196 Lady Isabel Browne. 
of the sporangium, while the projection is at the insertion and bears 
the sporangium. Miss Sykes suggests that the absence of a ligule 
in Spencerites and Lycopodium may be due to the fact that the 
sporangia of these genera are sufficiently protected by the sporophyll 
from too great evaporation and that the mucilage, once secreted by 
the ligule, is no longer necessary (20). This hardly seems a sufficient 
reason to account for the absence of a ligule, which appears to be a 
very persistent organ, constantly present in the ligulate cycle of 
affinity. The sporangia of Lepidocarpon and of those species of 
Isoetes that possess a velum appear to be more protected than the 
sporangia of the Lycopodiaceae; yet their ligules show no indications 
of reduction. Thus even if, as seems probable, the Lycopodiaceae 
are descended from Spencerites or allied forms there is at present 
little to connect that genus with the Lepidodendracese, though 
the discovery of its vegetative organs may bring it nearer to them. 
The Lycopodiaceae show perhaps even less indication of affinity 
to the Isoetaceae; this is what we should expect if the latter order 
is reduced from the Lepidodendraceae. Isoetes approaches the 
Lycopodiaceae in point of size, and resembles certain species of 
Lycopodium in being non-strobiloid. But this condition may well be 
secondary in a form clearly so reduced as Isoetes, while the absence 
of a ligule and of secondary growth in Lycopodium and its biciliate 
spermatozoids are important differences between the two genera. 
The Selaginellaceae are perhaps the order of the phylum coming 
nearest to the Lycopodiaceae, though even they are very remote 
from the latter. The anatomical evolution of the stem has evidently 
run a completely independent course from the time when the exarch 
protostele, characteristic of the primitive members of every order 
of the phylum, was laid down. The ligule and the heterospory even 
of the Palaeozoic Selaginellaceae mark out the two orders as having 
been distinct from a very early period; but the general habit, the 
absence of secondary growth in the members of both orders (except 
Selaginella spinulosa), the apparently constant production of a 
suspensor (except in Selaginella apus ), and, above all, the biciliate 
spermatozoids, may be indications of a real, though very distant, 
relationship between the two orders. Though it is difficult to 
distinguish certainly between impressions of Selaginellites and 
Lycopodites it is probable that forms similar to the Lycopodiaceae 
and referable to the latter genus existed even in the Palaeozoic age. 
This may explain the isolation of existing Lycopodiaceae. 
