354 
Bower . — A Theory of the 
ance of the sporangia. If the arrested sporangia be accepted 
as vestigial, then the correlatively larger foliage-leaves which 
subtend them must be regarded as sterilized sporophylls, and 
the conclusion follows that in some cases at least foliage-leaves 
are sterilized sporophylls . 
Before leaving this subject it may be remarked that a similar 
view has been applied by Prantl and others to the stamens 
and perianth-whorls of Ranunculaceae, &c., viz. that the 
latter owe their origin to sterilization of staminal members. 
It seems not unlikely that this view may be applicable also in 
many other cases among the higher plants. 
We have thus arrived at the position that sterilization of 
potential sporogenous tissue is a common phenomenon, re- 
curring frequently throughout the Archegoniatae and Phanero- 
gams. We find it affecting single cells, groups of cells, or 
even whole sporangia. In certain cases the sterile cells may 
remain isolated, or be absorbed by the developing spores, 
thus appearing to serve a directly nutritive function : in other 
cases permanent sterile rods (trabeculae, or columellae) or 
plates of tissue may be formed, and there is reason to think 
that a grouping together of sterile cells may have resulted in 
certain cases in the formation of sterile tissue-masses (Antho- 
ceroteae). In others, again, complete septa may partition off 
a previously concrete potential sporogenous mass into isolated 
portions : such tissue-masses as trabeculae and septa may 
serve the purposes of mechanical support, and of assistance in 
bringing nutrition to the masses of developing spores. We 
have further learned from the resumption of spore-production 
by certain sterile septa, coupled with the facts of sterilization, 
that there is no fundamental difference between sterile septa 
and fertile tissue, either being convertible into the other. 
With these conclusions before us, drawn from both higher 
and lower forms, we may now address ourselves to the 
problem of forming some idea of the probable methods of 
morphological advance of the simpler homosporous Vascular 
Cryptogams. On grounds of comparison it has been gene- 
rally held that they originated from some Algal-Bryophytic 
