THE SPORE-PRODUCING MEMBERS 693 
may have come about. In Chapter VII., which deals with sterilisation, 
examples have been brought forward showing how widespread is the 
conversion of individual cells, and even tracts of tissue from the fertile 
to the sterile state, and that in some cases septation of spore-sacs has 
actually been the result. It was concluded (p. 102) that plants show 
not uncommonly to-day such a conversion of cells from the propagative 
to the vegetative state as the antithetic theory would demand. Further, 
in Chapter VIII. (p. 112) it was shown that commonly the archesporium 
of Vascular Plants is not strictly circumscribed, but that the sporogenous 
groups have often ragged edges: this suggests on the basis. of structure 
that each fertile tract is a residuum left by advancing sterilisation; in 
fact, the sporangia may in the simpler cases be regarded as islands of 
fertile tissue which have retained their spore-producing character. In 
Chapter XI. (p. 140), on the theory of the strobilus, it was shown how 
the disposition of the parts in some of the simplest Pteridophytes suggests 
as a prototype, prevalent though perhaps not general, an upright, radial, 
strobiloid structure, consisting of a predominant axis showing continued 
apical growth, and bearing relatively small and simple appendages formed 
from it by enation. Associated with these are sporangia each containing 
as its essential feature an island of fertile tissue. It is impossible to 
bring proof how a simple strobilus such as this actually originated; but 
it can be claimed that all the structural and developmental facts 
described in Part II. accord readily with a theory of origin ky septation 
from a continuous spore-sac and enation of appendages. So also is 
physiological probability, for the sporangial types are better fitted for the 
mechanical protection, the nutrition, and the dispersal of the numerous 
spores than those with the non-septate sac: and in homosporous forms, 
which all the most primitive types were, the larger the number of germs 
the greater the probability of survival and of spread. 
Passing, however, from such hypotheses, which are not susceptible of 
actual proof under present conditions, to matters of direct observation, a com- 
parison of the fertile shoots of all the known homosporous Pteridophytes 
shows them to be composed of three constituent parts: (i) the ax7s, which 
the embryological comparison as well as the facts of development in 
the growing shoot have shown to be the pre-existing part; (ii) the. dracts 
or sporophylls, which are appendages produced by outgrowth from the pre- 
existent axis; and (iii) the spore-producing-members, under which general 
term are included sporangia and sporangiophores, with their phyletic products. 
These may be inserted either on the axis or on the sporophyll. It is 
believed that (ii) and (iti), though they: show commonly a local relation to 
one another, have actually been distinct organs throughout descent : neither 
has been the result of metamorphosis of the other. In further support of 
this it will be shown that they do not bear any obligatory relation one to 
another: either may exist without the other: while either may show fission 
independently of the other, though in some forms both are alike in this. 
