168 



PRINCIPLES OF PL ANT- TERATOLOGY . 



but deeply lobecl. The leaf of the orange (Citrus) nor- 

 mally consists of a petiole bearing a single leaflet ; it is 

 a case of a compound leaf reduced to its simplest form; 

 abnormal cases are known, especially in seedlings, in 

 which there are several leaflets present, which is an 

 indication of what the ancestral leaf was like. Other 

 compound leaves are known to become still more com- 

 pound ; in some Leguminosse it has been observed 

 how simply-pinnate leaves, e. g. those of the carob 

 tree (Geratonia Siliqua), become bipinnate, owing to 

 the replacement of all or some of the leaflets by 

 pinnate leaves. 



Then again, in some of the Australian acacias in 

 which normally phyllodes, or flattened petioles, only 

 are produced, these, on some twigs, may be replaced 

 by bipinnate leaves. This feature always appears 

 normally in the seedling. 



b. Terminal Fission or Forking. — This is in all cases 

 due to a kind of hypertrophy, and an attempt on the 

 part of the organ to reproduce itself. The phenomenon 

 of leaf -forking is one of the most widely-spread in 

 the vegetable kingdom, occurring in nearly all orders of 

 plants. It will be referred to again under the head of 

 Phyllotaxis. Bifurcation of the leaf may be only par- 

 tial or it may be complete ; in the latter case it results 

 in the formation of two ] eaves where previously there 

 was only one. 



Partial bifurcation of the main leaf is beautifully 

 exemplified in the ferns, where we see how common 

 it is for the main frond to be forked more or less 

 deeply at the apex, the forking rarely if ever ex- 

 tending to more than half the length of the frond, 

 e. g. in the hart's-tongue. Each of the two portions 

 thus produced may themselves become forked, and the 

 products forked again, and so on, until the "crested" 

 form is attained (PI. XIV, fig. 1). 



Forking of the sterile portion of the frond has been 

 observed in the moonwort (Botrychnnn), but it is a rare 

 phenomenon in the Ophioglossacese. 



