CHAPTER V 

 ASSIMILATION OF CARBON BY PLANTS 



37. General Concept of Photosjmthesis. Products of Assimi- 

 lation of Carbon. Connection with the Assimilation of the 

 Elements of Water. — At germination, the rapidly developing 

 seedling draws upon the reserves stored in the cotyledons or in 

 the endosperm for the nutrients indispensable for its growth. 

 A considerable part of these reserves is expended in the process 

 of respiration to provide the energy necessary for growth. As 

 has already been emphasized, the beginning of germination is 

 therefore a time of rapid loss of dry weight of the seedling. If 

 germination is carried on in darkness for too long a time as when 

 seeds are sown deeply, exhaustion may result in the death of the 

 seedlings. 



With the emergence of the first leaflets above the surface of 

 the soil, an abrupt alteration occurs in the character of the 

 nutrition of the young plant. The leaves that were yellow under- 

 ground become rapidly green under the influence of light and 

 start energetic photos3nithesis of organic substances. The 

 expenditure of food reserves, characteristic of the first stage of 

 germination is replaced by their accumulation, and the seedling, 

 which first was nourished at the expense of its reserves, now 

 changes into a plant independently procuring its nutrients from 

 the surrounding medium. 



In discussing the elementary composition of the plant, it has 

 already been noted that almost half of the dry substance is 

 made up of carbon. But the significance of carbon is not limited 

 to its quantitative prevalence only. Atoms of carbon, as is 

 known, form the basic skeleton of all organic compounds. The 

 capacity of carbon to give various combinations with other 

 elements explains the enormous diversity of these compounds. 

 When the plant begins its independent existence, the first and 

 most important problem is the absorption of carbon dioxide from 

 the surrounding medium and its synthesis into complex sub- 

 stances to form the structural part of all organs. The primary 



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