SPORE-ENUMERATIONS 643 
sporangium was uniformally large. Passing from these to the prevalent 
Leptosporangiate Ferns of the present day, the output falls with some 
degree of consistency, and the striking fact emerges that, so far as 
observation goes, no Polypodiaceous Fern has a larger output than 64, 
while in many cases it is smaller. The evidence points to a general 
reduction in the course of Descent. 
But variations occur within near circles of affinity, and in accordance 
with the above generalisation these have a special interest in those 
families which hold an intermediate position. This is seen within 
moderate limits in the Gleicheniaceae, Osmundaceae, and Schizaeaceae, 
in which the numbers approximate for the most part to the typical 
numbers 512, 256, 128. In some cases it is difficult to see any circum- 
stances which serve as an explanation; thus the variation between 
Gleichenia flabellata (512-1024) and GZ. dichotoma (256 or more), is not 
susceptible of ready explanation, unless it be connected with the more 
numerous sporangia in the sorus of the latter. Nor is that within the 
genus Lygodium. In other cases, however, the conditions of life and 
the structure of the assimilating apparatus throw some light on the 
matter; thus TZodea barbara, with its thick. assimilating leaves, gives 
higher numbers per sporangium than Z. superba and hymenophylloides, 
with their thinner pellucid leaves. But the most interesting series in 
this connection are the Hymenophyllaceae, for the limits of variation in 
number are here very wide, ranging between such numbers as 421 and 
32. Here the variation in number of spores per sporangium runs fairly 
parallel with the size of the sporangia, the smaller number of spores 
being contained in smaller sporangia (compare 77. ventforme and 
Tr. pinnatum). Further, there is a correlative elongation of the recep- 
tacle, on which there is thus space for a larger number of the smaller 
sporangia to be inserted and produced in succession. On these characters 
the Hymenophyllaceae may be laid out as a series, extending from types 
chiefly of the less specialised genus Aymenophyllum with short receptacle, 
large sporangia, and large output of spores from each, to types of the 
more specialised genus Zrichomanes, with longer receptacle, smaller spor- 
angia, and smaller output of spores from each. The former link on 
naturally to the Gleicheniaceae in the characters named, as well as in 
the general appearance of the sporangia: it seems not improbable that 
in the Hymenophyllaceae we may see a series of specialisation in which 
the “filmy” character is the most salient vegetative feature, and that 
this has carried with it, as in the genus Zodea, a decrease in size of 
the sporangia, and in the number of the spores produced from each. 
Taking the facts thus acquired from the Simplices and Gradatae they 
show that within circles of near affinity there may be a wide margin of 
variation in output of spores per sporangium, such as a theory of phyletic 
reduction of the sporangium would demand: and this goes parallel with 
the increase in number of the sporangia in the sorus, a decrease in size 
