THE ROLE OF THE CYTOPLASM 231 



ter cells. In a unicellular green alga the daughter individuals ai-e like tlu^ 

 mother in being green because the chloroplast of the mother ('(>11 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 indi- 

 A'iduals is a character transmitted directly by the repeated division of 

 chloroplasts. 



A somewhat similar interpretation has been placed upon the inherit- 

 ance of chlorophyll characters in the higher plants, the supposition being 

 that plastids, multiplying only by division, are responsible for the dis- 

 tribution, in the individual plant and through successive generations, of 

 those characters which manifest themselves in these organs. Abnormali- 

 ties in chlorophyll coloring, such as pale greenness, whiteness, and 

 variegation, are accordingly attributed to an abnormal condition in the 

 chloroplast or the surrounding cytoplasm. Since the color itself is not 

 present in the plastids of angiosperm gametes, this character may resem- 

 ble ordinary Mendelian characters in being developed anew in each 

 generation, but it differs from them in depending upon the reproduction 

 and distribution of differentiated cytoplasmic organs, the plastids. 

 Indeed, it has been shown that the various known chlorophyll characters, 

 even those appearing much alike, fall into two categories: (1) those 

 inherited according to ordinary Mendelian rules, which is taken to mean 

 that the processes concerned in their color development are under the 

 influence of differential nuclear factors; and (2) those not so inherited 

 and therefore having their differential in the cytoplasm. Both types may 

 appear in the same genus or species, as in maize. It is to be emphasized 

 that the characters in both categories are developed under the influence 

 of both nucleus and cytoplasm but that they differ with regard to the 

 location and nature of the factors acting differentially. 



A classic example of the non-Mendelian type is a variegated four- 

 o'clock, Mirahilis jalapa albomaculata. Plants of this kind have some 

 branches with normal green leaves, some with white leaves, and some 

 A\T.th variegated leaves. Flowers are borne on branches of all three types. 

 Crosses between unlikes result in seedlings with the color of the maternal 

 l^arent or branch. For instance, when a flower on a green branch is 

 l^ollinated with pollen from a flower on a white branch, the oft'spring are 

 all green. In the reciprocal cross the offspring are all white and soon die 

 ])ecause of the lack of chorophyll. If flowers on variegated branches are 

 l)ollinated, offspring of all types may result. In no case does the pollen 

 affect the color of the progeny. 



The hypothesis proposed to account for these facts is that there is 

 present in the plant an abnormal cytoplasmic condition that prevents 

 the normal development of the chloroplasts. It is delivered directly 



