14 SPHENOPHYLLALES [CH. 



lobe, as Scott ^raggested. Professor Thomas draws attention to 

 the resemblance between Tmesipteris sporophylls and the foliage- 

 leaves of Sphenophyllum, which are either simple with dicho- 

 tomously branched veins or the lamina is deeply divided into 

 two or more segments. In some types of Sph&nophyllostachys 

 the bracts are simple {S. Dawsoni), but in others {Sphenophyllum 

 majus, fig. 113, C) they are forked like the foliage-leaves 

 and bear a close resemblance to the abormal sporophylls of 

 Tmesipteris. Moreover, in Sphenophyllostachys Romeri (fig. 

 113, A) each ventral lobe of a sporophyll bears two sporangia, 

 a condition almost identical with that represented by the 

 occasional occurrence of a synangium on a comparatively 

 long stalk in Tmesipteris. Similarly the more elaborate 

 sporophylls of Cheirostrobus may be compared with the branched 

 sporophylls of Tmesipteris (fig. 120). This agreement between 

 the sporophylls of the Palaeozoic and recent genera acquires 

 additional importance from the very close resemblance between 

 the exarch stele of Sphenophyllum and that of the genus 

 Psilotum, which conforms to the Palaeozoic type not only in 

 the centripetal character of the primary xylem and in its exarch 

 structure, but also in the occasional occurrence of secondary 

 xylem ^, and in the stellate form of its transverse section. The 

 occasional mesarch structure of the stele of Cheirostrobus finds 

 a parallel in the mesarch xylem groups in the stem of Tmesipteris. 

 It is thus on the strength of these resemblances that Thomas 

 and Bower would remove the Psilotaceae from the group 

 Lycopodiales and unite them with Sphenophyllum and Cheiro- 

 strobus in the Sphenophyllales. While admitting the validity 

 of the comparison briefly referred to above, I prefer to retain 

 the Psilotaceae as a division of the Pteridophyta including only 

 Psilotum and Tmesipteris. 



In his recent book on The Origin of Land Flora, Prof. Bower 

 raises objection to the use of the term ventral lobe in speaking of 

 the sporangium-bearing stalk or sporangiophore borne on the 

 sporophyll of Sphenophyllum. He points out that the use of 

 this term implies the derivation of the sporangiophore by 

 metamorphosis of part of a vegetative leaf, an opinion untenable 



^ Boodle (04) ; seepoitea p. 21. 



