THE GENETICAL ARCHITECTURE OF HETEROSTYLY IN PRIMULA SINENSIS

Abstract

Flowering plants show a variety of mechanisms which encourage, or even ensure, the setting of seed by crossrather than by self-pollination. Heterostyly is such an outbreeding device and it is widespread in its occurrence. Heterostyled species may be classified into two groups, the distylic and tristylic, the former being the commoner type. In a distylic species the plants are of two kinds, differing in the flowers that they bear. The one type, termed is recognizable by its anthers being borne above the level of its stigmata, while the other, or type, has the position of stigmata and anthers reversed, the former now being above the latter (see fig. 2). This is the type of heterostyly which -occurs in species of Primula and with which the present account is concerned. Species showing the second type of heterostyly, tristyly, have plants of three kinds, distinguished by the level of the stigma being below, between, or above the levels of the two sets of anthers which are borne in such flowers. The details of tristyly need not, however, detain us in our consideration of distylic Primulas. Distyly in Primula species is a complex character. The morphological duality involving position of anthers and position of stigma, is obvious to mere inspection, but the differences of structure do not stop there. The two types of stigma differ also in the, size of their papillae, the pin stigma having larger ones than the thrum, and the stamens differ in the size of the pollen grains they produce, thrum pollen being larger in mean size and also less variable than pin.. Finally, in addition to these morphological differences there is, as Darwin (1877) showed, a physiological distinction. Pollen, whether from pin or thrum flowers, is more successful in giving rise to seed on a stigma of the opposite type of flower than on one of its own type. There is an incompatibility reaction of pin pollen on pin stigmata, or perhaps more correctly of pin pollen tubes in pin styles, and of thrum pollen on thrum stigmata. We can thus recognize six s'ub-characters in heterostyly (Lewis 1949):