Acacia (Leguminosae)


Acacia baileyana
© Australia Nat.Bot.Gard


Acacia baileyana
James Canright


Acacia constricta
University of Arizona

Plant:
Acacias include over 800 species of spiny shrubs and trees, occurring throughout the tropics and the southern hemisphere. Such a large genus displays variability in both foliage and flowers. The leaves are typically compound (pinnatifid or bipinnatifid) but simple leaves, phylodes, and cladodes also occur. The flowers are in compound heads from spherical to cylindrical. Typically the heads are yellow to white.

Pollen light micrograph:
Polyads of 8, 16, or 32 cells, ca. 20-70 µm, cohesion of cells varies, with detached cells occasional in fossil preps. Apertures of individual cells obscure to colporate. Sculpture smooth to microreticulate. Tectum forming distinctive patterns in some species.

Key to the pollen of selected North American taxa.
After Caccavari & Dome, 2000.

1. Polyads COLPORATE colpi Y- or H- shaped
    2. Polyads 16 cells
    A. cochilacantha
    A. constricta
    A. farnesiana
    A. pennatula

    2'. Polyads 32 cells

1'. Polyads PORATE
    3. Polyads 16 cells

      4. Structure uniform
      A. greggii
      A. willardiana

      4'. Structure pseudocolpate

    3'. Polyads 8 cells
    A. angustissima

Pollen scanning electron micrograph (SEM)
Note "squares" formed by the tectum structure.

Production and Dispersal:
Moderate production, low dispersal.

Preservation:
Poorly preserved due to crumpling and fragmentation.

Fossil Occurrence:
The earliest occurrences are from the southern hemisphere: Eocene (Cameroon, Africa), Oligocene (Puerto Rico), Miocene (Australia, Mexico, Central & South America) and Pliocene (New Zealand, now extinct) (Muller, 1981). Acacia appears to have diversified and expanded into the Northern Hemisphere from the Miocene onward as global climate cooled and became drier (Tsudy and Scott, 1969; Traverse, 1988).

References:
    Barreda, V. and Caccavari, M.A. 1992.
    Mimosoidea (Leguminosae) occurrences in the early Miocene of Pategonia (Argentina) Paleo. Paleo. Paleo. 94: 243-252.

    Caccavari, M., and Dome, E. 2000.
    An account of morphological and structural characterization of American Mimosoideae pollen. Part I: Tribe Acacieae. Palynology 24: 231-248.

    Coetzee, J.A. 1955.
    The Morphology of Acacia Pollen. Suid-Afrikaanse Joernaal vir Watenskap 23-27.

    Graham, A. 1991.
    Studies in noetropical paleobotany IX. The Pliocene communities of Panama - Angiosperms (Dicots). Annals Missouri Botanical Gardens 56: 186-189.

    Gueinet, P. 1990.
    The genus Acacia (Leguminosae, Mimosoideae): its affinities as borne out by its pollen characters. Plant Systematic Evolution (suppl.) 5: 81-90.

    Muller, J. 1981.
    Fossil pollen records of extant angiosperms. Botanical Review 47: 1-42.

    Traverse, A. 1988.
    Paleopalynology. Allen and Unwin. 600 p.

    Tsuchdy, R.H. and Scott, R.A. 1969.
    Aspects of palynology. Wiley Interscience.

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