292 The Physiology of Plants BOOK HI 



The plastid, or chlorophyll corpuscle, appears therefore 

 to be the essential part of the machinery. Its structure 

 was made the subject of study by Pringsheim in 1874. 

 He treated the plastids of leaves with dilute alcohol, in 

 which the pigment is soluble, and found a colourless stroma 

 left behind. Others he exposed to the action of steam, 

 when he found the chlorophyll exuded in viscid droplets. 

 The stroma which remained he described as a spongy or 

 trabecular structure, whose meshes are normally occupied 

 by the pigment. The latter he held to be only mechanically 

 associated with the framework. 



In 1883 the subject was further studied by A. Meyer, 

 who agreed with Pringsheim that the stroma is spongy in 

 character. In the colourless stroma he distinguished little 

 specks or ' grana ' of green colour, and he described them 

 as filling up the meshes of the network of the corpuscle. 

 He considered the pigment to be oily in its composition. 



Reinke in the same year agreed, from the absence of 

 fluorescence of the plastid, that there must be some kind 

 of combination between the latter and the pigment, fluor- 

 escence being a property of chlorophyll solutions. 



A lacunar structure was suggested by Chodat in 1892. 



At the end of the century Timiriazeff determined that 

 the colouring matter is limited to a thin layer on the 

 surface of the plastids. When the latter are examined by 

 red light, the so-called ' grana ' of Meyer can be seen as 

 small black specks in this layer, whose thickness he esti- 

 mated at -i of a micron. 



Timiriazeff found also that if chlorophyll grains oiPhajus, 

 which are rather larger than most, are burst by admitting 

 water to a solution of sugar in which they are mounted, 

 preparations showing this structure can be made with- 



be able to show that a strong solution of chlorophyll can decompose 

 carbon dioxide in the absence of the plastid. Their results have 

 not been confirmed. 



