218 BOTANY part i 



would thus give back to the air 1200 million kilos of COg (340 niilliou kilos C). 

 The COj discharged into tlie air from all the chimneys on the earth is an enormous 

 amount. Credner calculated that 460,000,000 kilos of coal are burnt annually, 

 yielding to the atmosphere about 1,265,000 million kilogrammes of carbonic acid 

 gas. These sources of COg are, according to Beyerinck, insignificant compared 

 with the enormous amounts jiroduced by the Bacteria of the soil in putrefactive 

 processes. The whole carbon supply of the atmosphere is at the disposal of plants, 

 since the COg becomes uniformly distributed by constant diffusion. 



Submerged water-plants absorb the CO2 dissolved in water. Its amount varies 

 considerably according to the temperature. At 15° C. a litre of water contains about 

 as much CO^ as a litre of atmospheric air. 



Artificially conducting carbonic acid through the water increases, to a certain 

 degree, the evolution of oxygen, and the assimilatory activity. Similarly an 

 artificial increase of carbonic acid in the air is followed by increased assimilation. 

 According to Kreusler assimilation in sunshine attains its maximum in air con 

 taining about 10 per cent of carbonic acid ; with a higher percentage it begins to 

 decrease. If the amount of carbonic acid gas be increased three hundred times 

 (from 0"03 per cent to 10 per cent in the atmosphere), the formation of starch is 

 only increased 4-8 times, while an increase of the COo to six times the normal 

 proportion results, according to H. BiiowN, in the formation of six times as much 

 starch (^-). 



Carbon monoxide (CO), though it can be utilised by certain Bacteria {Bac. 

 oHc/ocarbophilus), can probably not be utilised by green plants ; it cannot thus 

 replace the carbon dioxide. 



The CO.,-assimiIation, like all vital processes, is dependent on the 

 temperature. It begins at a temperature a little above zero, reaches 

 its maximum at about 37° C, and again stops at about 45° C. 



These cardinal points not only have different positions in dilferent plants but 

 do not remain constant for any particular plant. This is especially true of the 

 optimum which in the course of a few hours may sink from 37° C. to 30° C. This f;ill 

 indicates the compound nature of the optimum curve (cf p. 178). In bright warm 

 weather assimilation does not reach its full possible value since the su^jply of 

 carbon dioxide is then insuflacient. 



Other less important factors need not be considered in detail. It 

 may be mentioned, however, that many substances can bring about a 

 temporary, or ultimately a permanent, limitation or arrest of the 

 assimilatory process. 



Products of the Assimilation of Carbon Dioxide. — It Avas assumed 

 above that sugar was formed from the carbon dioxide, and analysis 

 in fact shows that the amount of sugar in a foliage leaf is increased 

 after exposure to sunlight. It is true that grape-sugar is neither 

 always nor only shown to be present ; usually other more complex 

 carbohydrates appear. These can all, however, be traced back to 

 hexoses like grape-sugar, and arise by the union of two or more 

 molecules of hcxose and the loss of the elements of water. Prominent 

 among them are cane-sugar (Cj.,H.„Ojj^) and starch (C,JH-^|j05),^. The 

 occurrence of starch in the chloroplasts of illuminated foliage leaves is 



