THE ANATOMY AND AFFINITY OF STROMA TOPTERIS MONILIFORMIS, METT. 14 7 
figs. 29, 30, and 31. From them it will be seen that the division is initially unequal, 
but that the two steles finally formed are of almost equal dimensions. The somewhat 
smaller stele supplies the erect left-hand branch. Throughout the greater part of 
this branch it remains relatively small (fig. 32), but distally it widens and divides 
into two similar portions (figs. 33, 34, and 35). Of these, the right portion will be 
followed first. As the branch which it supplies becomes free, the stele, at first 
slender and condensed, becomes wide and parenchymatous (figs. 36 and 37). Within 
the nodular swelling it expands and divides unequally (figs. 38, 39, and 40). Of the 
two steles thus formed, the smaller left-hand member is rapidly reduced, and dies out 
without even the accompaniment of a protuberance on the surface of the nodule. 
It is believed that this aborted stele should have supplied a laterally displaced 
axis, and it would not be surprising if an anatomical study of the nodular swellings 
which Mettenius observed at the leaf-bases of Stromatopteris revealed a branching 
of stele and abortion of axis similar to that just described. 
The tapered columnar structure which is the direct upward continuation of the 
nodule is supplied by the surviving protostele (figs. 40 and 42), but it will be noted 
that the phloem of the latter has become ill defined. A sudden transition to a 
Y-shaped arrangement of the xylem follows almost immediately the entry of the 
stele into the branch (fig. 43), but this condition is in turn quickly replaced by a 
return to protostely (fig. 44). In these fluctuations is seen the same uncertainty as 
to stelar structure as characterised the base of the dominating right petiole of 
the first plant, and the petiolar nature of the branch under discussion seems obvious. 
The final establishment of the horse-shoe leaf-trace is gradually accomplished as 
the petiole is ascended (figs. 45 and 46) ; but as the distal portion is approached, 
reduction of the trace, both as regards dimensions and internal specialisation, takes 
place. This reduction fundamentally affects the phloem and xylem. The former 
can no longer be clearly recognised, while the latter becomes gradually disintegrated, 
and when the tip of the column is reached there survive only three small groups 
of tracheides. The steps in this reduction are illustrated in figs. 47 to 52. The 
tissues of this branch are fully matured, and it is regarded as a petiole which 
should have been continued by a pinnate portion. But the abortion which stunted 
the stem, and led to the formation of the nodule, has extended to the leaf also. 
Pinna formation was entirely suppressed, and the leaf expanded as a relatively short 
filiform structure consisting entirely of rachis. It cannot be argued here whether or 
not filiform leaves are common in Stromatopteris , but an arrestment of the leaf, 
similar to the one just described, is figured by Diels ( loc . cit.), and on the specimen 
in Edinburgh Herbarium there is one leaf showing the condition depicted in text- 
fig. 16. It is of a slender and reduced type, and bears distally a few incomplete 
pinnae. 
As regards the left-hand branch of the specimen which has been under discussion, 
it will suffice to state that at its ba,se the stele at first shows signs of adopting the 
