348 



BOTANY OF THE LIVING PLANT 



ventilating system, wliile 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 amphihiouSf 

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

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



SPORE 



ZYGOTE 





Oametophyte- 



7f 





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 Athyyium, 14^ lor Lastraea pseudo-mas, but only 32 for MarsiHa). 

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

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

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

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

 Plant being in all cases the diploid sporophyte. 



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



