SECTION VI. 



PIGMENTS. 



CHLOROPHYLL. 



As is well known, chlorophyll is contained in the chloro- 

 plasts which are universally present in green plants and vary 

 considerably in their size, shape, and number within the cell. 

 With regard to their structure there has been much dispute. 

 It is, however, generally agreed that the structure of the 

 plastids is either reticulate or vacuolate. 



The pigment itself is variously stated to be dissolved in 

 some oily substance which is held in the channels and meshes 

 of the plastids, or to exist in the form of a precipitate ; and 

 with regard to the distribution of the pigment within the 

 plastid there is again some dispute. According to many, it 

 is distributed evenly throughout the stroma, whilst, on the 

 other hand, others maintain that it is restricted to the peri- 

 pheral layers of the plastid. 



Amongst the most recent contributions to the subject is 

 the investigation of Priestley and Irving * on 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 TimiriazefFs 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. 



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 nova 

 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 



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

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