EVOLUTION OF THE PLANT KINGDOM 421 



Further Expressions of Sexuality. In the evokition of sex the differen- 

 tiation of gametes was soon followed by a differentiation of sex organs. A 

 further stage was the differentiation of structures bearing the sex organs, 

 while a final stage was a sexual differentiation of entire individuals. 



In most liverworts belonging to the Marehantiales the antheridia are 

 borne on male receptacles and the archegonia on female receptacles. 

 These show a marked structural differentiation. In some members, such 

 as Marchantia, there is also a differentiation of individuals, the male 

 plants bearing antheridial receptacles and the female plants archegonial 

 receptacles. In Sphaerocarpus, belonging to another group of liverworts, 

 the male plants are much smaller than the female. Certain species of 

 Oedogonium have dwarf male filaments consisting of only a few cells. 



Although Spirogyra has not reached the level of heterogamy, some spe- 

 cies show a differentiation of sexual individuals. This expresses itself 

 only in the behavior of the gametes, those of one member of a pair of con- 

 jugating filaments being active, while those of the other are passive. The 

 occurrence of distinct male and female individuals is a feature of many 

 heterogamous algae, such as Cutleria, Dictyota, Polysiphonia, and many 



others. 



In plants with an alternation of generations the gametophyte is com- 

 monly called the sexual generation and the sporophyte the asexual one. 

 This misconception arises from failure to regard fertilization and meiosis 

 as complementary processes, both of which are integral parts of a complete 

 sexual life cycle. Vegetative spores are asexual but meiospores are not. 

 Where the zygote directly gives rise to four meiospores, as in Oedogonium, 

 it is easy to associate their formation with fertilization. Where meio- 

 spores are borne on a sporophyte, the time interval between fertilization 

 and meiosis is longer, but the relation between them is the same. 



In the bryophytes and homosporous pteridophytes the sporophyte does 

 not express any sexual characters. But, with the establishment of het- 

 erospory, sexual differentiation becomes extended from the gametophyte 

 to the sporophyte. The significance of heterospory lies in the production 

 of a male gametophyte by the microspore and a female gametophyte by 

 the megaspore. The occurrence of two kinds of gametophytes is reflected 

 in a visible differentiation of the spores. This differentiation may be 

 extended to the sporangia, sporophylls, strobili, and even to the entire 

 sporophyte. In Selagindla the visible consequences of heterospory are 

 not as far-reaching as in seed plants, where the organs associated with the 

 production of microspores (stamens and pollen sacs) and of megaspores 

 (carpels and ovules) are as highly differentiated as the sex organs of bryo- 

 phytes and pteridophytes. Stamens and carpels are not sex organs, but 

 their differences are associated with a sexual differentiation that has been 

 extended to them from the gametes. 



