78 Worsdell . — The Structure and Morphology of the 4 Ovule ! 
Fig. 26. Lepidocarpon : dia- 
grammatic. lateral view of sporo- 
phyll, showing ‘ integument ’ and 
its mode of attachment to sporo- 
phyll ; position of ligule and 
sporangium are seen through the 
supposedly transparent ‘ integu- 
ment.’ 
from the lower surface of this modified leaf-lobe. He regards the velum as 
4 unquestionably equivalent to the indusium of the Ferns/ £ The ligule 
along with the velum and sporangium is homologous with a leaf-segment 
of Cyathea with its integumented sorus, and in 
both cases the development of the leaf-segment 
precedes the formation of the integumented 
sorus.’ ‘As now a leaf-segment of a Fern 
with inferior indusium is homologous with a 
doubly-integumented ovule ; thereby is also the 
homology of Isoetes set forth : the velum corre- 
sponds to the inner integument, the ligule to 
the basal lamina of the half-proliferated ovule, 
the leafy equivalent of the outer integument.’ 
The present writer regards the 4 integument ’ of 
Lepidocarpon as homologous with the velum 
of Isoetes ; being more especially comparable 
with this organ in I. echinospora , where it com- 
pletely envelopes the sporangium. In Lepido- 
carpon he regards the ligule as really (i.e. morphologically or ideally) 
situated outside the ‘integument,’ which is here open at its distal end 
(Figs. 26, 27). In other genera either the velum or both this and the 
ligule have quite aborted or never developed : the result, probably, of 
an efficient protection of the sporangia being afforded 
by the peltate ends of the sporophylls, as in Lepido - 
dendron and Spencerites. The same may be said with 
regard to the Equisetaceae. 
Before proceeding further it is necessary to direct at- 
tention to one very important consideration. Celakovsky, 
until comparatively recently, regarded the virescent con- 
ditions of the Angiospermous ovule as retrogressive 
phenomena, as reversions to the primitive ancestral 
structures of the organ. After a deeper study, how- 
ever, of the phylogenetic relationships of the various 
organs of plants, from which he gathered that the primitive position 
of the sporangium on the sporophyll was a terminal one, he was eventually 
led to regard the normal structure of the ovule as representing a more 
ancestral state of affairs, inasmuch as the nucellus or sporangium occupies 
a position terminal to the ovular leaflet. The more vegetatively developed 
is any structure, the more modified will it be in the direction of an advance 
away from the primitive reproductive condition. The virescent condition 
of the ovule merely reveals to us the homologies of the latter ; it tells us 
nothing as to its phylogenetic origin ; this can alone be determined by 
the comparative method of research. 
Fig. 27. Lepido- 
carpon : diagrammatic 
transverse section taken 
through dotted line in 
preceding figure, int 
= integument ; sph = 
sporophyll. 
