ENDOTHECIAL THICKENINGS IN ANTHERS OF PORATE MONOCOTYLEDONS

A survey of the presence of endothecial thickenings in 20 genera from 11 families of porate monocotyledons tested the long-standing hypothesis that porate anther dehiscence is correlated with the absence of thickenings. Although this hypothesis is widely repeated in texts, it has been supported by a limited, biased data base from studies of Ericaceae and Melastomataceae, two large families of dicotyledons. The results of our broad study of monocotyledons show that endothecial thickenings are present in most porate monocotyledons. Thickenings are absent in only two of the 11 porate families surveyed, Mayacaceae and Commelinaceae, both in the Commelinales. Thickenings are also absent in Xyris (Xyridaceae), a nonporate genus. These findings indicate that in monocotyledons porate anther dehiscence and the absence of thickenings are not as closely related as previously supposed. THE ENDOTHECIUM is a hypodermal layer that is considered to promote anther dehiscence by differential shrinkage of its prominent wall thickenings (Fahn, 1967; Foster and Gifford, 1974). Two major patterns of anther dehiscence are recognized: apical dehiscence by pores or slits, and longitudinal dehiscence (Coulter and Chamberlain, 1903; Esau, 1965; Foster and Gifford, 1974). Plant anatomy and embryology texts (Maheshwari, 1950; Eames, 1961; Esau, 1965; Davis, 1966) commonly state that endothecial thickenings are generally absent from anthers that dehisce apically, and are present in anthers with longitudinal dehiscence. This generalization is apparently based on a relatively small sample of porate taxa, almost exclusively from dicotyledons. In addition, among dicotyledons two large porate families, Ericaceae and Melastomataceae, have received the most attention, and are generally given as examples. Numerous families of both monocotyledons and dicotyledons with porate anthers have either not been examined for endothecial thickenings, or no synthesis of published information has been made. In a recent study of Araceae, which is the largest assemblage of porate monocotyledons, virtually all porate genera were found to have endothecial thickenings (French, 1985a, b, 1986). These findings prompted a survey of the occurrence of wall thickenings in all genera of ' Received for publication 14 January 1987; revision accepted 8 April 1987. Research supported by NSF Grant BSR 85-00337. We gratefully acknowledge plant material supplied by Royal Botanic Gardens, Kew, Missouri Botanical Garden, New York Botanical Garden, and U.S. National Herbarium. porate monocotyledons. Porate genera were identified using published accounts (Harris, 1905; Hutchinson, 1978; Cronquist, 1981; Dahlgren and Clifford, 1982; Buchmann, 1983). A more complete study of the endothecium was made in Rapateaceae (Gerenday and French, unpublished data), which is the second largest assemblage of porate genera, next to Araceae. MATERIALS AND METHODS-Dried anthers were removed from herbarium sheets and treated for six hours at 50 C with a 3:1 solution of 30% H202 and glacial acetic acid. Anthers were rinsed in water, spread open on slides and mounted with glycerin. Observations were made with a Nikon microscope equipped with differential interference contrast optics. Table 1 includes a list of all species examined. RESULTSEndothecial wall thickenings were found in virtually all of the genera examined in this study. The sample includes 46 species from 11 families of monocotyledons. Although the organization of the endothecium is highly variable among the taxa examined, thickenings are consistently present over the entire length of the anther. Exceptional genera with no wall thickenings include Mayaca (Mayacaceae) and Dichorisandra (Commelinaceae), in which four of the six species examined lack wall thickenings. These results are summarized in Table 1. DISCUSSION-These results together with those of previous studies of Araceae (French, 1985a, b, 1986) and Rapateaceae (Gerenday and French, unpublished data) clearly dem-