THE LICHEN LIFE-CYCLE 21 i) 



interest lies in the determination of the lines of progressive failure in 

 older established mechanism once initiated in the sea 1 . 



The general aspect of the life-cycles of all these series is sufficiently 

 clear. No plant-phylum has made yood in suhaerial environment, 

 unless it had previously, in the sea, attained to a two-phase life- 

 cycle, in which the asexual spores, no longer water-demanding, might 

 be utilized as perennating, more or less air-dried, and protected 

 dispersal units. But in the rigour of the new conditions of desicca- 

 tion, and possible failure of nutritive metabolism over long periods, 

 every phase of somatic and reproductive deterioration becomes 

 possible. With the ultimate failure of the original aquatic mechanism 

 of cross-fertilization, conditions of autogamy may still be successful 

 in lesser degree ; as the latter in turn may be abbreviated by pseudo- 

 gamy, affording a last possibility in retaining a certain amount of 

 meiotic benefit of the subsequent spore-stage. If this cannot be 

 done, the asexual generation may continue as a deteriorated haploid 

 expression, or else it is obliterated. With it goes the initial possi- 

 bility of free dispersal by airborne-spores, and the further progression 

 of the organism as a ' higher ' land-plant is closed for ever. 



The gametophyte generation alone can only endure as a land- 

 plant by vegetative methods of propagation ; and that such may be 

 possible to a practically indefinite extent is shown by the prepon- 

 derance of adaptive ' conidial ' stages in many saprophytic Fungi 

 {Penicillium, Eurotium), as also by the full multiplication of many 

 Lichens by soredia, — -a lucky solution of the problem which probably 

 just saves many races ; though, as in the case of the conidium- 

 producing fungus, leaving the type in a wholly elementary phase 

 of reproductive progression, without the benefit of sexual fusion and 

 its meiotic consequences. As the gametophyte by itself is thus 

 useless for further upward progression in competition with better 

 equipped races, so the sporophyte of such fungus-series fails to make 

 any further advance when left alone (Exoascus) ; in that it had 

 already lost in the sea (under conditions parallel with the case of the 

 Florideae) the somatic organization of this decadent and parasitic 

 phase following fertilization in situ 2 . It remains interesting to note 



1 For a good account of the substitution of vegetative growths and fusions 

 for the original sexual organs, cf. Polystigma rubrum (Blackman and Welsford, 

 Ann. of Bot. 1912, p. 765), which probably gives the key to a wide range of 

 phenomena of pseudogamic deterioration. 



For interesting example of a mechanism for secondary autogamy in Collema 

 ptdposvm, cf. Bachmann, Ann. of Bot. 1912, p. 753. 



- Thus Exoascus has admittedly lost its sexual phase and appears now as a 

 mere normal hymenium of asci. The spores readily germinate in sprouting form, 

 the sprouts being readily-detached conidial-ramuli of algal mechanism. Yeast- 

 fungi (Saccharomycetes) apparently represent a retention of this last stage of 

 vestigial somatic organization, and are by no means ' elementary ' organisms. 

 It is certainly a far cry from a complex autotrophic marine alga to a vestigial 

 heterotrophic sprouting fungus, living most precariously for a short time in the 

 year on the surface of fruits of higher land-flora ; but it may be also pointed out 

 that the fusions of ' conjugating ' Yeasts, often described as ' hologamic ' (cf. 

 Guilliermond (1913), Rei Progressus,p. 434) are preferably to be regarded as 

 indicating the fusions of ramuli, which come directly into line with the generally 

 accepted phenomena of paendugamij. 



