ASSIMILATION OF CARBON BY PLANTS 167 



a somewhat roundabout way to ascertain the added weight of 

 dry matter. A leaf showing as far as possible a symmetric 

 structure is taken for the experiment. One-half of the blade is 

 cut away along the midrib. From this half, a portion of a pre- 

 cisely 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, remain- 

 ing in connection 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 simultaneously in the leaf. These processes are the trans- 

 location of the substances that are produced in assimilation, from 

 the leaf to the stem, and respiration, which is a continuous 

 oxidation 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 be 

 considerably higher than those given above. Sachs has deter- 

 mined the total dry weight of the organic matter formed in 1 hr. 

 per square meter as 1.7 to 1.9 g. for the sunflower leaf and 1.5 g. 

 for the pumpkin leaf. These experiments have been repeated 

 by other investigators with similar results. Hence, it may be 

 accepted, as average values at least, that during 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 later. 



39. Green Plastids as Special Organs of Photosynthesis. 

 Chlorophyll, Its Chemical Composition, Structure, and Prop- 

 erties. Pigments Accompanying Chlorophyll. — The assimilation 

 of carbon dioxide takes place in special cell structures, the green 

 plastids. An indirect proof of this is the fact that only the green 



