5°6 
Stiles The Podocarpeae. 
pletely, into distinct bundles.’ 1 In such a series we have the ancient 
Lepidodendron petty cur erne? with its solid xylem core, L. selaginoides 
( = L. vasculare ), 3 where parenchyma was intermingled with the tracheides 
in the middle of the stele, and Z. Wunschianum , 4 where a large pith occupied 
the middle of the stem, and the primary xylem was reduced to a com- 
paratively small ring surrounding the pith. A further reduction of primary 
xylem is exhibited in Sigillaria spinulosa 1 by the partial division of the 
narrow ring of primary xylem into bundles, and ultimately in S. Menardi 
the xylem was completely split up into separate bundles, the primary parts 
of which were very small compared with the size of the stem, for in 
Brongniart’s specimen the pith was about 15 mm. across, while the ring of 
bundles, the greater part of which was secondary wood, was only about 
1 mm. in thickness. 5 
Thus in the evolution of the Sigillaria Menardi type of stele from the 
exarch protostele of the earlier Lepidodendrons, such as L. rhodamnense 
and Z. pettycurense , the chief tendency has been towards elimination of 
the primary centripetal wood, which was replaced by secondary centri- 
fugal wood. If we suppose this tendency to have full play we shall at 
length reach a condition in which the centripetal xylem of the stem has 
disappeared altogether. The stele in its primary state will then consist of 
a ring of bundles, each consisting of little more than the proxylem elements, 
the great mass of wood in the stem being secondary. Under these circum- 
stances it is not only possible but likely that the mode of insertion of the 
leaf-traces will differ from their mode of insertion on a stele containing much 
primary centripetal wood. 
Thus it appears to the writer that the presence of leaf-gaps in the stele 
of the Coniferales is a character that cannot be legitimately used as an 
argument against the derivation of this group of plants from an ancestor 
without leaf-gaps in the stele. 
As regards the structure of the elements themselves the nature of the 
pitting of the tracheides furnishes an argument against the Lycopodialean 
ancestry of the Conifers, but here it seems that in Renault’s Sigillariopsis 
Descaisnei there were pitted tracheides as well as scalariform ones, 6 though 
the approach to a coniferous type does not seem very near. 
The presence of a double leaf-trace in the Abietineae has been looked 
upon as furnishing an important piece of evidence in favour of the Cor- 
daitalean relationship of the Coniferae. But in a similar way the presence 
of a single leaf-trace in the Podocarpeae (except Nageia ) may be regarded 
as furnishing equally important evidence in the opposite direction. 
Gametophytic and embryonic structures have not been considered in 
1 Seott (’09), p. 634. 2 Kidston (’07), p. 207. 
3 Scott (’08), p. 146. 4 Seward and Hill (’00), p. 907, and Seward ('10), p. 164. 
3 Scott (’08), p. 215. 6 Renault (’79), p. 270; Scott ('08), p. 230. 
