Date of Award


Degree Type



Biological, Geological and Environmental Sciences

First Advisor

Krebs, Robert

Subject Headings

Plants -- Reproduction, Hedyotis, Rubiaceae, Plants -- Self-incompatibility, Plant morphology, Distyly, heterostyly, plant mating system, plant development, plant morphology, morphometrics, self-incompatibility, plant reproduction


Distylous species of flowers possess two distinct floral morphs that are generally found in equal numbers in naturally occurring populations. The flower form that has a relatively long style is called a "pin" the form with a short style is a "thrum." Within the distylous mating system, selfing and intramorph mating are prevented due to the spatial separation of stigmas and anthers and by a self-incompatibility system that is inherited along with the dimorphic floral structure and ancillary polymorphisms. However, a breakdown of distyly has been documented in several families, either through the development of completely separate sexes (dioecy), or, more frequently, through the development of monomorphy and self-fertile homostyles. Here several populations of Hedyotis. caerulea, a perennial spring-flowering herb native to eastern North America, were surveyed in the Cleveland Metroparks and sampled to investigate the degree of distyly present and to detect any movement of the mating system towards selfing or dioecy. Morphometric analysis of the variation within and across populations of stigma height and anther height indicate that this species exhibits reciprocal herkogamy. Nearly all populations surveyed were isoplethic (i.e., contained a morph ratio of pins to thrums that was not different from 1:1), which is found when the species' mating system promotes intermorph pollen transfer (dissortative mating) with nearly 100 outcrossing. There was no indication that the self-incompatibility system has broken down with no movement towards homostyly. Variation was present, however, in these primary traits as well as in stigma length, corolla tube width, pollen diameter, pollen count, dry weight, and seed set of the flowers collected among populations. Several ancillary floral traits, including corolla tube shape, pollen diameter, and length of stigmatic papilla, likewise varied. Across populations, the two morphs invested a similar amount of biomass in the production of pollen and seed, suggesting that each morph had