Seed Impressions on Plastic Films

Microscopic details on the hard surfaces of seeds or other parts of fruits are frequently sufficiently characteristic to permit the accurate identification of related species. These taxonomic features have hitherto been largely neglected, however, because of the difficulty of seeing them directly under the microscope. The thickness of the fruit structures, the presence of pigmentation, and the reflection of light from smooth surfaces are difficulties that may be overcome by making impressions of the structures on plastic films. This new method is an outgrowth and modification of a method of obtaining impressions of the surface structures of fur fibers recently developed by Hardy and Plitt (1). Seeds were placed in carbitol acetate1 in a small dish for a minute or two. They were then removed by means of forceps and placed between two pieces of transparent cellulose acetate film, 0.005 inch thick and 1 inch square. Adequate padding to obtain clear impressions without crushing the seeds was built up by placing on each side of this double stratum the following: 4 additional strips of plastic film, one strip of sponge rubber (insulation stripping) approximately 0.25 inch thick, and a smooth steel plate. The whole stack was held together by means of a wide clamp which was screwed down until the rubber was compressed to J or of its original thickness. This unit was placed for 5 minutes in an electric oven maintained at approximately 90? C. Upon removal from the oven the central double stratum containing the seeds was taken out of the unit and allowed to cool and harden before removing the seeds with a brush or needle. The two plastic films bearing the impressions were mounted face down on glass slides with very narrow strips of transparent adhesive film around the edges. Thus the impressions are positive images of the surfaces. The impressions are best viewed through the microscope using oblique transmitted