An Historical Sketch . 
75 
ship between the outer integument and the ovular leaflet is described as 
follows : the former is an upgrowth out of the lower surface of the lower 
portion of the leaflet, after this latter has become inverted and folded in 
towards the upper surface . There exists, therefore, no essential difference 
between the case of Aquilegia and that of the other plants mentioned 
above. 
As demonstration of the fact that these variable structures, known as 
the ‘ ovular leaflet ’ or virescent ovule, are all governed in their development 
by definite laws of growth and sequential differentiation, and are not the 
fortuitous and haphazard result of ‘ sportive 5 tendencies on the part of 
the plant producing them, it may be mentioned that precisely identical 
phenomena have been observed as curious modifications of the entire 
foliage-leaves of the common Lilac ( Syringa vulgaris ). 
Again, exact homologues both of the metamorphosed ovule and of the 
unaltered ovule or ‘ ovular leaflet 5 may be found as normal structures in 
other departments of the vegetable kingdom. Celakovsk^ finds such in 
the apparatus of the female ‘flower’ of the Coniferae. The Taxaceae 
present the case of the normal ovule along with its two integuments ; the 
remaining groups that of the semi-proliferated ovule, of which the semi- 
niferous scale (or rather one-half thereof, seeing that each scale possesses 
two ovules) is the vegetatively developed outer integument bearing the 
involuted nucellus-producing inner integument on its lower (dorsal) surface. 
The case of Cupressus , in which a single seminiferous scale bears several 
such inner integuments on its lower surface (Fig. 14), finds its counterpart, 
as we have seen, in Hesperis ; such a structure as this might conceivably 
arise out of a compound ovular leaflet , the terminal segment of each lobe 
becoming, as in the simple three-lobed leaflet, the inner integument borne 
on that lobe’s lower surface. Descending lower in the scale, precisely the 
same set of structures (although naturally modified in accordance with the 
idiosyncrasies of the special group of plants in which they occur) are 
exhibited as normal stages of development in the sporophylls of the Ferns. 
In Thyrsopteris and Hymenophyllaceae (Figs. 15, 16) we see the case of 
the normal ovule of Angiosperms and of Taxaceae, in which the receptacle 
bearing its numerous sporangia (homologue of the nucellus), terminal in 
position, is ensheathed by the integuments, of which the indusium is the 
morphological equivalent of the inner, while the outer integument is 
represented by the laminar extension (when present) of the pinnule- 
segment on either side of the indusium. If now this structure be compared 
with the virescent ovule in Trifolium repens , its similarity to the bilobed 
funicular lamina enclosing the inner integument is obvious : the two 
structures are to be regarded as homologous although naturally presenting 
differing degrees of development of the respective parts. In Dicksonia the 
sorus is also terminal to the leaf-segment, but the indusium is here two- 
