348 BOTANY OF THE LIVING PLANT 



ventilating system, while the object of its existence, viz. fertilisation, 

 can only be secured in the presence of external fluid water. As 

 regards the water-relation the whole life-cycle of a Fern, or of Pteri- 

 dophytes generally, might not inaptly be designated as amphibious, 

 since the one phase is dependent on external fluid water for achieving 

 its object of propagation, while the other is independent of it. 



^ l>uadim 



SPORE 



ZYGOTE 



A 



/Irrfheru/ium 



Oametophyte 



■l/u/Jina 



Fig. 291. 

 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 Lastraea pseudo-mas, but only 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 Hable to certain modifications. Some are by 



