SEED IMPRESSIONS ON PLASTIC FILMS
THORA M. PLITT
(WITH
ONE
PLATE)
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 acetate' 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 i or i of its original
thickness. This unit was placed for 5 minutes in an electric oven maintained at approximately 900 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
1 Carbitol acetate may be obtained from the Carbide and Carbon Chemical Corporation, 30 E. 42d Street, New York, N. Y.
PLATE V
Photomicrographs of impressions (100 x) of the achenes of (1) Polygonum punctatum, and (2) P. persicaria; of the seeds of (3) Brassica juncea, and (4) B. napusand of the lemmas of (5) Poa compressa, and (6) P. pratensis. Photomierographs of
the correspoDing parts of the fruits taken directly (100 x)': (la) Polygonum punctatum,
(2a) P. persicaria, (3a) Brassica juncea, (5a) Poa compressa. Enlargements of the
above impressions (300 x): (lb) Polygonum punctatum, (5b) Poa compressa, and (6b)
P. pratensis. (7) Photograph of equipment used in making impressions.
422
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PLANT PHYSIOLOGY
PLATE V
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Copyright © 1941 American Society of Plant Biologists. All rights reserved.
424
PLANT PHYSIOLOGY
light, with an amber filter. The lOx objective has considerable depth of
focus; magnifications of 100 to 300 diameters are obtained by interchanging
suitable oculars.
In this procedure the purpose of placing the seeds in carbitol acetate is
merely to use a minimum quantity of the liquid to soften the film. Variations in the procedure for convenience may easily be introduced to suit the
material or the equipment at hand, such as the thickness of film, thickness
of padding, time, temperature, and manner of controlling the temperature.
The accompanying illustrations show the microscopic details of some
seeds or other fruit structures. These materials were furnished by the
Section of Food Habits of the Fish and Wildlife Service. The three sets
of related species were selected because of their similarities and attendant
difficulties of identification, because of their importance in food-habit
studies, and in agricultural problems. It may be noted that unsuspected
details are revealed by the impressions, and furthermore that the arrangement of characteristic features is visible over a greater area than by direct
inspection of the seeds. Clear definition of fine details at higher magnifications may be obtained by use of the impression method. Patterns once
established at high magnifications are subsequently more readily recognized
at lower magnifications.
This method is applicable in taxonomic studies, in the determination of
certain foods and drugs, and in the identification of difficult seeds and fruits
in food-habit studies. Impressions may be obtained in a few minutes;
furthermore many impressions may be made simultaneously on one set of
films.
U. S. DEPARTMENT OF THE INTERIOR
WASHINGTON, D. C.
LITERATURE CITED
1. HARDY, J. I., and PLITT, THORA M. An improved method for revealing
the surface structure of fur fibers. Wildlife Circular 7. U. S.
Department of the Interior. 1940.
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Copyright © 1941 American Society of Plant Biologists. All rights reserved.