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 J;hat 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 Helminthostachys ; 

 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 protection, 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 circumstances. 



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 of 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 XL). 



