Somatic Embryogenesis in the Cycadales

Abstract

The cycads (Fig. 1) constitute remnant species of an ancient class of gymnosperms, the cycadophytes, that evolved from the free-sporing progymnosperms, which also gave rise to the coniferophytes. According to Gifford & Foster (1989), the cycadophytes have included 3 orders of plants, the extinct Cycadeoidales and Pteridospermales (seed ferns), that are known only from the fossil record, and the Cycadales, that includes the cycads. The cycadophytes are supposed to be ancestral to the Gingkoales and possibly, the Gnetales (Crane & Upchurch, 1987; Taylor & Taylor, 1993). The Cycadales are believed to have originated during the Permian era, and to have flourished during the Mesozoic period, probably peaking in distribution and success during the Jurassic period. They have been referred to as “living fossils” (Gilbert, 1984). Because of their great antiquity, the living cycads have been considered to be invaluable for the study of developmental events (Norstog, 1987). Currently, the Cycadales are comprised of only 3 families, the Cycadaceae, the Stangeriaceae (monogeneric) and the Zamiaceae.