18 



PROTOPLASM 



manufacture of food in plants. A fundamental distinction 

 between plants and animals is the ability of the former to manu- 

 facture food (organic matter) from inorganic material. This 

 is possible because of the presence in plants of chlorophyll which, 

 acting as a catalyst, or stimulator of chemical reactions, converts 

 water and carbon dioxide into sugar with the aid of the sun's 

 energy. Chloroplasts do not occur in typical animal cells. 



Fig. 10. — a, A cell (fibroblast in culture, boundary indistinct below) with six 

 nuclei, all of which have two or more nucleoli; b, animal cell (fibroblast, a con- 

 nective tissue cell from a chick embryo culture) with mitochondria: 1450 X 

 (from Warren Lewis); c, chromosomes of pine (cambium) beginning to migrate 

 from the equatorial plate; d, chromosomes of pine (cambium) at the equatorial 

 plate, (c and d from I. W. Bailey.) 



The chloroplast is one type of plastid, or "small body," in cells. 

 Other more minute ones are ynitochondria, or chondriosomes, 

 which have enjoyed considerable scientific publicity. They 

 are granules, rods, or threads of varying size and shape occurring 

 in practically all cells (Figs. 5, 106). What they really are no 

 one knows. Guilliermond has made a very extensive study of 

 mitochondria and believes them to play an important part in the 

 life of the cell. Being motile, they were once believed to be 

 independent organisms, like a colony of bacteria enjoying the 

 hospitality of a host, but this belief lacks convincing evidence. 

 The names given to them are legion. At least half a hundred 



