Feb., 1922 sexual evolution in the plant kingdom 109 



Examples : Acer platanoides, Acer rubrum. 



There are, of course, interesting intermediate developments, 

 as for example one type of individuals may be monecious and 

 the other purely staminate. An example of this condition is 

 Arisaema dracontium. 



22. Diecious sporophytes with complete or nearly complete 

 suppression of the opposite set of organs, in normal cases in all 

 the flowers of an individual, and having a more or less decided 

 vegetative dimorphism. These diecious sporophytes often have 

 a considerable percentage of intermediate individuals or are 

 known by experiment to show frequent reversals in sexual 

 expression, either through mutations, or through the influence 

 of the environment. 



Examples: Arissema triphyllum, Morus alba, Morus rubra, 

 Cannabis sativa, Humulus japonicus, Mercurialis annua. 



23. The last and most extreme type with diecious sporo- 

 phytes apparently without intermediates and not readily or 

 not at all reversible in sexual expression. When the sexual 

 state is once established, the staminate plants remain pure 

 staminate and the carpellate pure carpellate. 



Examples: Acer negundo, Populus deltoides, Fraximus 

 americana. 



Note : As stated, by such species is represented the extreme 

 limit of evolution in relation to sexual expression. The gameto- 

 phyte consists of pure male and female individuals apparently 

 irreversible in the normal environment, and the sporophyte 

 consists of pure staminate and carpellate individuals whose 

 sexual state is apparently fixed in their normal environmental 

 conditions. It is probable, however, that all diecious sporo- 

 phytes will be found reversible as to sex under the proper 

 environment during development. In these plants both the 

 gametophytes and sporophytes are dimorphic. The sexual 

 state of the gametophyte is determined in the previous sporo- 

 phyte generation and the sexual state of the sporophyte is 

 determined either in the gametes before fertilization, in the 

 zygote at the time of fertilization, or in the embryo after the 

 germination of the zygote. 



