30 Saxelby . — The Origin of the Roots in Lycopodium Selago. 
enlarged. Fig. 2 shows a root origin and base of a leaf-trace, at the same 
level in the stem, both being cut transversely. 
The vascular bundle in an older root is usually seen in transverse 
sections to consist of either one continuous xylem group, in the shape 
of a horse-shoe, with protoxylem at the ends of the two arms, and meta- 
xylem in the centre, or else to consist of two separate parallel bands of 
xylem. In the latter case there are four protoxylem groups, one at each 
end of each band of xylem. Both types are shown in Fig. 7, and the first 
type is seen in more detail in Fig. 10. The root is so orientated that the 
line between the two xylem groups in the latter case, or through the centre 
of the horse-shoe in the former, is radial in the stem cortex. The concave 
side of the horse-shoe is always turned away from the stem (see Fig. 7). 
Jones 1 finds that a very large percentage of the roots are tetrarch, but that 
the diarch type occurs fairly commonly also. Bruchmann, 2 on the other 
hand, describes the type with two separate xylem bands as that typical 
of most roots, and he finds that only the first and second embryonic roots, 
and the root from the hypocotyl, have one group of tracheids and one 
of phloem cells. My observations agree more with those of Jones, in that 
I find most roots with diarch xylem, but I also find a close connexion 
between the two types. At the highest level at which the root xylem 
is seen separate from the stem xylem in any particular root, the xylem 
of the root may be in one group or in two. An intermediate stage may 
also be seen in which the xylem is still horse-shoe shaped, but the part 
opposite to the open end of the horse-shoe is composed of protoxylem cells, 
and by slight separation in this region the tetrarch type would be produced 
(see Fig. 11). This intermediate type is therefore really triarch, but I find 
no triarch type in which the xylem is drawn out into three rays. Lastly, 
in some roots scattered protoxylem elements are found round the outer 
side of the metaxylem bands (cf. Figs. 9 and 10). The type lower down 
the root is not entirely dependent on that at the base. A diarch root may 
become tetrarch or vice versa, though more often the same type is retained 
throughout the root. Even close to the meristematic apex, where the 
xylem cells can be distinguished by their size and position, though no 
lignification has yet taken place, the root may be either tetrarch or diarch. 
On the average, more roots show the diarch structure. Of those breaking 
through to the outside I find all diarch, none tetrarch. That the diarch 
type is more usual may be gathered from the fact that out of 25 roots 
in one stem, traced from the base of the root to the apex (or point of break- 
ing out from stem cortex), 17 were diarch, 3 were tetrarch, throughout ; — 
4 were first tetrarch then diarch, and 1 was first diarch (or rather inter- 
mediate) then tetrarch. The protoxylem cells are distinguished from the 
metaxylem by their smaller size, but more especially by their earlier 
1 Jones, loc. cit., p. 29. 2 Bruchmann (b), loc. cit., p. 81, 102. 
