124 Outline of Genetics 



to white individuals only, and would evidently continue to breed 

 true to the white condition if white individuals could be matured. 

 In like manner the variegated hybrid may give rise to a pure green 

 branch, which would start a line of pure green individuals. 



A suggested explanation of this situation is that the white 

 condition results from the occurrence of purely colorless plastids 

 in the tissue, while the green condition has the normal green 

 chloroplasts. A cross between the two types will introduce into 

 the hybrid zygote a mixture of green and white plastids ("plastid 

 primordia"); and the same result will be obtained whichever way 

 the cross is made, since the male as well as the female parent con- 

 tributes plastids to the zygote. During somatogenesis in the plant 

 which develops from such a zygote, there will be an inevitable 

 segregation of green and white plastids, since there is no mecha- 

 nism provided for a perfectly even distribution to daughter-cells 

 of those cell components which lie outside the nucleus. If the 

 number of plastids per cell be not too large, sooner or later, 

 through the operation of the laws of chance, cells will arise which 

 contain plastids entirely of one sort or the other, and these will 

 produce tissues which are pure green or pure white. 



B. The chlorophyll character governed by the distribution 

 of other and finer cytoplasmic elements than the plastids them- 

 selves. 



Again male and female parents both contribute the effective 

 extra-nuclear elements to the hybrid zygote, and again a tendency 

 tow^ard irregular segregation appears during somatogenesis of the 

 resulting plant. In this case, however, the effective units are so 

 small and numerous that a complete segregation of units of the 

 two types is never achieved, but merely the production of rela- 

 tively paler and relatively greener regions on the plant. 



The behavior of Ikeno's (7) albomaculaia type of Capsicum 

 might be interpreted on this basis. Paler and greener patches 

 occur on the albomaculata plants, and the average paleness of the 

 whole individual may be greater or less. Since this "average 

 paleness" of the parent is reflected in the nature of the offspring, 

 from any sort of a cross, it is felt that such a parent produces 

 gametes having a characteristic proportion of green and white 

 elements or units, whatever these elements or units may be. 



