An Historical Sketch . 
77 
while the leaflet of the latter is simple. The outer wall of the sporocarp is 
homologous with the upper surface of the outer integument of the ovule ; 
here, as in Hesperis and Cupressus , it is polysorous, bearing a number of 
indusia or inner integuments (each with its enclosed sporangia) on the 
lower surface (Figs. 22-25). 
Fig. 24. 
Fig. 25. 
Fig. 21. Azolla : megasporangium enclosed by indusium (after Campbell). Fig. 22. Pilularia\ 
diagram of morphological structure of sporocarp from two pairs of leaflets, each bearing a sorus on 
its lower surface. Fig. 23. Pilularia : transverse section (taken through dotted line in preceding 
figure) of leaflet, showing sorus and indusium; ma = mega-, mi = microsporangia. Fig. 24. 
Marsilia : diagram of morphological structure of sporocarp, from several pairs of leaflets, each 
bearing sorus on its lower surface. Fig. 2 5. Marsilia : transverse section (taken through dotted 
line in preceding figure) of leaflets here supposed to be folded together from opposite sides of 
midrib to form the sporocarp ; sorus shown. 
For a further elaboration of this subject of the homologies in the 
different groups the reader is referred to our authors exhaustive paper in 
‘ Pringsheim’s Jahrbucher.’ 
In the Lycopodiaceae are again found equivalents for our ovule and its 
various parts. The ligule of the Selaginelleae our author, in the above- 
mentioned paper, regards as the same leaf-lobe which later in the Schi- 
zaeaceae grows out over the sporangia, but here, owing to the ventral 
position of the sporangium, the ligule is also ventral, and as such is so 
weakly developed that it is partly fused with the carpel, so that it de facto 
springs from the carpel ; the sporangium, as in the Schizaeaceae, springing 
