Structure and histochemistry of embryo envelope tissues in the mature dry seed and early germination of Phacelia tanacetifolia

The embryo envelope tissues in both mature dry seed and during early germin ation of Phacelia tanacetifolia were investigated by bright-field and fluor escence light microscopy and scanning electron microscopy. The ruminate see d had an irregularly reticulate surface owing to the presence of polygonal areas, corresponding to the cells of the seed coat. The raised margins of t hese cells joined at the lobe tips, where radially arranged thickenings occ urred. The unitegmic seed coat was made up of three distinct layers: the fr ayed outer layer, the middle layer with portions rising outwards to form th e radial thickenings, and the inner layer, the thickness of which was great est in the micropylar zone. The endosperm tissue had two regions, the micro pylar and the lateral endosperm, which differed in polysaccharide compositi on, thickness and metachromasy intensity, and presence (in the lateral endo sperm) or absence (in the micropylar endosperm) of birefringence of the cel l walls. Moreover, in the micropylar region, where the embryo suspensor rem nant was round, Ca-oxalate crystals were scarce or absent. The presence of a partially permeable cuticle covering the seed endosperm was observed. Inc ubation of seeds in Lucifer Yellow CH indicated that water was able to pene trate quickly into the seed coat along the pathway formed by the radial thi ckenings, the raised margins of the polygonal cells and the middle layer. A fterwards, LY-CH readily infiltrated the apical portions of the seed lobes and then the whole endosperm. Following imbibition, morphological changes w ere found in the micropylar endosperm, such as the initial digestion of pro tein bodies. In addition, both in the seed coat and in the endosperm. a wea ker fluorescence, probably due to leaching of polyphenolic substances, was observed. Once the seed coat was broken at the micropylar end of the seed, the endosperm cap surrounding the radicle tip had to be punctured by it so that complete germination could occur. Weakening and rupture of the micropy lar endosperm are briefly discussed. (C) 2000 Annals of Botany Company.