ENATION OF LEAF 141 



or enation ' of regions of the superficial tissue of the axis to form them, 

 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. The 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 he not homogenetic 

 in the strict sense. Thus 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 suggestions 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. 



An essential feature in the theory of the strobilus is that it involves the 

 phyletic pre-existence of the axis. This 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 



1 The term "enation'' has long been used in Vegetable Teratology. See Masters, 

 "Vegetable Teratology,'' l\oy. iVv., 1869, p. 443: it connotes the exogenous outgrowth of 

 an appendage from a previously vacant surface. 



