292 LYCOPODIALES 
which Spring placed first of all was Z. Se/ago; and though this may not 
be actually the most elementary living species in the genus, still it is the 
best known of those which show a low degree of differentiation. 
The plant of Z. Se/ago is shrubby, with dichotomously branched axes, 
bearing numerous leaves of approximately equal size and simple form 
(Frontispiece). There is usually a sterile region at the base of the plant: 
this is followed by the well-known alternating sterile and fertile zones, 
the length of which corresponds with a high degree of exactitude on the 
several branches. They are stated to be determined by successive seasons, 
the middle region of each year’s increment of growth being fertile. These 
zones are not definitely marked by any distinction of the leaves themselves, 
but by the presence or absence of sporangia: nor are they strictly delimited 
in this respect, for occasionally a single sporangium may be found in an 
otherwise sterile region. About the limits of these zones sporangia of 
smaller size may be found, which sometimes remain closed when all those 
near them have dehisced. These are those incomplete sporangia which 
have already been referred to in Chapter XIII., in connection with the 
argument for sterilisation as affecting the balance of the sterile and fertile 
regions. The condition thus seen in Z. Se/ago is shared in more or less 
complete degree by about 40 living species, which constitute the section 
Selago: they are mostly ground-growing plants. An examination of them 
shows that while most of them have, like Z. Se/ago, a sterile basal region 
of considerable length, still in certain species (Z. compactum, Hook., and 
L. Trencilla Sodiro) sporangia have been found in the leaf-axils down to 
the base of the mature plant: this has been noted also, but less completely, 
in L. firmum, Mett., and ZL. rigidum, Gmel. Unfortunately these species 
are unknown in the embryonic state, so that it is impossible to tell how 
early in the individual life the formation of sporangia actually begins ; 
but practically the whole of the mature plant. is a fertile strobilus. The 
incomplete differentiation of the sterile and fertile zones is seen in all the 
40 species: isolated sporangia are frequently found in an otherwise sterile 
zone, and occasionally sterile leaves occur in a fertile zone: these facts, 
together with the occurrence of incompletely developed sporangia at the 
limits of the zones, and the very uniform character of the leaves whether 
sterile or fertile, have their direct bearing on the theory of sterilisation 
enunciated in Chapter XIII. 
Some ten other species were grouped by Baker under the heading 
Sub-Selago, and are characterised by having the sterile leaves a little 
different from the fertile, but passing into them gradually, while the 
sporangia are aggregated into indistinct terminal spikes. All the species 
thus grouped have a sterile basal region: above this follows a recurrence 
of sterile and fertile zones, as in § Sedago (ZL. Dalhousiaeanum): reversion 
from the fertile strobilus to a permanently sterile state is more common 
(L. carinatum, gnidioides). A progtessive diminution of size of the fertile 
leaves upwards is seen: it is sometimes gradual (Z. sguarrosum), but 
