NUTRITION 45 



bon-dioxide wholly extracted to supply the tree. Does a 

 line drawn through the periphery of a tree weighing 11,000 

 pounds when dry enclose this volume of air? Certainly not, 

 but this is immaterial, for the air is constantly in motion, 

 agitated by mechanical means and by the winds. Further- 

 more, the air, a mixture of gases, is subject to the laws 

 of diffusion of gases. The absorption or decomposition of 

 one volume of any gas in a mixture will produce a diffusion 

 current of that gas to make good the loss. Hence the tree, 

 whenever it is absorbing carbon-dioxide, is the centre to- 

 ward which numberless diffusion currents of carbon-dioxide 

 molecules are moving. The rapidity of their movement is 

 directly proportional to the rapidity of the absorption. 



Noll's figures, giving the amount of carbon contained in a 

 tree and the volume of air required to furnish this, may well 

 be supplemented by Brown's iigures of the hourly absorp- 

 tion of carbon-dioxide.* According to Brown's determina- 

 tions there is an increr.se of one gram of dry substance 

 (mainly carbohydrates) to every square meter of leaf sur- 

 face of Catalpa bignonioides per daylight hour. To form 



1 gr. starch 1.628 gr. C0„ must be absorbed 



1 " glucose 1.466 " " " " " 



1 " saccharose 1.543 " " " " 

 From these data 1.545 gr. of carbon-dioxide may be taken 

 as the mean amount needed to form one gram of carbo- 

 hydrate. This at normal temperature and pressure equals 

 784 cubic centimetres of carbon-dioxide, the volume of 

 carbon-dioxide absorbed per hour by each square meter of 

 leaf surface. If we take into consideration the fact that 

 stomata occur only on the under side of Catalpa leaves, 

 and that they occupy only 1% of the total leaf surface, we 

 see that the absorbent power of the leaf is very great. 



The daily absorption of such large volumes of carbon- 

 dioxide from the air suggests the query as to the means of 

 maintaining the supply. We have already seen that the 



* Brown, H. T. The fixation of carbon by plants. Address to Chemical 

 Section, Brit. Assoc. Adv. Science, 1899. Published in Nature, Sept. 14, 

 1899. 



