Fig. 7

The formation of carapace in Cypripedium plectrochilum seeds. A At the proembryo stage, the cell wall of the inner seed coat (arrowhead) has become thickened, and the cytoplasm of the inner seed coat is filled with dark materials. The cell wall of the outer seed coat (arrow) remains thin-walled without lignification, as judged by the purple color of the TBO stain. Scale bar = 30 μm. B Light micrograph showing maturing globular embryo. At this stage, the cells of the inner seed coat begin to shrivel, resulting in the formation of a black and thickened carapace. At the same time, the outer seed coat is still alive with starch deposits within the cytoplasm. Scale bar = 30 μm. C In the mature seeds of C. plectrochilum, the embryo is enclosed by a black and thickened inner seed coat, i.e., carapace (arrowhead), and then enveloped by the thin-walled outer seed coat (arrow). Scale bar = 200 μm