Bud Variation 



121 



gous. This may be true, but it would not be safe to conclude that 

 there is any mechanism ever provided in somatic tissue which 

 corresponds to the normal Mendelian mechanism for segregation 

 during gametogenesis. If "somatic segregation" ever takes 

 place, it is through the operation of some quite different mecha- 

 nism, as indicated in the examples given below. 



1. Chromosome elimination. — A theoretical illustration 

 would be as follows. A plant heterozygous for linked genes, A-B 

 a-b, has an irregular mitosis take place in some part of its somatic 

 tissue. One of the daughter-nuclei of this mitosis fails to receive 

 its full complement of chromosomes, the A-B chromosome hav^ing 

 somehow been eliminated. This cell and its progeny, haploid now 

 with respect to this chromosome pair, which is represented only by 

 the a-b chromosome, will form tissue in which the recessive char- 

 acters a and b will become manifest. 



This would be the principle underlying somatic segregation 

 through chromosome elimination. As a matter of fact, there are 

 really only two clearly demonstrated cases of this sort, and both 

 of these are limited to rather special situations. In one of these 

 cases, " gynandromorphism " in the fruit fly, a special chromo- 

 some set is involved, the sex chromosomes. This will be taken up, 

 therefore, in the chapter on "Sex determination." In the other 

 case, endosperm "mosaics" in corn, a special triploid tissue, the 

 endosperm, is involved. This case will be discussed in the chap- 

 ter on "The endosperm in inheritance." 



2. Chimaeras. — A chimaera is a plant in which some of the 

 tissues have all of the characteristics of one variety or species, 

 while the rest of the tissues on this same plant are characteristically 

 those of a different variety or species. The most famous chimae- 

 ras are the "graft-hybrids" of Solanum produced by W'inki.kk 

 (lo). This investigator made grafts of two distinct species of 

 this genus, the tomato and the nightshade. After the tissues of 

 stock and scion had been given time to "weld" together, Wink- 

 ler cut the stem in such a way that the exposed cross-section 

 was made up partly of tissues of the stock and partly of scion 

 tissues. From this cut surface, adventitious buds would arise, 

 and at times these buds came at the exact point where stock and 

 scion tissues were in contact. Such buds developed branches which 



