In suspension cultures of carrot, embryoids are initiated at or near the surface of characteristic cellular aggregates (embryogenic clumps) and are released from these aggregates as free-floating structures capable of further development into plantlets. Embryoid development is promoted by transfer from a medium containing 2,4-D to one from which it is omitted. The structure of the embryogenic clumps, in these two media, has been studied by the thin section technique involving fixation in glutaraldehyde and embedding in glycol methacrylate. The superficial cells of the clumps are clearly marked off from the central cells and contain large starch grains, a large central nucleus, abundant cytoplasm and a number of vacuoles. It is from individual superficial cells that the embryoids are seen to arise in the 2,4-D omitted medium but it has not been possible to distinguish, in advance of embryogeny, such cells from within the superficial cell layers. The fragmentation of the proliferating clumps in 2,4-D containing medium and the release of embryoids into the 2,4-D omitted medium is promoted by some of the superficial cells becoming free of starch and undergoing pronounced enlargement. A sequence of segmentations leading from single superficial cells of the clumps to globular embryoids has been traced. This differs from that reported to occur during early embryogeny from the egg cell of carrot. The resemblance to zygote embryogeny is closer from the late globular stage of embryoid development; embryoids then differ from zygotic embryos in their shorter suspensors. Attention is drawn to the initiation of additional embryoids from the epidermis of some of the embryoids developing in culture.
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McWILLIAM et al. (1974) studied this question.