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Environmental Determinants of Outcrossing in Impatiens capensis (Balsaminaceae)

Donald M. Waller
Evolution
Vol. 34, No. 4 (Jul., 1980), pp. 747-761
DOI: 10.2307/2408029
Stable URL: http://www.jstor.org/stable/2408029
Page Count: 15
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Environmental Determinants of Outcrossing in Impatiens capensis (Balsaminaceae)
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Abstract

By producing functionally and morphologically distinct cleistogamous and chasmogamous flowers, Impatiens capensis (jewelweed) is capable of adjusting its degree of outcrossing in response to local environmental conditions. Adequate soil moisture and higher light intensities favored growth and subsequent outcrossing in natural populations. Drought, deep shade, or injury often restricted jewelweed to producing only cleistogamous flowers. The proportion of total, aboveground biomass invested in selfed flowers and fruits remained independent of plant size, but the outcrossed proportion increased significantly with increased plant size. This pattern of facultative chasmogamy is in contrast to the situation where the proportion of resources devoted to cleistogamy increases in response to adverse conditions. Greenhouse experiments confirmed the developmental nature of this response and the primary importance of plant size in determining a threshold at which outcrossing begins. The point at which plants began to produce outcrossing flowers depended only on their size, while the quantitative level of outcrossing depended on both size and light intensity. This response is sufficient to explain patterns of outcrossing observed within and between populations. The pattern of increased outcrossing in large plants may be a response to predictable high levels of sibling competition, a situation agreeing with one of the models proposed by Williams (1975) to account for sex.

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