In the primrose, two flower forms differ in the length of anthers and stigma. What is this called?

1. Plants have developed many physical barriers to self-pollination. 2. The primrose has evolved two flower types differing in anther and stigma length. 3. The pin-eyed form and the thrum-eyed form place these organs at different heights, and insects seeking nectar therefore cross-pollinate readily. 4. This phenomenon is known as heterostyly. 5. Apomixis is the production of seeds without fertilisation, which is a quite different route. 6. Other species separate the timing of pollen and ovary maturity instead, which is a barrier of time rather than of organ length. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 32 "Plant Reproduction", section 32.2 Pollination and Fertilization_