30 THE SYNTHESIS OF CARBOHYDRATES 



The internal temperature of assimilating leaves will vary 

 considerably in accordance with such conditions as the inten- 

 sity and quality of the light, the character of the leaf surface 

 and so on ; Blackman and Matthsei * demonstrated, by 

 thermoelectric means, an excess of 7 C. to 16 C. in the leaves 

 of Prunus laurocerasus in bright sunlight above the adjacent 

 shade temperature. 



ILLUMINATION. 



The importance of light in the formation of chlorophyll 

 is a commonplace in physiological experience ; chlorophyll 

 may, however, be formed in the dark, or in comparative dark- 

 ness, as was shown by Schimper f for Alg?e and Burgerstein % 

 for conifers, and in other plants. § In the majority of plants, 

 however, some exposure to light is necessary to produce the 

 green pigment, presumably by its action on a precursor, termed 

 chlorophyllogen by Monteverde and Lubimenko,|| according 

 to whom plants grown in darkness for an excessive period 

 lose their ability to form chlorophyll when exposed to light. 

 In passing it may be mentioned that in addition to light, 

 other conditions are requisite for chlorophyll formation, es- 

 pecially an adequate temperature, again a fact of common 

 experience, and a supply of carbohydrate.^] Mention also 

 may be made of the well-known fact that a high light intensity, 

 presumably owing to its destructive action on chlorophyll, 

 may bring about a cessation of photosynthesis.** 



Of the light falling on a leaf, a portion only is available 

 for the chloroplasts since varying amounts, according to the 

 characteristics of the leaf surface, such as the presence of 

 cuticle and of hairs, the thickness of the leaf and so on, will 



* Blackman and Matthaei : loc. cit. 



f Schimper: " Jahr. wiss. Bot.," 1885, 16, 1. 



% Burgerstein: " Ber. deut. bot. Ges.," 1900, 18, 168. 



§ See Vol. I., p. 311. 



|| Monteverde and Lubimenko : "Bull. Jard. Imp. Bot. St. Peters- 

 bourg," 1909, 9, 27; "Bull. Acad. Sci. St. Petersbourg," 1912, 609. 



1[ See Palladin : "Ber. deut. bot. Ges.," 1891, 19, 229. Mansky : 

 " Biochem. Zeit." 1922, 132, 18. 



** See Ewart : "Ann. Bot.," 1897, 11, 439; 1898, 12,379. Panta- 

 nelli: "Jahr. wiss. Bot.," 1903, 39, 167. Ursprung : " Ber. deut. bot., 

 Ges," 1917, 35, 57. 



