582 
DR JOHN M‘LEAN THOMPSON ON 
result of reduction from the former. Behind the reduction theory lies the idea that 
all normal ferns with tubular steles must have stelar gaps, for unless this be so, a 
fern such as Platyzoma with an isolated pith, if considered normal and unreduced, 
would be a stumbling-block to the generalisation that “ the pith must in all cases 
be regarded as a derivative of the cortex which has become more or less completely 
sequestered within the stele” (ll). Such a view as that advocated by the theory 
of the cortical origin of pith would seek to exclude the alternative view that ferns 
with pithed tubular steles may in certain cases illustrate a condition in which stelar 
gaps have not yet been formed either in the ontogenetic or phyletic sense. But if 
in any fern a pith has arisen without the creation of stelar gaps , it cannot have been 
formed by cortical intrusion. It may be noted further that on the upgrade theory 
the presence of ill-defined or scattered phloem, or of incipient leaf-gaps, might con- 
ceivably be considered evidences of further amplification of an original protostele, 
provided it be granted that tissues of one kind or another can be formed where 
required and are not necessarily referable in origin to one original and continuous 
tract which has been extended and disintegrated in the phylogeny. 
The stelar structure in the large specimens of Platyzoma will be referred to first. 
In each case there was throughout a well-defined pith enclosed by a continuous 
tubular endodermis which was at no point in connection with the outer endodermis. 
Neither inner phloem nor any structure suggestive of inner phloem was found. 
There were neither leaf-gaps nor evidences of the prior existence of leaf-gaps. The 
only points which these plants added to the facts already acquired were the following. 
The pith fluctuated in bulk from point to point. In some plants it was a relatively 
thick strand where the traces of large leaves were most numerous, and more slender 
where the stem bore mainly small leaves. In other plants no such fluctuations were 
observed, even although the leaf-zonations were well shown. In some instances the 
pith was sclerotic throughout ; in others it was in part sclerotic, in part parenchy- 
matous. A number of dichotomies were cut in serial sections, and in each instance 
the division of the stele into two followed the plan illustrated in text-figure 1. The 
stele of the parent stem became expanded laterally (a, b), and as preparation for 
division of the xylem proceeded the inner endodermis became separated into two 
distinct and complete tubes (c). Between these a bridge of xylem was established 
( d ), so that the two parenchymatous cores now separated from each other were 
not placed in open parenchymatous connection with the pericycle. The stele 
then divided into two similar steles (e) by division of the xylem-bridge, the com- 
pletion of phloem and pericycle around the now isolated tubes of xylem, and the 
separation of the outer endodermis into two complete cylinders. 
It will be seen that neither ramular gaps nor other breaks in the continuity of 
the stele existed in the materials examined, and that the pith of the parent stem, 
in dividing to form the pith of the branches, was not brought into contact with the 
cortex. The investigation of these stems has added nothing to our knowledge which 
