THE MORPHOLOGY OF THE STELE OF PLATYZOMA MICROPHYLLUM, R. BR. 579 
certain statements which, seem to have a bearing on the subject in hand. On page 
241 it is stated : “ There are two phylogenetic types of tubular central cylinder, 
namely, that in which only ramular gaps are present, and that in which both 
ramular and foliar gaps occur.” “ The use of these constant and characteristic 
anatomical features results in the division of the Vasculares into two great primitive 
stocks — the Lycopsida which are cladosiphonic and palingenetically microphyllous, 
and the Pteropsida which are phyllosiphonic and palingenetically megaphyllous.” 
“ The Lycopsida include the Lycopodiales and Equisetales ; the Pteropsida include 
the Filicales, Gymnosperms, and Angiosperms” On page 242, in amplification of 
the above statement, he continued : “ Pteropsida. Palingenetically megaphyllous 
vascular plants with dorsi-sporangiate sporophylls, the tubular central cylinder when 
present characterised by foliar gaps or interruptions in the fbro-vascular tissue 
immediately above the outgoing foliar traces .” In this way it is implied that a 
fern possessing a tubular central cylinder like that of Platyzoma, devoid of foliar 
gaps or other interruptions in the vascular tube, is uncharacteristic for ferns, and 
can reasonably be regarded only as of downgrade and reduced type. The alternative 
upgrade view was not even considered. 
This attitude was still more clearly shown in 1910 when Dr Jeffrey’s paper 
on “The Pteropsida ” appeared (ll). In it the “morphological status” of pith 
was regarded as “ external tissue included by the stele in the course of evolution.” 
On page 408 it is stated : “ It appears evident, since the stelar system of the stem 
in the case of certain of the lower Pteropsida is able to include tissues and sub- 
stances which are beyond question extrastelar in their origin, that no difficulty 
arises in regarding the pith present within the siphonostelic central cylinder of the 
lower as well as the higher Pteropsida as morphologically equivalent with the 
fundamental tissues of the cortex, with which it is often continuous through the 
gaps in the stelar walls resulting from the exit of the vascular supply of leaves and 
branches.” “ A further argument in favour of this view is the frequent textural 
similarity between pith and cortex, even where the former as the result of upward 
evolutionary tendencies is no longer continuous with the fundamental tissues 
without the stele.” On page 412 he concludes : “ It now in fact appears much more 
clearly than formerly that the primitive condition of the vascular system in both 
stem and leaf in the Vasculares as a whole was what the present writer has 
designated protostelic, i.e. a condition in which the fibro-vascular tissue harboured 
no pith. Following this condition was one in which the fibro-vascular system 
became transformed especially in the more progressive stem into a stelar tube lined 
both externally and internally with phloem and endodermis. In the process of time 
the internal phloem became degenerate, probably on account both of the absence 
of direct relation to the leaves and of the appearance of secondary growth, advan- 
tageously localised ultimately on the outer surface of the stele. The internal 
endodermis more slowly followed the internal phloem into oblivion, and is often 
TRANS. ROY. SOC. EDIN., VOL. LII, PART III (NO. 22). 88 
