592 
DR JOHN M‘LEAN THOMPSON ON 
upgrade intrastelar differentiation they might be held to demonstrate how entirely 
intrastelar in nature as in location are the pith and inner endodermis. 
On the other hand, a small — and possibly young — but incomplete specimen 
revealed some interesting features which are illustrated in the accompanying plates. 
It is shown that as its stele was followed forward from the broken base the pith and 
inner endodermis decreased until the latter was_ narrowed to a vanishing point and 
a medullated protostele was locally established (figs. 1-5). The pith again expanded 
and within it there arose de novo an inner endodermis, which, at first of irregular 
and indefinite construction, was subsequently definitely tubular (figs. 6-11). By 
disintegration of this wide tubular inner endodermis an irregular formation again 
arose within the pith, but when the latter had once more diminished the inner 
endodermis was an unbroken tube narrowing on a vanishing point (figs. 12-15). 
For at least the second time in the history of this plant a medullated protostele 
was established locally (fig. 16), and on subsequent re-expansion of the pith inner 
endodermis was again created de novo, and was subsequently maintained to the 
apex of the stem as an unbroken cylinder (figs. 17-25). 
A study of the stelar differentiation showed that the xylem was roughly divisible 
into two concentric cylinders. The outer of these was differentiated first and was 
alone involved in supplying xylem to the traces. In it the order of differentiation 
was indefinite and no spiral stelar protoxylems were recognised. The inner cylinder 
showed more or less clearly a centripetal order of differentiation. Spiral protoxylems 
were occasionally found in the departing leaf-traces but were not continuous into the 
stele. The theoretical position of the stem-protoxylems is mesarch. 
The new facts of stelar-structure having been recorded, their bearing on the 
stelar problem is considered. It is recognised that Platyzoma is at points protostelic, 
and may augment from within the stele itself an attenuated pith. In like manner 
it has been seen to develop an intrastelar inner endodermis. Among the results 
secured there is nothing which can reasonably be considered evidence in favour 
of the reduction theory unless it be that protostely recurs where small leaf- 
traces predominate. 
On the other hand, the theory of upgrade internal differentiation has not been 
proved, though the general trend of the structural evidence might reasonably be 
considered in its favour. 
It is considered that any one of three definite states may exist in the stele of a fern, 
(i) The original protostely may persist undisturbed throughout the entire 
ontogeny. 
(ii) 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. 
(iii) The original protostely may recur in the ontogeny in ferns in which 
the advanced stelar state is not yet permanently established. 
