646 STUDIES IN FOSSIL BOTANY 



became extinct about the close of the Palaeozoic 

 period. 



In the series of the Lyginodendreae it appears 

 evident that Heterangium represents, from an anatomical 

 point of view, the most primitive stage known (p. 401). 

 Lyginodendron itself is anatomically a Heterangium. 

 which has acquired a pith and isolated its leaf-traces. 1 

 It thus lies on the direct way to the stem-structure of 

 the higher plants. 



The view that the medullate stele of Lyginodendron 

 was derived, by suppression of the central xylem, from 

 the protostele of the Heterangium type is materially 

 confirmed by the observations of Kidston and Gwynne- 

 Vaughan on the fossil history of the Osmundaceae. 

 In this analogous series they are led, by the investiga- 

 tion of various Permian Osmundaceae with tracheal 

 tissue occupying the centre of the stele, to the con- 

 clusion that the pith of this family has been derived 

 directly from an originally solid xylem mass. 2 



The Calamopityeae (p. 476) evidently form a 

 parallel series to the Lyginodendreae ; they are, perhaps, 

 of even greater antiquity, and we have no light, as yet, 

 on their derivation. 



In later Palaeozoic times we find, in the Cycadoxyleae 

 (p. 483), plants which appear to have affinity with the 

 Lyginodendreae, and may even have sprung from them. 



1 On the question of the anatomical evolution, my article on " The Old 

 Wood and the New," New Phytologist, vol. i. 1902, p. 25, may be 

 referred to. 



3 " On the Fossil Osmundaceae,'' Part iii. Trans. Royal Soc. Edin- 

 burgh, vol. xlvi. Part iii. 1909, p. 663. More direct evidence appears to be 

 afforded by certain newly discovered stems of Lyginodendreae from the 

 Moravian coal-balls (see below, p. 663) which further connect the structure 

 of Lyginodendron with that of Heterangium. 



