252 Bower . — Imperfect Sporangia in certain 
defined, and certain members of the group illustrate a further 
specialization in accordance with their ground-habit, the 
strobilus being carried up on an elongated peduncle with small 
scale-leaves, an obviously useful device to secure a wider 
distribution of the spores. Another line of specialization 
results in the dorsiventral vegetative shoot, as in Selaginella , 
and this culminates in the climbing species, L. volubile , with 
its large and branched foliage system and clearly defined 
strobili. The whole genus shows in its living species the 
lines of specialization fairly indicated by gradual specific 
steps, starting from an undifferentiated strobilus, and attaining 
first a clear differentiation of the strobilus from the vegetative 
region : the latter may become in a high degree adapted to 
its environment ; the former probably retains more truly the 
primitive condition of the whole shoot. Thus a comparison 
of the living species indicates that there has been a shifting 
onwards of the spore-producing zone and progressive inter- 
calation of a vegetative zone, comparable to that indicated 
in the Mosses. In both series there is a strong physiological 
probability that such a differentiation should take place, as 
a nutritive advantage is gained, and in some cases the better 
provision for dispersal of the spores is secured ; both advan- 
tages being the result of comparatively slight morphological 
changes. 
The facts relating .to partially or completely abortive spo- 
rangia in the genus Lycopodium are fairly intelligible, owing 
partly to the undifferentiated state of some of the species 
of the Selago group, partly to the considerable number of 
species in the genus. It is not to be expected that the matter 
will be as obvious in other Lycopodineae ; still, abortive 
sporangia are found in other genera, which are susceptible 
of similar interpretation. Such examples as have been found 
will now be noted. 
Selaginella. 
In this genus the strobili are definitely marked off from 
the vegetative region ; no case of alternating sterile and fertile 
zones is recorded, but in some species the fertile spikes revert 
