Ultrastructure and histochemistry of seeds of Colophospermum mopane during imbibition

An Environmental Scanning Electron Microscope (ESEM) was used to study the interaction between water and the seed surface of Colophospermum mopane during imbibition. A network of vein-like ridges that is caused by overlapping, compressed epidermal cells, traverses the dry seed surface. The areas where the epidermal cells overlap are characterized by a series of fine grooves. Temperature and pressure parameters in the specimen chamber were adjusted to obtain a relative humidity (RH) of 100%. Water precipitated onto the seed surface and a series of micrographs were taken over a period of approximately 8 h to capture the structural changes taking place. The entire seed surface is eventually covered with a thin, smooth mucilaginous layer as the outermost seed surface reacts with water. During imbibition, mucilage is also extruded from the epidermal grooves. Based on morphology and histochemistry, three different types of mucilage were identified in mopane seeds. Transmission Electron Microscopy (TEM) revealed that the inner cell wall surfaces of the seed coat and aril cells were lined with a thin mucilaginous layer that appeared to play a significant role during the unfolding and expansion of the compressed aril and seed coat cells.