1218 THE TEMPERATURE FACTOR CHAP. 31 



less rapid, and more or less irreversible "chill injury" or "heat injury." If 

 the rate of a specific biochemical process in vivo is measured as a function of 

 temperature, one often finds a region of temperatures in which the effect of 

 heating is reversible, and similar to that observed in the study of simple 

 reactions in vitro. However, at the two ends of this range, the reversible 

 influence of temperature becomes obscured by irreversible, destructive 

 processes, such as changes in the colloidal structure of the protoplasm, 

 which affect indirectly all chemical reactions in the living organism. 



In the case of photosynthesis, the region of reversible changes extends, 

 for plants adapted to moderate climates, only from about 0° to about 30° C. 

 Below +5°, and above +25° C, a slow chill injury or a slow heat injury 

 may set in, so that the observed rate of photosynthesis depends not only 

 on the momentarily prevailing temperature, but also on how long the plant 

 has been exposed to it. 



The exact limits of the biokinetic range of photosynthesis depend on 

 the individual (ontogenetic) adaptation of the plants, as well as on the 

 phylogenetic adaptation of the species. Analogous to the existence of 

 heliophilic and umbrophilic species and individuals (described in chapter 

 28), certain plants exhibit thennophilic, others cryophilic properties. This 

 "thermal adaptation" will be discussed in section 3. 



1. Lower Temperature Limit and Chill Injury 



Conifers, mosses and lichens retain their chlorophyll in winter, 

 even though the temperature of the air may drop to — 50° C. It has been 

 observed (c/. Ehlers 1915) that the starch deposits in conifers sometimes are 

 replenished in winter (while those of evergreen deciduous plants are used 

 up during the same period) ; this points to continued photosynthetic ac- 

 tivity of conifers in cold weather. How low the temperature must be to 

 inhibit all photosynthesis has not yet been determined by reliable experi- 

 ments under laboratory conditions ; while observations made in the open 

 gave contradictory results. Boussaingault (1874) found no photosynthesis 

 below the freezing point, not even in conifers. Kreusler (1887, 1888), on 

 the other hand, noticed photosynthesis in Ruhus at -2.4° C, and Jumelle 

 (1891) claimed to have observed oxygen evolution by conifers {Picea ex- 

 celsa, Juniper communis), and lichens (Evernia prunastri) even at -30° 

 and -40° C. These extreme results found little acceptance; Ewart (1896, 

 1897), among others, rejected them as erroneous, asserting that tropical 

 plants cease to reduce carbon dioxide at temperatures as high as +4° or 

 +8° C, that subtropical and aquatic plants stop reduction at 0° or +2° C, 

 and that land plants from temperate, arctic and alpine zones do so slightly 

 below the freezing point. Ewart noted that oxygen production ceases 



