ENATION OF LEAF 141 
or_enation! of resions—of—th ficial—tissvre—of—theaxistoLonithemy 
CECI Pe rice bs 
and this would occur not simultaneously but successively, the origin of 
the appendages following the continued apical growth of the axis, as it does 
in the developing shoot of the present day. Lhe—axis_would_pre-exist_in_ 
descent, as it actually does in the normal developing shoot. The origin 
of these appendages may have occurred independently along divers lines 
of descent, and the appendages would in that case be not homogenetic 
in the strict sense. ‘lhus there would be no common prototype of the 
leaf, no morphological abstraction or archetypic form of that part. More 
than one category of appendages might even be produced on the same 
individual shoot, differing in their function on their first appearance. 
Such has perhaps been the case in the Calamarian strobilus, where, as 
will be seen later, the leaf-tooth cannot be readily homologised with the 
sporangiophore. These suygestions will suffice to indicate how elastic a 
strobiloid theory is, and how its-application -will cover various types of 
construction, even such as are shown by the most complex cones of 
Pteridophytes. 
The objection to a theory of enation will probably be raised that it 
contemplates an origin of new parts rather than a modification of parts 
already present, and that experience indicates the latter as the usual source. 
The reply to this is a double one: first, that the appendages actually 
appear in the ontogeny by enation: a leaf arises as an outgrowth from the 
previously smooth surface of the pre-existent axis: the theory reads the 
descent in terms of the individual life. But secondly, an origin of new 
parts upon a smooth surface of a pre-existent part must necessarily have 
taken place frequently in the formation in isolated genera of emergences 
and prickles, often of large size and with vascular supply. Thus the origin 
of new appendages is not without frequently recurring precedent among 
Vascular Plants. 
Ag_essential feature in the theory of the strobilus js that it involves the, 
phyletic pre-existence of the axis. [his is a point upon which embryo- 
logical evidence can be adduced, both that of the primary embryology 
and of the continued embryology of the growing shoot (see Chapter XIV.). 
Thus far nothing has been said of the sporangia in relation to this 
theory of the strobilus. It remains to trace the relation of these to the 
appendages. On the above hypothesis the shoot originated from a body 
having a fertile upper region and a sterile base. It is not necessary to fix 
upon any type of sporophyte represented in any living, plant as a prototype: 
what is contemplated is an acropetally growing body, with already some 
distinction of a sterile base, and a terminal fertile region endowed with 
apical growth. In two or more types of living Bryophytes the relegation of 
spore-production towards the outer surface is seen, with the formation of a 
1The term ‘‘enation” has long been used in Vegetable Teratology. See Masters, 
“Vegetable Teratology,” Woy. Soc., 1869, p. 443: it connotes the exogenous outgrowth of 
an appendage from a previously vacant surface. 
