The Structure of the Spores of Gigaspora Margarita. I. The Dormant Spore

SUMMARY Histochemical tests, light and electron microscopy have been used to investigate the structure of the dormant spore of Gigaspora margarita (Endogonaceae). There is a plug in the neck of the mature spore consisting of a dense matrix of highly osmiophilic material bounded by wall‐like depositions. The complex spore wall is composed of four prominent layers, an inner and outer layer that contain polysaccharide, protein and lipid; a laminated chitinous layer and an ill‐defined‘cementing’layer. The cytoplasm is composed of amorphous material and a variety of granules and particles that form a fine network between large oil droplets. In certain areas the cytoplasm is concentrated and forms two dense regions named the 'synthesizing’and‘nuclear’centres which contain different proportions of various organelles. Two unusual types of organelles found in the cytoplasm of G. margarita spores have been previously described in honey‐coloured, sessile spores of Acaulospora laevis.