PLANT CELL GROWTH AND NUTRITION 465 



seem to be a retrograde step, since salt accumulation seems to be in- 

 dissolubly linked to the whole cellular organization and, in addition, 

 to the ability of the cell to grow. The mitochondrion in situ is doubt- 

 less the site of production of active phosphorylated compounds, but it 

 seems erroneous to conclude that, for this reason, it should be either 

 an active seat of salt accumulation or necessarily an active site of pro- 

 tein synthesis. Both ideas have been suggested— the former by Robert- 

 son et al. ( 1955 ) , the latter by Webster ( 1955, 1957 ) . Neither of these 

 ideas, insofar as they relate to isolated mitochondria, now seems profit- 

 able, especially because they divert attention from what may be the 

 more essential fact. This is that cells are highly compartmentalized. 

 Reactions and stimuli at one point result in actions and responses else- 

 where, and the chief challenge of modern plant cell physiology is to 

 determine how this highly discrete system operates in an integrated 

 way. The true role of the mitochondrion would seem to be that, 

 through its activities as a center of oxidation, the high-energy phos- 

 phate compounds may be made only to be released to the cytoplasm 

 and thus convey the energy to the point of application. 



In this context a recent study (Sutcliffe, Bollard, and Steward, 

 1960) of particles, obtained under aseptic conditions from cultured 

 carrot cells by the methods used to isolate mitochondria, presents some 

 significant features. The cultured carrot cells, growing on a medium 

 containing the growth-stimulating substances of coconut milk (see 

 Figure 1 ) , not only synthesized protein readily but synthesized a 

 special protein moiety which incorporated C^^-proline very avidly 

 ( Figure 5 ) and there converted it progressively into hydroxyproline 

 (Pollard and Steward, 1959). The ratio of C^^-proline to C'^-hydroxy- 

 proline in the hydrolyzed total protein tends to come to a value of 

 about 0.7. Thus one can use this technique as a sensitive and direct 

 measure of protein synthesis in the growing cells. In this way it has 

 been shown that exogenous C^^-proline appears appreciably in the 

 protein after as little as 15 minutes, and it increases there linearly with 

 time thereafter. The work of Sutcliffe, Bollard, and Steward ( 1960 ) 

 showed quite convincingly that some C ^-proline could be incorporated 

 into the protein of the supposed mitchondria as they were isolated from 

 the same type of cells. However, if one measured both the protein 

 synthesis and proline incorporation by the intact cells and by the 

 particles isolated from them, they were not even of the same order of 

 magnitude. Furthermore, there was an important qualitative difference, 

 for the particles which, from their method of preparation, would be 

 called mitochondria never formed hydroxyproline in their protein, 

 although they did incorporate some C^^-proline. Again, studies have 

 been made of the absorption of Cs^'" by the intact cells and by the iso- 

 lated particles. If the latter (regarded as mitochondria from their 



