THE MORPHOLOGY OF THE STELE OF PLATYZOMA MICROPHYLLUM, R. BR. 589 
But if the reduction theory has received little support from the recorded facts, 
on the other hand the theory of upgrade internal differentiation has not been proved, 
though the general trend of the stelar fluctuations which have been described in 
these pages might reasonably be considered in its favour. Thus the fact that within 
a protostele a pith has been shown to grow and to become more or less included in 
an inner endodermis created de novo, points directly to the course which a supporter 
of the theory of intrastelar origin of the pith in Platyzoma would expect to be 
followed in the ontogeny. In the case described such an intrastelar development 
would be considered to have occurred at least three times, and on all but the last 
occasion had not been maintained, but had reverted directly to the original proto- 
stelic state. The inference would be that any one of three definite states may exist 
in the stele of a fern, and that these states are continuous in the evolutionary sense. 
First, the original protostely may persist undisturbed throughout the entire ontogeny. 
Primitive protostelic ferns would be examples of this. Second, the original protostely 
may be transformed at an early stage in the ontogeny once and for all into one or 
other of the more elaborate stelar states. Third, the original protostely may recur 
in the ontogeny in ferns in which the advanced stelar state is not yet permanently 
established. Whatever be the actual course of stelar amplification followed in 
individual cases, the second state appears to have been widely exemplified in modern 
ferns. Platyzoma may provide an illustration of the third and apparently excep- 
tional state in which the stelar structure of the ancestry has not yet been definitely 
restricted to the “ sporeling ” stages, but tends to persist in the mature organism. 
If this be so, then the structural evidence dealt with above may indicate a remini- 
scence of the steps taken in the initial transformation of the ancestral protostele. 
Such steps would be, first, the growth of the pith within the stele itself, and second, 
the inclusion of the bulk of the pith within an independent internal endodermis 
formed de novo. It has been shown by Dr Lang that endodermal cells may be 
formed where required in the stem of Helminthostachys ( 16 ). And further, Flas- 
kaemper has shown that pith may be locally present and locally absent in one and 
the same root according to the general conditions provided for the organism (17). 
He found that after a seedling of Vicia had been grown until the radicle had begun to 
elongate, if the cotyledons were removed, the growth of the plant was checked, 
and the root, pithed in its older part, was devoid of pith in the part subsequently 
formed. But when the plant had become strong under continual exposure to 
favourable conditions, a pith was reorganised in the later parts of the same root. In 
like manner the stelar pith of the stem of Platyzoma may be viewed as open to fluctua- 
tion either by decrease or increase under the varying conditions of life, and need not 
be regarded as the relic of a cortical intrusion which has persisted under xerophytic 
conditions. A parallel may possibly be found in the case of Lepidophloios Scottii, 
Gordon (18), in the stem of which the pith may come and go, though indeed no 
evidence has been advanced to show that in it this fluctuation is a sign of reduction. 
