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 " Selago" 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 in 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 



