[52 SPORANGIOPHORES AND SPOROPHYLLS 



Spore-bearing bodies, or sporangiophores, or sori such as those above 

 named, have then the following characters in common : They are out- 

 growths of varying length, which bear one or more sporangia : these are, 

 when numerous, more or less closely related one to another, and frequently 

 synangial : they are usually disposed in a circle round the periphery of a 

 disc-like expansion at the distal end of the more or less elongated stalk, 

 but other arrangements may be found. A vascular strand usually runs 

 through the stalk to its distal end, where it may divide into branches 

 which terminate in close relation with the sporangia. The similarity in 

 all essentials of structure of the sporangiophores in these early Pteridophytes, 

 notwithstanding the diversity of their position, points to the conclusion that 

 they are the result of similar response to similar requirements, carried out, 

 it may be, in various distinct stocks in the various positions in which 

 they are now seen. Their structure suggests that they are simply the 

 outcome of placental growth, which has the advantage of securing freedom 

 of dehiscence of the sporangia which they bear. In that case there is no 

 obligation to hold that they were the result of "metamorphosis" of any 

 pre-existent appendage or part. And herein lies the importance of the 

 introduction of the term sporangiophore which is now applied to them ; 

 for it simply implies the fact that the part so called bears sporangia, but 

 does not suggest any view of its further morphological character, nor does 

 it impose any limitation upon the position which the sporangiophores 

 may hold. 



It remains to consider what relation, if any, subsists between such 

 sporangiophores and the " leaves." The difficulty in finding mutual ground 

 for discussion of this question lies in the preconceived ideas which the term 

 "leaf" carries with it in the minds of many botanists. It is often assumed 

 that the vegetative leaf was pre-existent in. descent to the appendages of 

 the strobilus, the mind naturally translating the successive events of the 

 individual life directly into the history of its evolutionary organisation ; in 

 fact, the sporophyll has habitually been regarded as a vegetative leaf which 

 has become fertile. In the following chapter reasons will be given for 

 holding that the converse is nearer the truth, i.e. that foliage leaves are 

 often the equivalent of sporophylls which have become in the course of 

 descent sterile. If this be so, then signs of the origination of a vegetative 

 system are to be sought in the fertile zone rather than the converse. 



A second preconception which is commonly entertained is that "the 

 leaf," whether sporophyll or foliage leaf, is a part having a common 

 evolutionary origin in all plants in which it appears ; but on grounds 

 explained above we shall be prepared to contemplate as possible a 

 polyphyletic origin of those parts which are designated collectively 

 " leaves." 



A third preconception, which is a common one also, is that those 

 bodies which are designated sporangiophores are necessarily of the nature 

 of sporophylls, or of segments or parts of sporophylls. Any sporangiophore 



