152 



PRINCIPLES OF PLANT-TERATOLOGY. 



both in the number and size of organs, has played a 

 large part in evolution in the past ; this becomes clear 

 when we consider the derivation of the Cycads from 

 the ancient Pteridosperms (Cycadofilices). In these 

 the foliage-leaves were large and fern-like ; in their 

 descendants they are reduced in size and in complexity 

 of organization. It is possible, although we know 

 nothing of the ancestry of Angiosperms, that in these 

 also reduction has played a large vole. In this section 

 of the work it will be assumed that a well-developed 

 foliage-leaf is the type or ancestor from which all other 

 kinds of vegetative foliar organs of the present day 

 have been derived, and so far this view agrees with that 

 of Goebel, although he cannot surely have any right to 

 assume that foliage-leaf nature inheres in every rudi- 

 ment, for the modification into bract, scale-leaf, etc., 

 must in most cases be congenital. 



Now, if it be a true theory that the foliage-leaf is the 

 original ancestor of all other forms of vegetative leaves, 

 we should expect to find here and there tendencies to 

 reversion to the jDarent-form. And this is what we 

 actually find. 



All leaf -abnormalities may be grouped under three 

 main headings: Retrograde Metamorphosis, Progres- 

 sive Metamorphosis, and Change of Position. 



1. DIFFERENTIATION. 



The reduction in size and complexity of organization, 

 and the alteration of consistence, have produced many 

 and various forms of vegetative foliar organs which 

 have become fixed and stereotyped in the life-history 

 of the plants producing them; and it requires an ex- 

 ceptional disturbance either in the individual plant or 

 organ, or in the environment, or both, to obtain a 

 release from this fixity, and a reversion, partial or 

 complete, towards the ancestral form. 



1. Ph^llody of Bracts. — Bracts are the leaves which 

 subtend a flower or an inflorescence. They may be 



