360 Bower . — A Theory of the 
compared with the columella ; the continuous archesporium of 
the Bryophyte is replaced by the discrete archesporia of the 
Pteridophyte, while these are carried out on appendicular 
organs, the sporangiophores. Thus the strobilus would appear 
to be the correlative of some body like a sporogonia! head, in 
which the archesporium is septate and borne outwards on 
eruptive growths. Applying these ideas to the strobilus of 
Equisetum , we see the probability of them reflected in the 
development. The surface of it when young is almost smooth : 
the isolated cells which are to form the archesporia, can be 
recognized almost as soon as the surface becomes undulated 
by the eruption of the sporangiophores: these cells are 
proportionately nearer the surface than in the Bryophyta : 
but it has already been pointed out that the very formation 
of the columella was a step in the direction of relegating the 
spores to a superficial position, while such a position is 
necessary for the dispersal of the spores from small loculi. 
I do not mean to suggest that by such comparisons as these 
the hypothesis of origin of a strobilus from a body of the 
nature of a sporogonial head can be proved : the comparison 
may be, and indeed probably is, merely a tracing of analogies 
in parts which have advanced along somewhat similar lines : 
and our endeavour is, as explained at the outset, not so much 
to trace homologies as to recognize the methods of advance 
in archegoniate plants : the chief points which have been 
recognized thus far, and are believed to have been the 
important factors in advance are (1) sterilization of potential 
sporogenous tissue , (2) formation of septa , (3) relegation of the 
spore-producing cells to a superficial position , and (4) eruption 
of outgrowths {sporangiophores) on which the sporangia are 
supported. 
In the Lycopods also the whole strobilus may be recognized 
as the result of similar methods of advance : regarding Phyllo- 
glossum as probably their simplest representative, we see that 
the plant consists of the protocorm, bearing protophylls, 
which may have been the result of direct vegetative out- 
growth from the protocorm. Borne upon an elongated 
