cytology: the study of the cell 145 



plastids in the evening primrose are known to have mutated on rare occa- 

 sions, as a result of which they have taken on new characteristics (Schotz, 

 1954). 



Plastid inheritance is therefore a well-known phenomenon and has been 

 studied intensively in a number of organisms. Since plastids or their pre- 

 cursors are usually transmitted through the egg but not the sperm, their in- 

 heritance is usually maternal in character; i.e., the offspring receives its 

 plastids only from the mother. There are exceptions, however, where plastids 

 may also be introduced into the offspring along with the sperm. 



But it is not only the plastids which are of interest from the standpoint of 

 cytoplasmic inheritance. The work of Sonneborn and others has drawn at- 

 tention to the fact that certain hereditary characteristics require the inter- 

 action of both the genes and certain elements in the cytoplasm. The "killer" 

 characteristic in Paramecium, a one-celled animal, is a case in point (Sonne- 

 born, 1946). Some individuals in Paramecium aurclia are capable of excreting 

 a substance into the surrounding water which will kill other "sensitive'' ani- 

 mals. To be a killer a cell must have a certain gene (K) and in the cyto- 

 plasm it must have minute particles which are known as kappa particles. If 

 both are present, the animal is a killer. If, however, the K gene is present and 

 the kappa particles absent, the cell cannot produce the killer substance. 

 Kappa particles are self-reproducing bodies, but they can live and repro- 

 duce only in the presence of the K gene. In the absence of this gene they soon 

 vanish. The K gene, on the other hand, cannot synthesize new kappa par- 

 ticles. If none of the particles are present in a cell possessing K, none can be 

 formed. In other words, kappa particles must come from preexisting kappa 

 particles. The conditions under which these particles can divide and function 

 are set up by the K gene, but the K gene cannot create new particles. The 

 particles are therefore dependent upon the gene for the proper environment 

 in which to continue existence, but not for their origin; the gene is de- 

 pendent on the particles to carry out the process of killer-substance forma- 

 tion. Thus a Mendelian or hereditary character is dependent for its expres- 

 sion on both a gene and a type of cytoplasmic particle. A number of other 

 cases of this type of inheritance are known. 



Another type of particle in the cytoplasm is the chondriosome, or mito- 

 chondrion. Chondriosomes have been known for 50 years or more, but they 

 have not received much attention until recently. They are ordinarily minute 

 spherical or rod-shaped bodies, about the size of bacteria, with which they 

 have been confused by some investigators. For a long time their function 

 remained obscure, although their universal presence in all plant and animal 

 cells indicated that they had a vital role to play. In recent years, however, 

 a combined biochemical and cytological approach has shown that they are 

 the chief centers of respiration, the regions where most of the enzymes are 

 situated which together bring about the liberation of the energy needed by 



