164 STERILE AND FERTILE REGIONS 
the initial vegetative stage is soon closed by the appearance of sporangia 
in the axils of the leaves; but alternate fertile and sterile zones, merging 
into one another imperceptibly as regards form, succeed one another at 
irregular intervals throughout the upper region of the plant. This may 
be styled the Se/ago condition, as it is seen conspicuously in Lycopodium 
Selago (Frontispiece). In others, and especially in those in which the 
fertile region appears in the form of a definite terminal strobilus, the 
initial vegetative phase is more extensive, though still essentially similar to 
the strobilus in its construction; there is, however, a prevalent difference 
of form between the sterile and the fertile leaves, but the relation of the 
sporangia to the latter is the same as in the Se/ago form. It seems 
natural to conclude that the Se/ago type is the more primitive, and the 
definitely strobiloid type the derivative. 
The question in either of these cases is, what gémetic relation. has 
existed between these sterile and fertile regions which are so similar in 
plan, but differ in the absence or: presence of the sporangia. The 
clew is given by examination of the basal limits of the fertile zones in 
either case; for here, at the point of transition from the sterile to the 
fertile, imperfectly developed sporangia are often found, occupying the 
place normally taken in the fertile region by those fully developed. 
Applying to these the same argument as in the case of an imperfectly 
developed ovule or pollen-sac in an Angiospermic flower, they will be 
held to be vestigial representatives of sporangia, normally present, and 
actually initiated, but not completely developed. Passing from these 
to the vegetative region, where no vestigial sporangia are present, though 
the arrangement, character, and in the Se/ago type even the form of 
the leaves is the same as in the fertile region, the question arises 
whether these are not essentially sporophylls, in which the sporangia 
are completely suppressed? ‘$he result of a broad consideration of the 
question will be an answer in the affirmative. The facts indicate that 
in the simple Lycopod type progressive sterilisation has been effective, 
and that it has involved the partial abortion, or even the complete 
suppression, of whole sporangia; the result is that leaves originally in 
the race fertile have become sterile, and have thus contributed to the 
enlargement of the vegetative region. The fact that the Se/ago condition 
is seen represented in certain Lycopod fossils of the Coal period is 
important evidence of the validity of this progression as an early evolu- 
tionary factor. 
Such sterilisation as that believed to have occurred in Lycopodium in 
the course of descent has been experimentally induced by Goebel in 
Selaginella, by cutting off young strobili, and treating them as cuttings: 
the sporangia of the upper region aborted, and the sporophylls of the 
newly formed parts of the shoot developed as foliage leaves. Thus the 
result theoretically contemplated may follow from experiment. 
1 Organography, p. 657. 
