INTRODUCTION 



13 



The basic life-cycle, common to bryophytes and pterido- 

 phytes, is represented diagrammatically in Fig. i. Under 

 normal circumstances there is a regular alternation between 

 a gametophyte (sexual) phase and a sporophyte (asexual) 

 phase. The male gametes, produced in numbers from an- 

 theridia, are known as antherozoids, since they are flagel- 

 lated and are able to swim in water, while the female gametes 

 (Qgg cells) are non-motile and are borne singly in flask- 

 shaped archegonia. Fusion between an egg cell and an 



Gametophyte- 



Egg ("V (E) Spermatozoid 



II 



2n 



Zygote Sporophyte 



++^ ® ^ Spores 



-ri • ^(n) 



Meiosis 



Fig. 1 



Life cycle of a homosporous pteridophyte 



antherozoid results in the formation of a zygote, which 

 contains the combined nuclear material of the two gametes. 

 Its nucleus contains twice as many chromosomes as either 

 of the gamete nuclei and it is therefore described as diploid. 

 The zygote develops directly by mitotic divisions into the 

 sporophyte which is, Ukewise, diploid. Uhimately, there are 

 released from the sporophyte a number of non-motile spores, 

 in the formation of which meiosis brings about a reduction of 

 the nuclear content to the haploid number of chromosomes. 

 The life-cycle is then completed when these spores germinate 

 and grow, by mitotic divisions, into haploid gametophytes. 

 In mosses and liverworts, the dominant phase in the life- 

 cycle is the gametophyte, for the sporophyte is retained upon 

 it throughout its Hfe and is either partially or completely 



