AMPLIFICATION OF THE SPOROPHYTE 247 
It is possible to conceive of an indefinite increase of the sporophyte, 
by continued cell-division and progressive sterilisation, in a body main- 
taining a simple form; but mechanical and physiological checks impose 
a moderate limit. The Bryophytes illustrate in some of their forms such 
progress in the sporophyte successfully carried out to a relatively high 
degree of complexity. But in all their more advanced types there is a: 
distinction of apex and base, the basal region being sterile and the apical 
region fertile. Their sporogonia, however, always retain a simple form, 
and with few exceptions the radial type of construction: they are all 
alike also in having a single continuous spore-sac. This is plainly a type 
of construction which has its limits imposed by mechanical and physio- 
logical conditions. Reasons such as these have tended to prevent the 
Bryophytes from developing their sporogonia beyond a very moderate 
size. They show, however, very clearly on comparison the successive 
steps by which progressive sterilisation may advance the complexity of 
a simple type of sporophyte (Chapters III. and IX.). 
But the Vascular Plants, while showing the same plan of life-cycle, 
have been able to continue development without those mechanical and 
physiological checks operating upon their spore-output. The outstanding 
features in which they are more free than the Bryophytes follow from 
the segregation of sporogenous tissue in distinct sporangia, and the 
formation of appendicular organs. The biological advantages thus attained 
are obvious: a plurality of sporangia makes possible the separate, and 
more efficient nutrition of each: thereby also the mechanical difficulties, 
which act in limiting the Bryophyte sporogonium, are effectively avoided. 
On the other hand, the development of appendicular organs makes 
independent self-nutrition of the sporophyte really effective, while the 
position of the sporangia on the appendages facilitates the dispersal of 
the spores. The palaeontological record shows conclusively that both of 
these features were of very early date, and their consequences are 
illustrated in the earliest fossils of which there is any detailed knowledge 
(Chapter XVIII.). The advantages secured by an unrestricted type of 
development were doubtless such as to lead to a rapid advance. It 
can therefore be no matter for surprise that connecting links between 
the two states are absent, even supposing the two phyla, in which they 
are characteristically shown, to have had some degree of community of 
origin. 
The Pteridophytes show diversity of type, according to the size of 
their appendages: those which are smaller-leaved, as in the Lycopods, 
Equiseta, and Sphenophylls, have as a rule a terminal strobiloid fructifica- 
cion, though this is not always clearly differentiated from the vegetative 
region. In the Fern-like types the fructification is disposed more 
generally over the enlarged leaves. As in the Bryophyta so in the 
strobiloid Vascular Plants, a sterile basal region precedes the terminal 
' fertile strobilus. This vegetative region may be held to be a phase 
