2.5% (v/v) glutaraldehyde (0.1 mol L1 phosphate buffer, pH ¼ 7.2)
for at least 48 h once cut from plants. Then the samples were
immersed in 1% (v/v) osmium acid for post-fixation, resin embedding
and sectioned ultrathin for transmission electron microscope
(H7650, HITACHI, Tokyo, Japan).