76 WorsdelL — The Structure and Morphology of the ‘ Ovule! 
lipped instead of cup-shaped. In Davallia and Microlepia the first stage 
in the projection of the sorus on to the lower surface is seen, this being 
caused by the elongation of the upper side of the indusium, which becomes 
green and is (in part) an extension of the pinnule-segment (the outer 
integument (Fig. 17). 
The final stage is seen in Cystopteris , where the sorus, with the lower 
lip of the indusium, is projected completely on to the lower surface of the 
pinnule-segment (Fig. 18). In Cibotium and Cyathea there is a distinct 
cup-shaped indusium situated on the lower surface of the segment (Fig. 
19); in the former this is at first terminal and marginal, becoming sub- 
Fig. 15. Thyrsopteris : sorus with indusium {ind) and terminal receptacle bearing sporangia 
{spg). Fig. 16. Trichomanes : receptacle and cup-shaped indusium (= inner integument) seated 
in sinus between two lobes of leaflet (= outer integument) (cf. Fig. io). Fig. 17 . Davallia : 
early phylogenetic stage in projection of indusium on to lower surface of leaflet ; indusium one-sidedly 
developed. Fig. 18. Cystopteris : later stage in same process. Fig. 19. Cyathea : same stage, 
but indusium here completely formed ; spg, receptacle bearing sporangia. Fig. 20. Hesperis : 
proliferated outer integument bearing inner integument on its lower surface; this is precisely the 
same structure as that in Cyathea. (All after Celakovsky.) 
sequently displaced into an inferior position ; in the latter it is inferior 
from the first. In these two latter cases we have the exact counterpart of 
an inner integument situated on the lower surface of a ‘basal lamina’ or 
proliferated outer integument or funicle, such as occurs in Alliaria or 
Reseda (Fig. 20). 
As regards the Rhizocarps : in the Salviniaceae the fruit is equivalent 
to an ovule with one integument ; the indusium represents the inner 
integument and the leaf-lobe bearing the sorus is probably homologous 
with the outer integument. There is a striking resemblance, admitted by 
most botanists, between the monangic sorus of Azolla and an ovule 
(Fig. 21). Prantl regarded the two as homologous structures, and Campbell 
suggests the same thing. In the Marsiliaceae the fruit has the value of 
a compound fruit of Salviniaceae. In Pilularia it is homologous with 
a pinnately quadri-foliolate leaflet of the entire leaf of Marsilia ; in Marsilia 
with a pinnately multifoliolate leaflet ; in these cases the leaflet of the 
sporangiferous leaf possesses the compound structure of the vegetative leaf, 
