XIl] PSILOTACEAE 13 



in the Sphenophyllales. The shoots of Tmesipteris bear simple 

 foliage leaves spirally disposed on a slender axis, and in 

 association with these occur sporophylls consisting of a short 

 axis bearing a pair of small lobes and a bilocular synangium* 

 (fig. 120, B). The synangium is seated on a very short stalk 

 given off from its sporophyll at the base of the pair of laminae: 

 the synangium with its short stalk may be spoken of as the 

 sporangiophore. In most cases the synangium appears to be 

 sessile on the sporophyll, but occasionally the much reduced stalk 

 is prolonged and forms an obvious feature. Dr Scott ^ suggested 

 that the Tmesipteris synangium with its axis may correspond 

 to the ventral lobe (or sporangiophore) of Sphenophyllum. In the 

 latter genus the whorled sporophylls consist in most species of 

 a dorsal and a ventral lobe, the latter serving as a sporangio- 

 phore bearing one or more sporangia ; in Tmesipteris the 

 sporophylls are spirally disposed and each consists of a bilobed 

 sterile portion bearing a septate sporangium or bilocular 

 sjTiangium on a very short ventral lobe. Professor Bower^ in 

 his account of the development and structure of the sporophylls 

 of Tmesipteris, drew attention to the comparatively frequent 

 occurrence of abnormal sporophylls and spoke of the plant as 

 unstable. More recently Professor Thomas^ of Auckland has 

 carefully examined living plants, with the result that variations 

 of different kinds are proved to be exceedingly common. He 

 finds that sporophylls occur which exhibit repeated dichotomy 

 of the axis (fig. 120, T>, F) and thus each may bear four instead 

 of two leaf-lobes and three synangia, one at the first fork and 

 one at each of the forks of the second orders- 

 Other abnormalities occur in which the synangium is raised 

 on a distinct stalk instead of being more or less sessile at 

 the point from which the leaf-lobes diverge. A third form of 

 departure from the normal is that in which there is no synangium 

 on the bilobed sporophyll, its place being taken by a leaf-lobe. 

 The deduction from the occurrence of these abnormalities is 

 that the synangium of Tmesipteris represents a ventral leaf- 



1 See p. 19. 2 Scott (00) p. 499. 



3 Bower (94) p. 545. * Thomas (02). See also Sykes (08). 



6 Sykes (08). 



