212 'carbon-fixation,' 'assimilation' 



(vi) 'Carbon-fixation' is also influenced by the presence or 

 absence of certain mineral substances, especially compounds of 

 potassium obtained from the soil, but the particular part which 

 these substances play in the process is not known. 



The ' fixation of carbon ' is a vital process and ceases with the 

 death of the plant. 



Plants grown in air from which the carbon dioxide has been 

 extracted do not increase in dry weight, and after a time death 

 takes place from starvation. They are not able to live in an 

 atmosphere of pure carbon dioxide, but are able to carry on 

 ' carbon-fixation ' in air containing as much as 20 or 30 per cent, 

 of the gas. According to the experiments of Montemartini the 

 formation of carbohydrates is carried on best and most rapidly 

 in air containing 4 per cent, of carbon dioxide, an amount six 

 or seven times as great as that normally present in the atmosphere. 



' Carbon-fixation ' is apparently carried on only by specialised 

 portions of the protoplasm of the cells, namely, by the chloro- 

 plasts, for it only occurs in the leaves and parts which are green. 

 The roots, the petals of flowers, and the white portions of 

 variegated leaves from which chloroplasts are absent take no 

 part in the process, and parasitic and saprophytic plants which 

 are devoid of these structures are also incapable of utilising 

 carbon dioxide for the formation or synthesis of carbohydrates. 



The leaves of the copper-beech, purple cabbage, red beet 

 and many other plants have reddish cell-sap which disguises the 

 green colour of the chloroplasts : the latter are nevertheless 

 abundant in the palisade and spongy parenchyma of such leaves, 

 and the plants as readily carry on the process of 'carbon- 

 fixation ' as those having ordinary green leaves. 



The chloroplasts are small structures imbedded in the cyto- 

 plasm of the cell; their substance is permeated with a green 

 pigment named chlorophyll, associated with which is a reddish 

 orange substance known as carotin, and a yellow material 

 termed xanthophyll allied to the latter. 



