Thin cross section through a black mustard (brassica nigra) seed grain, prepared in water, showing the outer surface of the seed grain. On the outer epidermis the cells can uptake lots of water showing swelling and birefringent mucilage production, seen here in the upper part of the image. The middle of the image shows the palisade parenchyma cell layer with the characteristic surface seen macroscopically on the seed grain surface. The bottom of the image shows the aleuronic cell layer. Microscopic contrast technique polarization contrast. Magnification 180x when printed 10 centimetres wide.