46 DIMORPHIC LEAVES IN RELATION TO HEREDITY. 



not only with respect to their products of pollen grains and ovules, 

 but in other characters. The same freedom of change and contrast of 

 characters apparent in the external visible features may be supposed 

 to exist in mternal characteristics of the germ cells. 



Plants that produce both stamens and pistils in the same flowers 

 are often described as hermaphrodite, ])ut this normal bisexual 

 condition should not be confused with an abnormal, partial, or 

 intermediate expression of the characters of both sexes in the same 

 individual, as sometimes occurs among sexually differentiated ani- 

 mals. In normally bisexual ])lants, on the contrary, the characters 

 of both of the sexes are fully expressed in the separate individual 

 members of the colony. Abnormal hermaphroditism, like that of 

 animals, is shown in plants in the rare cases of malformed organs 

 intermediate between stamens and pistils. The abnormal organs 

 heretofore mentioned (p. 22) as intermediate between stamens and 

 petals represent a similar failure of complete change in the expression 

 of contrasted characters, as also occurs in abnormal intermediate 

 forms of branches. 



Morphologists may object that the higher animals, as well as the 

 higher ])lants, have a segmental or metameric structure in the sense 

 that their bodies are made up by the union of structural elements 

 corresponding to the more distinctly segmental bodies of the lower 

 groups of animals. But whatever stress may be laid upon this 

 idea from the standpoint of morphological theories, it is evident that 

 the physiological differences are profound, involving different rela- 

 tions among the primitive segments and different requirements for 

 changes in the expression of the hereditary characters during the 

 processes of development. The processes of heredity, as shown in 

 the formation of the segments, might be described as simultaneous 

 in animals and successive in plants. 



The segmental growth of the animal body is determinate at a very 

 early stage, long before the growth in size is completed. In the 

 higher animals the determinate condition is shown most definitely 

 in the female sex, the whole complement of ovules being formed 

 while the animal is still in an embryonic stage of development. In 

 bees and related insects the male sex is more determinate than the 

 female. The plant body, on the other hand, begins ^nth only one 

 or two segments and adds the others gradually during the process of 

 growth. The individual stamen or pistil of a })lant is determinate, 

 but most plants can produce an indefinite succession of stamens and 

 pistds as well as of vegetative internodes. 



Plants grow chiefly by successive additions of segmental imits. 

 The striking fact about the successive additions of new structural 

 units to the plant body is that they are not all alike but are capable 



221 



