SPORE-PRODUCING MEMBERS 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 “Selago” 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 Lycopodium Selago and others; while the 
amplifications noted by Thomas about the middle of the fertile zones in 
Tmesipterts 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 OF THE SPORE-PRODUCING ParRTS. 
The apical cone of Zmesépierts 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 wedgelike 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 
