ASSIMILATION OF CARBON 35 



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. 1 



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 2 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. 3 



§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 

 by temperature. According to the investigations of Kreusler, 4 the decomposi- 

 tion of carbon dioxide begins at temperatures almost as low as the freezing point 

 and continues up to 5o°C. His data are presented below. 



Tempera- Amount of Tempera- Amount of 



ture C0 2 De- ture CO3 De- 



Deg. C. composed Deg. C. composed 



2.3 1.0 20.6 2.6 37-3 2.3 



75 i-7 25.0 2.9 41.7 2.0 



11. 3 2.4 29.3 2.4 46.6 1.3 



158 2.8 33.0 2.4 



If the amount of carbon dioxide decomposed in a unit of time at 2.3 be repre- 

 sented by unity it is seen that this rate is not yet equal to 3 at 25 . Such a rise 

 of temperature increases the rate of respiration to many times its original value. w 



1 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.-P6tersbourg Ser. 

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



2 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.] [See also note 1, p. 288.] 



3 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- 

 sauren mit Rhamnose bzw. 5-Methyl-furfurol. Ibid. 14: 349-350. 1908. Idem, Bemerkung iiber die 

 "Glucothionsauren." Ibid. 16: 250-253. 1909. Idem, Notiz iiber Phytin. Ibid. 16: 406-410. 1909. 



1 Kreusler, U., Beobachtungen iiber die Kohlensaure-Aufnahme und -Ausgabe (Assimilation und 

 Athmung) der Pflanzen. II. Mittheilung: Abhangigkeit vom Entwicklungszustand — Einfluss der Tem- 

 peratur. Landw. Jahrb. 16: 711-755. 1887. [Idem, same title. III. Mittheilung: Einfluss der Tempera- 

 tur; untere Grenze der Wirkung. Ibid. 17: 161-175. 1888. Idem, Beobachtungen iiber Assimilation 

 und Athmung der Pflanzen. IV. Mittheilung: Verhalten bei hoheren Temperaturen; Kohlensaure-ausschei- 

 dung seitens getodterer Exemplare; Kohlensaure Verbrauch, wenn Ober- und Unterseite der Blatter dem 

 Licht Zugewendet. Ibid. 19: 649—668. 1890.] 



w But Gabrielle Matthaei's very careful studies (Matthaei, Gabrielle L. C, Experimental 

 researches on vegetable assimilation and respiration. III. On the effect of temperature 

 on carbon dioxid assimilation. Phil, trans. Roy. Soc. London B197: 47-105. 1905) show 

 that the influence of temperature upon photosynthesis in leaves of Prunus laurocerasus 



