XIII] SOURCE OF THE CARBON 131 



Now since there is no carbon in the water, beyond 

 traces of carbonic acid which result from the respiration 

 of the immersed roots, it is obvious that the assimilation 

 of the carbon is due to the activity of the green leaves in 

 the light of sufficient intensity. It is possible to carry 

 the proof even further. It will be noted that a trace of 

 iron salt was added to the water : this is because experi- 

 ments have shown that the green substance chlorophyll 

 is apt to remain in abeyance unless a trace of iron is 

 present. If we omit the iron, and so deprive the seedlings 

 of their green chlorophyll, even those fully exposed to 

 light fail to increase their dry weight : they cannot fix 

 the carbon, and soon die, because the carbonic acid at 

 their roots is of no use to them. 



It is clear then that the process of carbon-assimilation 

 is dependent on the co-operation of sufficiently intense 

 light, with the chlorophyll of the leaves, in air containing 

 carbon-dioxide. Naturally, the astounding statement that 

 green leaves waving in the air can snatch from the latter 

 the minute quantities of carbon-dioxide it contains 

 about '03 '04 per cent. with sufficient avidity and 

 rapidity to accumulate the large amounts of carbon- 

 material in the plant, awakened vigorous scepticism when 

 first promulgated. On reflecting that 10 cubic metres 

 i.e. 10,000 litres of ordinary air contain only three or four 

 litres of carbon-dioxide, only T 3 T ths of which are carbon 

 and weigh less than 2 grams, while a fully grown Sun- 

 flower may contain 1000 grams of carbon fixed during 

 its one summer's development, and an Oak tree many 

 thousand times as much in proportion, the matter is 

 certainly sufficiently marvellous. 



The facts are, however, completely established by ex- 

 perimental evidence ; and we have simply to correlate the 

 enormous area of thin leaves, exposed to bright sunshine, 



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