984 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



The relation between the kmetic properties of sun-adapted (heho- I 



phiHc) and shade-adapted (umbrophiHc) plants will be discussed on page | 



987; that between warmth-adapted (thermophilic) and cold-adapted 

 (cryophihc) plants, in chapter 31. As shown by Harder 's data in Table 

 28. Ill, the effects of adaptation to weak light and low temperature differ 

 in sign — the first one reduces respiration and thus shifts h toward weaker 

 illuminations, while the second one enhances repiration, and thus shifts h 

 toward more intense light. 



Algae that live deep under the sea, particularly red algae, are adapted 

 both to weak light and to low temperature. The effect of umbrophilic 

 adaptation predominates, however, and the compensation points of these 

 algae generally are lower than those of the surface algae. Without low 

 compensation points, these organisms could not develop 100-120 meters 

 under the sea (the lowest level from which organisms have been recovered 

 by dragnet) because the intensity of illumination at 120 meters depth is of 

 the order of only 200 lux (see data of Seybold 1936, in chapter 22). 



As discussed in more detail in chapter 15 (page 424), the deep-sea algae 

 are adapted not only to low light intensity, but also to predominantly blue- 

 green light. If the compensation points of the green surface algae and 

 the colored deep-sea algae were compared in blue-green light, the lower 

 compensation points of the latter probably would appear even more strik- 

 ingly than in Table 28. III. 



In general, the compensation points of different species shown in the 

 table are comparable only if the experiments were carried out under closely 

 similar conditions (same carbon dioxide concentration, temperature and 

 previous history of the plants), since otherwise the intensity of respiration 

 of a given species may vary wddely. Plants allowed to photosynthesize 

 efficiently for some time often accumulate assimilates and then respire 

 many times stronger than similar plants "starved" for an extended period 

 of time (c/. Harder's data in Table 28.III). Such special conditions may 

 perhaps explain the very low /, values found by Plaetzer for some aquatic 



plants. 



The ratio between respiration and photosynthesis at low light intensi- 

 ties is generally changed in favor of respiration by an increase in tempera- 

 ture (cf. chapter 31); thus, higher temperature must cause an upward 

 shift of the compensation point (of. fig. 28.15 and Table 31. III). Narcotics 

 have a similar influence, since they, too, reduce photosynthesis (at all light 

 intensities) much more effectively than respiration (c/. chapter 12). En- 

 zyme poisons (e. g., cyanide) may have a lesser or even opposite effect, 

 because their influence on photosynthesis in weak light usually is rather 

 small (c/. figs. 28.9A, 28. 11 A), while most of them strongly inhibit 

 respiration. In certain algae (e. g., some Scenedesmus strains), the effect 



