Studies in the Morphology of Spore-producing Members. 97 



These divisions are primarily based on the order of appearance of the 

 sporangia in the sorus, the Simplices having all the sporangia of the 

 sorus formed simultaneously, the Gradatae having them disposed in 

 basipetal succession, and the Mixtae having the sporangia of different 

 ages intermixed. But it is found that other important characters run 

 parallel with these : thus the Simplices and Gradatae have an oblique 

 annulus (where definitely present), the Mixtse (with very few excep- 

 tions) have a vertical annulus. None of the Mixtae have been f ound to 

 have a higher spore-output per sporangium than sixty-four, but this 

 number is exceeded by some of the Gradatae, and large numbers are the 

 rule in the Simplices. The Simplices and Gradatae have relatively short 

 thick stalks, the Mixtae usually have long and thin stalks. The 

 orientation of the sporangia in the Simplices and Gradatae is usually 

 definite, in the Mixtae it is indefinite. The receptacle is often elon- 

 gated in the Gradatae, but not in the Simplices or Mixtae. The sum of 

 these characters, which for the most part run parallel to one another, 

 appears to give a substantial basis to the classification. 



Evidence as to the transition from type to type has been collected. 

 In the case of the transition from a simultaneous to a successive sorus 

 it does not amount to a demonstration : but it is specially pointed out 

 how slight a step it is from such a sorus as that of Gleichenia dichotoma 

 to that of an Alsophila : that given a basal indusium and marginal 

 position, the similarity of sporangial structure and dehiscence between 

 Gleichenia and Loxsonia is suggestive ; as also the sporangial structure 

 and high spore-output in Hyrnenophyllum. Though we may recognise 

 these lines of similarity, they do not focus upon any one genus as the 

 actual transitional link from the simultaneous to the basipetal. But 

 the transition from the basipetal to the mixed sorus can be followed in 

 detail ; intermediate steps are seen in the Dennstaecltiinae, while the 

 fully mixed type is seen in the closely allied Damllia. Probably this 

 is only one of several such lines of transition from the basipetal to the 

 mixed type. 



It is shown that a biological advantage would be gained by the 

 suggested transitions. In the Simplices the few sporangia are large, 

 and, arising simultaneously, make a demand all at once on the nutri- 

 tive resources of the part. In the Gradatae the smaller sporangia are 

 produced in succession upon an elongating receptacle, and the drain on 

 the part is spread over a longer period. But with the assumption of 

 the mixed character the drain may be spread over an equally long time, 

 while, as the elongated receptacle disappears, the surface from which 

 nourishment can be derived is enlarged, and the distance through 

 which it has to be transferred is shortened. Thus it appears biologi- 

 cally reasonable that the succession should be as suggested. 



It is shown how the various types of dehiscence, and the action of 

 the annulus stand in close relation to the orientation of the sporangia, 



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