plastids increase in size but do not develop grana. These plastids can 

 synthesize chlorophyll in the presence of light but it is broken down as 

 fast as it is formed. In the barley mutant, xantha-3, the plastids differ- 

 entiate to the extent of developing some internal layered structures, but 

 no true lamellae are ever found and large numbers of globuli accumulate 

 within the plastid. Some chlorophyll and carotenoid pigments are synthe- 

 sized and localized in the globuli, but since there is no extensive lamellar 



Figure 3-18. Schematic Diagram Depicting the Sequence of Changes in 

 Plastid Structure of Etiolated Primary Leaf of Phaseolus vulgaris L. Grown 

 in the Dark and Light. It can be seen from the diagram that exposure of 

 10- to 16-day dark-grown leaves to light results in rapid reorganization of 

 plastid structures and formation of the grana. (Courtesy of Dr. D. von 

 Wettstein, Forest Research Institute, Stockholm, Sweden.) 



structure formed these cannot be properly distributed as in the normal 

 plastid. In the xantha-10 mutant, lamellae are developed, but rather than 

 being arranged in the more typical parallel fashion, are concentrically 

 orientated. The structural organization of plastids in this mutant is similar 

 to that found in plastids of etiolated plants grown in the dark (Figure 

 3-19). Biochemical studies indicate that this concentric arrangement of 

 lamellae is correlated with a deficiency in the synthesis of chlorophyll 

 precursors in the xantha-10 mutant. The fact that plastids in this particu- 

 lar mutant develop a lamellar structure in spite of the apparent block in 

 chlorophyll synthesis, suggests that development of lamellae may be 

 independent of the synthesis of chlorophyll. 



STRUCTURE AND FUNCTION OF CYTOPLASMIC ORGANELLES / 45 



