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PHYSIOLOGY
III. ASSIMILATION AND RESPIRATION
A. INFLUENCE OF TEMPERATURE
a. HIGH TEMPERATURE. It has been proved that plants without chloro-
phyll are less affected by great heat than those that contain chlorophyll.
Lichens in which both types are present are more capable of enduring high
temperatures than the higher plants, but with undue heat the alga succumbs
first. In consequence, respiration, by the fungus alone, can go on after
assimilation (photosynthesis) and respiration in the alga have ceased.
Most Phanerogams cease assimilation and respiration after being sub-
jected for ten minutes to a temperature of 50 C. Jumelle 1 made a series of
experiments with lichens, chiefly of the larger fruticose or foliaceous types,
with species ofRamatitia, Physcia and Parmelia, also with Evernia prunastri
and Cladonia rangiferina. He found that as regards respiration, plants
which had been kept for three days at 45 C., fifteen hours at 50, then five
hours at 60, showed an intensity of respiration almost equal to untreated
specimens, gaseous interchange being manifested by an absorption of oxygen
and a giving up of carbon dioxide.
The power of assimilation was more : quickly destroyed as a rule it
failed after the plants had been subjected successively to a temperature of
one day at 45 C., then three hours at 50 and half-an-hour at 60. The
assimilating green alga, being less able to resist extreme heat, as already
stated, succumbed more quickly than the fungus. Jumelle also gives the
record of an experiment with a crustaceous lichen, Lecidea (Lecanora) sul-
phurea, a rock species. It was kept in a chamber heated to 50 for three
hours and when subsequently placed in the sunlight respiration took place
but no assimilation.
Very high temperatures may be endured by lichen plants in quite natural
conditions, when the rock or stone on which they grow becomes heated by
the sun. Zopf 2 tested the thalli of crustaceous lichens in a hot June, under
direct sunlight, and found that the thermometer registered 55C.
b. Low TEMPERATURE. Lichens support extreme cold even better than
extreme heat. In both cases it is the power of drying up and entering at
that enables them
any season into a condition of lowered or latent vitality
to do so. In winter during a spell of severe cold they are generally in a
state of desiccation, though that is not always the case, and resistance to
cold is not due to their dry condition. The water of imbibition is stored in
the cell-walls and it has been found that lichens when thus charged with
moisture are able to resist low temperatures, even down to 40 C. or - 50
as well as when they are dry. Respiration in that case was proved by
1
Jumelle 1892.
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Zopf 1890, p. 489.