68 Sykes. — The Anatomy and Morphology of Tmesipteris . 
composed of narrow scalariform tracheides, united by large scalariform 
metaxylem elements. The endodermis is still distinct, and the brown 
material increases in amount. A few specimens showed fibres in the 
phloem at this stage, but the ordinary phloem-cells are unlignified. 
In some examples monopodial branching took place near the apex of 
the rhizome, but this was rather an unusual occurrence. Branching is 
frequent in the middle portion of the rhizome and is generally monopodial, 
but one case was examined in which both limbs appear equal, and this may 
be true dichotomy. In this case (Text-fig. Ill, E, F), just before the forking 
of the rhizome, the stele divided into two equal parts, each containing one 
protoxylem group, after which each of the protoxylem groups divided again, 
restoring the bi-polar structure to both steles. Generally (Text-fig. Ill, A-d), 
at the formation of a branch, only one protoxylem group of the main axis 
divides in order to give rise to the stele for the branch, while the other 
remains unaffected. 
A 
O O 3 
E 
o° o- 
<3 0 
Figure III. Illustrating equal and unequal branching of rhizome; = protoxylem. A-D 
= successive stages in the unequal division of the xylem. E-F = successive stages in the equal 
division of the xylem. G = portion of rhizome ; a-f = levels at which sections A-F were taken. 
(b) As the transition region is approached, the amount of starch in the 
cortex diminishes and the fungus and absorbing hairs disappear. In one 
case only a branch was found to be given off during a very early stage of 
the transition. The changes in the stele during transition are as follows. 
At first one, and then several, thin-walled, unlignified cells appear in the 
centre of the xylem (Text-fig. IV, a), and soon these communicate with 
the phloem, the cells of which they strongly resemble (Text-fig. IV, b). 
The xylem then becomes crescent-shaped, and meanwhile one or both 
protoxylem groups have divided. No distinctly triarch stage, such as that 
described by Boodle in P silo turn 1 is present. Each protoxylem group 
gradually becomes mesarch. 
The metaxylem is then broken across and two distinct groups are 
formed (Text-fig. IV, c) ; sometimes these unite again and form a crescent 
or ring-shaped mass of xylem, surrounding a large amount of thin-walled 
tissue (IV, d), and reminding the observer of a reduced solenostele in which 
1 L. A. Boodle, 1904. 
