540 Cytogenetics and Evolution 



ful there since the pedigree culture method cannot be applied 

 to human beings. 



Nonchromosomal Inheritance 



Up to this point we have studied the behavior and importance 

 of genes and chromosomes in heredity and evolution. However, 

 there seems to be considerable evidence that certain bodies in the 

 cytoplasm such as plastids and mitochondria are autonomous 

 bodies, reproducing by division and arising apparently only from 

 the division of preexisting plastids and mitochondria. We might 

 well inquire, therefore, whether they are of any importance in 

 evolution and heredity. 



There is definite evidence that the inheritance of chloroplasts 

 sometimes is merely a matter of the division of the plastids them- 

 selves. The discovery of this type of inheritance dates back to 

 the early days of genetics to the work of Correns on the four- 

 o'clock, Mirabilis jalapa. The normal chloroplasts in the leaves 

 of this plant are dark green in color, but in one particular strain 

 large areas of the leaves have plastids which have considerably 

 less than the normal amount of chlorophyll. Areas with these 

 chlorophyll-deficient chloroplasts are consequently very pale 

 green, pale yellowish, or white. In regions where the normal 

 green and the white areas are contiguous, both cells with normal 

 chloroplasts and with defective chloroplasts are to" be found. 

 Plants with green and wliite areas in the leaves are variegated, 

 and frequently this condition may apply to branches as well as 

 to leaves. In Mirabilis, for example, occasionally entire branches 

 may be white on an otherwise green or on a mottled green and 

 white plant, and some branches also may contain patches of both 

 green and white cells. 



Apparently independently of the genotype of a variegated 

 plant, all the seeds developed on wholly green branches produce 

 only plants which are entirely green whereas those seeds that 

 develop on a white branch of the same plant produce only plants 

 entirely deficient in normal chlorophyll. When seeds that de- 

 velop on the branches which are a mosaic of green and white 

 tissue are planted, some of the resulting plants are completely 

 green, some are variegated, and others are wholly white. Nor- 

 mally, chloroplasts are developed in the seed but not ip the 

 pollen grain or pollen tube so that all the chloroplasts a plant 



