APPENDIX 



POSTSCRIPT TO CHAPTER VIP 



In a remarkable paper on The Symbiosis of Lichens'^, Dr A. Henry Church 

 has presented a new and striking view of the origin and development of 

 lichens: he has sought to link them up with other classes of vegetation that, 

 in the great transmigration, passed from sea to land. As we know from his 

 Thalassiophyta^ and the subaerial transmigration, he holds that primeval 

 algae of advanced form and structure were left exposed on dry land 

 by the gradually receding waters, and those that successfully adapted 

 themselves to the changed conditions formed the basis of the land flora. 

 A certain number of the algae lost their surface tissues containing chlorophyll 

 and they had perforce to secure from other organic sources the necessary 

 carbohydrates : they adopted a heterotrophic existence as saprophytic or 

 parasitic fungi. Fungi are a backward race (deteriorated according to 

 Dr Church) as regards their soma, but in number, distribution and variety 

 of spore-production, they are eminently successful plants. 



Lichens are similarly regarded by Dr Church as derived from stranded 

 contemporaneous types of marine algae — crustaceous, foliose and fruticose, 

 that had also lost their chlorophyll, but by taking into association green 

 algal units of a lower grade they established a vicarious photos3mthesis. 

 But, to quote his own words*, " as the alga-lichen-fungus left the sea, so it 

 remained : it might deteriorate, but it certainly never advanced, once the 

 sea factors which produced it were eliminated, it simply stopped along 

 these lines." 



And again^ : " Lichens thus present an interesting case of an algal race 

 deteriorating along the lines of a heterotrophic existence, yet arrested, as it 

 were, on the somatic down-grade, by the adoption of intrusive algal units 

 of lower degree to subserve photosynthesis (much in the. manner of the 

 marine worm Convoluta). Thus arrested, they have been enabled to retain 

 more definite expression of more deeply inherent factors of sea-weed habit 

 and construction than any other race of fungi; though closely paralleled 

 by such types as Xylaria (Ascomycete) and Clavaria (Basidiomycete), 

 which have followed the full fungus progression as holosaprophytic on 

 decaying plant residues." 



Dr Church's theory is of vivid interest and might be convincing were 

 there no possibility and no proof of advance within the symbiotic plant, but 



' See p. 302. 2 journ. Bot. Lvni. pp. ■213-9; 262-7, 1920. ^ Bot. Memoirs, 3, Oxford, 1919. 

 •> Church !K lift. ^ Journ. Bot. I.e. 



