The Developmental Anatomy of Cryptogeal Germination in Bunya Pine (Araucaria bidwillii).

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  • Bunya pine (Araucaria bidwillii Hooker) produces large seeds that consist of a massive megagametophyte that surrounds an embryo 25-30 mm long and 3-5 mm in diameter. The seeds germinate on the soil surface, but elongation of the cotyledonary tube pushes the hypocotyl, with its associated plumule and radicle, into the soil (cryptogeal germination). The hypocotyl subsequently develops into a large paren- chymatous tuber that is supplied with carbohydrates translocated from the megagametophyte through the cotyledonary tube. Later the epicotyl grows upward through the overlying soil to commence photo- synthesis. Elongation of the cotyledonary tube from 3 to 9 cm in less than 7 d results entirely from cell elongation, principally in the proximal regions of the tube. Cell elongation is also important in the tenfold elongation of the hypocotyl in less than 2 wk, but a substantial number of anticlinal divisions followed by cell elongation also occur. The sevenfold increase in hypocotyl diameter involves both increase in cell diameter and periclinal cell divisions. When elongation of the cotyledonary tube ceases, the distal portion that remains embedded in the megagametophyte remains structurally unaltered for several weeks. In contrast, the tube outside the seed quickly develops tannin deposits in the surface layers, and between weeks 2-4 extensive collapse of the ground parenchyma cells occurs, leaving the vascular bundles sus- pended within a framework of collapsed cells. Unlike other cryptogeal species, bunya pine forms an abscission zone at the base of the cotyledonary tube, and thus an organized detachment of the tube from the tuber can occur. The initial divisions leading to abscission zone formation commence between weeks 2-3, and by week 4 the zone extends across the ground tissues but not the vascular bundles. The vascular system of the tuber consists of four to six pairs of vascular bundles that run parallel to each other, without anastomoses, for most of the length of the hypocotyl. At week 0 the vascular bundles were completely undifferentiated, but at week 1 they had typical collateral bundle tissue distribution and proportions, although abutting the abaxial side of the primary phloem was an unusual group of relatively large-diameter (25-50 Am) cells that never accumulated starch grains and elongated to over 2,500 ,gm in length. After several months the cambia in each pair of vascular bundles differentiate through the intervening paren- chyma toward each other, eventually forming four to six small cambial rings. This results in some highly unusual pattems of secondary growth as several cylinders of secondary vascular tissues begin to develop within the storage parenchyma of the tuber.

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    Article (Published)
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    Copyright 1994 by The University of Chicago
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  • Citation for previous publication
    • Burrows, G.E., and Stockey, R.A. (1994). The Developmental Anatomy of Cryptogeal Germination in Bunya Pine (Araucaria bidwillii). International Journal of Plant Sciences , Vol. 155, No. 5, 519-537.