250 



PRINCIPLES OF PLANT-TERATOLOGY. 



gamy, and the adventitious prothalli occurring on the 

 sporophylls of Ferns gives rise to aposponj ; both of 

 these represent short cuts in the life-history, and can 

 no more be regarded as constituting a support for 

 the theory of homologous alternation of generations 

 in Vascular Cryptogams, than can the occurrence of 

 adventitious shoots on leaves be regarded as showing 

 an identity in morphological nature between these 

 t wo organs. 



The whole series of adventitious shoots and adven- 

 titious prothalli stand on precisely the same footing, 

 ami must be understood and explained along the same 

 lines. Such shoots are merely those which arise at a 

 later period than their usual time, and out of their 

 normal position. 



The Leaf. 



The German poet Goethe intuitively recognised the 

 root-principle of modern comparative morphology 

 when he regarded, in his ' Metamorphoses of Plants,' 

 all leaves as variants of a typical leaf, though he 

 missed the idea of their common evolutionary origin. 

 Alex. Braun and Goebel are also cited in this con- 

 nection. 



Leaf-abnormalities are grouped under the main 

 headings of Differentiation, Simplification, Adnation, 

 and Change of Position. 



Under " Differentiation " the following phenomena 

 are described : Phyllody of Bracts ; Reappearance of 

 Bracts or Foliage-leaves ; Sarcody of Scale-leaves ; 

 Phyllody of Scale-leaves ; Phyllody of Tendrils ; Phyl- 

 lody of Thorns ; Bracteody of Glands ; Petalody of 

 Scale-leaves ; Staminody of Scale-leaves ; Sporophyl- 

 lody of Scale-leaves ; Division of Simple Leaves, com- 

 prising lateral fission or laciniation and terminal 

 fission or forking ; with the exception of some cases 

 due to depauperization and leaf -forking, the pheno- 

 mena of leaf-division are regarded as reversionary in 



