16 PLANT STRUCTURES 



In order to distinguish them the large and passive female 

 gamete is called the oosphere, which means " egg sphere," 

 or it is called the egg ; the small but active male gamete is 

 variously called the spermatozoid, the antherozoid, or simply 

 the sperm. In this book egg and sperm will be used, the 

 names of similar structures in animals. 



In isogamous plants the mother cells (gametangia) 

 which produce the gametes are alike ; but in heterogamous 

 plants the gametes are so unlike that the gametangia which 

 produce them become unlike. Accordingly they have re- 

 ceived distinctive names, the gametangium which produces 

 the sperms being called the antheridium, that producing the 

 egg being called the oogonium (Fig. 10). 



The act of fusing of sperm and egg is called fertiliza- 

 tion, which is the common form of the sexual process. The 

 sexual spore which results from fertilization is known as the 

 oospore or " egg-spore," sometimes called the fertilized egg. 



It is evident that heterogamous plants are bisexual, and 

 bisexuality is not only attained among Algas, but it prevails 

 among all higher plants. Among the lowest forms there is 

 only vegetative multiplication ; higher forms added sexu- 

 ality ; then still higher forms became bisexual. 



16. Summary. Isogamous forms produce gametangia, 

 which produce similar gametes, which by conjugation form 

 zygotes. Heterogamous forms produce antheridia and 

 oogonia, which produce sperms and eggs, which by fertiliza- 

 tion form oospores. 



