1230 THE TEMPERATURE FACTOR CHAP. 31 



20° C. The specimens showed no difference in appearance, but their 

 responses to temperature were quite different. An example is given in 

 Table 31.11. In weak light, the photosynthesis of cold-adapted Fontinalis 

 plants declined between 8° and 18° C. while that of the warm-adapted 

 plants increased; in strong light, it increased in the same range, but propor- 

 tionately much less than the photosynthesis of the warmth-adapted in- 

 dividuals. 



Since the figures in Table 31.11 appear to represent true photosynthesis, and not net 

 gas exchange, the negative sign of the temperature effect in the case of cold-adapted 

 plants in weak light is noteworthy. 



The analogy between light and temperature adaptations caused 

 Harder to ask whether cold-adapted plants have an enhanced capacity 

 for photosynthesis at low temperatures (analogous to the higher efficiency 

 of shade-adapted plants in weak light). He carried out no experiments 

 with plants of equal dry weight (or equal leaf surface), but attempted in- 

 stead to answer this question by analyzing the figures given in Table 31.11 

 for plants of unknown weight and leaf area. He concluded that at low 

 temperatures cold-adapted individuals actually are more efficient than 

 warmth-adapted ones. Theoretically, the situation in cryophilic plants is 

 somewhat different from that in the umbrophiles; in the latter case, an 

 improved efficiency (rate per unit area) in weak light could be easily ex- 

 plained by a higher chlorophyll content and consequent enhanced absorp- 

 tion of fight (c/. page 422). For cryophilic plants to have an enhanced ef- 

 ficiency at low temperatures (in strong light), they should contain a higher 

 concentration of rate-limiting enzyme ; and if this is the case, one can ask : 

 why have they not a superior efficiency at the higher temperatures as well? 

 One conceivable explanation is to assume that two different enzymatic 

 processes limit the rate at the two temperatures. 



In contrast to the low respiration of umbrophilic plants, the respira- 

 tion of cryophilic plants is, according to Harder, not weaker, but even 

 stronger than that of the thermophilic individuals; consequently, their 

 compensation points are higher. 



Plants that do not interrupt photosynthesis in winter experience a re- 

 versible adaptation to cold during this season as shown, e. g., by meas- 

 urements of Staifelt (1937, 1939) on hchens. The temperature curve of 

 respiration is practically unaffected by the season, while the optimum of 

 photosynthesis, and consequently also that of net organic synthesis, is 

 shifted in winter toward lower temperatures. For the eight species inves- 

 tigated by Stalfelt the average shift of the optimum of net photosynthesis 

 was from 18.5° in summer to 14.1° C. in winter. The fact, observed by 

 Beljakov (1930), that barley plants suddenly cooled to 3-10° interrupt 



