CHLOROPHYLL AND THE CHLOROPLASTS 379 



will and which can be subjected to almost any conditions, and a microscopic 

 living cell of the most complex structure, containing a large number of 

 substances and rendered inactive by relatively slight alterations in external 

 conditions. 



In reactions taking place in a living organism, as for example, in 

 photosynthesis, there are substances which play a very important role 

 about which, from the chemical viewpoint, we know virtually nothing 

 and for the study of which no adequate means have been found. There 

 is some evidence that in the photosynthetic process a reaction of enzymatic 

 character plays an important part. This enzyme is presumably a product 

 of "protoplasmic activity."' We also know that photosynthesis appar- 

 ently does not take place in the entire cell, but centers in or about small, 

 well defined bodies within the cytoplasm of the cell. These minute 

 bodies, the chloroplasts, contain the chlorophyll and are the centers of 

 photosynthetic activity. The chloroplasts consist of a colorless portion, 

 the stroma, and a colored portion containing the pigments. They have 

 been the subject of much micro-anatomical study and much attention 

 has been given to their origin and development as far as can be observed 

 under the microscope. The results of those studies have proved to be 

 of only limited value for an understanding of the chemistry of photo- 

 synthesis and as yet we can ascribe to the chloroplasts no definite chemical 

 function. 



The size, form and number of chloroplasts in each cell varies con- 

 siderably in dififerent species. In some of the simplest forms each cell 

 contains one large chloroplast, often the shape of a disc or saucer. With 

 higher differentiation of the plants, the number of chloroplasts in each 

 cell increases. Haberlandt " attempted to show that there was a close 

 relationship between the number of chloroplasts in a leaf and its rate 

 of photosynthesis. He counted the chloroplasts in the mesophyll and cal- 

 culated the number of chloroplasts per unit area of leaf surface. For 

 example, in a leaf of Ricinus communis he found an average of 36 chloro- 

 plasts in the palisade cells and 20 in each cell of the spongy mesophyll. 

 There are about twice as many palisade cells, so that there may be ascribed 

 about 403,200 chloroplasts to the palisade tissue and 92,000 to the spongy 

 parenchyma i>er square millimeter. That is, about 82 per cent of the 

 chloroplasts are in the palisade tissue and 18 per cent in the spongy 

 mesophyll. The ratio of the number of chloroplasts in the two portions 

 of the mesophyll differs with the species. The total number of chloro- 

 plasts per unit area also varies, e.g., according to Haberlandt, TropcEohim 

 tnajus has 383,000 chloroplasts per square millimeter, Phaseolus mnlfi- 

 flonis 283,000, Ricinus communis 495,000, HeUanthus annuus 465,000. 

 Haberlandt endeavored to establish a proportionality between the num- 

 ber of chloroplasts and the rate of photosynthesis or the "specific photo- 



'•' Haberlandt, Jahr. wiss. Bot., 13, 74 (1882) ; "Physiologische Pflanzenanatomie," 

 1924, p. 256. 



