Preparation techniques and student evaluation of
bioplastic specimen blocks for use in
crop science teaching1
A. W. Burger and R. D. Seif2
ABSTRACT
Plastic specimen blocks enhance crop science
lecture and laboratory teaching according to student reaction measured three semesters in the Department of Agronomy, University of Illinois. Students indicated that these visual aids are useful,
interesting, important, and successful in promoting teaching objectives. Similar positive student
responses to the use of plastic-embedded crop
specimens were found among all classes and
majors enrolled in the introductory crop science
course. Agronomic specimens which have been
successfully embedded include: seeds of weeds
and crops, grass collars, flowers and typical
leaves of plants, grass spikelets, plant inflorescences, ears and ear sections of corn, soybean
pods, and nodulated soybean roots. Special
"freeze dry" techniques are essential in preparing
wet plant specimens for embedding since plastic
resin does not mix with water. Advantages of preserving agronomic specimens in bioplastic blocks
include: (a) minimum breakage because of fragile
nature of dry specimens (b) easy stereomicroscopic examination, and (c) preservation of plant materials in natural color for repeated use.
Additional index words: Visual aids in crop science, Freeze drying of plants.
Messersmith3 and others at North Dakota State University have prepared a number of different kinds of
agronomic specimens in plastic for classroom and
laboratory use in recent years. Embedding fragile vegetative specimens in plastic preserves these materials for
repeated use. Seeds of various species and cultivars of
crops can be more easily compared to each other in the
rigidity of the plastic matrix. Stereoscopic examination
if facilitated because specimens are held "in place" in
the plastic while they are being observed. The purpose
of this paper is to report on (a) various methods and
techniques used in preparing bioplastic blocks of agronomic biological specimens and (b) student reaction to
the use and value of these specimen blocks in crop science teaching.
MATERIALS AND METHODS
The technique of embedding a bioplastic is well documented
in flyers and literature distributed by various commercial biological stores which sell the resin. (For example, "Embedding
in Bio-Plastic," Ward's Natural Science Establishment, Inc.,
P.O. Box 1712, Rochester, N.Y. 14603). The resin may be purchased in bulk lots from a one gallon pail to a 55-gallon drum.
Opened containers have a short shelf life; thus, we worked
with gallon bulk packs and 8-pint gallon packs. The latter is
more expensive but also extremely versatile since it contains
' Contribution from the Univ. of Illinois, Urbana, Campaign.
Professor of agronomy and professor of biometry, agronomy, respectively.
'Unpublished report. Permanent plant mounts by embedding in
plastic. Dep. of Agronomy, North Dakota State Univ., Fargo.
2
E
MBEDDING biological and geological specimens is
not new. However, embedded specimens of agronomic crops in various forms are quite rare. C. A.
24
JOURNAL OF AGRONOMIC EDUCATION
eight pint bottles of resin and eight individual vials (1 for each
pint of resin) of catalyst in one carton. Other materials used in
the preparation of the blocks include: mold release compound,
hand polishing kit, aluminum “take-a-part” bioplastic molds,
home-made galvanized tin molds, liquid abrasive, liquid
polish, and bioplatic solvent, bioplastic grinder-polisher, complete with buffing compound, polishing belts in four texture
grits (100, 200, 320, and 600), wet strength abrasive cloth,
lyophilizer, dry ice, asbetos gloves, and laminator.
Agronomic specimens used for embedding include: many
varieties of soybean seeds, assorted field crop and weed seeds,
podded soybeans, nodulated soybean roots, inflorescences,
flowers, and vegetative parts of several grasses and legumes,
ears of corn, grain and grass spikelets, both intact and disassembled.
Evaluation of the merit of bioplastic specimen blocks in introductory crop science teaching was carried out during three
semesters (Spring 1977, Fall 1977, and Spring 1978) involving
266 students at the University of Illinois. A survey of student
opinion at the beginning (pre-survey) and at the end of the
semester (post-survey) using an evaluation form on which the
student rated the use of bioplastic specimen blocks in crop science teaching was made. The students were asked to circle
numbers 1 = (strongly disagree), 2 =(disagree), 3 = (neutral),
4 = (agree), and 5 = (strongly agree) concerning their reaction
relative to eight descriptive adjectives, namely, useful, important, unusual, positive, active, successful, new, and interesting as these adjectives applied to the use of bioplastic specimen blocks in crop science teaching.
The student reaction ratings were statistically analyzed using
the standard paired t test. Differences noted are significant at
the 1070 level.
S P EClMEN
SUPPORTING LAYER
Fig. 1. Three major layers and alternate trapping layer in
the plastic embedding procedure.
Embedding Procedure
The basic principle of casting plastic specimen blocks is
hardening of a liquid synthetic resin in the presence of a
catalyst, namely, hydrogen peroxide. The reaction between the
resin and catalyst is indicated by a color change from blue to
green. As the reaction goes to completion, the resin polymerizes or gels to a crystal-clear solid. Many different variations of embedding procedures have been used by the authors;
however, the general principle of embedding dry agronomic
specimens involves the preparation of (a) a specimen-supporting layer (b) a specimen-retaining layer and (c) a specimencovering layer (see Fig. 1).
The amount of catalyst used is inversely correlated with final
block thickness. Blocks up to 0.25 inch in thickness require
about 80 drops of catalyst per 100 cc of resin whereas 0.25 to 1
inch and 1 to 2 inch blocks require, respectively, 40 and 10 to
20 drops, respectively. The thicker blocks must harden more
slowly to avoid cracking. In addition, because the chemical reaction of catalyst with resin generates heat, the higher
temperature generated by an excess of catalyst will tend to discolor the specimen. However, maximum proportions of
catalyst must be used to: (a) obtain a clearer and harder block
and (b) reduce the line visibility between successive layers of
Douring.
Most of our blocks are 1/4 inch to 1 inch in thickness and
the general procedure for embedding most of our specimens is
as follows:
1. Preparation of the molds. We use either 3 x 2 x 3/4 inch
or 4 x 2-3/4 x 7/8 inch rectangular “take-a-part” aluminum molds (see Fig. 2). The take-a-part feature refers to a
rectangular aluminum mold consisting of removalbe ends
and a U-frame. Before use, the end pieces are clamped to
Fig. 2. Aluminum “take-a-part” mold showing the
frame and removableend plates.
U-
the U-frame with rubber bands. The inside of the now
rectangular aluminum mold box (top side open) is completely covered by brushing on a thin coat of mold release
compound. After the mold release compound dries, the
mold is poured. Where mold sizes or shapes are not commercially available we have found galvanized metal fitted
to desired specifications to work very satisfactorily.
2 . Mixing the resin with a catalyst, One hundred cubic
centimeters of resin are poured into a disposable unwaxed paper cup. Forty drops of catalyst are added to
the resin and the contents are stirred gently with a clean
glass stirring rod, being careful to avoid stirring bubbles
into the mixture.
3. Pouring the specimen block. Just enough resin is poured
into the mold to form the base layer. Air bubbles can be
teased out using a dissecting needle. Air bubbles usually
surface automatically unless high amounts of catalyst resulting in quick set are used. These air bubbles tend to
obscure the specimen and distract from clear observation
of the specimen under the stereomicroscope and should
be teased out with a dissecting needle before hardening
of the matrix at all stages of pouring.
As soon as the base layer firms to the point of supporting the mounting specimen, the specimen is “stuck” to
the base layer. A small amount of liquid mix is poured
over the specimen to trap it in a few minutes of firming
time. This prevents the specimen from floating when the
BURGER AND SEIF:
SPECIMEN BLOCK PREPARATION
BIOPLASTIC
SPECIMEN BLOCKS-their use in crop science teaching
USEFUL
~
[]
PRE
UNUSUAL
POSITIVE
ACtiVE
SUCCESSFUL
N~w
~NTERSS~
I
strongly
2
disogree o~ee
~
4
5
ogree
Fig. 3. Studentreactionto the useof bioplastic specimenblocksin introductorycrop scienceteachingat
the beginning
(pre) andthe end(post) of the semester.
coveringlayer is pouredto finish the block in the next
step. Trappingthe specimenwith a small amountof the
resin mix avoidsthe needfor pouringthe specimenlayer.
The specimenand coveringlayer are pouredat nearly the
sametime, thus speeding up the entire pouring procedure. Anylabeling that needs to be done should be carried out at the time of "trapping" the specimen.Weuse
Avery and/or Dennison self adhesive or Pres-a-ply
labels. Back-to-back
labels are usedso that identification
is possible on both sides of the specimenblock. The use
of letters and/or numbersinstead of actual wordshelps
to render the specimenblocks useful for study and examination purposes from both top and bottom sides of
the block. Thickpaper or cardboardlabels can be used;
however,the neatness of the specimenblock is enhanced
by using commerciallyprepared labels. External legends
are used to identify the numberand/or letter embedded
in the block.
Smallgaugeblack wire or even thread is used to bracket
or identify different features of the specimenwithin the
block. Small gaugeblack wire is rigid and moreeasily
teased into position than thread. The completelypoured
blockis allowedto set overnightin a dust-free room.The
newly poured blocks should be covered with a paper
canopy. The cast can be removedfrom the moldthe next
day. After removingthe end plates, the cast will come
out easily because of the low adhesive property of the
mold release compoundand someshrinkage of the bioplastic itself during the hardeningprocess. The curing
process or hardeningof the block is hastenedthroughuse
of overnightovencuring at 140to 160F.
4. Grindingand polishing. Becausethe grinding and polishing workis very time-consuming,we have chosen to use
an electric motor-drivenbioplastic grinder-polisher. This
device allows for rapid, easy, and perfect polishing of
cast bioplastic blocks. Thedesignof this grinderparallels
that of a shop belt sander. The sander is vertically
oriented and the sanding belt runs through a water bath
permitting wet block sanding. Wet grinding of the six
block surfaces eliminates dust, providesa moreevencut,
and permits an easy backand forth motionof the block.
The sanding belts are changedfrom a coarse 100-grit
initial belt to rapidly grind the veryroughinitial surface
of a cured block throughgrits of 200, 320, and 600. The
600-grit belt providesa very smoothfinal surface. Asthe
25
600 belt is used, frequent inspections under runningtap
water will display the degree of grind. After finishing
with the 600-grit sanding, the finish of the block can be
improvedby polishing the block in a back and forth motion on a felt polishing boardtreated with a quarter of a
teaspoon of liquid abrasive. Thefinal glossy finish is
achieved by a back and forth motion of the block to a
spinning buffing wheelwhichis mountedon the grinderpolisher unit and treated with a cake of buffing compound. The finished block should have a glossy, clearly
transparent appearance.
Special techniques in the preparation of wet, opaque,
and~or colored specimens for embedding. Fresh plant
specimenscannot be embeddeddirectly since water and
resin will not mix and a water-resin mixturns cloudyand
opaque. Water must be removedbefore embedding.Drying the specimen will removewater but the specimen
loses color and shrinks beyond use. To maintain the
natural condition of the moist specimen, we use the
freeze-dry method.In this technique wet specimens,for
example, nodulated soybeanroots, are placed into the
vacuumchamberof a lyophilizer unit and covered with
dry ice to instantly freeze the specimen.Careshould be
taken to removeall free water with paper towelingbefore
the dry ice is used to cover the entire sample.The chamber is placedinto the lyophilizer unit and hookedinto the
vacuumsystemfor 48 hours. Theoriginal life-like condition of the specimenis preserved, and it maythen be embeddedas described above. Greencolor of leaves can be
preserved by press drying followed by laminating the
leaves betweentwo layers of adhesive plastic. The extraneous plastic can be trimmedwith a scissors and the
specimen maybe embeddedas described before. Laminating flat leaf tissue helps in handlingthe fragile specimenafter press drying. Whenfleshy specimensof leaves,
flower buds, stems, etc. are to be embedded,the freezedry methodis recommended.
STUDENT EVALUATION OF SPECIMEN
BLOCKS IN CROP SCIENCE TEACHING
Student reaction to the use of bioplastic specimen
blocks in introductory crop science classroom and
laboratory
teaching was measured during three
semesters in 1977 and 1978 at the University of Illinois.
These visuals are used as displays in the autotutorial
carrels, crop science laboratory, and the lecture classroom depending on the appropriateness of the specimens for reinforcement of the subject matter. When
used in the lecture classroom the specimen blocks are
circulated amongstudents seated in a given row of seats.
Sufficient duplicates (1 block per l0 students in a given
row) are used in order that all students mayobserve the
specimen blocks in minimal time during the lecture
hour. An evaluation was conducted to reflect the opinions of 266 students at the beginning (pre-survey) and
the end of the semester (post-survey) using an evaluation form on which the student: (a) strongly disagreed
1, (b) disagreed = 2, (3) was neutral = 3, (d) agreed
and (3) strongly agreed=5 that the use of bioplastic
specimen blocks were useful, important, unusual,
positive, active, successful, new, and interesting (Fig.
3). While student opinion on the use of these specimen
JOURNAL OF AGRONOMIC EDUCATION
26
blocks was nearly neutral at the beginning of the
semester, there was a highly significant increase in
agreement that these teaching aids were useful, important, unusual, positive, active, successful, new, and interesting by the end of the semester.
The increase in agreement on the successful use of
bioplastic block specimens in crop science teaching: (a)
was not significantly different for freshmen than that
for sophomores or upperclassmen and (b) not significantly different for agronomy majors than that for students majoring in other disciplines.
SUMMARY
The use of various bioplastic agronomic specimen
blocks during three semesters of lecture and laboratory
teaching in the Department of Agronomy, University of
Illinois, shows increasing student agreement from the
beginning to the end of the semester that these visual
aids are useful, interesting, important, and successful in
promoting the teaching objectives. Similar positive responses to the use of these specimen blocks wee found
among: (a) all classes i.e. freshmen, sophomores,
juniors, and seniors, and (b) students in all curricula,
i.e., agronomy majors, animal science majors, etc.
Successful bioplastic embedding of agronomic specimens include seeds of weeds and crops, nodulated soybean roots, flowers, leaves, grass collars, grass spikelets—both complete and disassembled, inflorescences,
ears of corn and sections of corn ears, soybeans pods—
opened and closed. Special freeze dry techniques are essential in preparing wet plant specimens for embedding
since bioplastic resin does not mix with water.
Advantages of preserving agronomic specimens in
bioplastic blocks include: (a) minimum specimen breakage because of fragile nature of dry specimens (b) easy
stereomicroscopic examination because specimens are
held "in place" in the plastic matrix while being
observed (c) preservation of life-like plant materials in
natural color for repeated use.