-U) TIIK .TO UK VAT. OF HnTAXY 



THE LICHEN AS TRANSMIGRANT. 

 By A. II. CfiUBCH. 



(Concluded from p. 13.) 



On tracing back to the sea it begins to be clear that phenomena 

 of intrusion at an unprotected surface are the commonplace of marine 

 biology, otherwise epiphytism is equally prevalent ; since in the 

 abundant life of the sea, as in a tropical rain-forest, everything grows 

 on top of or inside anything else available, in the increasing struggle 

 for some sort of substratum. But the possibilities of penetration are 

 limited by the constitution of the limiting membranes of the organism. 

 This is again but a part of the wider problem which gives so many 

 animal races an external membrane of chitin, becoming an impene- 

 trable exoskeleton, while an internal permeable surface is retained as 

 the alimentary chamber or canal for purposes of food-absorption. 

 Though, in the case of plant-forms, the earliest soft mucilaginous 

 membranes of polysaccharide waste. may be readily penetrable (as in 

 the case of intrusive green algal cells in Schizonema Diatoms), the 

 more resistant polysaccharides of higher plants restrict further pene- 

 tration, and thus acquire a ' protective ' value. Hence, since the 

 provision of a firmer and more coherent and resistant polysaccharide, 

 including a greater percentage of celluloses, follows the evolution of 

 the higher alga in rougher strata of surface-water, so the possibilities 

 of the intrusion of lower alga? become less and less, until its excep- 

 tional occurrence occasions remark ; just as land-plants under natural 

 conditions are practically immune to bacterial attack, though many 

 become susceptible to mass-infection under cultivation, and so appear 

 as a novelty. Further it becomes evident that intrusion will be less 

 and less a general phenomenon as cell-walls become more efficiently 

 protective (though always open to special cases, as in the attack of 

 definitely parasitic epiphytoid races and specialized endophytes), ami 

 will be less and less practicable in higher grades of marine vegetation ; 

 again just as higher animals are attacked most readily from the 

 unguarded alimentary canal or lungs. From this it follows that 

 successful utilization of such a means of existence will not occur 

 in the normally autotrophic races of the sea. That is to say, the 

 Lichen-habit was never evolved in the open sea ; but must be the 

 product of subaerial and transmigrant conditions. That a few minor 

 forms (cf. Lichina) can endure partial submersion does not affect 

 the argument; so can many Angiosperms. Zostera and Posidonia, 

 for example, live permanently submerged, and in deep water ; but no 

 one supposes that they were evolved there originally. The evolution 

 of the Lichen in encouraging and utilizing phenomena of algal intru- 

 sion, en masse, and not by mere chance ' infection,' is thus based on the 

 possibility of penetration at the surface ; and this will become the 

 more possible as the older plant-surface, originally presenting a 

 frontage of 'photosynthetic units in closest contact, or in palisade- 

 series, begins to lose its essential function, and with it the special 

 attributes given by active growth and division. A deteriorated 

 cortical system prepares the way for successful intrusion. It remains 



