340 Prof. A. P. W. Thomas. 



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. 



