THE ROLE OF THE CELL ORGANS IN HEREDITY 331 



general type of development and the sperm and egg nuclei supply only 

 the details" (p. 176). 



Plastid Inheritance. Certain cases of "plastid inheritance" have 

 been brought forward to show that the character of an organism may not 

 be entirely due to factors delivered to it by the gamete or spore nuclei. 

 It has been pointed out that two successive generations of cells repro- 

 ducing by division resemble each other for the obvious reason that the 

 organs of any given cell may actually become the corresponding organs of 

 the daughter cells. Thus in the case of a unicellular green alga the 

 daughter individuals are like the mother individual in being green because 

 the chloroplast of the mother cell is divided and passed on directly to 

 them. In those algae in which a swarm spore germinates to produce a 

 multicellular individual (Ulothrix etc.), or associates with others of its 

 kind to form a colony (Hydrodidyon, Pediastrum; Harper 1908, 1918a6), 

 the color of the successive colonies or multicellular individuals is a charac- 

 ter that i's transmitted directly by the repeated division of chloroplasts. 

 Thus, as Harper urges, the nucleus is not required here to account for the 

 resemblance between successive generations of cells or individuals, so far 

 as this character is concerned. 



A similar interpretation has been placed by some geneticists upon the 

 inheritance of "chlorophyll characters" in the higher plants, the supposi- 

 tion being that plastids, multiplying only by division, are responsible 

 for the distribution, in the individual plant and through successive 

 generations, of those characters which manifest themselves in these 

 organs. Abnormalities in chlorophyll coloring are accordingly held to be 

 due to an abnormal condition or behavior of the chloroplasts. 



Such a case is that of Mirabilis jalapa albomaculata, described by 

 Correns (1909). In plants of this race there are some branches with 

 normal green leaves, some with white leaves, and some with "checkered" 

 (green and white) leaves. Flowers are borne on branches of all three 

 types. In all cases crosses between unlikes result in seedlings with the 

 color of the maternal parent: inheritance is strictly maternal. For 

 instance, if a flower on a green branch is pollinated with pollen from a 

 flower on a white branch the offspring are all green. In the reciprocal 

 cross the offspring are all white, and soon die because of the lack of 

 chlorophyll. In neither case does the pollen affect the color of the 

 resulting individual. The explanation offered by Correns for the color- 

 less condition is that it is due to a cytoplasmic disease which destroys 

 the chloroplasts. It is therefore delivered directly to the next generation 

 in the egg cytoplasm, and is not transmitted by the male parent because 

 no male cytoplasm is brought into the egg at fertilization. If it had been 

 due to nuclear factors it would have been transmitted by both parents, 

 since the nuclear contributions of the two are equal. This condition is 

 analogous to that occasionally found in animals, in which bacteria may 



