ABSORPTION OF CARBON 13 



is taken for the experiment. One-half of the blade is cut away 

 along the midrib. From this half a portion of a precisely defined 

 area is cut out, dried, and weighed. By dividing the weight by 

 the area, the initial dry weight of a surface unit of the leaf is 

 obtained. The other half of the leaf blade, remaining in connec- 

 tion with the midrib, is exposed to light. After several hours, the 

 weight of a surface unit is determined in the same way. The 

 increased weight in comparison with the first half gives the increase 

 in dry matter per surface unit. 



The determinations made by Sachs, according to this method, 

 have shown that a sunflower leaf in bright light accumulates in 

 1 hr. 0.914 g. of dry matter per square meter; a pumpkin leaf 

 accumulates 0.680 g., a leaf of rhubarb, 0.652 g. The exact values 

 of assimilation, however, must be considerably higher, as two other 

 processes, leading to the loss of dry matter, are going on simulta- 

 neously in the leaf. These processes are the translocation of the 

 substances which are produced in assimilation from the leaf to the 

 stem, and respiration which is a continuous combustion of organic 

 matter. If this loss of substance, during the experiment, is taken 

 into account, by placing in the dark a second leaf similar to the 

 first one, and then determining the loss in dry matter per surface 

 unit, the values of assimilation will prove to be considerably 

 higher. Sachs has determined the total dry weight of the organic 

 matter, formed in an hour per square meter, as 1.7 to 1.9 g., for 

 the sunflower leaf, and as 1.5 g. for the pumpkin leaf. These 

 experiments have been repeated by other investigators with simi- 

 lar results. Hence, we may accept, as average values at least, 

 that in 1 hr. of a summer day the plant assimilates 1 g. of dry 

 matter per square meter of its surface. 



Besides these principal methods of studying the absorption 

 of carbon dioxide by the plant, there are other methods of a more 

 special character. Some of these will be mentioned farther on. 



3. Products of Carbon Assimilation. Connection with the 

 Assimilation of Elements of Water. — The carbon dioxide entering 

 the plant is completely utilized. One of the end products of this 

 utilization is oxygen, which is given off by the plant to the sur- 

 rounding atmosphere. An exact determination of the volumes 

 of carbon dioxide absorbed and oxygen eliminated shows that these 

 volumes are precisely equal. On the basis of this equal volumetric 

 ratio, it may be said that the plant splits up the carbon dioxide, 



