SUMMARY 429 
for consecutive argument, but their correspondence with those in 
Lycopodium, where the argument can be more fully developed, points to a 
clear analogy. Accordingly the facts may be held to indicate a probability 
that here also a progressive differentiation of sterile and fertile regions 
from an indifferent shoot which performed both functions has been 
effective, and that abortion of sporangia has played its part. That the 
strobilus which we see is the residual and now specialised fertile tract, 
while the vegetative system below has been initiated, or at least extended, 
by abortion of sporangia: this results in deferring the spore-production to 
a later period. Appendages of two sorts are recognised throughout, viz. 
the leaves and the sporangiophores: the former remain effective for 
assimilation or for protection in the lower vegetative region: in the 
strobilus the two may exist together, and even show intimate relations: 
or the sporangiophores only may be present, as in Zgudsetum, the true 
leaves being absent, perhaps by abortion. 
As regards spore-output, continued apical growth and branching have 
served as a set-off against the progressive sterilisation in the region below. 
But in addition these plants illustrate a probable amplification of the 
spore-producing members themselves, by septation and upgrowth of the 
vascular placenta, resulting in the sporangiophore itself: this is a more 
effective spore-producing member than a single sporangium. Another 
method of advance has probably been by fission, which increases the 
number of sporangiophores; but this appears to have brought in its train 
a corresponding reduction in the number of the sporangia, as seen in 
Sphenophyllum Dawsont. Thus both evidences of increase and of decrease 
in number of sporangia are illustrated in this, as in most other phyla of 
Vascular Plants. 
Some idea of the probable origin and nature of the sporangiophore 
has been gained by the study of the Equisetales and Sphenophyllales. 
It is a member attaining a considerable size, and endowed with a vascular 
system, while it carries out the important function of spore-production. 
There is no prima facie reason why such a member should show any 
strict limitation of size. The larger it grew, the greater would be its 
capacity for producing fresh germs: there would thus be an inherent 
probability of its increase, rather than the reverse. When the question 
is asked whether such increase is illustrated in any forms of Vascular 
Plants, a debatable ground is reached in which the Ophioglossales are 
the subject of dispute. It is only by a careful study of their details 
that an opinion can be formed: this will now be taken up. 
