BOTN 201 - Department of Biology - TAMU
Images of the Navasota Flora - 7 May 1995
The genus Sabatia includes 17 species that are limited to North America and the West Indies. One of 6 Texas Sabatia species ('Rose Gentian' or 'Rose-Pink - Gentianaceae), S. campestris is now in full flower in open sites of the Navasota Valley. Angiosperms take many adaptive tacks to maximize pollen flow between plants and minimize 'selfing'. This insures high levels of genetic variation that are required for selective response to changing conditions. This is accomplished in S. campestris populations by sequential anthesis. Anthers dehisce shortly after the flower opens but, as this is happening, the two stigmatic branches of the style are twisted together and unexposed. Once pollen has been removed from a given flower, the stigmatic braches untwist and take on a more receptive position. A look at these flowers on side view suggests epigyny in that there is an expansion between the perianth-part extensions and the pedicel. However, a close look reveals the presence of an hypanthium that is not adnate to the ovary. Thus, the 'Rose-Gentian' is perigynous with a superior ovary.
Sequenced anthesis is a central adaptive theme of the largest dicot family, the Asteraceae (Compositae). A common and conspicuous element of this family in the Navasota flora, the genus Coreopsis shows a radiate inflorescence with the central disc florets surrounded by ray florets. The cental mass of disc florets show sequenced anthesis in that those positioned toward the outside of the capitulum (closest to the ray florets) are the first to open. This is followed by a 'wave' of anthesis that moves toward the center of the inflorescence. Each disc floret shows a sequence as well. The emerging style pushes pollen from a tube that is formed by the connate anthers. This produces a 'pollen presentation mass'. After this mass has been removed, and perhaps taken to another Coreopsis inflorescence, the style continues to elongate. The style branches eventually split and become receptive for pollen that has been produced by another flower.
A look at the base of a Coreopsis inflorescence reveals an unusual feature of the bracts that subtend the inflorescence (Phyllaries). They are not, as is the case with many elements of the family, a uniform series of bracts. They, instead, form series or whorls that are different from one another in general structure. This is a useful marker for Coreopsis and a related cluster of genera in the Tribe Heliantheae of the Asteraceae.
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Last updated by HDW on 10 May 1995