CHAP, ii Photosynthesis 291 



it took place much earlier in the animal than the vegetable 

 kingdom ; it has almost disappeared from the former, and 

 is still slowly leaving part of the latter. In the remainder, 

 however, it has found its great development and its per- 

 manent abode. 



The development of knowledge of the chlorophyll 

 mechanism will be seen to afford a basis for the view just 

 stated, which has really gradually come into existence as 

 the result of the researches of the latter half of the century. 



The first advance in our knowledge of the mechanism 

 was made by Boussingault in 1864 and confirmed by him 

 in 1869, when he demonstrated that the appropriation of 

 the carbon dioxide is exhibited in full activity only by 

 normal green chloroplastids, or, as they were at first called, 

 chlorophyll corpuscles. Though these plastids had been 

 seen by Von Mohl, up to this time their activity had not 

 been asserted. 



In 1881 Pfeffer showed that the plastid, whose structure 

 had been meanwhile studied by Pringsheim, is not able to 

 work in the absence of the colouring matter which it normally 

 contains. In his experiments he made use of leaves which 

 did not develop their chlorophyll on account of their not 

 being supplied with iron, a relation which had been dis- 

 covered much earlier by Gris. 



Zimmermann, in 1893, confirmed Pfeffer's observation, 

 while in 1897 Kny showed that the chlorophyll without the 

 plastid is equally inefficacious. Kny made use of Engel- 

 mann's bacterium method for the detection of oxygen ; he 

 exposed to light drops of oil to which the colouring matter 

 had been added ; the bacteria were immersed in the solution 

 in air containing carbon dioxide, but no exhalation of 

 oxygen followed. Nor was Czapek more successful at the 

 end of the century. He introduced his coloured oil drops 

 into living cells, but they were without effect. 1 



1 Since the end of the century Usher and Priestley have claimed to 



T 2 



