530 



The Journal of Heredity 



LIKE A FINGER IN A GLOVE 



Diagram of edges of two geranium leaves in cross-section, after Baur. (a) is 

 that of a variegated geranium ; the outer part is white, but inside there is 

 a mass of green tissue, entirely overlaid by white, (b) is a normal green 

 leaved geranium, no white cells being present. The section enclosed in 

 the small rectangle in each one is shown enlarged in Fig. 9, the following 

 illustration. (Fig. 8.) 



erous ways in which experiments with 

 them may yield commercial results, as 

 well as throw light on problems of plant 

 physiology and heredity, and the study 

 of chimeras is probably destined to be- 

 come a regtilar part of experimental 

 horticulture. It is not always easy, 

 however: Winkler made 268 grafts, 

 which resulted in the production of more 

 than 3000 sjjrouts, in order to get the 

 five chimeras which stand to his credit. 



THE TEST OF BREEDING. 



Discovery of the nature of these 

 chimeras was made possible only by the 

 association of the microscope with 

 genetics, for mere observation of their 

 behavior when propagated would hardly 

 have cleared up the mystery. If they 

 arc to be considered really hybrids, they 

 ought theoretically to represent the 

 hybrid generation (Fi) in spite of the 

 fact that they were produced by grafts 

 rather than seeds. Their own seedlings 

 should then represent the critical second 

 filial (F2) generation, and either show a 

 new type that bred true (a behavior 

 that, in hybrids of distinct species, 

 would occasion no great surprise) or 

 else S])lit up into a variety of forms, as 

 most hybrids do in that generation. 

 It has already been mentioned that the 

 seedlings of Solanum tubin^ense were 

 nothing more nor less than little night- 

 shades, which bred true in all succeeding 

 generations. Similar results were found 



with the other three chimeras, each one 

 breeding true to the parent which it 

 most resembled. 



Another suggested test was to com- 

 pare them with the normal sex-hybrids 

 produced by cross-pollinating the to- 

 mato and nightshade. This could not 

 be done, however, because the investi- 

 gators were not able successftilly to 

 cross those two species. There was one 

 line of attack left — namely, a study of 

 the cell contents. Those who possess 

 an elementary knowledge of biology 

 know that each cell has a nucleus con- 

 taining certain little rods of easily- 

 stained material called chromosomes, 

 that the ntimber of chromosomes varies 

 from species to species, but that within 

 each species it is constant. This, there- 

 fore, gave a means of identifying either 

 one of the parents found in the sectorial 

 chimeras. The tomato possesses 24 

 chromosomes, the nightshade 72. In a 

 real hybrid, the number of chromo- 

 somes would, by expectation, be the 

 sum of half of each of these numbers, 

 that is, 12 + 36 = 48. On the other 

 hand, as this hybrid was created by 

 vegetative means, the reduction to one- 

 half the normal number, which occurs 

 in the chromosomes in sexual conjuga- 

 tion, might conceivably not take place; 

 if it did not, the sectorial chimeras 

 would be expected to possess 24-|-72 = 96 

 chromosomes. So the microscope was 

 brought out, and the chromosomes of 



