38 DIMORPHIC LEAVES IN RELATION TO HEREDITY. 



may take place in adjoining internocles of the stalk of the same indi- 

 vidual plant. There can be no question, in such a case, regarding 

 the separate transmission of the units of the two contrasted char- 

 acters to different plants. The same plant not only inherits both of 

 the contrasted characters, but brings them both into expression. 

 Such facts may be considered as additional reasons for believing that 

 Mendelian inheritance may be looked upon as a phenomenon of alter- 

 native expression of characters. It no longer seems necessary to 

 predicate an alternative transmission of characters, as often assumed 

 in the study of Mendelism. 



That the phenomena of Mendelian inheritance are of much sig- 

 nificance in the study of heredity need not be questioned, but what 

 the significance may be is still in doubt. It is possible to interpret 

 the facts of Mendelism in at least two very different ways. The 

 mathematical relations of Mendelism are equally well explained, 

 whether ascribed to an alternative transmission of contrasted char- 

 acters or to alternative expression. Neither transmission nor expres- 

 sion is understood in its essential nature — that is, as a physiological 

 process — but this only makes it the more desirable not to confuse the 

 two processes in attempting to understand them. The importance 

 of distinguishing between expression and transmission is not so 

 obvious, perhaps, as long as investigation is limited to cytological 

 and statistical studies of typical cases of Mendelism, but collateral 

 evidence of other kinds should not be neglected. On this question 

 plants seem to afford better evidence than animals because of their 

 habits of growth by the vegetative multiplication of internodes. 

 Among the internode members of the same plant body there can be 

 no question of differences of transmission, yet definitely contrasted 

 expression remains the rule of development. Not only are there 

 abrupt transitions from one class of internodes to another, but the 

 tendency to contrasted expression is accentuated by dimorphic 

 specializations within the same class. 



DIFFERENT TYPES OF DIMORPHIC SPECIALIZATION. 



If the internodes of plants be thought of as individuals, the definite 

 differences that exist between the various kinds of internodes of the 

 same plant appear closely analogous to the contrasted characters of 

 the sexes of the higher animals or the several castes that compose the 

 highly organized colonies of bees, ants, and termites. 



Species that are composed of two or more sexes, castes, or other 

 distinct kinds of individuals have been called "ropic," a term that 

 denotes a definite tendency to contrasted expression of the characters^ 



221 



