives rise to 1', 1', 1' in the stem stele, and 2 gives rise 
to 2', 2' in the stem stele.) 
(c) The Aerial Stem. Gradually (Text-fig. IV, f) five distinct groups 
of protoxylem are formed, sometimes remaining separate, sometimes partly 
connected by metaxylem. Until now the endodermis has been quite 
clear, but here it loses its distinctive markings, and its existence becomes 
hypothetical. Several scale-leaves are given off from the lower part of the 
stem or from the uppermost part of the transition region, but these re- 
ceive no vascular supply. At the base of the larger and later of these, 
elongated thick-walled cells are found, which are continuous with similar 
cells in the cortex of the stem, and probably serve as conducting elements. 
The last scale-like leaf receives a very small trace (Figs. 2 and 3, PI. VII). 
A cross-section taken from the lower part of the stem of Tmesipteris 
