Ultrastructure of the Vegetative Cells of Nostoc flagelliforme Prepared with High Pressure Freezing and Freeze Substitution Technique Electron Tomography in Plant Cell Biology Cryo-electron microscopy and its application in studies of plant (animal) materials Ultrastructure of the Floral Nectary of Arabidopsis thaliana L. Prepared from High Pressure Freezing and Freeze Substitution