THE NATURE OF PHOTOSYNTHESIS 101 



tensity. Exact values for the light intensities were not obtained. But they 

 were able to determine what proportion of direct sunlight was required 

 to give the maximum possible photosynthesis at a definite temperature. 

 Thus the photosynthetic rates determined at 29.5° at mid-day of August 

 8 and 16 respectively were : 



Cherry-laurel: 0.28 sunlight gives 0.0116 g. CO2 per 50 sq. cm. per hour. 

 Helianthus: 0.62 " " 0.0224 " " " " " " " 



These values then represent the minimum light intensity which is suffi- 

 cient for a maximum rate of photosynthesis at that temperature and as- 

 suming there is no other factor limiting. Helianthus is therefore capable 

 of a much higher rate of photosynthesis than cherry-laurel at the same 

 temperature, and neither of the plants is capable of utilizing for photo- 

 synthesis the whole of the solar radiation. The highest photosynthetic 

 rate observed by Blackman and Matthaei equaled about 2900 cc. CO2 per 

 square meter per hour for Helianthus at 29.5°. According to the Black- 

 man view, in order to determine whether photosynthesis as observed at 

 any given time is Hmited by light intensity, temperature or some other 

 factor, it is necessary only to alter the intensity of each factor separately. 



It has just been stated that the two ditTerent species of leaves, cherry- 

 laurel and Helianthus, exhibit different maxima of photosynthetic activity 

 at the same temperature. This raises the question whether different types 

 of leaves have different specific photosynthetic characteristics. Blackman 

 and Matthaei have also investigated this question using different plants as 

 Helianthus, Tropaeolum, Bomarca and Aponogeton. They conclude that 

 "leaves in general have the same coefficient of economy in the photo- 

 synthetic process." While at 29.5° Helianthus can assimilate twice as 

 much carbon dioxide as the cherry-laurel, at low temperatures, they have 

 similar photosynthetic maxima. Blackman and Matthaei harmonize the 

 latter statement with their assumption of "equal coefficients of economy" 

 by pointing out that Helianthus requires twice as much light to attain 

 the double assimilation at 29.5°. "The fundamental existing specific differ- 

 ences would seem to lie in their different coefficients of acceleration of 

 activity with increase in temperature." For a rise of 10° the increase with 

 cherry-laurel is 2.1 and with Helianthus it is about 2.5. 



The question of the specific photosynthetic value of leaves requires 

 further investigation before Blackman and Matthaei's dictum can be en- 

 tirely accepted. There are two points which suggest themselves at once. 

 The first of these is that the differences of acceleration due to temperature 

 of photosynthetic activity are probably related to enzyme action or what 

 we here include under internal factors, which, under the experimental 

 conditions, were governing the rate of photosynthetic activity. Of this 

 we know as yet very little. Secondly, it must be kept in mind that leaves 

 differ greatly in their photosynthetic activity according to their chlorophyll- 

 content, but that there is probably no direct relationship between the 



