588 
DR JOHN M‘LEAN THOMPSON ON 
The fuller facts of stelar structure have now been recorded. It remains to con- 
sider the effect of their recognition upon the stelar problem. It is at once admitted 
that they are inadequate to allow of a solution of the problem, for they lack the 
support of the evidence of the “ sporeling ’’-structure, and of the earlier ontogenetic 
stages generally. But the following points have been proved : — 
(i) Platyzoma is at points protostelic. 
(ii) It has been .shown to greatly augment an attenuated pith from within 
the stele ; on the other hand, the pith may be drastically diminished. 
(iii) It has been shown to develop entirely from within the stele an inner 
endodermis around a portion of the pith. 
(iv) It has neither foliar nor ramular gaps ; nor does it show any structural 
evidence of having possessed at any time either stelar gaps or 
internal phloem. Thus an original solenostele has not been 
demonstrated. 
Among the results of this investigation there is not one fact which could reason- 
ably be considered evidence in favour of the reduction theory of the stelar structure, 
unless it be that protostely recurs in those zones which are apparently the bearers of 
the majority of the small leaf -traces. This alone could be considered evidence of 
reduction if it be granted that the stelar structure is liable to be reduced under 
xerophytic conditions. It might be held to demonstrate the extent to which 
reduction may go. But no evidence of the original solenostely upon which the 
reduction theory hangs has emerged ; much less have the materials shown “ pocket- 
ing ” in an original protostele by which the cortical ground-tissue has “ invaded ” the 
protostele and established a pith. It will be evident that if the course of stelar 
transformation and reduction proposed for Platyzoma has actually been followed in 
descent, this is the first recorded example of a fern which has reached solenostely 
by amplification of an original protostele, and has returned to protostely by reduc- 
tion. On the other hand, it will be clear that, even supposing the so-called ectophloic- 
siphonostely of Platyzoma be the result of reduction from solenostely, no evidence 
has been advanced to show that this solenostely arose by a process of cortical 
“ intrusion.” If, as seems the case in the materials discussed above, both diminution 
and augmentation of pith and inner endodermis are purely intrastelar phenomena, 
there is no initial reason why inner phloem should not arise within a medullated 
protostele not only where required, but also independently of the outer phloem. 
But whatever be the possibilities, the fact remains that in promulgating their view 
of the stelar structure of Platyzoma the advocates of the reduction theory have seen 
neither the solenostelic stage they presume existed nor the protostelic stage which 
has been described above, and from which arises a purely intrastelar pith. It will 
be of interest to see what bearing the fluctuations of the inner endodermis above 
recorded may have on the problem raised by the isolated endodermal spindles which 
Mr Boodle has recorded from the stele of Schizsea dichotoma (15). 
