Callogenesis and callus maintenance. Callus tissue is induced from young sporophytes (a). Growing sporophytes on 1× MS 2% sucrose 0.7% agar medium (pH 5.8) containing 5 μM BAP (MS-BAP) causes development of callus at the shoot apex within 2 weeks (arrowhead
b). Callus tissue is then removed from the progenitor sporophyte using forceps and transferred to fresh MS-BAP medium (c). Inset shows individual callus piece, scale 1 mm. Callus tissue on MS-BAP maintains an undifferentiated fate and will grow independently. After 2 weeks (d) callus is typically large enough for bombardment and/or sub-culturing to fresh medium to maintain callus stocks for future bombardments.