THE DEVELOPMENTAL ANATOMY OF CRYPTOGEAL GERMINATION IN BUNYA PINE (ARAUCARIA-BIDWILLII)

Bunya pine (Araucaria bidwillii Hooker) produces large seeds that cons ist of a massive megagametophyte that surrounds an embryo 25-30 mm lon g and 3-5 mm in diameter. The seeds germinate on the soil surface, but elongation of the cotyledonary tube pushes the hypocotyl, with its as sociated plumule and radicle, into the soil (cryptogeal germination). The hypocotyl subsequently develops into a large parenchymatous tuber that is supplied with carbohydrates translocated from the megagametoph yte through the cotyledonary tube. Later the epicotyl grows upward thr ough the overlying soil to commence photosynthesis. Elongation of the cotyledonary tube from 3 to 9 cm in less than 7 d results entirely fro m cell elongation, principally in the proximal regions of the tube. Ce ll elongation is also important in the tenfold elongation of the hypoc otyl in less than 2 wk, but a substantial number of anticlinal divisio ns followed by cell elongation also occur. The sevenfold increase in h ypocotyl diameter involves both increase in cell diameter and periclin al cell divisions. When elongation of the cotyledonary tube ceases, th e distal portion that remains embedded in the megagametophyte remains structurally unaltered for several weeks. In contrast, the tube outsid e the seed quickly develops tannin deposits in the surface layers, and between weeks 2-4 extensive collapse of the ground parenchyma cells o ccurs, leaving the vascular bundles suspended within a framework of co llapsed cells. Unlike other cryptogeal species, bunya pine forms an ab scission zone at the base of the cotyledonary tube, and thus an organi zed detachment of the tube from the tuber can occur. The initial divis ions leading to abscission zone formation commence between weeks 2-3, and by week 4 the zone extends across the ground tissues but not the v ascular 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 v ascular bundles were completely undifferentiated, but at week 1 they h ad typical collateral bundle tissue distribution and proportions, alth ough abutting the abaxial side of the primary phloem was an unusual gr oup of relatively large-diameter (25-50 mu m) cells that never accumul ated starch grains and elongated to over 2,500 mu m in length. After s everal months the cambia in each pair of vascular bundles differentiat e through the intervening parenchyma toward each other, eventually for ming four to six small cambial rings. This results in some highly unus ual patterns of secondary growth as several cylinders of secondary vas cular tissues begin to develop within the storage parenchyma of the tu ber.