72 BOTANICAL GAZETTE [JULY 
Although it is theoretically attractive, its failure to satisfy these two important 
details of the hybrid vigor situation has condemned it. 
Jones’ has ingeniously modified the dominance hypothesis so as to avoid 
these difficulties. At first consideration his theory seems to be clearly the most 
reasonable explanation of hybrid vigor that has yet been presented, although 
in time it may encounter destructive criticism. The argument is essentially the 
same as that for the old dominance hypothesis, with the following important 
modification. Assume that one parent contains the dominant determiner A, 
linked with the recessive c; on another chromosome it contains B linked with d. 
The total formula may be expressed conveniently as Ac, Bd. The other parent 
has the formula aC, bD. The hybritl is more vigorous than either parent 
because it combines all 4 dominant determiners. The attractiveness of this 
scheme is that it escapes the objections that were made to the older dominance 
hypothesis: (1) the fact that 100 per cent hybrid vigor cannot be fixed is quite 
in accordance with JoNnEs’ scheme, for it is obviously impossible to isolate a 
homozygous race, combining the 4 dominant determiners, A, B, C, and D 
(unless crossing over occurs); (2) a simple mathematical derssnatention will 
show that the distribution of F. individuals (with respect to hybrid vigor) is 
quite what it should be, represented by a symmetrical curve, similar to the 
curve of probabilities. In fact, this new theory, ‘‘the dominance of linked 
factors,” seems altogether sound. We should reasonably expect that each 
chromosome would contain one or more dominant determiners (conducive to 
vigor) linked with one or more recessives. In this day of factors and determi- 
ners such a hypothesis is quite appropriate. It may be, however, that in the 
future such a phenomenon as hybrid vigor may be explained on the basis of 
the stabilities and reactivities of the constituents of specific protoplasts.— 
MERLE C. COULTER. 
Taxonomic notes.—BLAKE‘ has published a fascicle of papers containing 
descriptions of new species. In the paper dealing with Compositae new species 
are descri in A poorer’ SP saint aaat: Verbesina, Liabum, and 
Cirsium. Collections from Vene and Curacao contain new species in the 
following genera: Ruprechtia @), Metis Bauhinia, Croton (2), Mayltenus, 
Zizyphus, Vismia, Hecatostemon (a new genus of Flacourtiaceae), Passiflora, 
Jacquinia, Bumelia, Aspidosperma, Plumeria, Marsdenia (2), Lycium, Tabebuia, 
Dianthera, Oxycar pha (a new genus of Compositae), Simsia, and Verbesina. 
The new species from Oaxaca are referred to Iresine (2), Amyris, Guarea, Tri- 
3 Jones, D. F., Dominance of linked factors and heterosis. Genetics 2:466-479- 
ie 
4 Brake, S. F., II. Further new or noteworthy Compositae. Contrib. Gray 
Herb. N.S. no. 53. pp. 23-30. 1918. 
, New Spermatophytes collected in Venezuela and Curacao by Messrs. 
aI 
Curran and : - Pp. 30-55. 
cree plants from pn Ibid. pp. 55-65. 
