D. F. Jones and W. R. Singleton, of the Connecticut Agricultural Experiment Station, report the occurrence of spontaneous changes, or mutations, in lines of corn that have been inbred continuously for many generations. The mutated lines have reduced vigor, but when they are crossed with the pure lines from which they arose, the resulting hybridsfrequently are stronger than the parental pure lines. This remarkable finding does not by any means disprove the prevailing theory that hybrid vigor is due primarily to the action of dominant favorable-growth genes. It does suggest, however, that at least a part of the vigor may be due to something else—namely, to the complementary action of different forms of the same gene. It suggests also the practical possibility of using in the seed-production program the comparatively vigorous hybrids produced by crossing the mutated lines to their parental inbreds. Since a small number of genic differences (possibly only one) are involved in these crosses, the "hybrids" are actually pure lines in most respects. Crosses between two of these "hybrids" would be essentially single-crosses, with greater uniformity than possessed by "double-crosses" (crosses between two single-cross hybrids). Double-cross hybrids are now used by farmers almost to the exclusion of single-crosses, because seed for the latter is produced on low-yielding inbreds and is therefore very expensive.

A second recent contribution to corn breeding is the "gamete selection" technique originated by L. J. Stadler at the Missouri Agricultural Experiment Station. His plan emphasizes the greater genetic variability among gametes (unfertilized sex cells) than among plants, and hence the greater chance of recovering an exceptionally good gamete than an exceptionally good plant. Gametes from open-pollinated varieties are recovered by crosses to existing inbred lines; and performance tests of these hybrids indicate which received superior gametes. These best plants are then inbred, with further performance tests to insure retention of the genes for high yield. Gamete selection seems to be particularly well adapted to the replacement of individual inbreds in existing double-cross combinations. There is no theoretical reason, however, why the inbreds obtained through gamete selection could not be combined into valuable, new, double-cross combinations.

Another recent development in genetics is concerned with polyploid plants—how they behave genetically and how to produce them artificially. Polyploidy means the possession of some multiple of the so-called basic number of chromosomes, the minute bodies that carry the genetic factors, of the group to which the particular plant or animal belongs. For example, the basic number of chromosomes in the wheats is seven pairs—each kind of chromosome is present in duplicate, one being maternal and one paternal. That basic number is possessed by the einkorns, the diploid wheats. Another group, the emmers and durums, with 14 pairs, are the tetraploid wheats. The common wheats, with 21 pairs, constitute the hexaploid group.

Two different types of polyploids are recognized, autopolyploids and allopolyploids (or amphidiploids). In autopolyploids the increases in the, number of chromosomes are the result of doubling the chromosomes of a single species, so that in the tetraploid, for example, each chromosome is present four times instead of only twice. In allopolyploids the doubling of chromosomes is preceded by hybridization between two different species, so that, as in the diploid, no chromosome is present more than twice.

By use of the drug colchicine or certain other chemicals, plants with doubled chromosome number are now easily obtained. By treating seeds, or the growing tips of established plants, with colchicine in solution or in a paste mixture, polyploid sectors or entire polyploid plants are produced. Recently, Department scientists have simplified colchicine treatment of large numbers of plants by developing an aerosol, a sort of chemical fog, containing colchicine.

When A. F. Blakeslee and A. G. Avery, of the Carnegie Cold Spring Harbor station in New York, first announced the effectiveness of colchicine in doubling chromosome number, it was immediately obvious that here was an important new tool for plant research. Plant scientists immediately sensed its practical applications. Subsequently many polyploids have been produced, the majority of them being autotetraploids. For crops where the setting of seed is an important factor, few, if any, autopolyploids have proved of practical value, for seed set is usually considerably lower than in the diploid. Furthermore, while some autopolyploids may attain a final size greater than that of their diploid parent, this is not true of all; and autopolyploids tend to grow more slowly than diploids. Consequently, higher yields of forage can be expected only infrequently from autopolyploids.

But in some respects autopolyploids commonly excel diploids, and flower size is perhaps the most conspicuous of these characteristics. Flower gardeners have taken advantage of this circumstance, with the result that there are now on the market tetraploid marigolds, snapdragons, and pinks. Tetraploid lilies have been produced experimentally. Autopolyploids, too, frequently show an increased proportion of special substances. L. F. Randolph and D. B. Hand, of the Department of Agriculture and Cornell University, found, for example, an increased vitamin A content in tetraploid corn; and the content of gums and essential oils is known to be higher in autotetraploids of certain plants.