250 SUMMARY OF THE WORKING HYPOTHESIS 
during -its descent. The problem will therefore be to assign its proper 
place in the evolutionary history to any or each of these factors. But 
to do this presumes a knowledge of that history more complete than is 
at present accessible: still it is well thus to formulate the problem, with 
a view to clearing the points at issue. 
The sporangia are rarely inserted directly on the axis, but usually on 
appendicular organs of various form and size: these have been designated 
in some cases sporophylls, in others sporangiophores. Reasons have 
been assigned in Chapter XII. for the opinion that all these appendages 
are not to be held as referable to any single original category of parts, 
such as the formal morphology of the higher plants would recognise. 
According to a strobiloid theory there is no need to assume that all 
appendicular organs were alike in their initial character, though circum- 
stances may have led to their ultimately settling down to a more or less 
uniform type among plants of advanced development. 
The term sporangiophore is applied to certain appendages which bear 
one or more sporangia, and are traversed as a rule by a vascular strand 
for their supply. Their position may be directly upon the axis, as in 
the Equisetales ; or upon some lateral appendage, as in Aelminthostachys ; 
or on the surface or margin of a leaf, as in Ferns, where they are commonly 
called sori. The sporangiophore, wherever found in primitive forms, may 
be held to be itself a primitive structure, and is not to be assumed to 
be a result of modification of any other sort of appendage (Chapter XII.). 
The position which “foliar” parts hold relatively to sporangia or sporangio- 
phores is frequently that of subtending them, as though determined by some 
function of protéction, or, in some cases, of nutrition. It is illustrated in 
the Lycopods, the Sphenophylls, and the Ophioglossaceae ; and with less 
regularity in the Calamarians. These relations are probably due to some 
common causal circumstanees. 
Such discussions naturally open up the question of the nature and 
origin of those parts which are comprehended under the term “leaf.” 
So long as the fossil record remains as imperfect as at present, there 
can be no certain knowledge on these points, since the foliar development 
was present in the earliest vascular fossils of which there is certain or 
detailed evidence: accordingly the question can only be approached on 
grounds of comparison. There is reason to believe that the Bryophytes 
acquired their leaves polyphyletically, and this consideration would suggest 
that the foliar appendages cf Vascular Plants may also have been poly- 
phyletic; this position, which accords with their differences of character, 
is quite compatible with the strobiloid theory (Chapter XII.). One point 
which follows naturally from the observation of the earliest stages of 
development of foliar organs, whether in the sterile or the fertile shoot, 
is their lateral origin below the apex of the axis which bears them. In 
the ontogeny the axis pre-exists the youngest leaves: this is believed 
to have been the case also throughout descent (Chapter XI.). 
