SPORE-PRODUCING MKMHKKS 413 



amplification beyond what is held to be the normal for them. It is a 

 different question whether these were ever effectively realised in the past, 

 and thus figured as normal features in any ancestral race. Nevertheless it 

 is hardly possible to avoid the comparison of the forked sporophylls of 

 the Psilotaceae, and these extra branchings, also with forked leaves, which 

 are so prominent a characteristic of the Sphenophylleae. 



From the study of the external characters of the living Psilotaceae it 

 appears that the sporophyte is readily referable to a strobiloid origin. The 

 rootless condition and the leafless rhizomes present no difficulty, but 

 rather the reverse. It may, however, be a question whether this condition 

 was primitive in them, or the result of reduction in accordance with their 

 peculiar habit. As regards the lax shoots, the dichotomous branching is 

 reminiscent of the Lycopodiales rather than the Sphenophyllales. The 

 vegetative development of the lower parts of the aerial shoots, as well as 

 the " Se/ago " condition so clearly seen in their upper regions, corresponds 

 to that of the simpler Lycopods, while it finds its correlative also in 

 Sphenophyllum majus. The chief points of divergence as regards external 

 form are the shape of the leaves and sporophylls, and their alternate 

 arrangement, though they share the latter with most of the Lycopods. 

 The reduction or abortion of sporangiophores about the limits of the 

 fertile zones compares with the imperfect development of abortion of 

 sporangia in a similar position in Lycopodmm Selago and others ; while the 

 amplifications noted by Thomas about the middle of the fertile zones in 

 Tmesipteris only accentuate the recognition of those zones as distinct from 

 the sterile parts. Accordingly the general reference to a strobiloid origin 

 will apply to the Psilotaceae with equal force to that in the case of 

 Lycopodium, and this will be so upon the facts themselves, whatever the 

 genetic relations may have been between the Psilotaceae, the Lycopods, 

 and the Sphenophylls. 



DEVELOPMENT OK THE SPORE-PRODUCING PARTS. 



The apical cone of Tmesipteris is very variable in bulk : in strong 

 young shoots it may be a broad dome, while in weaker specimens, or 

 those in which the apical growth is beginning to fail, it may be com- 

 paratively narrow. Passing from the actual apex the sides of the cone are 

 covered externally by deep prismatic cells, which are of somewhat irregular 

 origin, depth, and arrangement. When a leaf or sporophyll is about to be 

 formed, certain of these increase in size, and undergo both periclinal and 

 anticlinal divisions so as to form a massive outgrowth, the summit of 

 which is occupied, as seen in radial section, by a single cell of a wedge- 

 like or prismatic form : it is not improbable that the latter passes over to 

 the wedge-like form as the part develops. In these early states it is 

 impossible to say whether the part in question will be a vegetative leaf or 

 a sporophyll, and even when older it is still a matter of uncertainty, so 



