3 22 
Notes . 
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 Mixtae (with very few 
exceptions) have a vertical annulus. None of the Mixtae have been 
found 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 elongated 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 Loxsoma is suggestive ; as also the sporangial structure 
and high spore-output in Hymenophyllum. Though we may recognize 
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 Dennstaedtiinae, while the 
fully mixed type is seen in the closely allied Davallia. 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 elongatingreceptacle, and the drain on 
the part is spread over a longer period. But with the assumption of 
