AND THEIR BEARING ON STELA R THEORIES FOR THE FEB NS. 
723 
of destination of procambial cells from point to point mature elements of different 
category may be variously grouped within the stele. To explain them, neither 
phyletic nor dynamic flow of tissues is necessary, nor indeed is there evidence of 
the latter. 
On the evidence already dealt with the following conclusions are reached : — 
(i) The species considered have in the adult state medullated protosteles. 
(ii) In each the medulla is of purely intrastelar origin. 
(iii) All the evidence shows that endodermal pockets through xylic gaps are 
intrastelar structures, and arise by change of procambial destination 
within the stele. 
(iv) Endodermal spindles partially or completely isolated within the stele may 
be regarded in some cases as potential and not vestigial pockets, in others 
* as independent though sporadic formations of inner endodermis which 
has not been continued permanently in the ontogeny. 
(v) By pockets and endodermal spindles the endodermal area for the stele as a 
whole is enlarged, and physiological control may be enhanced. 
(vi) That the contents of endodermal pockets and the adjacent cortex are 
commonly of the same histological character may be expected, since they 
are continuous through the mouths of the pockets. Thus united, they 
behave as one tissue which need not be expected to resemble the pith 
either in structure or function. 
Gleicheniacese. 
Of Gleicheniaceous plants Gleichenia jlabellata, Gleichenia pectinata , and 
Platyzoma microphyllum bear directly on the subject in hand. 
The internode of Gleichenia Jlabellata shows a cylindrical stele with a solid 
parenchymatous xylem-core. At the nodes shallow endodermal pockets associated 
with the leaf-traces are decurrent within the stele. The ontogenetic development of 
these pockets has not been traced, but by them the area of stelar endodermis is 
enlarged. On the view of static procambial change within the stele, the pockets 
would be considered intrastelar in origin and independent of any possible medullation. 
On the theory of cortical origin of pith, their contents would be viewed as cortical 
tissue by confluence of which within the stele any future medullation would be 
attained. 
The ontogenetic evidence for Platyzoma microphyllum is still wanting, but the 
adult stele has been examined (4), (49), (61), (62). It is commonly a medullated 
cylinder with both outer and inner endodermis, and phloem only on the outer face of 
the xylem. It is devoid of leap-gaps and perforations. Thus the pith is completely 
isolated from the cortex, and the inner endodermis is independent of its outer cor- 
relative. Jeffrey (30) has affirmed that the medulla of .Platyzoma is “in reality 
TRANS. ROY. SOC. EDIN., VOL. LII, PART IV (NO. 28). 112 
