A comparative developmental study of the selfing and outcrossing flowers of Clarkia tembloriensis (Onagraceae)

Clarkin tembloriensis (Onagraceae) contains both highly outcrossing and hig hly self-pollinating populations. Except for the ovary, flowers on plants f rom self-pollinating populations typically have shorter floral organs and h ave anthers that produce less pollen. A comparative study of flower develop ment, including organogenesis, histogenesis, organ growth, and allometry, w as undertaken to determine the stage in development at which differences in floral form arise. Growth rates of flower buds from 3.0 mm in length to ma turity are the same in the two flower types. Earlier stages were not access ible for direct measures of growth. Growth rates of ovaries from 1.0 mm in length to anthesis are the same for both flower types. Since ovaries achiev e the same final size in both flower types at anthesis, the ovary was used as an index organ by which to measure the growth of other organs in the flo wer from a bud size of 3.0 mm. The size of sepals, petals, long anthers, an d styles in selfing flowers are smaller than in the outcrossing flowers of the same ovary length as measured using dissected buds, scanning electron m icrographs, and histological sections. Flower primordia are initially the s ame size in the two floral types, so divergence between them can be documen ted in a developmental study. The earliest detected differences in organ si ze and form between flower types occurs after all the organs are initiated in a bud ca. 1 mm long with an ovary length of 0.3 mm. Since the allometric relationship of the organs to the ovary is the same in both flower types i n buds >3 mm, the mature differences must arise during this developmental w indow after organ initiation and just before organ differentiation. Signifi cant differences in pollen production in the two flower types can be attrib uted to formation of fewer sporogenous cells in the self-pollinating flower .