C. J. Bond 347 



The type of inflorescence and the type of flower may vary in the 

 same individual plant, and the question arises whether the Begonia 

 (and other monoecious plants) may not have attained their present 

 monoecious condition in two ways: (1) by diverging from the herma- 

 phrodite stage along primarily female lines as far as the flower is concerned, 

 and (2) by developing along primarily male lines as far as the inflor- 

 escence is concerned. In the walnut and some other monoecious trees 

 the occasional occurrence in some individual trees of a rudimentary 

 gynoecium in a central position in a male floret, suggests that these 

 plants have passed through a hermaphrodite stage in which the flowers 

 were built up on a primarily female type. 



Sex Dimorphism thus becomes a problem of (1) the kind of units 

 among which the sex differentiating process takes place, and (2) the 

 period in the life history of these units at which this process occurs. If 

 the diffei-entiating cell division occurs in the germ cell stage the 

 dioecious variety of sex dimorphism results, if it occurs a little later 

 during the development of the flowers the monoecious form results, while 

 if it occurs at a still later stage during the development of the sex 

 organs on the common floral axis the hermaphrodite flower is formed. 

 We are ignorant of the factors which determine the stage at which this 

 all-important segregation shall occur, but of the three stages at which 

 it has up to the present taken place in the history of plant evolution, 

 the middle or monoecious stage seems to have been the most unstable 

 of the three. For it is in monoecious plants that we find the greatest 

 number of vestigial remains or rudimentary organs of the opposite sex. 

 In some monoecious plants, e.g. the Box, indications exist that the 

 plant is now on the way to or away fi-om the hermaphrodite condition. 



It is a matter of considerable interest to find that in plants as 

 in animals a close association exists between a sex abnormality like 

 hermaphroditism and a delayed occurrence of those differentiating cell 

 divisions which determine sex. Postponement of the sex differentiating 

 cell division from the germ cell to the zygote stage may not only bring 

 about hermaphroditism in the individual, but the actual period in the 

 development of the zygote at which the differentiation of sex organs 

 occurs also influences the type of hermaphroditism which results, for 

 instance, if the differentiation of the factors for maleness and femaleness 

 is postponed to the stage of blastomeric division, in which the right and 

 left halves of the animal body are laid down, then a lateral gynandro- 

 morph is produced. Many such cases have been recorded among insects, 

 and quite a number have now been recorded among the vertebrates. 



