Collins: Reversion in Composites 



133 



bear a single imperfectly developed 

 floret or may have in the place of a 

 floret a smaller capitulum which has 

 proliferated in exactly the same way on 

 a smaller scale. In the latter case each 

 smaller pedicel bears an imperfectly 

 developed floret. The capitulum has in 

 such a case produced a compound 

 umbellate inflorescence. On the recep- 

 tacle each pedicel which replaces an 

 achene appears to be subtended by a 

 palea or bract. 



The appearance of palea-like bracts 

 on the receptacle subtending achenes 

 (Fig. 16) may be considered as a parallel 

 change to a still further remote ances- 

 tral condition if we assume that in the 

 precomposite, umbellate progenitor the 

 palea-like bracts on the receptacle, 

 excepting those of the outer florets, had 

 disappeared already. (Fig. 16.) 



The appearance of such reversions 

 cannot be predicted but it is possible 

 to ofi"er a very logical explanation for 

 their appearance. Such characters in 

 the ancestral form may have been the 

 somatic expression of a number of 

 interacting genetic factors. During 

 evolution of the family, these factors 

 became separated into diff^erent indi- 

 viduals and perhaps even into diff'erent 

 species so that they no longer were able 

 to produce their typical combined 

 somatic efl"ect. The factors still exist, 

 but in a separated and inactive condi- 

 tion at least insofar as the palea are 

 concerned. As a result of cross fertili- 

 zation and the operation of the law of 

 chance there are brought together in a 



few cases all the necessary factors 

 within a single plant and the character 

 is again produced as a result of their 

 recombination. 



The cross of two white Emily 

 Henderson sweet peas, which in the 

 Fi hybrid produced a purple sweet pea 

 like the native sweet peas of Sicily, is 

 well known as a typical case of rever- 

 sion to an ancestral character. In this 

 case two factors operating together in 

 the same plant produced the purple 

 color typical of the peas of Sicily, but 

 either factor by itself could produce 

 only plants with white flowers. Each 

 parent had contributed to the hybrid 

 plant what the other parent lacked. 



The most convincing evidence for 

 this factor explanation of the sudden 

 appearance of reversionary types is 

 furnished from the work with Droso- 

 phila melanogaster. It is possible to 

 take two mutant types^ which have 

 bred true to the mutant character for 

 many generations, each differing mark- 

 edly in at least one character from the 

 wild type, and in the first generation 

 from a cross between them produce the 

 wild type of fly. 



Another possibility which may be 

 considered is that a mutation occurs 

 which restores to a single plant a 

 genetic factor which had been eradi- 

 cated from the hereditary material by a 

 loss mutation during the evolution of 

 the species. The final conclusions must, 

 in any case, be drawn from the results 

 of breeding tests and experiments. 



2,Types of flies which are different from the normal wild flies from which they originated and 

 which have appeared as a result of a sudden change in the constitution of the hereditary germmal 

 material. 



The Farmer's Botany 



Text- Book of Pastoral and Agri- 

 cultural Botany, for the study of 

 the injurious and useful plants of 

 country and farm. By John W. 

 Harshberger, Ph.D., professor of 

 botany. University of Pennsylvania. 

 With 121 illus. Pp. 294. Philadel- 

 phia, P. Blakiston's son and Co., 

 1920. 



Dr. Harshberger has compressed 

 within small compass much useful in- 

 formation and references on poisonous 

 plants, and also on the principal eco- 

 nomic plants. The material was origi- 

 nally presented by him to classes of 

 veterinary students. It will be valued, 

 however, by anyone with some scien- 

 tific education, who is interested in 

 agriculture. — P. P. 



