ASSIMILATION OF CARBON 39 



that have been previously deprived of starch are able to assimilate various 

 organic substances from solution and thus to form starch in darkness. In 

 this manner starch can be formed from saccharose, glucose, fructose, lactose, 

 glycerine, dextrine, mannite, melampyrite, and adonite. 1 Sapozhnikov 2 

 investigated this matter quantitatively. Leaves of Astrapcea wallichii, 

 previously rendered starch-free, formed in seven days from 4.6 to 5.3 g. of 

 starch, per square meter of leaf surface, when floating upon a 20-per cent, solu- 

 tion of cane sugar in darkness. Here assimilation is not limited for forma- 

 tion of starch, however; the amount of proteins also increases when leaves are 

 grown upon cane-sugar solution in darkness, and respiration is accelerated. The 

 ability to absorb organic compounds is even more pronounced in roots than 

 in leaves. Many green plants possess mycorhiza (see Chapter IV) and grow 

 on humus soils, and these probably assimilate organic materials. Light 

 influences the absorption of organic compounds by green plants. 3 



According to the experiments of Reinhardt and Sushkov 4 the accumulation 

 of starch in leaves floating upon cane-sugar solution depends upon a variety of 

 conditions. This process occurs rapidly only at medium temperatures, while 

 starch that was previously present disappears at higher or lower temperatures, 

 in spite of the supply of sugar. Among poisons, some (quinin) hasten the first 

 appearance of starch but prevent its continued accumulation; others (0.5 per 

 cent, of caffein) favor the accumulation of starch. 



Experiments in which green plants were supplied with organic nitrogenous 

 compounds, in a chamber free from carbon dioxide, gave negative results. 5 



Summary 



1. Importance of Carbon Assimilation by Green Plants. — Green plants form 

 organic compounds from inorganic ones. Non-green plants and animals are unable to 

 do this and are therefore all ultimately dependent on green plants for organic sub- 

 stances. The study of plant physiology may begin by inquiring about photosynthesis 

 of carbohydrates by the green parts of plants. These organic compounds are formed 

 from carbon dioxide and water, by means of solar energy that is absorbed and trans- 

 formed in the green tissues. Carbon dioxide is of course a carbon compound, but it is 

 not combustible and is usually classed as inorganic. Combustible carbon compounds 

 derived from organisms are capable of being burned in air because they are incom- 

 pletely oxidized; when completing their oxidation these compounds absorb oxygen and 

 produce carbon dioxide and water, and this process of combustion liberates energy 



1 Treboux, O., Starkebildung aus Adonit im Blatte von Adonis vernalis. Ber. Deutsch. Bot. Ges. 

 27: 428-430. 1909. 



2 Saposchnikoff, W., Ueber die Grenzen der Anhaufung der Kohlenhydrate in den Blattern der Weinrebe 

 und anderer Pflanzen. (Vorlaufige Mittheilung.) Ber. Deutsch. Bot. Ges. 9: 293-300. 1891. P. 298. 

 Idem, 1890, 1893. [See note 2, p. 32. 1 



3 Lubimenko, W., Influence de la lumiere sur l'assimilation des matieres organiques par les plantes 

 vertes. Bull. Acad. Imp. Sci. St.-Petersbourg VI, 1 : 395-426. 1907. 



4 Reinhard, [L. V.] and Suschkoff, Beitrage zur Starkebildung in der Pflanze. Beih. Bot. Centralbl. 

 18: 133-146. 1904-1905. 



5 Grafe, Victor, Untersuchungen tiber die Aufnahme von Stickstoffhaltigen organischen Substanzen 

 durch die Wurzel von Phanerogamen bei Ausschluss der Kohlensaure. Sitzungsber. (math.-naturw. Kl.) 

 K. Akad. Wiss. Wien. 118': 1135-1153. 1909. 



