68 S. S. COHEN 



chloroplasts tend to have fewer of these units than cells possessing smaller 

 plastids. It has been reported that division occurs by construction of an 

 elongated plastid, culminating in a pinchmg-off process in the center. 



The reproduction of the chloroplast and inheritance of its structures are 

 controlled in part by nuclear genes in a Mendelian fashion. Nuclear mutations 

 may lead to disturbances in chlorophyll synthesis and to the production of 

 colorless plastids, as in the iojap maize character of chlorophyll striping 

 (Rhoades, 1946). The reproduction of these mutant plastids, once formed, 

 may now proceed and, despite the reintroduction of the normal nuclear gene, 

 the cell may continue to produce colorless plastids. Thus, certain genetic 

 characters determining chloroplast structure may be transmitted by the 

 plastid relatively independently of the nucleus. 



The relation of the chloroplast to mitochondria has been mentioned earlier. 

 It has been noted that once the cell loses all its plastids, following growth in 

 the dark or growth in the presence of streptomycin (ProvasoH et al., 1948), 

 they are never regained. Chlorophyll may disappear m some instances as a 

 result of growth in the dark, but if the plastid remains, it can generate the 

 pigments again in the presence of light. This process has recently been 

 studied in Euglena by means of electron microscopy (Wolken and Palade, 

 1953). 



A poor supply of N or Fe can also reduce the rate of chloroplast production. 

 On the other hand, a supply of UNA and amino acids selectively promotes 

 chloroplast growth without affecting the rate of cellular multiplication 

 (Deken-Grenson, 1957). 



Chloroplasts may also orient themselves within the cell in response to light. 

 It is evident that their synthesis, turnover, movement, and reproduction 

 present many important unsolved problems, other than those related 

 directly to photosynthesis. 



It has proved to be rather difficult to obtain homogeneous preparations of 

 chloroplasts. Careful studies of preparations of these particles have revealed 

 that homogenization of plant tissue tended to yield materials heavily con- 

 taminated with chromatin threads, mitochondria, and other ceU fragments. 

 The botanists have been somewhat provoked by the ease with which enzymo- 

 logists and other biochemists have designated their preparations as "chloro- 

 plasts" and it is felt that many studies on the distribution of enzymes in the 

 plant cell are not to be taken seriously in the absence of careful cytology, side 

 by side with the chemical studies. 



During isolation, structures known as "grana" frequently become visible 

 within the chloroplasts. These optically dense, wafer-shaped bodies are 

 embedded in a clear protein-containing matrix or "stroma." The grana are 

 about 6000 A in diameter and 800 A thick, and seem to be rather imiform in 

 size. It is probable that they contain the concentrated pigments and are 



