INTRODUCTORY ON PTERIDOPHYTA 289 



study of the Bryophytes. Norton the other hand, does it justify the 

 initial assumption that the origin of the sporophyte in Vascular Plants 

 differed essentially from that in the Bryophytes. Accordingly, the theory 

 of progressive sterilisation will here be applied to the study of the 

 Pteridophytes also, along lines parallel to those observed for the Bryo- 

 phytes. It is not to be expected that the facts will amount to a complete 

 demonstration : the present object will be to see how far they accord with 

 a theory which has its more obvious application in the simpler series of 

 Archegoniate Plants. 



The most important evidence will naturally be obtained from the study 

 of the spore-producing members themselves ; and these will be described 

 in detail in the several types of Pteridophytes. But facts of value bearing 

 indirectly on the general hypothesis, are also to be derived from the form 

 and structure of the vegetative parts, as well as from their origin and 

 early development. In fact, the whole sporophyte is to be studied in 

 relation to the question of its origin, just as much in the more complex 

 as in the simpler Archegoniate forms. One guiding line must constantly 

 be maintained, and it is this : that however late in the individual life the 

 production of spores may appear, still spore-production was on our general 

 hypothesis the first office of the sporophyte. By various means the vege- 

 tative phase may have attained a large size, and great complexity of 

 structure : but however preponderant it may appear, still we should be 

 prepared to regard it theoretically as secondary, that is, as a phase 

 intercalated between the events of nuclear fusion in the zygote and 

 reduction in the spore-mother-cell. 



It will be well to observe some regular order in the discussion of 

 the large area of fact involved. The several groups of the Pteridophytes 

 will accordingly be taken in succession, starting from those with relatively 

 small appendages and strobiloid habit, and proceeding to those with 

 appendages of larger size. The fossil representatives will be included in 

 the discussion, together with the living forms. In each group a pre- 

 liminary section will deal with the external characters of the mature 

 organism, 'with special reference to the balance of the vegetative and 

 reproductive regions. It will be followed by a detailed examination of 

 the spore-producing members, and lastly, certain facts of anatomy and 

 of embryology will be considered in their bearings on the general ques- 

 tion. The characters of the gametophyte will only be referred to 

 incidentally, so far as they affect the biological circumstances of the 

 young sporophyte. 



If then the Pteridophyta be arranged according to the complexity of 

 the appendages, and especially of their spore-producing parts, the Lycopodiales 

 will come first, since in them each isolated sporangium is attached in the 

 median plane to its subtending sporophyll. 



A second series is characterised by having one or more sporangia 

 borne on a vascular pedicel : when the number is more than one they 



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