PLASTIDS 



With the possible exception of the very lowest groups of plants, 

 plastids are found in the cells of most tissues of members of the plant 

 kingdom. Plastids vary in size, number, shape, and chemical organization 

 with the tissue and organism concerned. "Mature" plastids can be shown 

 to arise from more or less undifferentiated plastid primordia which occur 



Figure 3-14. Electron Micrograph of Section of the Photosynthctic Bac- 

 terium, Rlwdospirilluin nihriini. Note the discrete photosynthctic particles 

 (chromatophores) which pack the entire cytoplasm of the cell when it is 

 exposed to light during growth. (From Murray, R. G. E., 1960. "The In- 

 ternal Structure of the Cell," //; "The Bacteria," I. C. Gunsalus and R. Y. 

 Stanier (Eds.), Vol. I, Academic Press, New York, N. Y., Fig. 10. p. 49. 

 Courtesy of Dr. R. G. E. Murray. University of Western Ontario.) 



in meristematic cells, or those of the developing embryo. Such primordia 

 range in size down to the virtually invisible, so that it is difficult, indeed, 

 to determine their origin. In general, plastids appear to be centers of 

 carbohydrate synthesis and metabolism. They are generally classified on 

 the basis of color, though such a classification is not necessarily related 

 to function. Noncolored plastids are usually referred to as leucoplasts, 

 while colored ones are called chromoplasts. Since leucoplasts may give 

 rise to various types of chromoplasts, such classification is of only super- 

 ficial value. Leucoplasts often may function as storage organelles by 



STRUCTURE AND FUNCTION OF CYTOPLASMIC ORGANELLES / 39 



