the Anatomy of the Genus Selaginella. 457 
The roots arise singly at each forking of the stem, and are 
partly aerial, party subterranean. The root is at its origin 
about equal in thickness to the weaker branch, and at once 
bends downwards and backwards. On the upper side of the 
stem there also arises at each forking a papilla which re- 
presents an abortive root. The branching of the root is 
dichotomous, the first forking being in a plane at right angles 
to the long axis of the stem, the next at right angles to the 
first and so on for several divisions, until external conditions 
interfere with the regularity of the branching (PL XX, Fig. 2). 
The vascular cords of the root are inserted on those of 
the larger branch, apically to the point of origin of the 
minor branch (Fig. 1). The vascular cord of the dorsal 
abortive root arises behind the point of origin of that of the 
functional root, its xylem ending halfway through the cortex 
of the branch. The lacuna surrounding the vascular system 
of the stem is continued up the vascular cords of the roots, 
both dorsal and ventral, for a short distance. A transverse 
section of the root in its unbranched portion shows a crescentic 
vascular cord with the protoxylem in the concavity. The 
protoxylem is broken up into patches in the larger bundles 
and distributed round the concave side (Fig. 4). The outer 
cortex is strongly lignified, but becomes less so inwards, again 
becoming lignified as the endodermis is approached. In the 
younger roots the endodermis shows the usual radial markings, 
but as age advances the cuticularization becomes complete. 
The pericycle is from two to four layers thick. There is one 
row of sieve-tubes (or, more rarely, two) round the convex 
side of the cord ending at the tips of the horns of the xylem. 
Three to four layers of phloem-parenchyma completely 
envelop the xylem. In young roots the crescentic form of 
the xylem is not so well marked, and the horns approximate 
so that the protoxylem becomes almost enclosed. 
The convex side of the bundle faces the apex of the stem, 
and the branching of the vascular cord of the root takes place 
before external branching. At the forking of the root the 
vascular bundles of the branch-roots undergo a change in 
