506 BOTANY OF THE LIVING PLANT 



the whole life-cycle of a Fern, or of Pteridophytes generally, might 

 not inaptly be designated as amphibious, since the one phase is 

 dependent on external fluid water for achieving its object of pro- 

 pagation, while the other is independent of it. 



fporvpfit/tic 

 buaefina 



POROPHYTE 



ZYGOTE 

 /t 



rfnf/iecu/ium 



/IreheaoMum 



Oametophyte 



■budlavuj 



Fig 400. 

 Diagram illustrating the cycle of life of a Fern. 



The normal cycle thus presented to the eye involves differences of nuclear 

 condition of the alternating phases, those differences being established re- 

 spectively by fertilisation and by the tetrad-division. The sporophyte or 

 Fern-Plant is diploid, and the number of chromosomes is usually very large 

 (about 90 for Athyrium, 144 for Dryopteris pseudo-mas, but 32 for Marsilia). 

 This number is reduced to one-half in the tetrad-division of the spore-mother- 

 cells, and the spores on germination produce the gametophyte which is haploid. 

 But in fertilisation, when the gametes fuse, the diploid number is restored. 

 This normal cycle corresponds to that seen in higher forms, the substantive 

 Plant being in all cases the diploid sporophyte. 



The cycle as thus defined is liable to certain modifications. Some involve 

 the introduction of new incidents, others the excision of certain phases. 

 For instance, buds may be produced either on the Fern-Plant or on the pro- 

 thallus, which repeat respectively the one or the other (Fig. 386, p. 494). These 



