244 Bower . — Imperfect Sporangia in certain 
vestigial parts, above by phylogenetic ally nascent or super - 
numer ary parts. Again, the comparison may be made with 
a Moss-sporogonium, and there also a fertile zone is found, 
with a vegetative region below : the series of cells of the arche- 
sporium is continued beyond the region of actual fertility 
both above and below : and the cells at the lower limit may 
similarly represent a phylogenetically evanescent arche- 
sporium, those at the upper limit a phylogenetically nascent 
region. As regards the succession of vegetative and repro- 
ductive phases, and their probable evolutionary origin in the 
sporophyte generation, the cases are alike 1 . What it is in 
such cases that determines where the limits of the fertile zone 
shall be is obscure ; but the cause is doubtless related closely 
to nutrition 1 . 
By comparison of living species of Lycopodium we see that 
the fertile zone is not always located at the same level of the 
plant as a whole : it is sometimes preceded by a shorter, 
sometimes by a longer vegetative region. An idea can thus 
be arrived at of the shiftmg of the fertile or spore-producing 
zone. The biological significance of this shifting is obvious, 
for any advance of the zone to a higher point by abortion 
of sporangia, while the sporophylls remain in a vegetative 
capacity only, provides a larger vegetative zone below for 
purposes of nutrition. 
In most species of the undifferentiated Selago group, as in 
all the more specialized species of Lycopodium , there is a con- 
siderable basal region which is sterile. This region is however 
variable in length ; indeed in some species of the Selago - 
group , notably in L. comp actum and Trencilla , sporangia are 
found quite down to the base of the mature plants ; here then 
the whole of the mature plant is virtually a strobilus. Com- 
parison of these extreme cases with those which show alter- 
nating sterile and fertile zones leads fairly to the conclusion, 
1 Parallel examples might be quoted from the cones of Gymnosperms, and the 
inflorescences of many Angiosperms ; but I prefer for the present to discuss only 
those simpler cases where the questions in hand are not complicated by high 
specialization. 
