34 PHYSIOLOGY OF NUTRITION 



Kilograms of 

 Carbon Assimilated 

 Kind of Vogetatbon per Hectare 



Forest in Central Europe 1800 



Well fertilized meadow 35°o 



Helianthus tuberosus (Jerusalem artichoke) 6coo 



From a series of calculations, Becquerel came to the conclusion that, in France, 

 plants assimilate less than 1 per cent, of the radiant energy that reaches them. 

 Timiriazev arrived at the same result, and Brown's 1 more recent determinations 

 give a still smaller value. In the latter case a Helianthus leaf received on 

 a sunny day 600,000 g.-cal. per square meter of leaf surface per hour. 

 In the same time an equal surface of leaf produced 0.8 g. of carbohydrates, for 

 the formation of which 3200 g.-cal. were necessary. Thus the leaf accu- 

 mulated, by the photosynthetic process, barely 0.5 per cent, of the solar energy 

 reaching it; viewed as a machine designed to produce organic compounds, its 

 efficiency is thus seen to be far from high." 



An excess of light has a retarding effect upon increase in dry weight. It 

 appears that different rays of the spectrum are effective in different stages of 

 the photosynthetic process. 2 



The importance of light to plants is not confined to the photosynthesis of 

 carbohydrate from carbon dioxide and water; light is necessary for very many 

 kinds of chemical reactions taking place in plants. Among the investigations 

 that already testify to this are those upon the influence of light in protein 

 formation. Numerous other reactions that are influenced by light and that 

 are purely chemical in nature furnish additional evidence upon this point. 

 Ciamician and Silber 3 were able to establish the fact that very many oxidations, 

 reductions, hydrolyses, polymerizations and condensations are effected by light ; 

 such changes may progress very rapidly when an inorganic substance is involved. 4 



§8. Influence of External and Internal Conditions upon Photosynthesis.— 

 One of the most important of the external conditions upon which various 

 physiological processes depend is the temperature of the surroundings. The 

 influence of temperature upon the velocity of the greening process has been 

 shown above. Photosynthesis, on the other hand, is only very slightly affected 



1 Brown, H. Т., Recherches sur la fixation du carbone par les feuilles et sur la diffusion de l'acide 

 carbonique. Traduit librement de l'Anglais par M. E. Demoussy. Ann. agron. 27: 428-438. 1001. 

 [The original paper is: Brown, Horace Т., Opening address by the President of Section В (Chemistry), 

 Brit. Assoc. Adv. Sei., Nature 60: 474-483. 1899. (See also correction: ibid. 60: 544. 1899.) Also 

 published in: Rept. Brit. Assoc. Adv. Sei. 1899: 664-683. 1900. See also: Brown, H. Т., and Escombe 

 F., Static diffusion of gases and liquids in relation to the assimilation of carbon and translocation in 

 plants. Phil, trans. Roy. Soc. London B193: 223-292. 1900. 1 



2 Liubimenko, V. N., La quantite de pigment vert dans le grain de chlorophylle et l'energie de la photo- 

 synthese. [Abstract in French, p. 263-266; text in Russian.] Trav. Soc. Imp. Nat. St.-Petersbourg Ser. 

 ///, Sect. Bot. 41: 1-266. 1910. 



3 Ciamician, G., Sur les actions chimiques de la lumiere. Bull. Soc. chim. France 4 (fasc. 15): i-xxvii. 

 1908. [A special appendix to this fasc, bound at end of vol., separately paged. 1 [See also note 1, p. 180.) 



* Neuberg, Carl, Chemische Umwandlungen durch Strahlenarten. I. Mitteilung. Katalytische Reak- 

 tionen des Sonnenlichtes. Biochem. Zeitsch. 13 : 305-320. 1908. Idem, Ueber die Reaktion der Gallen- 

 säuren mit Rhamnose bzw. г -Methyl-furfurol. Ibid. 14: 349-350. 1908. Idem, Bemerkung über die 

 "Glucothionsäuren." /6г'</. 16: 250-253. 1909. Idem, Notiz über Phytin. /61^.16:406-410. 1909. 



* In such calculations as this it is to be noted that the plant does not absorb nearly all the 

 energy reaching it and that all the organic material formed does not appear in the final deter- 

 minations. — Ed. 



