94 MEDULLOSEAE [CH. 



accessory steles in the stem of a recent Cycas, and the agreement 

 is emphasised by the presence of short square-ended tracheids 

 in the primary xylem. 



The roots branch freely and may attain a diameter of more 

 than 1 cm. : they are generally triarch and the triangular primary 

 xylem is enclosed by secondary xylem except opposite the proto- 

 xylem. The cortex is like that of Lyginopteris roots and a con- 

 spicuous double layer of superficial tissue is another feature common 

 to both (c/. fig. 410). The exceptionally well preserved specimens 

 described by Arber^ show very clearly the thick zone of periderm 

 which forms the covering of older roots, and in some of the sieve- 

 tubes groups of dark brown patches show the form and arrange- 

 ment of the sieve-plates. 



Refroductive organs. We have as yet no precise information 

 in regard to the reproductive organs of Medullosa anglica, but 

 there can be little or no doubt that the fronds bore seeds that 

 have long been known under the generic name of Trigonocarpus. 

 Many years ago Mr Hemingway noticed the almost constant 

 a,ssociation of the fronds of Alethopteris lonchitica with Trigono- 

 carfus, and Dr Kidston's discovery^ of seed-bearing Neuwpteris 

 pinnae considerably strengthened the evidence derived from mere 

 association. The structure of Trigonocarfus is described later 

 (p. 117) in a section devoted to reproductive organs attributed 

 to Medullosa. Nothing is known as to the microspore-bearing 

 organs. 



While in the structure of each of the steles Medullosa anglica 

 agrees very closely with Heterangium, it differs from that genus 

 in the presence of three steles and in the structure of the petioles 

 which are much less fern-hke than the simpler petioles of Heter- 

 angium and Lyginopteris. From the continental species the 

 British species is distinguished by its simpler stelar system, though 

 there is a close correspondence as regards individual steles. 



Medullosa pusilla Scott. 



This species, briefly referred to by Scott in 1909* and fully 

 described in a recent paper*, is founded on material from the 



1 Arber, E. A. N. (03). 2 Kidston (05^) B. 



3 Scott (09) B. p. 441 (footnote). • Scott (14). 



