118 THE PLANT CELL. 



Ill the Higher Pteridophyta (Ferns), the reproductive cycle is 

 somewhat more complicated, inasmuch as two separate genera- 

 tions are produced. One of these — the sexual form — is known 

 as the gametophyte, and is the product of the germination of 

 the spore, the latter arising in a special organ, the sporangium, 

 which occurs on the fern-plant proper. The other, or asexnal 

 generation, is the sporophyte, and is the result of the fusion of 

 two effective cells produced in special organs, which arise on the 

 under surface of the gametophyte, or prothallus, as it is some- 

 times called. Thus, tabulating as before, the following sequence 

 is noted : — 



>Spore 



I 

 Gametophyte = prothallus = the sexual generation 



$ Effective cell ? Effective cell 



Sporophyte = Fern-plant proper 



= the asexual generation. 



The arrows indicate in both tables the completion of the cycle. 



This reproductive cycle, then, includes two distinct genera- 

 tions, and, for this reason, the Higher Pteridophyta are said to 

 exhibit the phenomenon of an alternation of generations. 



With these few introductory remarks, it is possible to proceed 

 to the study of the formation and maturation of the essential 

 cells in Angiosperms, Gymnosperms, and Pteridophyta, and, in 

 all, the process of fertilisation and formation of the embryo- 

 plant will be shortly described. With regard to the Pterido- 

 phyta, it may be mentioned that two main types can be 

 recognised, viz., one in which only one kind of spore is produced 

 by the sporophyte, or spore-bearing plant, and the other in 

 which two kinds of spore are produced. In the latter, two 

 separate gametophytes or prothallia are formed, and the effective 

 cells thus arise on two separate sexual generations, there being 

 in this case a sort of double alternation of generations. The 

 two types are known respective!)' as Homosporous (one spore 



