CHAPTER XXV 

 CYTOPLASMIC HEREDITY 



In the foregoing discussions of heredity attention has been limited 

 to those characters which develop anew in successive generations under 

 the differential influence of nuclear factors. Since these factors are 

 carried by chromosomes, which are distributed in a definite manner 

 through successive life cycles, the characters dependent upon them are 

 inherited according to Mendelian rules. The characters in which 

 crossable organisms differ appear to be mainly of this kind (c/. p. 286). 

 In addition, there are some characters whose inheritance is non-Mende- 

 lian and depends rather directly upon peculiarities of the cytoplasm or 

 something it contains. Among these the best known are certain chloro- 

 phyll characters of plants. 



Chlorophyll Inheritance. — It is obvious that two successive genera- 

 tions of cells reproducing by division resemble each other partly because 

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

 of its 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 or associates with others of its kind to form a 

 colony, the color of the successive colonies or multicellular individuals 

 is a "metidentical" character transmitted directly by the repeated divi- 

 sion of chloroplasts.^ 



A similar interpretation has been placed upon the inheritance of 

 chlorophyll characters in the higher plants, the supposition 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, such as pale greenness, whiteness, and variegation, 

 are accordingly attributed to an abnormal condition or behavior of the 

 chloroplasts. Since the color itself is not present in the plastids of 

 angiosperm gametes, this character may resemble ordinary Mendelian 

 characters in being developed anew in each generation, but it differs from 

 them in depending upon the reproduction and distribution of differenti- 

 ated cytoplasmic organs, the plastids. Indeed, it has been shown that 

 the various known chlorophyll characters, even those appearing much 



' See Harper (1906, 1918a6) on Hydrodictyon and Pediastrum. 



414 



