CHLOROPLASTS 223 



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 distri- 

 buted evenly throughout the stroma, whilst, on the other 

 hand, others maintain that it is restricted to the peripheral 

 layers of the plastid. 



Amongst the most recent contributions to the subject is 

 the investigation of Priestly 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 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. 



On the other hand, other investigators hold that the 

 chloroplasts may arise from differentiated parts of the proto- 

 plasm, which parts are not plastids. Lewitski f draws atten- 

 tion to the presence of minute bodies occurring in the proto- 

 plasm, but not in the nucleus, which he calls mitochondria, 

 chondriosomes, etc. These structures, which he considers are 

 essential parts of the cytoplasm, increase by division, and give 

 origin to the plastids. For instance in the pea, Pisiim sativum, 

 and the asparagus. Asparagus officinale, the mitochondria of 

 the cells of the stem apex give rise to chloroplasts, whilst 

 those of the apex of the root are converted into leucoplasts. 



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

 t Lewitski: " Ber. deut. bot. Gesells.," igio, 28, 538. 



