FERTILITY AND HETEROSTYLISM Many plants and some hermaphrodite animals are self-sterile. In some species this is a purely individual matter, but in others the species falls into two or more groups, members of each of which are sterile when mated inter se, but fertile when crossed with another group. The sterility may be absolute, but is not always so. Plants may sometimes be obtained by unions within an appar ently self-sterile group if the end of the style is cut off before fertilization so that the pollen tubes have an abnormally short distance to grow. The mutually infertile groups are anatomically distinguishable in the case of heterostylous plants. Here one type has a short style and long stamens, the other a long style and short stamens. Not only is there a barrier to self-fertiliza tion, but unions between plants of the same class are generally relatively, sometimes completely, infertile. Where this infertility is not complete it is found that the short-styled (or thrum) type is dominant, most of such plants in nature being heterozygous. In some cases intermediate types of plant exist, also gen etically determined.
An analysis is often possible where no visible characters dis tinguish the classes. Thus in self-sterile plants of Nicotiana three allelomorphic (or strongly repelled) genes Si, S2, are found, no plant being homozygous for all three. A plant of composition is self-sterile, but if its pollen is used on one of composition the S, grains are functional. Thus the products of the various possi ble matings are : i : i S.S'3; 1 : i and so on (the pollen parent is shown on the right). A colour-determining gene exhibits partial linkage with these genes. In the same species other genes allelomorphic to S1, and S3 exist, and also a gene which renders plants possessing it self-fertile. Self and mutual sterility is of considerable economic importance in the case of apples, plums and cherries, in all of which fertilization is neces sary before fruit can be formed, though the fruit persists even if the embryos do not survive beyond a certain stage. Hence the conditions are highly complicated. In the plum it seems probable that fertility is determined by a system of genes like those of Nicotiana, their action being further complicated by the condition of polyploidy.
While allopolyploid plants are generally fully fertile inter se, autotetraploids are often less so than the corresponding diploids, though of course they are far more fertile inter se than in a cross with a diploid. But tetraploid forms of self-sterile species are commonly self-fertile. Very little is known concerning the in heritance of fertility in animals, except that genes involving physiological changes often depress it, and that it is usually in creased by a moderate degree of heterosis, and diminished in species-crosses. Of course intersexuality is commonly accom panied by lessened fertility (see SEx).