166 STERILE AND FERTILE REGIONS 
is, as a rule, a definite strobilus, which is fertile throughout ; but in S. majis 
this is not clearly differentiated by form either at base or apex from the 
Shoot of Pstlotusm. Natural size. 
condition in the bifurcate branch- 
vegetative : then follows a zone bearing synangia, then 
a vegetative zone, and higher up a second fertile zone. 
vegetative region. Such a condi- 
tion shows an interesting analogy 
with the less differentiated states 
of Lycopodium. 
Among the Equisetales, Agz?- 
setum and Bornia have, as a rule, 
a definite strobilus, composed 
entirely of sporangiophores borne 
on an axis, and clearly marked off 
from the vegetative region which 
precedes it in the ontogeny. But 
in the Calamarians, as also in 
the fossil known as Phylotheca, 
sterile leaf-sheaths are interspersed 
between the sporangiophores, a 
condition occurring also as an 
occasional abnormality in Zgue- 
selum (Fig. 88). The morphology 
of these cases will be more fully 
discussed below; meanwhile it may 
be held that while in Aywzsetum 
and Sornza the differentiation of 
the fertile strobilus from the vege- 
tative region is more distinctively 
marked, Phyllotheca or the Cala- 
marians show some degree of 
analogy with the Se/ago condition 
seen in Lycopodtum. 
The Ophioglossaceae provide 
clear cases where the argument 
of abortion leading to complete 
suppression will apply ; for various 
degrees of development of the 
fertile spike may be found borne 
upon the fully formed leaf, from 
that fully matured to small 
vestigial parts which do little more 
than mark the place where the 
normal spike would be inserted ; 
in other cases again the spike is entirely unrepresented. The facts here 
correspond to those in Lycopodium Selago or in Lsoetes, except as regards 
the size and number of the parts concerned. Such a condition in an 
Angiospermic flower would certainly be interpreted as abortion, and the 
