PDF - CES (IISc)

ASSIMILATION AND RESPIRATION 239
Jumelle 1 to continue to 10, but assimilation was still possible at a temperature
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 increasing
degree as the plant is further supplied with water until a certain
optimum is reached, after which the vital diminish.
processes begin somewhat to
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 Usiiea 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.