The peculiar fertile structures of Tmesipteris and Psilotum have given rise to much discussion as to their morphological nature. The foliage leaves in Tmesipteris, as is well known, are simple, whereas the fertile structures are forked, that is, each has a stalk bearing leaf-like lobes, a bilocular sporangium or synangium being attached to the stalk just below the lobes.
Three views have been taken as to the nature of these fertile structures
1. The whole structure has been interpreted as a branch of the
stem, being equivalent to a reduced Lycopodium spike. The lobes, according to this view, will be leaves. (Juraniyi, Stras- burger, Sachs, Goebel.)
2. It has been considered as equivalent to a single leaf. Brongniart
and Luerssen compared it with a Lycopodium sporophyll. Professor Bower, as the result of his investigation of the development of the organs of Tmesipteris, regarded it as a " single leaf with two lobes, bearing the synangium on its adaxial face."* He considered the synangium to be a septate sporangium.
3. Dr. Scott considers that an at least equally probable explana-
tion is, that the synangium with its axis may correspond to the ventral sporangiophore of the Sphenophyllales. t
I propose to adduce evidence to show that whilst Professor Bower's view that the whole fertile structure of Tmesipteris is equivalent to a single leaf is correct, Dr. Scott's suggestion that the synangium, with its axis, corresponds to a ventral sporangiophore, is also correct.
Professor Bower speaks of the whole fertile structure as a sporan- giophore, whereas Dr. Scott would restrict the term sporangiophore to the synangium and its axis, referring to the Sphenophyllales, in which sporangiophores spring from the iipper surface of bracts as long pedicels, bearing one or more sporangia at their ends. To avoid con- fusion, 1 propose to speak of the whole fertile structure as a fertile leaf or sporophyll, and will use the term sporangiophore in the sense of Dr. Scott and Mr. Seward.
The evidence I have to offer is derived from the study of the varia- tions which occur in the sporophylls. These variations are of striking
- 'Phil. Trans.,' B, 1894.
t ' Studies in Fossil Botany,' p. 499.
