530 
Gwynne- Vaughan.—Some Remarks on the 
All the earliest leaves, even the very first, have fully developed stipules, 
and in the upper region of the stipule of the later leaves (from about the seventh 
or eighth) a conspicuous patch of protophloem is to be seen at each side of the 
leaf-trace on its abaxial surface. Three groups of mucilage sacs also appear 
in the same region of the stipule; one just outside each of the above- 
mentioned protophloems and one median in the adaxial concavity of the 
trace. These features are characteristic of the stipular base of mature leaves, 
but they do not appear to occur in any part of the stipule of leaves below the 
sixth. The special tannin-containing cells that occur in the xylem-sheath 
of the leaf-trace in the base of the stipule of the early leaves have already 
been mentioned. It is to be noted that the xylem-sheath of the petiole of 
the fossil Bathypteris rhomboidea contains scattered sclerotic elements 
(Kidston and GWynne-Vaughan, Part IV, Fig. 53). 
Discussion. 
The study of the sporelings of Osmnnda regalis shows that in addition 
to and independent of the pithing of the stele, there is also a pocketing of 
the xylem-sheath in the axils of the leaves into the peripheral region of the 
xylem-ring. These xylem-sheath pockets account for the formation of the 
leaf-gaps or medullary rays in the originally continuous ring of xylem. In 
fact they break through the ring to meet the pith. In our paper on the 
Fossil Osmundaceae, Dr. Kidston and I did not fully appreciate the impor¬ 
tance of the axillary pockets in the formation of the medullary rays. We 
regarded the medullary rays rather as directly due to the removal of the 
elements of the xylem-ring by their passage outwards into the departing 
trace, and we looked upon the pockets more as subsequent downward 
prolongations of the medullary rays into the xylem of the ring. I now 
regard the formation of xylem-sheath pockets as the initial cause of the 
medullary rays. 
The interpretation of the vascular structure of the sporeling as indicat¬ 
ing the succession of the stages passed through in the phylogeny of the race 
is by no means a simple straightforward matter, but is one that requires the 
greatest caution in its treatment. It seems to me that each new modification in 
a series of progressive changes in the evolution of any vascular system would 
appear first of all in the mature regions of the fully adult plant only, as a new 
and an additional stage in its ontogeny. If it may be held that the longer 
any given modification has been in existence in the race the earlier it will 
appear in the ontogeny of the individual, it follows, theoretically, that it 
will eventually appear as the earliest stage of the sporeling. This is only 
possible, however, if the stele of the sporeling is able to become bulky 
enough to express the modification in question. If it is not, there must be 
a lower limit to the downward penetration of the modification into the 
sporeling stem which will be determined by the minimum number of 
