ANATOMY 419 
protoxylem makes its appearance: where the protoxylem is clear it is 
evident that the differentiation of the primary xylem is centripetal, as it 
is, with local exceptions, throughout the aerial shoot. Passing upwards 
along the aerial shoot, the peripherally projecting protoxylems increase 
in number, the whole stele enlarging: finally, in transverse section the 
xylem appears as a hollow, many-rayed star, while the centre is occupied 
by sclerotic tissue. Peripherally as before lie the phloem and the endo- 
dermis (Fig. 233). An examination of the lower part of the aerial shoot, 
and of some adjoining parts of the rhizome, shows a feeble secondary 
formation of xylem: there is no definite cambium, but the additional 
tracheides which arise from the tissue outside the primary wood often 
show signs of a radial arrangement. The secondary development fades 
off as the stellate structure of the upper region is attained. Thus 
Fic. 233. 
Psilotum triguetrum. Part of a transverse section of the central stele. 100. 
structurally the base of the stem of Psz/o‘wm recalls the stem of Spheno- 
phyllum, though with very feeble secondary growth, while the upper part 
of the stem resembles the axis of the cone of Chezrostrobus, though on a 
simple scale, and with fewer xylem-rays. 
In the stem of Zmesipteris no secondary development has hitherto 
been found. The rhizome exhibits much the same structure as that of 
Psilotum, with a solid group of tracheides at the centre, or in weaker 
branches an irregular xylem-ring, surrounded by phloem, and a very ill- 
defined endodermis. Often there is no distinct protoxylem; but, as the 
passage is made to the aerial stem, protoxylem may appear: its position 
in all the upper region is mesarch. In weaker shoots, and especially in 
the upper regions, the cauline tissue of the stele fades out: the leaf 
traces become the main feature of the vascular system, which in trans- 
verse sections is then represented by a ring of separate strands: each 
of these has its mesarch protoxylem corresponding to that seen in the 
leaf-trace (Fig. 234): in point of fact this becomes a phyllosiphonic type 
of structure. In strong axes, however, the xylem may still form a 
