INVERSION OF BALANCE 45 
protective wall of cells: it is dependent through life upon the gametophyte, 
and results in a limited number of spores. In more complex Bryophytes 
it is still short-lived and dependent, but larger, with distinction of apex 
and base, a brief apical growth, and a basal vegetative region distinct 
from the terminal capsule: there is entire absence of appendages, but 
a partial differentiation of tissues, with internal ventilating system and 
some assimilatory tissue. The spore-production is on a larger scale, but 
limited usually to the simultaneous development of one continuous 
spore-sac. 
In the Pteridophytes the mature sporophyte is an independent, self- 
supporting organism; but it is dependent in youth upon the parent 
prothallus: it is commonly perennial. It has theoretically unlimited 
growth of axis and root: the appendages vary greatly in size: there is 
high differentiation of the tissues, with an elaborate ventilating system: 
the plant thus constituted is capable of complete and continued self- 
nutrition. The spores are produced after a more or less prolonged 
vegetative. phase, and in perennial forms their production may be 
continued for an unlimited succession of seasons. They are borne 
in separate sporangia, which are commonly seated upon the appendages: 
the sporangia themselves are frequently produced in a continued succession. 
These arrangements are such as to lead to a high and even long-continued 
output of adequately nourished spores. The sporangia are frequently 
testricted to certain shoots, in which the parts are closely aggregated : 
these are termed strobili. 2 
The heterosporous state seen in all the highest Vascular Plants, 
introduced advantages conducing to certainty in nursing the embryo, and 
led in Seed-Plants to an infinity of special developments which secured 
that transfer of the microspores which is so necessary for fertilisation. 
But the essential plan of the independent, self-nourishing Vascular Plant 
‘once laid down was not departed from, even in the highest forms. The 
sporophyte, thus sprung from small beginnings, remains the dominant 
generation in all distinctively terrestrial plants. 
The entire inversion of the balance of the two alternating generations 
thus briefly sketched—the dwindling away of the one and gradually 
achieved dominance of the other—is a fact which requires some 
physiological explanation. We may be sure that such things do not 
happen without good reason. It will be our object later to enquire into 
this. Meanwhile we recognise the fact itself, and we shall see in the 
comparisons which lead to its recognition an enduring monument to 
the genius of Hofmeister who first pointed them out. 
