LIFE HISTORY OF A FERN 



173 



with the sperm, possesses a double or diploid nature.^ 

 In recognition of its dual nature it is called the oosperm 

 (egg and sperm). ^ The oosperm, however, Uke the un- 

 fertilized egg, is still only one cell, though its nucleus com- 

 prises substances contributed by both egg and sperm. 

 In some cases the egg and sperm that unite in fertilization 

 may come from different parents; their fusion is then 

 called cross-fertilization. 



Fig. 131. — Young embryo of a maidenhair fern {Adianlum concinnum), 

 still surrounded by the archegonium, which has grown in size. L, leaf; 

 5, stem; R, root; F, foot. (After Atkinson.) 



158. Development of the Fertilized Egg. — After fertili- 

 zation the egg begins to develop, undergoing a series of 

 nuclear and cell-divisions, accompanied by increase in 



1 As distinguished from the unfertilized egg, which is of a single, or 

 haploid nature. 



' The term oospore is often used here, but this term lacks the advan- 

 tage of indicating the real nature of the fertilized egg. 



