2 I 2 
On Secondary Thickening in Pteridophyta. 
PSILOTUM TRIQUETRUM. 
Stem. The formation of secondary xylem elements takes place 
in much the same manner as in Ophioglossum and they occur in the 
subterranean and aerial axes. No definite cambium is present, but 
it is possible, in some cases, to make out a radial seriation in the 
tracheides and the adjoining parenchymatous cells. The new 
tracheides are sometimes pitted in an irregular manner, but more 
often the pits are of the normal scalariform type. The secondary 
xylem elements are situated between the primary wood and the 
phloem, and, generally, are separated from the former by at least 
one row of parenchymatous elements. On the other hand they 
may be in direct contact with the central xylem-mass, and in great 
part they replace the zone of parenchyma which, in younger 
axes, occurs between the primary xylem and the phloem. 
The examination of longitudinal sections demonstrates that 
these secondary tracheides pursue a sinuous course, which fact is 
probably due to sliding-growth. 
Boodle, who was the first to discover these tracheides in 
Psilotum, supposes that their formation depends on the development 
of the aerial shoots, “ and appears to be due to a basipetal stimulus 
from the latter.” 
Tmesipteris. 
So far as the published accounts go, Tmesipteris does not exhibit 
any secondary thickening ; it would not be at all surprising, however, 
to find secondary tracheides in the axes of this plant, provided 
material sufficiently old were examined. 
Literature. 
SELAGINELLACE^E. 
Bruchmann.—Untersuchungen iiber Selaginella spinulosa. Gotha, 
1S97. 
Selaginella spinulosa. 
Stem. This is the only known species of Selaginella which 
shows the feature under consideration. At the base of the stem, 
which is a permanent structure, there is an indefinite growth in 
thickness. The phenomenon is very similar to what occurs in the 
same region of Isoetes ITystrix, but with this difference, that whereas 
in the last-named plant there is an addition of secondary parenchyma 
only, in the case of Selaginella spinulosa the later-formed elements 
consist of parenchyma and xylem. 
