ANATOMYOF A GREEN PEA

I employ the following laboratory exercise in my high school biology course as the culminatingexperi- ence of the botany unit. It correlatesthe flower to the fruit as well as demonstratingother botanicalconcepts. Becausethe pea pod is the product of a flower, it is a fruit; specifically,a typicallegume fruit. This exercise lends itself nicely to a classroom lab activity. Only simple readily available materials are needed. These include fresh green pea pods (string- JOW~~~~~~ beans or lima beans may be substituted), scalpel or razorblade, hand lens, glass slide, and microscope. 1. Observethe externalfeatures of the pea pod (fig. 1). The pea pod is an example of dry dihiscent fruit. When it is fully mature, it will split along very definite

lines. As it opens, its seeds may be dispersed.Note the Downloaded from http://online.ucpress.edu/abt/article-pdf/38/2/113/33642/4445487.pdf by guest on 29 September 2021 Fig. 2. For their independent study projects, Linda King and Steve Roller press plants for a herbarium collection for the college. two seams that run the full length of the pod. One of the seams is concave and runs into the pod. The seam independentstudy projects. Some studies, such as iden- on the opposite side is convex and either lies flat or tification of floweringplants, marinealgae, lichens, or rises away from the pod. of the The sepals from birds of an area, do not cost the instructor or the 2. Note the stem end pod. the flower remain. There are five sepals. Since the college any money. The students enjoy the field work pea is classified as a dicotyledon, we would and can do most of the identificationafter initial help pea plant its floral to be in fives or multiplesof five. from the instructor.With some money availableto the expect parts At the opposite tapered end of the pod, observe the instructor plankton nets, hydrometers,dip nets, and remnantsof the style. The entirepea pod is an enlarged other limnological equipment can be purchased for ovary. It has grown considerablyfrom its original size studies. Microbiologicalanalyses require the aquatic to accommodatethe seedsinside. of media. I have used Milli- purchaseand preparation 3. Note the green coloring of the pod. This color equipment for about half of pore (membrane filter) usually indicatesthe presenceof chlorophyllin plants. studieswith good reproducibilityof our bacteriological To prove this supposition, cut a very thin slice of the budgetswill allow the purchaseof boats results. Larger pod and observeit underlow power of the microscope. or spectrophotometricequipment, but these projects The spherical pale-green structuresthat you observe really need not be elaborate. Recently I have obtained are chloroplasts. Photosynthesis takes place in these funding that allows five students to go out weekly on a research vessel performing coliform counts, other bacterialsurveys, and fish and invertebratestudies on San FranciscoBay. Independentstudy projects have merit in the experi- ence they give to the student, whether in studying SEPAL bacterial degradationof crude oil or an enumeration of the lichens in a U.S. GovernmentSurvey quadrangle of farmland. I have found student interest is greater when their projects have meaning in relation to real environmentalproblems rather than biologicalabstrac- OUTER OVARY WALL SEEDOUTLINE tions. Pure researchhas merit and the data are mean- ingful, but the lower-divisioncollege student does not CONVEX SEAM CONCAVE SEAM have the scientific backgroundrequired for many such studies. Natural history and environmentaldata lend thenselves to these students' experience. The knowl- STYLE edge gained in independentstudy has led a numberof students to major in biology, but this is not the purpose. The purpose is to create an awarenessand Fig. 1. External view of the pea pod. concern in the student about his environment that resultsin an understandingof biological principles. structures.Pea pods not only protect the seeds inside ChristineL. Case but also contributeto their food supply by carryingon Biology Department photosynthesis. SkylineCollege 4. Now observe the internal structuresof the pea San Bruno, Calif. 94066 pod (fig. 2). Carefullyopen the convex side of the pod

TEACHERTO TEACHER 113 floral parts, as noted earlier, confirm the fact that the pea is a dicotyledon. SEPAL 9. Note the small nib between the cotyledons. This is the embryo plant (fig.3). It is the product of sexual FUNICULUS reproduction.The food stored in the cotyledon would CONVEX SEAM have been enzymaticallyconverted to usable nutrients to nourish the embryo until it was capable of making FIBROVASCULARBUNDLES its own food. 10. Make a completesketch of all the structuresyou SEED(RIPENED OVULE) have observed externally and internally. To review ] /OVARY your understanding,give the function of each structure namedin your diagrams. Ralph Postiglione South SeniorHigh School STYLE\ GreatNeck, N.Y. 11020

Fig. 2. Internal view of the pea pod.

INEXPENSIVE OCULAR Downloaded from http://online.ucpress.edu/abt/article-pdf/38/2/113/33642/4445487.pdf by guest on 29 September 2021 with the razor blade or scalpel. Remember that this is MICROMETERS the raised seam, also called the artificial groove. If the concave side is cut in error, the seeds will be disturbed High school biology teachers frequentlywant their from their natural position. studentsto accuratelymeasure the size of a particular 5. Note the cavity in which each seed lies. The inner microscopic specimen. Because of the high cost of ovary wall (as well as the outer ovary wall) is leathery ocular micrometers and stage micrometers, many and therefore inedible. The ovary wall in the string- teachers resort to having their students estimate the bean, however, is edible. Note the position of the peas numberof times a specimencould be placedend to end (ripened ovules). Observe the alternating pattern in the acrossthe diameterof the field of view and then divide opened pod. This is an example of the survival mecha- that diameter by the estimated number of specimens nism. Since the peas are attached to two fibrovascular for an approximationof specimensize. bundles, if one of the bundles is destroyed and unable A. Amaro (1965) presented a technique for pro- to bring nourishment to the seeds, the other bundle ducing relatively inexpensiveocular micrometers,but would be able to keep the remaining seeds alive. Note his article did not provide the necessary data to any undeveloped peas in the pod. There are two duplicate the method of production. The article reasons for this condition. Either one of the fibrovas- motivated me to experiment with a technique for cular bundles was destroyed, or the ovule was not making fairly good quality ocular micrometersat a fertilized and therefore could not develop into a cost of about ten cents each. The equipment and mature seed. materialsare as follows: 6. Count the number of peas in the pod. Are any Singlelens reflex camera(35-mm with 50-mmlens) two of them alike? The old adage "as alike as two peas Reflectedlight-light meter in a pod" is a stretch of biological truth. Since peas Copy stand with pair of flood lamps (3200 ?K) are produced as a result of sexual reproduction, we can HC 135-36 High Contrast Copy film ASA 64 always expect to find variation. Black matte finish constructionpaper (14 by 16 inch) 7. Note the structures to which the seeds are Whiteself-adhesive label cut into 11 strips(1/16 inch attached. These are the fibrovascular bundles, tubes by 1 inch) which bring nutritive materials to the peas as they are Microscopeslide developing. The stalk connecting the seeds to these CanadaBalsam bundles is the funiculus. The indentation on the seed Transparentmetric ruler cut into 1-cmincrements where the funiculus is attached is the hilum. 8. Remove a well-formed pea from the pod. Cut off The white strips are placed parallelto each other in the seed coat and lay open the two parts of the seed. approximatelythe center of the black construction Note the two fiat-faced halves. These are the cotyle- paper. The camera, on the copy stand, is focused on dons. Since there are two, the botanist has classified the white lines so that the lines pattern covers a very the pea as a dicotyledon. Two cotyledons and five small area on the viewer. Many viewers have a small focusingcircle which is about the correctlength for the PLUMULE white lines to cover. Each flood lamp is positioned so that the illuminationis even. (To determineif illumi- COYLDO *~ HYPOCOTYL nation is even, hold a pencil so that one end of the COTYLEDONRDICL pencil is touching a piece of paper. Then, adjust the flood lamps until the shadowscast on both sides of the Fig. 3. Split pea, containing the embryo. paper are equal in darkness.) Determine the correct

114 THEAMERICAN BIOLOGY TEACHER, FEBRUARY 1976