I] 



THE ALTERNATING GENERATIONS 



SPORE 



other the spore-bearing generation, or Fern-Plant. If the events above 

 detailed recur in regular succession the two phases of life will alternate. 

 Of these the one bears sexual organs, containing sexual cells or gametes, 

 and it may accordingly be called the 

 gainetophyte ; the other is non-sexual, 

 but bears sporangia containing the 

 spores, and is accordingly called the 

 sporophyte. The study of Ferns at 

 large leads to the conclusion that this 

 regular alternation is typical for them 

 all. These two alternating genera- 

 tions differ not only in form but 

 also in their relation to external 

 circumstances, and especially in the 

 water-relation. The sporophyte is 

 structurally a land-growing plant, 

 with nutritive, mechanical, and con- 

 ducting tissues, and a ventilating 

 system. Not only is it capable of 

 undergoing free exposure to the or- 

 dinary atmospheric conditions, but 

 dryness of the air is essential for the 

 final end of its existence, viz. the dis- 

 tribution of its spores. On the other 

 hand, the gametophyte is structurally 

 a plant ill-fitted for exposure, with 

 undifferentiated and ill-protected 



tissues and no ventilating system, while the object of its existence, 

 fertilisation, can only be secured in the presence of external water, 

 regards the water-relation the whole life-cycle of a Fern might not inaptly 

 be designated as amphibious, since the one phase is dependent on external 

 liquid water for achieving its object of propagation, while the other is inde- 

 pendent of it. 



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 in the sporangium. The 

 sporophyte or Fern-Plant is diploid, and the number of chromosomes is usually 

 very large, being about 90 for Athyriiun : it is 128 in Dryopteris Filix-mas, 

 but only 32 for Marsilia. These numbers are reduced to one half in the 

 tetrad-division of the spore-mother-cells, and the spores on germination 

 produce the gauietopJiyte which is haploid. But in fertilisation, when the 

 gametes fuse, the diploid number is restored. This normal cycle corresponds 



Diagram illustrating the cycle 

 of life of a Fern. 



VIZ. 



As 



