THE “SELAGO” CONDITION 165 
With this progressive sterilisation, and the consequent increase of the 
vegetative region, the apical growth of the axis keeps pace: it secures the 
initiation of additional sporophylls and sporangia to take the place of 
those transformed or aborted, and as there is no theoretical limit to the 
apical growth and branching, in such species as Z. Se/ago the balance 
can constantly be readjusted between the sterile and the fertile regions. 
This combination of sterilisation and continued apical growth provides, 
in a sense, a forward impulse, and it will be effective up to the limit of 
physiological supply. That it is so is seen in the fact that at the apex 
of any Lycopod strobilus imperfect sporangia are found, which are to 
-be recognised as supernumeraries, showing the continued exuberancé of 
initiation beyond the power of the plant to bring to complete maturity. 
We thus acquire the conception of a zone of reproductive activity—or in 
the Se/ago type it may be several interrupted zones—limited below by 
parts which are to be held as vestigial, and above by parts which are 
supernumerary. By comparison of living species of Lycopodium it is seen 
that the fertile. zone is not always located at the same level on the plant: 
it is sometimes preceded by a shorter, sometimes by a longer vegetative 
region. There has probably been a phylogenetic shifting of the fertile 
zone or zones: the biological significance of this is obvious, for any 
advance of the fertile zone to a higher point, by abortion of sporangia, 
while the sporophylls remain in a vegetative capacity as foliage leaves. 
provides a larger vegetative region below for purposes of nutrition. Such 
a manner of advance has probably been effective in the evolution of the 
Lycopods as we now see them. 
If the Lycopods stood alone in showing such features as those described 
the facts would be of limited interest, but they do not; conditions essentially 
similar are seen in the sporophytes of other Vascular Cryptogams, though 
varying in detail. The mature plant of Jsvezes is virtually of the Se/ago 
type: it bears fertile and sterile leaves intermixed: vestigial representatives 
of sporangia are found in the position normal ‘for sporangia upon many of 
the sterile leaves; further, the probability that the leaves actually sterile 
are so by suppression is as strong here as in the case of Lycopodium 
Selago. The mature plant is preceded by an embryonic vegetative phase, 
with leaves bearing no sporangia; but after the first sporangia appear, the 
whole plant may be regarded as a strobilus, imperfectly differentiated, as 
in the Sedago type, into fertile parts and parts sterile by abortion or by 
complete suppression. 
Similarly, in the Psilotaceae, the Se/ago condition, with irregular alter- 
nation of sterile and fertile zones, is seen in both Ps¢lotum (Fig. 87) and 
Tmesipteris, while imperfect synangia are found about the limits of the 
fertile regions. There is, however, a broad difference in form between the 
simpler sterile appendages and the more elaborate fertile ones; in this 
respect the differentiation of sterile and fertile parts has proceeded further 
than in the Lycopods. In the allied fossils, the Sphenophylleae, there 
