44 THE JOUHXAI, nl BOTAUT 



adequate absorption from the substratum. In this way the evolu- 

 tionary history of the Lichen-association which suggests its possi- 

 bilities, also equally defines its limitations, and at the same time 

 covers the range* of progression for all forms following a similar 

 sequence of biological factors. Summarizing the stages briefly, it may 

 be said that : — 



(1) Failure of oxygen -supply in standing pools of sea-water was 

 the most readily conceivable cause of the death of the autotrophic 

 surface-tissues of the original sea-weed, thus leaving the thallus 

 penetrable. 



(2) Competition for substratum gave the mantle of green intru- 

 sive ' gonidia ' of the type of Ghlorella. 



(8) The oxygen -problem may be also responsible for the greater 

 success of the associated organisms on leaving the water for sub- 

 saturated air ; though the cause of the plants being so exposed may 

 be merely the expression of the pools drying up. 



(4) The nitrogen-problem still keeps the plants impoverished; as 



(5) The water-problem tends to keep them small and restricted 

 to short seasonal periods, thus further delaying the rate of growth. 



The last again common to all land-vegetation of whatever grade, and 

 theoretically a factor of less importance than is generally admitted in 

 considerations of the progression of Land-Flora. 



Ability to withstand the extreme desiccation of wind and hot sun 

 to a condition of brittleness, often popularly regarded as distinctive 

 of the vitality of Lichens, not only from the standpoint of the 

 Fungus-constituent, as from that of the included 'protected' Alga?, 

 is by no means exceptional, or confined to members of this group. 

 Even high-grade Basidiomycetes of the Polyporus and Agaricus 

 model will similarly endure desiccation in the soma as a whole, as a 

 part of their normal biological equipment; rapidly recovering and 

 even renewing the delicate mechanism of spore-discharge on being 

 wetted. Thus Buller (1909) l describes Polystictus hirsutus and 

 Polyporus rigens renewing somatic growth afjter being kept air-dry 

 for a vear. Marasmius oreades was capable of recovery after weeks, 

 and renewed full activity of spore-discharge in a few hours after being 

 wetted. Other species of Merulius, Polystictus, Lenzites, and 

 Dadalea, kept dry for 2 years and months, only required about 

 4 hours' wetting to recover. 2 For marine alga? Borgesen (1908) 3 gives 

 Hildenbrandtia (Floridean and encrusting disc-type) grading into 

 lichen-mixtures at 200 ft. above high-water mark on exposed Atlantic 

 coasts. Enteromorplia intestinalis flourishes as a transmigrant in 

 fresh-water streams, attaining a length of 6 ft. in running water; 

 yet on exposed cliffs it covers large areas of rock-face brigbt green 

 where sea-birds congregate (Rockall), 3 and will endure desiccation 

 until it may be powdered. Unteromorpha and Prasiola (Chloro- 

 phycese) live abundantly exposed to fog, rain, and spray as the only 

 source of water and food-salts, 40 ft. above the sea at Faeroe ; and 



1 Buller (1909), Researches on Fungi, p. 106. 



2 Loc. cit., p. Ill, for list of times and species. 



3 Borgesen (1908), Botany of the Faeroes, p. 770. 



