ii8 METABOLISM 



LECTURE X 

 THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. II 



Let us summarize briefly what we have learned up to the present on the 

 subject of the assimilation of carbon : — the green plant decomposes the carbon- 

 dioxide present in the air or water, evolves oxygen, and constructs carbo- 

 hydrate. This process can take place only with the assistance of sunlight and 

 in the chlorophyll corpuscles. We will endeavour in the present lecture to 

 extend our knowledge of this phenomenon, more especially fixing our attention 

 on the manner in which external conditions affect carbon assimilation in the 

 plant, directly and indirectly. We shall take up the thread of our story where 

 we left off in the preceding lecture and study first the amount of carbon-dioxide 

 present in the air. 



The amount of this gas present in the air has been very frequently and 

 very accurately measured, and it has been found that it varies within very 

 narrow limits, indeed it might be said to be a constant quantity so long as we 

 avoid selecting for analysis the layers of the atmosphere immediately overlying 

 the soil. Brown's (1899 [and 1905]) latest researches may be quoted in this 

 relation, and he found on an average 2-8 parts of carbon-dioxide in 10,000 of air. 

 This number agrees with the previous results recorded, e. g. by Reiset, who 

 obtained2-9, and those of the Montsouris Laboratory, where a number of indepen- 

 dent observations gave, on an average, a proportion of 3 parts per 10,000. We 

 will not enter into a discussion of the variations in the amount of carbon-dioxide 

 in the air, for these are of no importance so far as the vitality of the plant is 

 concerned. We have already referred (p. 103) to the question as to whether the 

 carbon-dioxide of the air is necessary for the life of the green plant, and whether 

 it cannot also draw upon the stores of carbon-dioxide present in the soil. That 

 question was answered in the negative, still this is the proper place to refer 

 again to the experiments which have been adduced in proof of this fact. It 

 has been asserted by various investigators, most emphatically by Unger (1855), 

 that the carbon-dioxide of the air was insufficient alone, and that that present in 

 the soil must also form part of the supply. In opposition to this view, Moll 

 (1877) was able to show that a plant which was prevented from obtaining 

 carbon-dioxide save by its root, never succeeded in forming starch in its leaves, 

 and manifestly suffered from want of the gas. It is also obvious that since 

 the path from the root to the leaf is a long one, the transference must be slow, 

 and that most of the carbon-dioxide will be abstracted on the way up by the 

 chlorophyll bodies in the cortex of the stem. 



If, however, the carbon-dioxide of the air be the source of carbon to the 

 green plant, the question comes to be how is it possible that the amount in the 

 air remains approximately constant, notwithstanding the fact that plant 

 activity tends continually to diminish it. As a matter of fact, the amount of 

 carbon-dioxide which the plant world abstracts from the air is very considerable, 

 as the following statement will show. According to Sachs (1884), a sunflower 

 with a leaf surface of about 1-5 sq. m. increases in dry weight to the extent of 

 about 36 g. per diem (p. 114), and since 1-5 g. of carbon- dioxide in round numbers 

 is produced from i g. of dry weight, it must extract about 50 g. from the air 

 daily, or about 1-5 kg. in a month. If we assume the amount to be only i kg., 

 taking into account possibilities of unfavourable weather, and imagine the 

 whole surface of the earth covered with sunflowers, one to each square metre 

 or one million to the square kilometre, then the sunflowers existing on the 

 135 millions of square kilometres of land surface would consume 135 billion kg. 

 of carbon-dioxide in one month ; and since, according to the usual estimate, 

 there are 2,500 billion kg. of carbon-dioxide in the atmosphere, the supply 



