RELATION TO SEXUAL DIFFERENTIATION. 45 



Ileterohlasty, tlio ])r()du('ti()ii of ii distinct type of juvenile foliage 

 as in Eucalyptus, Juniperus, Pinus, Iledera, and Ficus. 



Homohlasty, the absence of a distinctive juvenile form of foliage. 



Dichogeny, expression of characters not completely determined in 

 early stages, allowing different characters to come into expression as a 

 result of accommodation to different conditions, as in Solanum tuhero- 

 suni, Ranunculus aquatilis, etc. 



Ropogeny, expression of characters completely determined in the 

 early stages, not subject to modification by differences of external 

 conditions, as in the fruiting branches of coffee, cacao, and Castilla 

 that are unable to regenerate vegetative shoots. 



RELATION OF DIMORPHISM TO SEXUAL DIFFERENTIATION OF 



PLANTS. 



Abrupt changes of characters during the development of plants 

 are not limited to these more or less exceptional cases of dimorphic 

 specialization of different kinds of leaves. Even where the leaves 

 are all of one type numerous changes in the expression of characters 

 a¥e required to form the different kinds of floral organs. This require- 

 ment of numerous changes of characters during the process of develop- 

 ment renders the phenomena of heredity in the higher plants some- 

 what different from those that are shown in the higher animals, 

 especially when viewed from a physiological standpoint. 



The fact that many of the higher plants are self-fertilized is often 

 taken to mean that the principle of sexuality is less important with 

 plants than with animals, but this idea represents only a partial 

 view of the facts. The pollen grains and ovules of plants are not 

 only as definitely dift'erentiated as the sex cells of animals, but they are 

 produced by plant individuals that have a sexual dift'erentiation quite 

 as definite as that of the higher animals. 



The plant individual is constituted in a different way from the 

 individual animal, being made up of a large number of internodes or 

 joints often capable of independent existence, if cut apart, or even 

 provided with natural means of separation. In other words, the 

 plant is to be considered as a compound individual or social organiza- 

 tion of numerous internode individuals. The stamens and pistils 

 also represent separate members of the series of internodes that 

 make up the compound plant body. 



The process of conjugation in plants involves the union of sex 

 cells derived from different individuals, no less than in animals. 

 Self-fertilization simply means that crossing is confined to germ cells 

 produced b}^ members of the same i)lant colony. The close association 

 of stamens and pistils in the same flower should not be allowed to 

 conceal the fact that these two types of organs are entirely unlike, 



221 



