THE STRUCTURAL COMPONENTS OF PROTOPLASTS 



29 



Plastids. — Plastids are protoplasmic bodies characteristic of the 

 vegetable kingdom. They are present in nearl}' all plant cells and play 

 an important role in metabolism. Plastids confront the biologist with 

 problems of three kinds: cj'tological, biochemical, and phylogenetic. 

 Cytologists have already learned much about plastids, but some of the 

 most important points regarding their structure, visible alterations, and 



Fig. is. ('lil(ii(ii.l:i~t,-< ill «ametophytes of fern {Polypodium). a. luinual gameto- 

 phyte. 6, c, <l, persi.^^tont modifications (chain-like, plate-like, and budding types) induced 

 in chloroplasts by X-ray treatment of .spores. (.After L. Knudson.) 



relation to other cytoplasmic differentiations remain obscure. Bio- 

 chemists are gradually approaching a better understanding of the exact 

 chemical changes that take place in plastids during the all important 

 process of photosynthesis. They are also studying the significant rela- 

 tion of certain plastid pigments to vitamins. Biologists in gencMal would 

 like to know more about the histoi'ical origin of i)l;istids and their role in 

 the divergent evolution of organisms with different tyi)es of nutrition. 

 The familiar green chloroplasts (Figs. 18, 19) occupy a i)e('uliarl\' 

 strategic position in the living world, for within them carbon dioxide 



