300 The Physiology of Plants BOOK in 



so to speak, from which all the organic substances of the 

 plant proceed by progressive chemical changes.' (Ibid., 

 P- 308.) 



The point which thus chiefly attracted Sachs' attention 

 during these researches was the appearance of starch. 

 Holding it to be, as he said, the first evident product of 

 assimilation, he gave his attention primarily to means of 

 demonstrating its formation and to determining the con- 

 ditions under which it makes its appearance. It was during 

 these researches that he devised his well-known macroscopic 

 test for the presence of the photosynthetic changes. By 

 shading with some opaque covering part of a leaf which 

 was being exposed to sunshine, and later, with proper 

 precautions, staining the contents of its cells with iodine, 

 he showed that starch is only formed in the cells which 

 receive the bright illumination. We must not, however, 

 think that the probable complexity of the formative pro- 

 cesses escaped his attention. To this point we shall return. 



The next point in the development of knowledge was 

 made by Godlewski, who was repeating and extending 

 Sachs' researches in 1873. Sachs quoted him as demon- 

 strating 



' By experiments as ingenious as they were simple that 

 in an atmosphere devoid of carbon dioxide no starch 

 is produced in the chlorophyll corpuscles, even in the light. 

 He also found that the starch produced in the chlorophyll 

 disappears not only in the dark, but even in intense light 

 when the surrounding air contains no carbon dioxide.' 



This point, that the air, and not the soil, is the source of 

 the appropriated carbon dioxide was further elaborated in 

 1877 by Moll, who found that unless particular leaves were 

 in contact with the gas, no starch appeared in them, even 

 though all the products of absorption from the soil could 

 reach them and they were adequately illuminated. Sachs 

 himself repeated the demonstration in 1882 and showed 



