238 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 cea.sed. 



Most Phanerogams cease assimilation and respiration after being sub- 

 jected for ten minutes to a temperature of 50° C. Jumelle^ made a series of 

 experiments with lichens, chiefly of the larger fruticose or foliaceous types, 

 with species o{ Ramalina, 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- 

 phttrea, 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^ tested the thalli of crustaceous lichens in a hot June, under 

 direct sunlight, and found that the thermometer registered 55° C. 



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 

 any season into a condition of lowered or latent vitality that enables them 

 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 condi,tion. 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 



"■ Jumelle 1892. - Zopf i8go, p. 489. 



