DR. D. H. SCOTT ON BOTRYCHIOXYLON PARADOXUM. 979 
development has led to some compression of the primary xylem. There is nothing so 
very surprising in finding that the substitution of secondary for primary wood has 
extended from the stele to the lower part of the leaf-trace; analogous cases, though 
scarcely known among Ferns, are familiar in Cycads and other Gymnosperms, as well as 
Dicotyledons. A second specimen, probably also referable to Botrychioxylon, confirms 
the view here taken in a rather striking manner, for it shows a leaf-trace in which the 
replacement of primary by secondary tissues has taken place in a more partial degree 
(Pl. 40. fig. 18). 
THE PETIOLE. 
The structure of the petiole is shown in two sections, that already referred to and the 
next above it. A third section (2466) is of little importance, as the vascular bundle is 
missing. In the lowest section the dimensions of the petiole are about 5:3 x 3:5 mm.; 
the outline is irregular, owing partly to the presence of a large aphlebia (PI. 37. fig. 4). 
The preservation is not perfect throughout, but the bundle is fairly well shown (PI, 39. 
fig. 11), as is also the case in the following section (Pl. 39. fig. 12), in which the petiole 
is less complete. The measurements of the bundle are about 1 mm. x 0'5 mm. There 
is a resemblance at first sight to Renault’s figure of his Dineuron pteroides (Renault, 
06, fig. 19, p. 23), the similarity being accentuated by the fact that the middle part of 
the xylem-plate is thin-walled (immature) in both. This, however, is clearly an 
accidental agreement, and a closer examination reveals important differences in the 
essential characters. ! 
In the lower section of the two (2464) the general outline of the bundle is well 
defined and is somewhat hourglass-shaped (Pl. 41. fig. 23). The xylem is only slightly 
dilated at the two ends, so that antenns can scarcely be said to be developed. The 
median band (“apolar ” of Dr. P. Bertrand) is straight and consists of large elements; 
the walls of the middle tracheides are still incompletely thickened. At the ends, where 
the xylem slightly spreads out, the elements are smaller; some of these also seem to be 
immature. There is a slight sinus at each end; the cells in the depressions are a little 
disorganized, there is no trace of any external xylem (filament) closing in the sinus, all 
the adjacent tissue being thin-walled ; neither are the cells small enough to be reasonably 
« filament ” in an immature stage (Pl. 41. fig. 23). ; 
some respects more characteristic. Here there is a 
and Pl. 41. fig. 24). One may 
oxylem, though I have not 
regarded as representing a 
The upper section (2465) is in 
quite definite cavity at each end of the xylem (Pl. "s 7 13 ; 
fairly suppose that these gaps mark the position of the pro ' 
E $i Ps mor nag No Pn the gaps are of the nature of delis 
loops or sinuses. I have endeavoured to ascertain which they are. z ao en z 
tissue bordering the gap externally is badly preserved, with broken cell-wa i ate d 
the broken walls are rather thick, but the appearance of thickening m on. fot "4 
effect of decay: the tissue resembles ill-preserved p rather = pics uy 
neither section is there any evidence of the presence of a ' c is P a a a 
of xylem, forming a peripheral loop; on the evidence die e — vost 
sinuses were open, though the preservation is not good enough to re 
