THE ANATOMY AND AFFINITY OF STROM ATOPTERIS MONILIFORMIS, METT. 145 
the last division (fig. 16). As the column is entered, the xylem becomes parenchy- 
matous (fig. 18), and as the broad apex is approached the stele is found to be immature, 
and apparently arrested in development. As will be seen from fig. 19, a preparation 
for a further division of the stele has been initiated, and it is to this that the terminal 
depression and the broadening of the apex of the column are due. 
The view which is taken of this short columnar branch is similar to that held 
regarding the small conical branch, associated with the right-hand petiole of this 
plant. It is believed to be an arrested axial branch, which initially bore the right- 
hand petiole but has now been displaced so that the latter appears to form the direct 
upward continuation of the main axial branch. The middle petiole shown on the 
specimen is likewise considered to have been initiated on the small columnar branch. 
But while on the right of the plant the formation of a single leaf is followed by 
arrest of the axis, on the left the formation of two leaves has ensued before the arrest 
occurred. Even then the formation of a third leaf upon the small columnar axis is 
indicated structurally. 
The following conclusions may, then, be drawn from the examination of the first 
specimen of Stromatopteris. The leaves are borne upon the distal branches, but 
during development these leaf-bearing branches are more or less arrested, while the 
leaves become increasingly dominant. Complete arrest of axis-formation is not 
shown at any point, but its accomplishment would lead to a truly terminal position 
for the leaf, and the transition from the protostele of the axis to the typical leaf- 
trace would then probably be gradual. As the matter stands, however, the leaf-trace 
has been shown to have been quickly and clearly defined when the formation of any 
one leaf has not hindered the subsequent successful development of another. But 
when leaf-formation is followed by arrest of the axis, the typical leaf-trace is not 
immediately established, but fluctuations occur, suggesting a partial imposition of 
the characters of the stele of the arrested axis upon the trace of the dominating 
leaf. The clearly defined leaf-trace with horse-shoe xylem and phloem, separating 
directly as such from the stele of the axis, may be considered the typical trace in 
Stromatopteris, while the trace which shows fluctuations in its structure, or passes by 
gradual transitions from a protostele to a foliar meristele, is held to be secondary 
and derivative. I do not think that the latter is to be interpreted as a primitive form, 
resembling in general the trace described by G-wynne- Vaughan and Kidston for 
Thamnopteris ( Proc . Roy. Soc. Edin., 1908). It is rather a consequence in a 
xerophytically reduced plant of the arrest of the axis, the dominance of the leaf at 
an early stage in development, and the consequent assumption by the leaf of a 
directly vertical position. Among flowering plants its condition finds a parallel in 
the aerial fertile branch of Juncus conglomerate, in which the flowering spike 
emerges obliquely from the massive leaf-sheath, and the cylindrical leaf is continued 
vertically upwards above the displaced axis. In this case the same general anatomical 
characters are shown by the leaf in its free cylindrical and erect distal portion as is 
