202 



PRINCIPLES OF PLANT-TERATOLOGY. 



With regard to the bracts, they are for the most part 

 probably modified foliage-leaves, and in the case 

 of the presence of extra spathes in some of the 

 Aroideae we seem to see the exact process by which 

 the normal spathe arose. 



As will be seen more clearly later on, the various 

 foliar organs of the flower are all probably derived 

 from foliage-leaves or leafy sporophylls, hence any 

 change whereby these latter assume the characters of 

 the former must be a progressive one. As regards 

 ferns, it is not at all improbable that the original type 

 of frond, as in so many modern ferns, was both assimilat- 

 ing and spore-producing, and that the differentiation 

 into sterile and fertile fronds on the same plant is a later 

 modification ; so that the change of a part of a sterile 

 frond into the fertile condition must be a reversionary 

 one. The case of Marsilia hirsuta, however, cannot 

 be wholly reversionary ; nor is it, probably, wholly 

 progressive in nature. 



All cases of lateral fusion of foliar organs, as also of 

 abortion and disappearance thereof, must of necessity 

 be progressive, and in no sense reversionary. 



As regards enations and ascidia, it is difficult to 

 assign an adequate cause for them. The balance of the 

 organ becomes, as it were, upset and the tissue-forming 

 energies materialize in these apparently fantastic shapes ; 

 we shall see later, however, when treating of the flower, 

 that all these structures have a definite meaning, and 

 that they are, in a sense, reversionary and probably not 

 to be regarded as progressive in nature. Moreover, as C. 

 de Candolle points out, they are not isolated and excep- 

 tional structures, but have their normal counterparts 

 here and there in the vegetable kingdom. We shall 

 find them recurring in certain organs of the flower. 



The pitcher or the bladeless extension of the midrib 

 emanating from the lower surface of the lamina must 

 in some instances be regarded as the terminal portion 

 of the leaf whose exceptional position is due to the 

 lateral fusion across the upper face of the leaf of 



