THE LICHEN AS TRANSMIGRANT 45 



JPorphyra umbilicalis (Floridean) at 30 ft. dries so that it crackles 

 when walked over. The desiccation of Pelvetia (Phseophycean), 

 almost to brittleness, is sufficiently familiar on sheltered British 

 coasts. Hence endurance of extreme desiccation on a rocky sea-cliff, 

 may be presumed to follow normal processes of natural selection in 

 any phylum of Alga? or Fungi. The Lichen-soma is by no means 

 unique in this respect, though it may be more familiar to the 

 collector. Endurance of complete air-dry desiccation may be attained 

 readily in the soma as a whole ; just as similar endurance of 

 desiccation becomes the normal condition of the air-borne spore in 

 every group of transmigrant plant-organism. 



Such observations again suffice to indicate the possibilities of 

 nutrition by drifted sea-salt which still characterize lichen-growths 

 facing the sea ; and no one who has seen the shaggy covering of 

 Lichen on the cliffs facing the Atlantic, can hesitate to believe that 

 this is the primary transmigrant habit of the Lichen-series as a whole. 

 They constituted one of the pioneer races of the older world, on the 

 first rock-surfaces exposed above the retreating sea ; and they have 

 held their station to the present time, since no other plant-organism 

 can compete with them in endurance on such a feeble food-supply. 



It will be noted that from the present standpoint, the precise 

 nature of the intrusive algae becomes a matter of relatively very sub- 

 sidiary importance, so long, that is to say, as it may be of discrete or 

 Protococcoid habit ; while, in the case of transmigrant races, a change 

 of helot intruders becomes not only conceivable but probable. Once 

 the lichen-habit is established and becomes obligatory for these special 

 fungus-migrants, it is possible that any readily available algal units 

 may be utilized for the purpose, with little consequent effect on the 

 morphology of the dominant fungus. These generalizations acquire 

 additional interest in view of recent observations by Paulson (J. L. S. 

 (1920), p. 504) on the utilization of forms of Chlorella in many 

 common Lichens, which have long passed as symbiotic with Plei/ro- 

 coccus ( Ci/stococcus), and what may be even in all cases the algal 

 component of Ph yscia parietina, as described in the classical synthesis 

 by Bonnier. The suggested evolution of a 'primitive' Lichen in 

 fresh-water ponds of the present day, 1 or the synthesis of Physcia on 

 the green dust of a tree-trunk in one's back garden, may serve to 

 visualize some points of the process, but affords no adequate appre- 

 ciation of the facts of the evolution of this remarkable race as a 

 whole, as probably the oldest surviving phase of land-vegetation, — and 

 also, in the more alga-like somata of fruticose forms, possibly the least 

 changed of any subserial types from a period beyond the range of 

 stratigraphical geology. It is considerations as these which lend 

 interest to what is otherwise one of the least considered groups in the 

 range of botany. As the Brown Seaweeds, admitted often grudgingly 

 and as a duty by old collectors to the repose of the Seaweed Album, 

 afford the clue to the evolution of the parenchymatous land-plant, so 

 the Lichen undoubtedly affords the best key to the development of 

 all Fungi ; since while the latter pass on to the land and fresh-water, 



1 Acton (1909), Annals of Botany, xxiii. p. 579, ' Botrydina, a primitive 

 Lichen.' 



