Page:Proceedings of the Royal Society of London Vol 69.djvu/363

Rh Leaving on one side obvious comparisons as regards some of these characters with certain ferns, I would venture to suggest that it is in the extinct Sphenophy Hales that we find the closest morphological parallel with the sum total of the characters now shown for Tmesipteris. The leaves in Sphenophyllum were frequently hetero- morphic. A common type of leaf was a wedge-shaped one, but it is important to notice that the veins were dichotomously divided, and the margin of the leaf frequently more or less notched or lobed in accordance with the venation. But the leaves varied much in form, from such as have been described to leaves with a lamina dissected into dichotomously branched linear segments, or to simple narrow uninerved leaves.* In 8. cuneifolium the upper cone-bearing branches show the finely cut foliage, whilst in some specimens entire and much divided leaves occur mixed together in the same specimen.

But it may be objected that such leaves are foliage leaves, whereas it is the sporophylls which alone are forked in Tmesipteris. The sporophylls of Sphenophyllum formed strobili, as a rule sharply marked off from the foliage-bearing regions, but in *S'. trichomatosum the cones were very lax, and not sharply marked off.f In the well- known form S. Duwsoni, the bracts were simple, but in others they were forked. In S. tenerriminn the cones were small, but the bracts narrow and dissected.

The force of the comparison with the Spenophyllales is of course intensified if it be admitted that the variations recorded above for Tmesipteris prove that the synangium is equivalent to a sporangio- phore with its sporangia.

In Sphenophyllum Dawsoni each bract carried two sporangiophores on its upper surface, each sporangiophore bearing a single pendulous sporangium. But in the Bownmnites lioi'uit'ri each sporangiophore bears two sporangia. If we may trust the variations recorded under Group 2 above, we have here a surprisingly close correspondence between Bowmanites and Tmesipteris. But some species of Spheno- phyllum present evidence of as many as four sporangia to a single sporangiophore. I have occasionally found trilocular synangia in Tmesipteris, as, indeed, others have done ; whilst in Psilotum the number of loculi, though normally three, may vary from two to five.

Perhaps one of the most interesting forms for comparison with Tmesipteris is the cone described by Scott under the name of Cheiro- strobus, and shown by him to belong to the Sphenophy Hales, f In Cheirostrobus the sporophylls are very elaborate; each is divided nearly to its base into three lower sterile segments and three upper fertile segments or sporangiophores. May we compare a sporophyll of

Seward, ' Fossil Plants,' vol. 1, p. 391. t Scott, ' Studies in Fossil Botany,' p. 105 ; I Scott, ' Phil. Trans.,' B, 1897.