EVOLUTION OF THE PLANT KINGDOM 419 



amy the differences between the two kinds of gametes merely become 

 apparent, so that they are recognizably male and female. 



Isogamy represents the original condition of gametic union. It has 

 been retained by many green and brown algae. These groups display 

 various degrees of heterogamy, however, indicating that this condition 

 has been derived from isogamy by a differentiation of gametes into sperms 

 and eggs. In isogamy, as a rule, both kinds of gametes are motile and 

 equally small, neither containing much food. In heterogamy there is a 

 division of labor, the sperm providing motility and remaining small, the 

 egg providing food and becoming large. Its advantage lies in the greater 

 supply of food available for the zygote and the young plant that develops 

 from it. In the red algae, where heterogamy is universal, the sperm is 

 nonmotile but much smaller than the egg. Heterogamy is estabhshed in 

 all the higher plants, with swimming sperms occurring in all bryophytes, 

 all pteridophytes, and a few gymnosperms. 



Evolution of Sex Organs. The production of gametes in ordinary 

 vegetative cells is characteristic of most of the green algae. In isogamous 

 forms these cells remain unchanged, while in nearly all heterogamous 

 forms they become modified in size and shape. Thus there are not only 

 two kinds of gametes but two kinds of gametangia, the sperms arising in 

 antheridia and the eggs in oogonia. A differentiation of gametes has been 

 accompanied by a differentiation of sex organs, but the gametes develop 

 from the protoplasts of vegetative cells. 



In a few green algae, such as Vaucheria, in the Charophyceae, and in 

 r. early all the brown and red algae a more advanced condition has been 

 reached. Here the gametes are borne in sex organs that have never been 

 a part of the vegetative body, but arise as special reproductive branches. 

 A differentiation has taken place between cells that remain entirely veg- 

 etative in function and those that are strictly reproductive. Although 

 this condition is found mainly among heterogamous algae, it occurs in a 

 few isogamous members of the Phaeophyceae, such as Ectocarpns, where 

 gametangia are developed on special branches. 



Thus among the algae three stages may be recognized in the evolution 

 of sex organs, depending upon whether gametes are produced in (1) an 

 unmodified vegetative cell, (2) a transformed vegetative cell, or (3) a 

 special reproductive cell distinct from the rest of the body. The first 

 stage is characteristic of isogamous forms, the second and third of het- 

 erogamous forms. 



The sex organs of the bryophytes and pteridophytes are more highly 

 developed than those of the algae in that both kinds are multicellular and 

 have an outer jacket of sterile cells usually forming a single layer. The 

 sterile jacket, which protects the gametes from drying out, was probably 



