For histological observations, embryogenic callus at different developmental stages were fixed in FAA (formalin:glacial acetic acid:ethanol, 5:5:90, v/v) for 24 h, dehydrated
through an ethanol–xylol series and embedded in paraffin wax. Tissues were sectioned at 3–4 mm and stained with 0.5% (w/v) fast green and 0.25% (w/v) safranin and examined under a microscope.