ASSIMILATION AND RESPIRATION 239 



Jumelle 1 to continue to 10, but assimilation was still possible at a tem- 

 perature of 40: Evernia prunastri exposed to that extreme degree of cold, 

 but in the presence of light, decomposed carbon dioxide and gave off 

 oxygen. 



B. INFLUENCE OF MOISTURE 



a. ON VITAL FUNCTIONS. Gaseous interchange has been found to vary 

 according to the degree of humidity present 1 . In lichens growing in sheltered 

 positions, or on soil, there is less complete desiccation, and assimilation and 

 respiration may be only enfeebled. Lichens more exposed to the air those 

 growing on trees, etc. dry almost completely and gaseous interchange may 

 be no longer appreciable. In severe cold any water present would become 

 frozen and the same effect of desiccation would be produced. At normal 

 temperatures, on the addition of even a small amount of moisture the 

 respiratory and assimilative functions at once become active, and to an in- 

 creasing degree as the plant is further supplied with water until a certain 

 optimum is reached, after which the vital processes begin somewhat to 

 diminish. 



Though able to exist with very little moisture, lichens do not endure 

 desiccation indefinitely, and both assimilation and respiration probably cease 

 entirely during very dry seasons. A specimen of Cladonia rangiferina was 

 kept dry for three months, and then moistened : respiration followed but it 

 was very feeble and assimilation had almost entirely ceased. Somewhat 

 similar results were obtained with Ramalina farinacea and Usnea barbata. 



In normal conditions of moisture, and with normal illumination, assimi- 

 lation in lichens predominates over respiration, more carbon dioxide being 

 decomposed than is given forth; and Jumelle has argued from that fact, 

 that the alga is well able to secure from the atmosphere all the carbon 

 required for the nutrition of the whole plant. The intensity of assimilation, 

 however, varies enormously in different lichens and is generally more powerful 

 in the larger forms than in the crustaceous : the latter have often an extremely 

 scanty thallus and they are also more in contact with the substratum rock, 

 humus or wood on which they may be partly saprophytic, thus obtaining 

 carbohydrates already formed, and demanding less from the alga. 



An interesting comparison might be made with fungi in regard to which 

 many records have been taken as to their possible duration in a dry state, 

 more especially on the viability of spores, i.e. their persistent capacity of 

 germination. A striking instance is reported by Weir 2 of the regeneration of the 

 sporophores of Polystictus sanguineus, a common fungus of warm countries. 

 The plant was collected in Brazil and sent to Munich. After about two years 

 in the mycological collection of the University, the branch on which it grew 

 1 Jumelle 1892. 2 Weir 1919. 



