CHLOROPHYLL 309 



Priestley and Irving * have investigated the chloroplasts 

 of certain species of Selaginella and Chlorophytum. They find 

 that the pigment is restricted to the peripheral regions of the 

 chloroplast, where it is held in the meshes of the network 

 of the matrix. They agree with Timiriazeff's views that the 

 function of the chlorophyll necessitates its distribution in very 

 thin layers in order that the amount of energy set free may be 

 as great as possible. 



Zirkle,f from his study of the chloroplasts of Elodea, 

 Phajus, Cabomha, Marchantia and other plants, concludes that 

 the plastid is a hollow, flattened, prolate spheroid, the 

 stroma of which is perforated by a large number of pores 

 connecting the central vacuole of the plastid with the sheath 

 of non-granular cytoplasm surrounding the plastid. This 

 sheath is more or less permanent but no differentiated mem- 

 brane of the plastid could be demonstrated. The pigments 

 are intimately mixed and are evenly distributed throughout 

 the stroma, coating the colloidal protein particles. In the 

 leaf, starch granules are included in the central vacuole of 

 the plastid which vacuole is thought to contain a dilute 

 aqueous solution of sugar and protein. Zirkle is of the opinion 

 that the chlorophyll is not in lipoidal solution since the pig- 

 ment can be extracted from chloroplasts by solvents which 

 cannot extract it from a lipoid solution. Further, the chloro- 

 phyll in plastids removed from a cell show a marked photo- 

 stability, whilst chlorophyll in solution is quickly destroyed 

 by light. 



With regard to the origin of the chloroplast there is also 

 some dispute. The general view, due originally to Schimper 

 and Meyer, appears to be that plastids do not arise de novo 

 within the cell, but by the division of pre-existing plastids, so 

 that, in this respect, there is continuity between parent and 

 offspring. This has led to the conception that originally the 

 chloroplasts once had a separate individuality, and that, in 

 a sense, ordinary plants are parasitic upon the imprisoned 

 plastids which have become permanent members of the 

 structures of the cell. 



* Priestley and Irving : " Ann. Bot.," 1907, 21, 407. 

 f Zirkle : " Amer. Journ. Bot.," 1926, 13, 301, 321. 



