Page:Proceedings of the Royal Society of London Vol 69.djvu/362

340 doubtless representing an abortive synangium. Professor Bower has also figured* a sporophyll, in which a long process arises in the place of the synangium. The process in the figure, though long in propor- tion to its width, is still only half the length of the leaf-lobes. This was interpreted as a correlative vegetative growth. A greater amount of material has shown what the correlative growth may become. I have found a large number of variations in which the growth reaches a greater development, until we get sporophylls in which the place of the synangium is occupied by a leaf-lobe of normal appearance. There can be no mistaking the appearance of this leaf-lobe the form and structure of the normal leaf-lobe or leaflet of a sporophyll are exactly reproduced even to the details of the peculiar mucronate termi- nation. What is the natural conclusion from the appearances described ? Is it not that the synangium of Tmesipteris is morphologically equiva- lent to a ventral leaf-lobe 1 Or some writers would possibly prefer to say that the structure which carries the sporangium (or sporangia) is a ventral leaf-lobe, the sporangia being held to be organs Kid generic, which may be carried in various positions, in the present case on a leaf-lobe. If so, we must suppose that the leaf-lobe in the normal synangium is reduced to the base, and probably the axis of the synangium.

It should l)e added that often two or three of the sporophylls at the distal end of a fertile zone may show this vegetative development of the ventral leaf-lobe, suggesting that after the primordium of the sporophyll has been developed to a certain stage, the particular nutritive conditions which lead to reproductive development have failed, and that there has accordingly been a relapse to the vegetative condition. Hence the primordium of the synangium has developed into a leaf-lobe of the ordinary structure.

There is little difficulty in stating the character of the variations to which Tmesipteris is subject, but to attempt to apply these for the purpose of tracing the affinities of the plant is a more hazardous matter, and there is doubtless room for differences of opinion.

Tmesipteris and its ally Psilotum have usually been classed with the Lycopodinae, though it has been admitted that they are rather out- lying members of the class. It has been the difficulty in reconciling the sporangium-bearing structures of the Psilotese with the usual simple sporangia and sporophylls of the typical Lycopodinae, which has caused so much morphological discussion as to their nature.

We are led, therefore, to inquire in what other group of Pterido- phytes do we find dichotomous division of leaves, sporophylls more elaborate than vegetative leaves, several synangia or groups of sporangia on a single sporophyll, and the presence of a special ventral lobe of the leaf as sporangiophore.


 * ' Phil. Trans.,' B, 1894, Plate 52, fig. 152.