A Morphological Investigation of the Unusual Cryptogeal Germination Strategy of Bunya Pine (Araucaria bidwillii)-An Australian Rain Forest Conifer.

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  • The morphology of the cryptogeal germination sequence of Araucaria bidwillii (bunya pine) was studied. Individual seeds were 5-6 cm in length and 2.5-3.5 cm in diameter and had an average fresh weight of ca. 16 g. In mature seeds the embryo consisted of a cotyledon tube, hypocotyl, and root cap that were 3-5 mm in diameter and 25-30, 3-4, and 5-7 mm in length, respectively. The embryo was surrounded by, but not fused into, a massive megagametophyte. Germination was rapid as a large diameter pseudoradicle emerged from the seed 48 h after the start of imbibition. At 1 wk the cotyledon tube had elongated to 3.5 times its original length. As cotyledon growth was positively geotropic the root and shoot meristems were forced out of the seed and buried in the soil below the initial position of the seed. At week 3 the hypocotyl was 12-15 and 3-4 times its original diameter and length, respectively, and had developed into a parenchymatous tuber. It appeared that starch stored in the megagametophyte was being mobilized, translocated down the cotyledon tube, followed by resynthesis in the expanding tuber. At week 4 the tuber had developed a thin reticulate periderm and had also initiated numerous lateral roots from its lower third. At this stage a distinct abscission zone had developed at the base of the cotyledon tube and after a further 2 -4 wk the cotyledon tube and megagametophyte were easily detached from the tuber at the abscission zone. In mature seed the epicotyl consisted of only a small apical dome, but at week 5 it was a small shoot that commenced its growth upward through the soil by either splitting or pushing away the cotyledon tube. Initially, the tubers possessed 4-6 pairs of strands of primary vascular tissue embedded in a parenchymatoums atrix. With the commencement of secondary growth each pair of strands developed into a cylinder of secondary tissues, which are eventually incorporated into the normal secondary growth increments. While extemal evidence of the tuber is lost as secondary thickening of the root and stem progresses the unusual nature of the hypocotyl can be ascertained by intemal examination. This germination sequence allows A. bidwillii to transform a large surface seed into a subterranean storage and perennation organ. Bunya pine is a rain forest emergent, and it appears that the carbohydrate reserve of the tuber allows seedlings to exist for several years under the low light levels of the forest floor, until a break in the canopy allows for rapid growth. The buried bud reserve created as the plumule grew through the soil would allow for regeneration if shoot damage occurred.

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    Article (Published)
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    Copyright 1992 by The University of Chicago
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    • Burrows, G.E., Boag, T.S., and Stockey, R.A. (1992). A Morphological Investigation of the Unusual Cryptogeal Germination Strategy of Bunya Pine (Araucaria bidwillii)-An Australian Rain Forest Conifer. International Journal of Plant Sciences , Vol. 153, No. 3, Part 1, 503-512