SIMULTANEOUS OR SUCCESSIVE 117 



earliest, the sporangia in near juxtaposition show a simultaneous origin ; 

 or some degree of succession may be seen from those earlier formed 

 near to the base of the shoot or leaf, and leading to the apical region, 

 where they appear later. Such simultaneity, or such acropetal succession, 

 may be regarded as a primary condition, and it is seen in the Lycopodiales, 

 Equisetales, and Sphenophyllales, as well as in those Ferns which are 

 designated below the Simplices (see Part II.). But in certain Ferns, which 

 the Palaeontological record, as well as comparison, would mark out as 

 secondary, the sporangia in near juxtaposition do not arise simultaneously: 

 sometimes, as in those which will be styled the Gradatae, there is a regular 

 basipetal succession within the sorus, those lowest on the receptacle 

 appearing latest. In others, again, there is no such regularity, and sporangia 

 of different ages are found promiscuously intermixed : these Ferns are 

 styled the Mixtae, and the Palaeontological record indicates that these 

 were the latest to appear. Such facts, which will be stated at length 

 below (Part II.), may be summed up into the following general statement. 

 In the most primitive forms the sporangia in near proximity to one another 

 develop simultaneously, though an acropetal succession may often be 

 traced on the shoot or strobilus as a whole. Those successions, whether 

 in regular order or irregular, which appear in various forms upon the 

 leaves, may be held to be later derived, and secondary. 



It will be readily gathered from the contemplation of those plants 

 in which sporangia are numerous that accurate comparison of individual 

 sporangia as identical bodies in parent and offspring, or in different, less 

 closely related specimens, is not possible in plants at large. For the most 

 part sporangia are merely examples of "essential correspondence" rather 

 than of "individual repetition." The actual sporangia of the offspring are 

 not coincident, as a rule, either in exact position or in number with those 

 of the parent. This is a consequence of that continued embryology which 

 is a leading feature in all vascular sporophytes. As a consequence the 

 individual sporangia of any one individual plant or species cannot be 

 held to be the exact ontogenetic correlatives of those seen on another 

 individual or species. The possibility of such a recognition is most nearly 

 approached in the Lycopods, where the sporangia are borne singly in 

 definite relation to the axis and leaf. It is departed from furthest in 

 the large-leaved Ferns : especially is this so in the Polypodiaceae, where 

 the mixed character of the sorus is the rule : but most of all in such a 

 case as that of Aspidium anomalum, Hk. and Arn., a Fern found on 

 the uplands of Ceylon, and sometimes regarded as a mere variety of 

 A. aculeatum, Sw. : its peculiarity consists in the appearance of sori upon 

 the upper surface of the leaf, where normally they do not occur. As 

 there is no question of mere inversion of the leaf, it can only be assumed 

 that there has been a transfer of the stimulus, whatever it be, to soral 

 development from the lower to the upper surface of the leaf. Clearly 

 the sori which result cannot be the ontogenetic correlatives of any 



