122 VARIATIONS IN NUMBER OF SPORANGIA 
the products have been rounded off: they will be best distinguished in 
the early stages of development. A good, clear example is found in the 
genus Gleichenia: the sorus of most species of the genus is of the radiate, 
uniseriate type, in which the sporangia form a series round the receptacle, 
while the centre of it is unoccupied by sporangia: there is good comparative 
reason to believe that this was the primitive type of sorus for the genus. 
But in G. a@ichofoma additional sporangia, one or more, are often seated 
in the central unoccupied space (Fig. 63). Examination of early stages 
of development shows that these sporangia are not a result of fission of 
sporangia typically present, but develop from new primordia not represented 
in other species (Fig. 64). The interpolation, of which this is a simple 
case, becomes a more prominent feature in the Ferns which palaeontology 
Tic. 63. 
a-h, sort of Gleichenia dichotoma, Willd. Showing one or more sporangia in the 
centre of the sorus, usually in this genus vacant. 7-4, sporangia of Gl. c’rcinata, Sed. 
é-n, sporangia of Gl dichotoma, a-h»about 14. 12% 50. 
teaches us to regard as the more recent. The succession of the sporangia 
characteristic of those Ferns which will be styled on that account the 
Gradatae, may be recognised as a repeated basipetal interpolation of 
sporangia on the receptacle of the sorus (Fig. 65 a). In the Mixtae it is 
still more prominent in the sorus, for new primordia, formed in continued 
uccession, appear scattered between those which are more advanced 
Fig. 66). But interpolation may take place not only of sporangia, but 
also of new spore-bearing organs between those typically present. This 
s 
( 
is exemplified in the interpolated stamens in certain flowers, the number 
of which is closely related to their bulk, and to the space available for 
them on the floral receptacle. Such interpolation of sporangium-bearing 
parts will clearly result in an increase in number of sporangia. The 
prevalence of interpolation thus seen in the Leptosporangiate Ferns, has 
given an entirely fictitious importance to this phenomenon in the general 
