784 LIFE : OUTLINES OF GENERAL BIOLOGY 



plants." Pre-Darwinian as it was, Hofmeister's picture of genetic 

 affinity between Cryptogams and Phanerogams was a brilliant 

 suggestion of Evolutionism. 



Much has been discovered since Hofmeister's day, but the general 

 result has been confirmatory. Among the steps may be noticed the 

 discovery that the sporophyte generation, resulting from a fertilised 

 egg-cell, has the diploid (2n) number of chromosomes, while the 

 gametophyte, resulting from an unfertiUsed spore, has the haploid 

 (w) number. A reduction or meiotic division, halving the number of 

 chromosomes, takes place in the formation of spores (usually four) 

 from a spore-mother-cell. 



Very interesting was the discovery, by Ikeno and Hirase, and by 

 Webber, of multiciliate sperm-cells which swim out from the pollen- 

 tube of Cycas and the Maidenhair Tree (Gingko) and link these 

 seed-plants back to ferns. 



Improvements in methods have made it possible to detect macro- 

 and mega-sporangia and dimorphic spores in many extinct types; 

 and while a unified genealogical tree of plants is still a dream- 

 picture, various distinct lines of evolution are becoming clearer and 

 more continuous. 



Of importance also has been the disclosure of such short-circuiting 

 as apospory and apogamy, and of strange phenomena like xenia — 

 where the pollen nucleus that unites with the endosperm nucleus 

 affects the character of the seed-tissue, just as its fellow, which 

 unites with the egg-cell, affects the embryo. 



THEORY. — The question rises as to the meaning of this wide- 

 spread phenomenon of metagenesis or alternation of generations, 

 and the problem must be considered: (i) in relation to other ways 

 in which life-histories are complicated by emphasising or sub- 

 ordinating, elongating or reducing, arcs in the ordinary curve of 

 life; (2) in relation to the fact that the sporophyte is diploid and the 

 gametophyte haploid; and (3) in relation to the advantages that 

 may be discovered in the alternation of sporophyte and gameto- 

 phyte. The difficulty of the problem is increased by the fact that 

 while the conspicuous chapter in the life-history of. say, a fern is the 

 sporophyte, the conspicuous chapter in the life-history of a moss 

 is the gametophyte. Furthermore, in some Algae and Fungi, where 

 the two phases live in similar conditions, there may be no contrast 

 between sporophyte and gametophyte except as regards the number 

 of chromosomes and the reproductive organs. How great the differ- 

 ence between this and the extreme dimorphism in higher forms, 

 whether Ferns or Flowers! 



It is possible that the primitive sequence was Sporophyte-Spore- 

 Sporophyte, and that the interpolation of a Gametophyte was 

 justified because it secured sexual reproduction and in particular 



