Feb., 1922 sexual evolution in the plant kingdom 105 



6. Hermaphroditic condition with a definite dimorphism 

 showing in the gametangia but not in the contiguous regions or 

 only to a very slight extent. In these plants the sex is deter- 

 mined in the incipient gametangia. 



Examples: Monoblepharis, Vaucheria sessilis, Coleochsete. 



7. Hermaphrodites having a greater or less area of tissue 

 immediately beyond the gametangia proper showing sexual 

 dimorphism. The sexual state is determined in the tissues 

 before the definite origin of the gametangia and so influences 

 the morphological expression in the tissues below them. 



Examples: Chara, Nitella. 



8. Hermaphrodites having considerable areas of their 

 bodies differentiated as male or female tissues or branches from 

 which only spermaries or ovaries are produced. The sex is 

 determined in the vegetative tissue or branch long before 

 gametangia are developed. 



Examples: Vaucheria synandra, Oedocladium protonema. 

 Fucus evanescens. 



9. The culmination stage in the series represented by uni- 

 sexual individuals of various degrees of intensity and stability 

 of the male or female state and showing some dimorphism in 

 the entire individual. The sexual state in such individuals is 

 determined either in the gametes before fertilization, in the 

 zygote, or in a very early stage after germination of the zygote. 

 There are probably two progressive phases, which, however, can 

 usually be determined only by experiment: (a) Species in 

 which sexuality is not strongly fixed, that is the hereditary con- 

 stitution is such that a reversal of the sexual state is readily 

 brought about in the individual, (b) Species in which the sexual 

 state, either male or female, seems to be strongly fixed in the 

 individual, so that it becomes difficult to change from one sexual 

 state to a neutral state or from one sexual state to the other. 



Examples: Oedogonium lautuniniarum, Fucus vesiculosus, 

 Mucor stolonifer (specialized sexual condition). 



HI. HOMOSPOROUS SERIES WITH ANTITHETIC ALTER- 

 NATION OF GENERATIONS. 



In this series the sporophyte shows no sexual differentiation 

 whatever, but in some species there may possibly be a physi- 

 ological difference in the spores with some cytological dimor- 



