The causes of low production of seeds capable of producing double-flowered plants and of high production of seeds capable of producing single-flowered plants were investigated in zinnias (Zinnia violaceu Cav.). Poor pollination was a major cause of the low seed set. A tubular floret produced abundant pollen; however, the pollen flow to ray stigmas was limited due to the infrequent visitation by pollinators. Moreover, in the double-flowered capitula, newly opened ray petals overlapped on the pistils that unfolded the previous day. These phenomena were considered to cause low seed set in double-flowered plants. Actually, capitula with more tubular florets produced more seeds than those with fewer tubular florets. Pollen germination and plants near zinnias had additional possible influences on seed production of double-flowered zinnias.
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