The Affinity of Tmesipteris with the Splwnophyllcdes. 345 



sporophylls perfectly developed. It would seem, then, as if the 

 greater development of the sporophylls occurred when the nutritive 

 conditions were most perfect. May we infer that, if the nutritive con- 

 ditions remained equally satisfactory throughout, the whole of the 

 sporophylls would show repeated dichotomy ? 



Professor Bower has observed that those abnormalities which occur 

 most frequently in a given species will be those which are most 

 worthy of consideration for morphological argument. If we accept 

 this statement as a reasonable one, the variations now under dis- 

 cussion appear to deserve serious attention. For they occur with 

 considerable frequency, as I will show ; and though we may, in a 

 sense, apply the term abnormality to them, it is in a sense with 

 which nothing of the nature of a pathological variation can be asso- 

 ciated. They are of healthy appearance, and occur in the most 

 vigorous parts of the best shoots. I have found as many as five or 

 six sporophylls with repeated dichotomy in a single fertile zone, 

 whilst shoots with three such sporophylls are fairly numerous. In 

 the course of half-an-hour's search in the forest I can always count 

 on finding several shoots with such variations. But this applies only 

 to those districts where favourable conditions for the growth of 

 Tmesipteris are best realised ; in less favourable localities the varia- 

 tions may certainly occur, but not in such numbers. It is probably 

 not going too far when we assert that, when conditions are most 

 favourable, the sporophylls of Tmesipteris normally show a repeated 

 dichotomy. 



2. In a second group of variations we find that the synangium, 

 instead of being sessile on the petiole of the leaf just below the 

 fork, that is, the point from which the leaf-lobes diverge, is raised 

 up on a longer or shorter stalk. I have found a considerable number 

 of these variations, and they show a good deal of diversity. Some- 

 times the synangium is carried up to a height equal to half the 

 length of the synangium, and lies transversely to the axis of the whole 

 leaf. The two lobes of the synangium then appear to be balanced on 

 the summit of the pedicel, hanging down somewhat, one on each side, 

 so as to suggest a peltate sporangiophore. In other cases the synan- 

 gium retains its direction between the leaf-lobes, but revolves on a 

 transverse axis, so that the longitudinal groove of the synangium, by 

 which dehiscence takes place, faces outwards between the leaf-lobes, 

 instead of looking rather towards the axis of the shoot. One would 

 think that the outward position would, on the whole, be more favour- 

 able for the dispersal of the spores. 



3. In the third group of variations no synangium appears, though 

 the leaf has otherwise the character of a sporophyll, and is forked. 

 Professor Bower has described, and I, too, have seen, many such sporo- 

 phylls in which a minute scar appears in a position below the fork, 



VOL. LXIX. 2 B 



