SPORANGIOPHORE A PART SUI GENI'R/S 153 



attached to an axis would thus be held to be a complete foliar structure, 

 and a sporangiophore attached to the upper surface of a bract would be 

 recognised as a "ventral lobe" of that bract. But if it be admitted that 



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spore-production in the sporophyte was prior to its vegetative development, 

 and was a constant phase throughout the evolution of the sporophyte, 

 then such a description seems not only unnecessary but highly improbable. 

 Moreover, it has been seen that sporangiophores may be present in positions 

 which are not those of the normal succession of known vegetative parts ; 

 in the endeavour to bring these into line with the customary position and 

 succession of vegetative parts, recourse has to be taken to almost fantastic 

 explanations. But there is no need for this if the sporangiophore be accepted 

 simply to be, what it structurally is, a placental growth. The object of 

 the morphologist should be not the forcible reduction of different organisms 

 to one scheme of construction, but to read in their diverse forms the 

 probably diverse story of their origin. This should proceed along the lines 

 of the least strained and simplest interpretation. Following these principles, 

 the sporangiophore in the Pteridophytes will be held to be a part sui 

 generis, itself primitive in its nature, in the sense that it is not the result 

 of modification or replacement of any other sort of appendage. 



Certain physiological limitations must necessarily have been operative 

 during the transition of the fertile region of any sporophyte from a simpler 

 to a more complex condition, such as has been figured to the mind in 

 the last chapter. As already pointed out, an increase in spore-production 

 is an advantage in homosporous plants, since it increases the chance of 

 survival and of distribution. But in any increasing body the formation 

 of separate loculi will facilitate the protection and nutrition of the increasing 

 mass of spores while young : thus segregation has its biological rationale. 

 Projection of the sporangia beyond the surface of the part which produces 

 them will facilitate the shedding of the spores, and makes possible those 

 mechanical devices which are seen in so many of the Pteridophytes. For 

 the protection of the sporangia while young, close juxtaposition of the 

 appendages of the strobilus is also important, and illustration of this is 

 seen in almost all strobiloid types. But at the same time any projection 

 of the spore-producing parts necessitates the conveyance of their nourish 

 ment through a longer distance, and by more restricted channels. Such 

 crossing of interests will have tended to keep the appendages which bear 

 the spores small, so long as they are themselves not active or essential as 

 nutritive organs ; in fact, there would in that case be a tendency to per- 

 petuate the strobiloid type. But if the appendages themselves carry on 

 efficiently the function of supply of organic material, then there need be 

 no limit to their size, provided that the water-supply to them can be 

 maintained; and they may accordingly bear an infinity of sporangia, as is 

 seen to be the case in the megaphyllous types. It is in connection with 

 these functions of protection and nutrition that the foliar development 

 would naturally come into prominence as a feature of the strobilus, and 



