154 SPORANGIOPHORES AND SPOROPHYLLS 
this may have taken effect in more than one of the several phyla of the 
early Pteridophytes. 
The relation of the sporangia and sporangiophores to the parts designated 
as bracts, sporophylls, or leaves is habitually, though not always, a close 
one. In the simpler strobiloid forms the leaf either subtends the spore- 
producing member, or the latter is borne upon its upper surface, commonly 
in a median position. The biological importance of this probably lies in 
the protection which is afforded, and in the ready supply of nourishment 
in cases where the leaf is an effective organ of assimilation. But it is an 
error to assume that there is any obligatory or constant relation for plants 
at large between the spore-producing members and the leaves. This is 
shown, first, by the fact that sporangiophores, even in very early fossils 
such as Bornia, may exist independently of the subtending leaves; and 
secondly, that when associated with leaves they may vary greatly both in 
numerical and local relation to them, even within near circles of affinity : 
this is seen in the Sphenophylleae with special clearness. Such examples 
taken from early fossils teach that the spore-producing members show a 
high degree of independence from the sporophylls. For the present these 
general remarks must suffice: but later, when the sporangiophoric Pterido- 
phytes have been described in detail, we may attempt some more exact 
recognition of the varying relations which .existed between the sporangio- 
phores and the sporophylls in early strobiloid types. 
In this connection the question may be raised whether sporangiophores 
and leaves have always been distinct categories of parts: whether leaves 
or foliar parts have ever developed into forms resembling sporangiophores. 
In the case of the Cycads there is little doubt that the parts usually 
designated female sporophylls or carpels are reduced foliar structures: it 
is shown on the basis of comparison that their form, so like that of many 
sporangiophores, has been attained by a process of reduction, and thus 
they may be held to be homoplastic with the primitive sporangiophores 
of Pteridophytes. 
Such considerations as these will deter the morphologist from any precise 
definition of the categories of parts borne upon the strobili of early Pterido- 
phytes according to experience derived from study of the Phanerogams. 
There is indeed no reason to assume that there was any initial uniformity 
of the development such as would lead to their always falling into 
strictly definite categories. Greater uniformity is, however, found among 
the higher forms, and it is this uniformity which has led to the establishment 
of those old morphological categories which are found to fit so ill upon the 
lower Vascular Plants. Each plant-type may be held to have worked out 
its own progressive development, while biological conditions common for 
them all would tend to reduce them to’ some common scheme. Such 
constancy as appears among the parts of the higher plants would then 
have been achieved by gradual evolution of order from: beginnings which 
were less constant: and as a matter of fact the exceptions from that 
