Origin and structure of embryoids arising from epidermal cells of the stem of Ranunculus sceleratus L.

Embryoids are initiated from single highly cytoplasmic cells of the stem epidermis of plantlets of Ranunculus sceleratus L. derived from callus cultures. The embryoids show embryological development of the Crucifer type except that the suspensor is reduced or suppressed. The cells of the young embryoids have a large central nucleus surrounded by small vacuoles, a cytoplasm rich in free ribosomes or ribosomes in small polysomes, prominent amyloplastids and abundant spherosomes. These cells also frequently contain multivesicular and myelin bodies. The spherosomes are observed in various stages of development; they are typically arranged in a layer below the cell wall, the space between the spherosomes and the wall being rich in spherical vesicles and microtubules. As embryoid development proceeds the plastids show internal differentiation; globular centres and, surrounding them, electron-transparent droplets are associated with the origin of the thylakoid membranes. The walls of the embryogenic cells are initially rich in plasmodesmata but as the embryoid develops cytoplasmic continuity with the embedding tissue is severed and the embryoid is clearly delineated. The embryology and fine structure of the embryoid cells is compared with that of zygotic embryos of Capsella and Ranunculus.