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XXII. — The Morphology of the Stele of Platyzoma microphyllum, R. Br. By 
John M‘Lean Thompson, M.A., D.Sc., F.L.S., Lecturer on Plant Morphology, 
Glasgow University. (With Three Plates and Three Figures in the Text.) 
(MS. received February 3, 1919. Read February 3, 1919. Issued separately September 29, 1919.) 
That the stelar problem considered in this paper may be more clearly visualised, 
it will be well to divest of purely theoretical considerations the facts of stelar 
structure already known for Platyzoma, and to summarise such structural features 
as bear on the subject in hand. 
In the first account of Platyzoma given by Robert Brown in 1810 (l), the habit 
of the stem and heterophyllic leaves, and the general form and position of the 
sporangia, were alone described. But in 1832 (2) a rough analysis of the structure of 
the stem was added, and the tubular nature of its stele was recognised. No material 
contribution to the knowledge of the stelar structure was made until 1893 (3), when 
Dr Poirault described the histology of the stele as shown by a small fragment 
of stem. In this material there was a sclerotic pith completely surrounded by an 
endodermis, outside which lay a zone of parenchyma surrounded by a broad ring 
of parenchymatous xylem. At the periphery of the xylem there was a narrow 
zone of phloem surrounded by a large-celled pericycle and an outer endodermis. 
Neither leaf-gaps nor perforations were found in the stele, and accordingly the 
complete isolation of the pith from the cortex was recognised. 
Fuller information regarding the stele was provided in 1901 by Mr Boodle (4). 
In particular, he described the origin of a leaf-trace by the nipping off of a group of 
phloem- and xylem-elements from the stele. The leaf-trace thus initiated departed 
from the stele without the formation of a leaf-gap affecting the endodermis;, and 
it was noted that frequently the exit of a leaf-trace scarcely affected the stelar- 
xylem beyond forming in it a small external pit. “ The nearest approach to a true 
leaf-gap ” which was observed was a parenchymatous ray running almost radially 
athwart the xylem-ring, but “ longitudinal sections through the insertion of a 
leaf- trace did not show any passage of leaf- trace -parenchyma towards the centre 
of the stele.” 
In 1916 (5) I described at some length the stelar structure as shown in part of 
a single specimen of Platyzoma microphyllum. From the central stele there departed 
a close succession of leaf- and root-traces. The leaf-traces varied considerably in 
size. The largest originated mainly from the upper surface of the stele, the smaller 
sprang from the sides and lower surface. The majority of the root-traces were 
attached to the lower surface of the stele. The pith was bulky and sclerenchy- 
matous, and — as in the preparations of Poiratjlt and Boodle — was surrounded by 
TRANS. ROY. SOC. EDIN., VOL. LII, PART III (NO. 22). 87 
