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The majority of pteridophytes are shown to have an amoeboid tapetum (Parkinson & Pacini, 1995), however, many homosporous ferns have globular bodies in their sporangial locules that were considered to be homologous to orbicules in spermatophytes (Lugardon, 1981).
In Brassicaceae, for instance, orbicules were never recorded; also in Arabidopsis thaliana they are absent, despite its parietal tapetum (Murgia et al.
A detailed developmental study at ultrastructural level of Anaxagorea brevipes (Annonaceae) showed that the parietal tapetum reluctantly invades the locule at the early tetrad stage and secretes both orbicules and other globular lipoidal concretions (Gabarayeva, 1995).
At present, identification keys are available to distinguish between tapetum types (e.
In the invasive tapetum (sensu Tiwari & Gunning, 1986a), the pollen mother cells are in the centre of the anther locule surrounded by layers of tapetal cells, several cells thick, as shown in Costus and Globba [ILLUSTRATION FOR FIGURES 18, 19, 32, 33 OMITTED].
Although some inconsistencies occur and undoubtedly there are areas where more data are required, the evidence points to tapetum type being a significant character in higher-level monocot systematics.
There are a few records for the "invasive" type of tapetum (sensu Tiwari & Gunning, 1986a), this has probably often been described as plasmodial in the literature (see below).
Undoubtedly, confusion has arisen in the extensive literature on the tapetum due to incorrect observations or differing interpretations.
References of ontogenetic studies of tapetum can be found in, e.
Both types of tapetum occur together in 12 families.
The distribution of the two main tapetum types in angiosperms was shown on dahlgrenograms: for dicots by Dahlgren (1991), and for monocots by Dahlgren and Clifford (1982) and Dahlgren et al.
The historical literature, from the first report in 1865 by Rosanoff up to the 1920s, concentrates mainly on anatomy and morphology of the anther, and often more specifically the tapetum.