348 Prof. A. P. W. Thomas. 







Tmesipteris with three synangia or sporangiophores, with the sporo- 

 phyll of Cheirostrobus 1 There is, indeed, considerable difference in 

 details ; thus the division of the sterile segment in Cheirostrobus is 

 palmatifid, whereas the division of the Tmesipteris sporophyll is 

 dichotomous. But this difference is probably of little importance, for 

 in any case we cannot expect to show more than a general corre- 

 spondence. It is much that we can show in each case an elaborate 

 sporophyll, with three ventral sporangiophores. 



Cheirostrobus, although a synthetic form, had probably the most 

 complex cone known amongst the Pteridophytes, and this was doubt- 

 less specialised in some particulars. It may be added that the number 

 of sterile segments and sporangiophores in Cheirostrobus might be 

 reduced to two, and we may compare this with those variations of 

 the sporophylls of Tmesipteris in which only two sporangiophores 

 occur. Further, the vascular bundle of each sterile segment bifurcates 

 at the base of the lamina, and the branches run out into two up-turned 

 processes, that is, the last division of the bundles is dichotomous. 



If we admit the homologies suggested here, we may draw an in- 

 teresting parallel between Tmesipteris and the Sphenophyllales on the 

 one hand, and such a simple Lycopodium as L. selago and Lepido- 

 dendron on the other hand. Tmesipteris arid L. xelago are both rela- 

 tively small forms, which show an alternation of zones of sterile and 

 fertile leaves, whereas the Paleozoic plants both showed more complex 

 vegetative structure, with secondary increase and specialised cones. 



There are certain other points, hoAvever, to be considered before 

 the affinity between Tmesipteris and the extinct Sphenophyllales can 

 be considered as established. As regards the anatomical structure, the 

 difficulty does not appear to be great. I do not propose to enter fully 

 into this question at present ; it will suffice to quote Scott and Seward 

 on this point. The former states that the Psiloteae " are anatomically 

 perhaps the nearest to the Sphenophylleee," whilst Seward considers 

 that " the anatomical characters of the Sphenophyllum shown are 

 such as one finds in some recent genera of the Lycopodinse, espe- 

 cially Psilotum." But Psilotum is unquestionably closely allied to 

 Tmesipteris. 



The character in which a greater contrast exists is the arrangement 

 of the leaves. In the Sphenophyllales the arrangement of the leaves, 

 both vegetative and fertile, in whorls, is a striking feature, whereas in 

 the Psilotese the leaves are scattered. This difference, however, can 

 hardly outweigh the evidence afforded by the other characters. 



The Spenophyllales have been recognised as intermediate in their 

 characters between the Equisetales and the Lycopodiales ; even on 

 the assumption, therefore, that the Psilotese are nearer the Spheno- 

 phyllales, it would not be surprising to find they possess some Lyco- 

 podinous characters. 



