GENERAL RESULTS— SPHENOPSIDA 627 



which have a triarch stele with the wood reaching 

 the centre ; in other parts of the Psilotum stem a 

 resemblance to the stele of Cheirostrobus has been 

 traced. Mr. Boodle's discovery that, at the base of 

 the aerial stem and in adjoining parts of the rhizome, 

 a distinct formation of secondary wood takes place 

 in old plants * materially strengthens the anatomical 

 analogy. The mesarch structure of Tmesipteris finds 

 some slight analogy in that of Cheirostrobus (p. 1 1 9). 

 Evidence from the anatomy is, however, insufficient by 

 itself, for on anatomical grounds a comparison of the 

 Psilotaceae with recent and fossil Lycopods is also ad- 

 missible. The morphology of the sporangial apparatus 

 is more significant, and must now be considered. 



The nature of the synangium of the Psilotaceae 

 has long been in dispute. In modern times there 

 have been two principal views : (1) that the synangium 

 is borne directly on the subtending leaf, a forked bract, 

 which is thus regarded as a true sporophyll ; and (2) 

 that the synangium represents a fertile branch, the 

 first two leaves of which are fused to form the forked 

 bract. On the second view the sporophylls are 

 assumed to be entirely suppressed. Celakovsky, who 

 upheld the first of these opinions, interpreted the 

 synangium as a fertile, ventral lobe of the sporophyll, 

 comparable to the spike of Ophioglossum, and this 

 comparison was adopted by Bower, who regarded the 

 synangium or fertile spike in both cases as ultimately 

 derived from a sporangium which had become septate. 2 



1 L. A. Boodle, "On the Occurrence of Secondary Xylem in Psilotum," 

 Ann. of Bot. vol. xviii. 1904, p. 505. 



2 For the most recent statement of Professor Bower's views, see his 

 Origin of a Land Flora, 1908, Chapters xxviii. and xxxi. 



41 



