40 Transactions. 



out by the fact that the vascular strand of the shoot is in close connection 

 with them. However, their early appearance in the young embryo is 

 noteworthy. Lawson's embryo presents an interesting stage slightly older 

 than those, described in the present paper, but there is still a gap in the 

 series which conceals the first differentiation of the young stem-apex, 

 although such very young plantlets as those shown in figs. 61, 64, and 65 

 in the present paper seem to indicate that the shoot arises from the hvpo- 

 basal portion of the embryo. 



Scott (1900, ]). 499) first pointed out the similarity between the sporo- 

 phyll of the Psilotaceae and that of the Sphenophyllales, and repeated 

 his statements more fully in the second edition of his Studies (1909, 

 pp. 626-31). Thomas (1902) strengthened this idea by showing that 

 the nature of the frequent abnormalities which occur in the sporophylls 

 of both Tmesipteris and Psilotum bring those structures liearer still to 

 those of certain of the Sphenophyllales and especially to that of Cheiro- 

 strobus. Miss Sykes (1908) has also supported this with additional evi- 

 dence by her elucidation of the vascular structure of the sporophyll and 

 synangium of Tmesipteris. Both Bower (1908) and Seward (1910, p. 14) 

 have accepted the suggestion of the affinity of the modern Psilotaceae 

 with the fossil Sphenophyllales. 



A general similarity in vascular structure in the mature plants of 

 Tmesipteris and Psilotum has been pointed out by various writers, and, 

 as described in the present paper, the study of the development of the 

 stele in both the rhizome and aerial stem of Tmesipteris helps to make the 

 nature of this structure more clear. Scott (1900) noted the similarity 

 between the stem-anatomy of the Psilotaceae and that of the Spheno- 

 phyllales, and Boodle (1904) has developed the idea and made it more 

 marked still by the discovery of what he believes to be reduced secondary 

 xylem in the subterranean parts of Psilotum. 



There is no need for me to recapitulate here all the details concerned 

 in this double correspondence between the Psilotaceae and the Spheno- 

 phyllale?, for they have been thoroughly co-ordinated and analysed by most 

 of those who have written recentlv on the subject, as, e.g.. Scott (1909), 

 Sykes (1908), and Boodle (1904). 



The peculiar features of the Psilotaceae are open to interpretation in 

 any of the following three ways : They may be regarded as primitive, or 

 as the result of reduction, or as being recent adaptations. This is so also, 

 of course, in other pteridophytic groups, such as, for example, the Lyco- 

 podiaceae and the Equisetaceae, and an instructive parallel may be drawn 

 between them and the Psilotaceae in this respect. Through our knowledge 

 of the fossil plants of the Carboniferous and succeeding periods we have 

 learned to look upon each of these two groups as being the modern repre- 

 sentatives — mere remnants — of families which dominated the forest of 

 the Palaeozoic age. The modern Lycopods and Equisetums do not show 

 the presence of secondary wood (except in one known instance), and this 

 may indicate either that they have lost it by reduction in their descent 

 from large Carboniferous ancestors which possessed it, or that they are 

 descended rather from humbler ancestors which existed side by side with 

 the tree forms but which had never attained to secondary growth. The 

 comparative study of the stem-stele in the modern Equisetums and the 

 fossil Calamites reveals the presence of a primary structure common to 

 both, so that the modern group in this particular, as also in external form 

 and in the nature of the strobilus, is regarded as preserving primitive 

 characters. The Lycopodiaceae may be read, according to two main 



