1052 
Benson.—New Observations on 
uniseriate hairs with a slightly enlarged base. The roots pass out of the 
stem rather abruptly, and resemble those of later species of Botryopteris. 
When the xylem of a stem stele is about to give rise to roots, the diagram¬ 
matic arrangement of protoxylems and metaxylem described above for the 
leaf-bearing stem (PL LXXXI,Figs. 1-3) is interfered with. Theprotoxylem 
becomes at first more scattered (Figs. 1 2 a and 13 a) } and small peripheral 
groups are formed at irregular intervals. Meanwhile the xylem increases 
considerably in bulk, and consists mainly of elements of a smaller size and 
less definitely porose marking than those supplying the leaf-trace ( vide 
Fig. 22). Different examples of such stems are shown in Figs. 4, 6 a , and 
9 a. It is. in this region that sections of the stem, e.g. Fig. 13 a , show 
considerable resemblance to those of the root-bearing regions in Botryopteris 
ramosa, and possibly admit of some comparison with those of Diplolabis 
and Metaclepsydropsis recently described by Dr. Gordon. 1 
VIII. Pseudo-secondary Thickening of the Stem Xylem. 
As already observed, the xylem of the stem increases its dimensions 
very rapidly after giving off a leaf-trace, especially if it is about to give off 
roots (cp. Figs. 11 and 13). 
In Fig. 13 two roots are being given off, and we may safely conclude 
that in Fig. 14 (cp. Figs. 15 and 14) and in P"ig. 9 (cp. Figs. 7 and 9) 
roots would have been demonstrated if the series could have been 
prolonged. 
This increase in size is brought about partly by the differentiation of 
parenchymatous elements which divide irregularly, as commonly occurs in 
primary xylem. Partly, however, the serial mode of division of a common 
mother-cell obtains, examples of which are seen in Figs. 4, 1 2 a, and 15. 
This mode of developing new xylem appears to be intermediate between 
the irregular division of normal primary mother-cells and the regular 
formation of secondary wood from a definite meristem. The resemblance 
is more striking when several contiguous cells lay down tangential walls in 
the same plane, as may be seen in Fig. 12 a. There is no reason to think 
this is a vestigial character, as no older nor contemporary form of the 
Botryopterideae has been described with normal secondary thickening. 2 
If primitive it is very suggestive of the way in which secondary meristems 
may have arisen, and thus seems worthy of record. For such cases of 
cellular increase as these the term ‘pseudo-secondary’ is suggested. They 
are of course exceedingly common, occurring in the stem of Metaclepsy¬ 
dropsis duplex and Diplolabis Romeri , 2 among contemporary Ferns and in 
many recent plants. 
1 Gordon: Diplolabis Romeri , 1910; Metaclepsydropsis duplex , 1911. Trans. Roy. Soc., 
Edinburgh. 
2 Gordon: loc. cit., 1911, Fig. 8. Since writing this Dr. Gordon tells me he has found 
a striking case in a new stem from Petty cur. 
