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 



