164 



D. H. Scott. 



of actual affinity between two groups so widely divergent in all their 

 remaining characters. Can we not find some other group possessing 

 ventral sporangiophores wdth which an affinity is more probable? 

 Such a group exists in the Sphenophyllales, in which the sporangia 

 are constantly borne on ventral lobes of the sporophyll, each fertile 

 lobe having its own vascular supplj^, arising as a branch-strand from 

 the bundle of the dorsal lobe. In Tmesipteris (the less reduced of the 

 two genera of Psilotaceae) the agreement with Sphenoplußlnm in this 

 respect is clear. A leaf-trace passes out from the stele and enters 



Fig. 8. Tinesii)teris. A. Diagrammatic radial section of syuangium and part of 

 sporophyll. showing vascular bundle, v. h. passing into sporangiophore from the sporo- 

 phyll ; on the right, part of one of the bundles of the forked sporophyll is shown. — 

 B. Transverse section of synangiura and sporophyll in a plane lying between the 

 sporangiophore and the axis. In the sporophyll 8 vascular bundles are shown, the 

 middle one belonging to the sporangiophore, and the two lateral strands to the 



forked sporophyll. 



the base of the sporophyll, where it divides into three (Fig. 8, B). 

 Two of its branches enter the two forks of the sporophyll itself, while 

 the third turns upwards, passes through the pedicel of the synangium, 

 and extends into the septum between its two constituent sporangia 

 (Fig. 8, A). Thus the position of the pedicellate synangium and its 

 anatomical relation to the subtending sporophyll correspond exactly 

 to the conditions in the Sphenophyllales. The comparison, which I 

 first pointed out in 1897 and further emphasized three years later, 

 has received valuable support from the observations of Prof. Thomas 

 of Auckland, New Zealand, w^ho has had more favourable opportuni- 

 ties for the study of Tmesipteris in its natural habitats than any 

 other observer. He has found (Thomas, 1902) that certain in- 

 teresting variations occur in Tmesipteris under normal conditions. 

 The most important of these consists in the repeated bifurcation 

 of one or both segments of the sporophyll, a synangium being present 

 at each fork. In the commonest case, when only one segment undergoes 



