In contrast to the high fertility of allotetraploids is the reduced fertility of autotetraploids, which arise following doubling of the chromosome number in a fertile diploid species.

Although most autotetraploids have larger seeds than do comparable diploids, there is a significant amount of seed abortion, and the potential yield is not realized. A large number of autotetraploids have been induced by colchicine treatment, but they have not yet been of much commercial importance in plants where the grain is the marketable crop.

The sterility of autotetraploids has been ascribed to their somewhat irregular chromosomal assortment at meiosis, and the consequent aneuploidy is assumed to cause seed abortion. It has also been suggested that the sterility is due to genitally determined pollen incompatibility.

Selection is possible for increased fertility of autotetraploids, however, and one tetraploid variety of rye with a satisfactory grain yield is grown commercially in Sweden. Work in progress in the United States indicates that similar success may be had for tetraploid corn.

Seeds normally arise from sexual reproduction, but in a number of plants found in families ranging from the grasses to the Compositae an asexual method has been substituted. Plants in which embryos arise without fertilization of the egg are called apomicts. The term "apomixis" is applied to this method of reproduction.

In plants with adventitious embryony, the young sporophyte arises directly from diploid somatic cells of the nucellus or integuments of the ovule. The progeny are uniformly alike, and all have the same genetic constitution. There is no segregation despite heterozygosity of the female parent. The female gametophyte, if formed, rarely functions to give an embryo of sexual origin.

There are three major variations of the course of events in adventitious embryony. In Citrus, for example, which has female gametophytes, fertilization of both egg and polars is necessary, and hybrids of sexual derivation are produced. These, however, cannot compete successfully with the adventitious embryos arising from the nucellus. Almost invariably it is the latter which are found in the ripened seed. In the second class, fertilization only of the polars is necessary for continued development of the adventitious embryos. Then there are those forms,such as Opuntia, in which neither egg nor polars are fertilized, but the adventitious embryos grow normally.

In pseudogamous and parthenogenetic apomicts, embryo sacs are formed, so there is an alternation of generations. The female gametophytes, however, are diploid and result from the failure of meiosis (diplospory), from development of an archesporial cell directly into a diploid female gametophyte (generative apospory), or from the division of a somatic cell (somatic apospory).

In pseudogamous apomicts, irrespective of the mode of origin of the diploid embryo sac, the egg cell divides without fertilization to form the embryo, but no endosperm is produced unless the polars are fertilized. Since an endosperm is essential for seed development in these forms, seed abortion occurs if there is no secondary fertilization.

In autonomous apomicts, where the diploid embryo sac can arise in the various ways cited above, both egg and polars are able to divide and form embryo and endosperm, respectively, without the stimulus of fertilization or even of pollination. Here fertilization has been entirely dispensed with and seed formation is wholly asexual.

It should be emphasized that in apomictic plants there is a substitution of meiosis and of the need of fertilization. Both substitutions must occur regularly, and the first must be followed by the second in plants that have apomixis as a recurrent method of reproduction.

It is of some interest that certain apomictic plants have progenies of sexual as well as of apomictic origin. These are called facultative apomicts. The proportion of sexual or apomictic embryos is known to be influenced by the constitution of the pollen parent. Obligate apomicts are plants that have offspring solely of apomictic origin. No sexual embryos are produced in them.

The precise genetic control of the mechanisms that make apomixis possible are not well understood, but most apomicts are both polyploid and are of hybrid ancestry. Neither polyploidy nor hybridity, however, alone will result in apomixis.

It is believed that genic combinations responsible for the substitution of meiosis and of fertilization are brought together by hybridization and that polyploidy brings about a more effective gene dosage for their expression. Since apomixis is genetically controlled, there will be a strong positive selection in sterile hybrids for any mechanism which affords an escape from sterility.

Despite the widespread occurrence of apomictic plants, they may well be doomed to eventual extinction since their asexual mode of reproduction precludes genic recombination. Barring somatic mutation, all of the offspring of an apomictic plant are as alike as identical twins. There is no opportunity to form new gene combinations that might enable adaptation to changing environments.

Many cases have been reported where an egg cell and occasionally a synergid, following a normal meiosis, underwent division to form a haploid embryo. In these embryo sacs, the polar nuclei were fertilized, so a 3N endosperm is formed. Since the haploid sporophyte is usually highly sterile the cycle is not repeated and nonrecurrent apomixis is of no significance as a method of reproduction.

Self-incompatibility, a widespread phenomenon, occurs in thousands of species distributed among at least 66 families of the angiosperms. A number of different genetic mechanisms for self-incompatibility, six in all, are known. They lead to the failure of fertilization. If seeds are necessary for normal fruit development in such nonapomictic plants as plums and cherries, it is essential that sources of compatible pollen be available.

M. M. RHOADES is professor of botany and chairman of the Department of Botany of Indiana University.