PHOTOSYNTHESIS RATK UNDER NATURAL PONDTTIONS 007 



This was denied by Boysen-Jensen and Miiller (1029); but one notices 

 in Table 28. VI that the newer measurements of the Danish school have 

 given somewhat higher values than those of 1018-1920, and thus reduced 

 the discrepancy between the averages in sections Ba and Be to a factor of 

 about 2. 



In section C, containing plants from arid zones, we find a similar dis- 

 crepancy l>et ween the result of Harder and co-workers in Algeria, and Wood 

 in Australia (1-10 mg./hr. 100 cm.-), and those of Kostychev and Kardo- 

 Sysojeva in Central Asia (20-70 mg.). 



In section D, practically all the listed values fall into the range 1-10 mg. (no Russian 

 measurements are listed here, c/. however, the data of Kostychev and Kui'saiiov for 

 the subtroiiical vegetation of the Black Sea littoral in .section Be). 



In the group of alpine jilants (section E), Monch (1037) and the Russians 

 agree in finding the highest yields ever recorded under natural conditions. 

 Earlier, Henrici (1018) had reported, for the alpine plant BelUs perennis, 

 a yield of 232 mg. C02/hr. 100 cm.^ This value appears so incredibly 

 high that we did not include it in Table 28.VI; but even the results of 

 Blagoveshchenskij (1035) and Monch (1037) (00-100 mg./hr. 100 cm.^) 

 indicate remarkablj^ high quantum jaelds (of the order of one CO2 mole- 

 cule reduced per twenty quanta, in light of 80 klux, and \vith not more than 

 0.03% CO2 present). 



It should not be assumed that the carbon dioxide concentration was exactly 0.03% 

 in all measurements listed in Table 28. VI. In Blagoveshchenskij 's experiments in the 

 Pamir, for example, the [CO2] assays varied between 0.01 and 0.02%, and the highest 

 yields were obtained at the latter concentration (which is. still considerably below the 

 normal value of 0.03%). Stocker found, in the undergrowth of the tropical forest, 

 [CO2] values up to 0.04%. (Compare also data given in Chapter 27, page 902.) 



To sum up, it is certain that plants growing in moderate climates can 

 reduce, in their natural habitats and under favorable conditions, 20 or 30 

 mg. CO2 per hr. per 100 cm.- of leaf surface; but it is much less certain 

 whether any plants — desert and alpine plants not excluded — are capable of 

 yields up to 100 mg./hr. 100 cm.^, as is suggested by the measurements of 

 Kostychev, Monch and Blagoveshchenskij. 



We will now discuss the relation between the maximum yield of photo- 

 synthesis of which leaves are capable under favorable natural conditions, 

 and the average production of organic matter by whole plants or large 

 plants assemblies. 



Land plants in the open air, exposed to the sun, can be expected to main- 

 tain the above-estimated rate of photosynthesis (about 20 mg./hr. 100 

 cm.^) for a considerable part of the day (barring such phenomena as the 

 "midday rest" ; cf. chapter 26). The intensity of illumination is sufficient, 



