590 
DR JOHN M'LEAN THOMPSON ON 
It seems more reasonable to raise ontogenetic evidence — even though incomplete 
— to the place of honour in the interpretations of adult structure, than to reject it 
in favour of distant lateral comparisons which necessitate mere assumptions. But 
it may be objected that the creation of an inner endodermis entirely independent 
of its outer correlative is a most exceptional occurrence, and that there is no 
apparent and reasonable need for it in a stele which is completely included in an 
unbroken outer endodermis. The assumption would then be that in the case of 
Platyzoma there is no reasonable explanation of an inner endodermis unless the 
stele has been at some time possessed of stelar gaps by which the pith and cortex 
have been directly connected. In thus assuming that the stele has been at some 
stage in its history on a more elaborate footing than at present, the supporters of 
the reduction theory would consider that the pericycle, phloem, xylem, and inner 
parenchyma should normally be delimited from the ground-tissue of both cortex 
and pith by endodermal barriers. ' To this it might reasonably be replied that the 
inner endodermis may have arisen when required in a growing intrastelar pith for 
purposes of intrastelar physiological control, the creation of the pith having rendered 
such a new endodermal formation necessary or advantageous. It may be asked, 
“ From whence arose the outer endodermis ? ” The presumption seems to be that 
it was independently evolved when required for the delimitation of a primitive 
stele. And further, if the formation of outer endodermis antedated the evolution 
of inner endodermis, may it not be because the delimitation of the stele from the 
general ground-tissue antedated medullation? These are questions which cannot 
yet be definitely answered, but their bearing upon the present problem is obvious. 
The conclusion to which this paper may be drawn is that in the case of Platyzoma 
the recorded ontogenetic facts would clearly allow of the tubular stele with inde- 
pendent outer and inner endodermis and only outer phloem being the upgrade 
development from within of an original protostele. At the same time the reduction 
hypothesis would meantime suffer exclusion merely from lack of evidence in its favour. 
The stelar problem presented by Platyzoma cannot be solved until the “sporeling ” 
stages have been fully investigated, and solenostely is proved or disproved. 
Summary. 
The introduction to this paper consists of a summary of the facts of stelar 
structure already recorded for Platyzoma. These have been provided in succession 
by Dr Poirault, Mr Boodle, and the present author. It is shown that in the 
materials so far examined the stele was an unbroken medullated cylinder devoid of 
either leaf-gaps or perforations. It was possessed of independent outer and inner 
endodermal sheaths, between which lay in centripetal succession pericycle, outer 
phloem, xylem, and inner parenchyma. The included pith was entirely isolated 
within the conductive cylinder, having no ground-tissue connection with the cortex. 
