ORIGIN OF THE FOLIAGE LEAF 251 
Though the evolutionary origin of the leaf must be still a matter of 
doubt, there is less uncertainty as to the relation of the sporophyll to 
the foliage leaf (Chapter XIII.), The idea of “ progressive metamorphosis ” 
from the foliage leaf to the sporophyll, as propounded by Goethe, is 
incompatible with the strobiloid theory as above stated: the converse of 
Goethe’s progressive metamorphosis will appear to have occurred, 
viz., that at least in some cases, and perhaps in all, the foliage 
leaf is a_ sterilised sporophyll: thus the vegetative region, though 
ontogenetically the first, is held to be phylogenetically the derivative 
not the primitive condition of the shoot. The evidence that this is 
so is primarily based upon broad comparison; but secondarily upon 
the existence of abortive sporangia in certain Pteridophytes, about 
the limits of the vegetative region. It is further pointed out that in 
cases of complete suppression, where no vestige remains of the undeveloped 
part, there is no structural evidence that the abortive part ever existed : 
this will account for the apparent deficiency of more direct evidence 
bearing on the origin of the- vegetative system. The result is a basal 
vegetative region, more or less clearly defined from a terminal strobiloid 
region, the latter retaining the primitive characters and the spore-producing 
function. A’ vegetative region thus established in any phylum as distinct 
from the fertile, may undergo a distinct progressive evolution of its own, 
according to its special nutritive or other function; and the result may 
be as wide a divergence in character of the two parts. But in many 
Pteridophytes the differentiation is not effectively carried out; as is seen 
in the “ Sedago” condition of Lycopodium; or in many Ferns, in which any 
of the vegetative leaves may bear sori. 
The anatomical characters of the shoot accord readily with the theory 
of the strobilus (Chapter XV.). The non-medullated monostele is generally 
accepted as the primitive type, and the more diffuse vascular conditions 
with medulla and ultimately with separate strands as derivative types ; 
and this holds even in the megaphyllous forms, for their individual life 
habitually opens with a protostelic condition of the axis, which may sub- 
sequently pass into some more diffuse structure. This is held to indicate 
a prior state of the shoot where the axis was structurally dominant, and 
the appendages small: the more complex vascular arrangements go along 
with an increasing influence of the leaf in the shoot, and are the internal 
expression of it. On the theory of the strobilus this is a secondary con- 
dition, as ih the anatomical history of the individual it is seen to be. 
The embryology of the sporophyte generation has figured largely in 
comparative argument. It is pointed out in Chapter XIV. that the im- 
portance of the earliest stages has been greatly overestimated. It has 
been shown that neither the initial segmentation of the embryo, nor the 
continued segmentation at the growing point bears any constant relation 
to the genesis of appendages, or of specific tissues. It thus becomes 
apparent that the early details of segmentation themselves are not 
