Pteridophytes. Are they vestigial? ' 251 
first leaves early exposed as assimilating leaves ; these, how- 
ever, do not differ in form from the early sporophylls with 
which they are probably homogeneous, though sterile by 
abortion of their sporangia. Starting from this prototype, 
the following modifications of the sporophyte are indicated 
by comparison of living species of Lycopodium. In those types 
with a protocorm (which are all more specialized sporophytes 
than Selago, having their sporangia formed later in the 
individual, and usually more strictly localized in strobili), 
the lowermost leaves, already sterile, have been developed 
as protophylls, which though appearing first in these indi- 
viduals, would represent a relatively recent modification of 
leaf structure 1 . The fertile region, which is continuous in 
some species, is in most of the Selago group interrupted by 
irregular sterile zones, an arrangement which provides for 
more adequate nutrition, for which frequently a long sterile 
basal region precedes the fertile (sp. 6, 7, 38, 39, &c.). The 
sterile and fertile zones usually differ only in the absence or 
presence of sporangia, but there is, in some species of the 
Selago group (sp. 9, 17, 39), evidence of a fining off of the 
sporophylls to smaller size than the sterile leaves, with the 
result of a partial definition of a fertile strobilus : this becomes 
more apparent in the sub- Selago group (sp. 42, 47, 49), but it 
is in the Lepidotis group that the strobilus becomes a definite 
terminal cone (sp. 50, 52), though still liable to be inter- 
rupted (sp. 51). In the Phlegmaria group the strobilus is 
marked off more sharply by the sporophylls being small ; 
but still occasional sporangia may be found outside the 
strobilus (sp. 60), while leaves of the strobilus may be barren 
(sp. 61, 67) ; the strobilus may also be continued into a shoot 
of the foliage type ; clearly the distinction of vegetative and 
fertile regions is not yet defined absolutely. But in the 
cernuum group the definition is more exact. In the clavatum 
group of ground-growing forms again the strobilus is well 
1 The terms 1 protocorm ’ and ‘ protophyll ’ may well be retained, but with the 
understanding that their priority is in the development of the parts of the indi- 
vidual, not of the race. 
