Preparation techniques and student evaluation of bioplastic specimen blocks for use in crop science teaching1 A. W. Burger and R. D. Seif2 3 ABSTRACT Messersmith and others at North Dakota State Uni- versity have prepared a number of different kinds of Plastic specimen blocks enhance crop science agronomic specimens in plastic for classroom and lecture and laboratory teaching according to stu- laboratory use in recent years. Embedding fragile vege- dent reaction measured three semesters in the De- tative specimens in plastic preserves these materials for partment of Agronomy, University of Illinois. Stu- repeated use. Seeds of various species and cultivars of dents indicated that these visual aids are useful, crops can be more easily compared to each other in the interesting, important, and successful in promot- rigidity of the plastic matrix. Stereoscopic examination ing teaching objectives. Similar positive student if facilitated because specimens are held "in place" in responses to the use of plastic-embedded crop specimens were found among all classes and the plastic while they are being observed. The purpose majors enrolled in the introductory crop science of this paper is to report on (a) various methods and course. Agronomic specimens which have been techniques used in preparing bioplastic blocks of agro- successfully embedded include: seeds of weeds nomic biological specimens and (b) student reaction to and crops, grass collars, flowers and typical the use and value of these specimen blocks in crop sci- leaves of plants, grass spikelets, plant inflores- ence teaching. cences, ears and ear sections of corn, soybean pods, and nodulated soybean roots. Special "freeze dry" techniques are essential in preparing MATERIALS AND METHODS wet plant specimens for embedding since plastic resin does not mix with water. Advantages of pre- The technique of embedding a bioplastic is well documented serving agronomic specimens in bioplastic blocks in flyers and literature distributed by various commercial bio- include: (a) minimum breakage because of fragile logical stores which sell the resin. (For example, "Embedding nature of dry specimens (b) easy stereomicroscop- in Bio-Plastic," Ward's Natural Science Establishment, Inc., ic examination, and (c) preservation of plant ma- P.O. Box 1712, Rochester, N.Y. 14603). The resin may be pur- terials in natural color for repeated use. chased 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 Additional index words: Visual aids in crop sci- more expensive but also extremely versatile since it contains ence, Freeze drying of plants. ' Contribution from the Univ. of Illinois, Urbana, Campaign. 2 Professor of agronomy and professor of biometry, agronomy, re- MBEDDING biological and geological specimens is spectively. E not new. However, embedded specimens of agro- 'Unpublished report. Permanent plant mounts by embedding in nomic crops in various forms are quite rare. C. A. plastic. Dep. of Agronomy, North Dakota State Univ., Fargo. 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, com- plete with buffing compound, polishing belts in four texture S P EClMEN grits (100, 200, 320, and 600), wet strength abrasive cloth, SUPPORTING LAYER lyophilizer, dry ice, asbetos gloves, and laminator. Agronomic specimens used for embedding include: many varieties of soybean seeds, assorted field crop and weed seeds, Fig. 1. Three major layers and alternate trapping layer in podded soybeans, nodulated soybean roots, inflorescences, the plastic embedding procedure. flowers, and vegetative parts of several grasses and legumes, ears of corn, grain and grass spikelets, both intact and disas- sembled. Evaluation of the merit of bioplastic specimen blocks in in- troductory 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 sci- ence 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, im- portant, unusual, positive, active, successful, new, and inter- esting as these adjectives applied to the use of bioplastic speci- men 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. Embedding Procedure The basic principle of casting plastic specimen blocks is Fig. 2. Aluminum “take-a-part” mold showing the U- hardening of a liquid synthetic resin in the presence of a frame and removable end plates. 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 poly- the U-frame with rubber bands. The inside of the now merizes or gels to a crystal-clear solid. Many different varia- rectangular aluminum mold box (top side open) is com- tions of embedding procedures have been used by the authors; pletely covered by brushing on a thin coat of mold release however, the general principle of embedding dry agronomic compound. After the mold release compound dries, the specimens involves the preparation of (a) a specimen-support- mold is poured. Where mold sizes or shapes are not com- ing layer (b) a specimen-retaining layer and (c) a specimen- mercially available we have found galvanized metal fitted covering layer (see Fig. 1). to desired specifications to work very satisfactorily. The amount of catalyst used is inversely correlated with final 2. Mixing the resin with a catalyst, One hundred cubic block thickness. Blocks up to 0.25 inch in thickness require centimeters of resin are poured into a disposable un- about 80 drops of catalyst per 100 cc of resin whereas 0.25 to 1 waxed paper cup. Forty drops of catalyst are added to inch and 1 to 2 inch blocks require, respectively, 40 and 10 to the resin and the contents are stirred gently with a clean 20 drops, respectively. The thicker blocks must harden more glass stirring rod, being careful to avoid stirring bubbles slowly to avoid cracking. In addition, because the chemical re- into the mixture. action of catalyst with resin generates heat, the higher 3. Pouring the specimen block. Just enough resin is poured temperature generated by an excess of catalyst will tend to dis- into the mold to form the base layer. Air bubbles can be color the specimen. However, maximum proportions of teased out using a dissecting needle. Air bubbles usually catalyst must be used to: (a) obtain a clearer and harder block surface automatically unless high amounts of catalyst re- and (b) reduce the line visibility between successive layers of sulting in quick set are used. These air bubbles tend to Douring. obscure the specimen and distract from clear observation Most of our blocks are 1/4 inch to 1 inch in thickness and of the specimen under the stereomicroscope and should the general procedure for embedding most of our specimens is be teased out with a dissecting needle before hardening as follows: of the matrix at all stages of pouring. 1. Preparation of the molds. We use either 3 x 2 x 3/4 inch As soon as the base layer firms to the point of support- or 4 x 2-3/4 x 7/8 inch rectangular “take-a-part” alumi- ing the mounting specimen, the specimen is “stuck” to num molds (see Fig. 2). The take-a-part feature refers to a the base layer. A small amount of liquid mix is poured rectangular aluminum mold consisting of removalbe ends over the specimen to trap it in a few minutes of firming and a U-frame. Before use, the end pieces are clamped to time. This prevents the specimen from floating when the BURGER AND SEIF: SPECIMEN BLOCK PREPARATION 25 BIOPLASTIC SPECIMEN BLOCKS-- 600 belt is used, frequent inspections under runningtap their use in crop science teaching water will display the degree of grind. After finishing USEFUL ~ [] PRE with the 600-grit sanding, the finish of the block can be improvedby polishing the block in a back and forth mo- UNUSUAL tion on a felt polishing boardtreated with a quarter of a POSITIVE teaspoon of liquid abrasive. Thefinal glossy finish is achieved by a back and forth motion of the block to a ACtiVE spinning buffing wheel whichis mountedon the grinder- SUCCESSFUL polisher unit and treated with a cake of buffing com- N~ w pound. The finished block should have a glossy, clearly transparent appearance. ~NTERSS~ Special techniques in the preparation of wet, opaque, I 2 ~ 4 5 and~or colored specimens for embedding. Fresh plant strongly disogree o~ee ogree specimenscannot be embeddeddirectly since water and resin will not mix and a water-resin mixturns cloudyand opaque. Water must be removedbefore embedding.Dry- ing the specimen will remove water but the specimen Fig. 3. Studentreaction to the useof bioplastic speci- loses color and shrinks beyond use. To maintain the menblocks in introductorycrop scienceteaching at natural condition of the moist specimen, we use the the beginning(pre) andthe end(post) of the semester. freeze-dry method.In this technique wet specimens,for example, nodulated soybean roots, are placed into the covering layer is pouredto finish the block in the next vacuumchamber of a lyophilizer unit and covered with dry ice to instantly freeze the specimen.Care should be step.