TYPES OF DIMOIU'HTC SPECIALIZATION. 43 



The phenomeiiii of dimorphisni ol' leaves and branches show a 

 general contrast with the phenomena of dichogeny, since they 

 appear to arise from a definite polarity or determination of ex- 

 pression or nonexpression of certain characters that may not be 

 subject to change through the influence of external conditions. In 

 such cases as the dimorphic branches of coftee, cacao, and the Central 

 American rubber tree (Castilla), it is evident that the nature of the 

 organ is definitely predetermined even in the earliest stages of its 

 development. From a bud in a certain position on the internode 

 only one kind of a branch can arise, while another kind of branch 

 comes quite as regularly from another bud in a different position. 

 The lateral or fruiting branches not only do not transform themselves 

 into vegetative limbs but may even be unable to produce new vege- 

 tative shoots from buds. In the cotton plant vegetative shoots can be 

 regenerated from axillary buds of the fruiting branches, but in coffee 

 the fruiting branches can produce only inflorescences or other fruit- 

 ing branches. The same is true of Castifla, except that the fruiting 

 branches nearly always remain simple. In the cacao tree two kinds 

 of branches are even more definitely specialized in their vegetative 

 characters and functions, though both kinds bear inflorescences. 



The word "ropogeny" may sei-ve as a general term to cover such 

 cases of definitely predetermined alternative expression of characters 

 resulting in dimorphism or polymorphism in the branches, leaves, 

 or flowers of the same plant. Ropogeny is to be contrasted with 

 dichogeny, in which the expression of the characters is not definitely 

 determined in the early stages but remains subject to change by 

 environmental influences during the development of the plant. 



Dichogeny and ropogeny, used in these senses, are strictly physio- 

 logical terms. One of the problems in the physiology of reproduction 

 is to understand, as far as possible, how the characters are determined 

 and brought into expression. It is evident from the facts of dichogeny 

 and ropogeny, as well as from the general nature of the processes of 

 development in plants, that expression is dift'erently determined in 

 difl'erent plants and even in different parts of the same plant. Not 

 only is there a general distinction to be drawn between transmission 

 and expression of characters, but different forms of alternative ex- 

 pression have to be recognized. 



The extent to which expression has been modified by specialization 

 does not appear to have any direct relation to the method of pre- 

 determination of the characters. Li some cases all gradations may 

 be traced between normal foHage leaves and minute bracts or bud 

 scales, while in other cases there are definite differences between two 

 kinds of large, expanded foUage leaves as shown in the cacao tree 

 and still more strikingly in the related species Theohroma hicolor. 



221 



