Tested Techniques Teaching the Evolution of the AngiospermCarpel Joseph E. Laferriere One of the most difficult morpho- logical concepts to teach introductory botany students is that of the carpel. The carpel is a highly modified leaf Downloaded from http://online.ucpress.edu/abt/article-pdf/54/6/362/370811/4449511.pdf by guest on 30 September 2021 endosing the ovules. As the ovules ma- ture into seeds, the carpeldevelops into a fruit, which may be either dry or fleshy. Two aspects of this structureare dif- ficult for students but can be taught successfully when properlyaddressed. First,the evolutionaryorigin of the car- pel as a modified leaf, and second, the nature of carpel fusion. The latter re- sults in compound pistils composed of several carpels. The number of carpels in a compound pistil is often difficultto determinewithout dissecting the pistil, Figure 1. Snow pea sliced open to show developing seeds. which is the structure visible during cursoryexamination. Most angiosperm carpels bear little resemblance to vegetative leaves, but ferriere1987). In this case each carpel effective in explaining the evolution- there are some exceptionswhich can be is considered a separate pistil. ary principles involved, but live fruits used to illustratethe point more clearly. Figure 2 shows the evolution of the help a great deal in illustratingthe idea One of these is the snow pea or sugar most common types of compound pis- even more clearly. Fruits are more pea, an immature legume fruit com- til arrangement from the ancestral effective than flowers since they are monly used in oriental cuisine. If the single-carpelmarginal type of placen- generally larger. Green peppers (Fig- pea is sliced down the side containing tation. Fusion of two or more such ure 3), cut crosswise, are one of the the developing seeds, it can be opened carpels results in axile placentation, best models for teaching students how up to reveal a green, leafy structure which then becomes modified into the fusion from three carpels can produce (Figure 1). It even has the seeds ar- other placentation types (Radford et a fruit with a few locules. The green ranged along the margin, thus resem- al. 1974).The numberof carpelscan be color also suggests modified leaves. It bling the ancestral seed ferns from determined by counting the "locules" is then easy to extrapolatethis to other which all other seed plants evolved. (compartments).Disintegration of the species such as oranges and tomatoes The fusion carpels have undergone partitions results in free central pla- in which the cavities have become in the course of evolution can be centation.Alternatively, dissolution of filled with liquid. A few fruits, such as taught effectively using certain plants, the axis and relocationof the ovules to the passion fruit, have parietalplacen- such as the strawberryand the mag- the periphery of the ovary results in tation. nolia. They have many separate car- parietal placentation. The number of These fruits can be extremely useful pels in the same flower. In the mature carpelsin parietalplacentation may be in teaching the evolution of the angio- strawberry the fleshy "fruit" is de- determinedby counting the numberof sperm carpel and are usually available rived from the receptacle;the botanical groups of points of ovule attachment at low cost from most grocerystores. fruits are achenes borne on the surface (e.g. four in the diagram in Figure 2). and commonly considered seeds (La- Basalor free-centralplacentation, which are rare among fleshy fruits, make it References difficultfor the student to count how are in the cross- Laferriere.J.E. (1987). Folk definitions in- many carpels present fluence the acceptance of technical vo- Joseph E. Laferribre is assistant professor section of the fruit. In such cases it is and acling director of the Marion Own- cabulary. The AmericanBiology Teacher, bey Herbarium, department of botany, usuallynecessary to count veins or stig- 49, 149-152. Washington State University,Pullman, WA mas to ascertainthe carpelnumber. Radford, A.E., Dickison, W.C., Massey, 99164-4309. Diagramming the evolution of car- J.R. & Bell, C.R. (1974). Vascularplant pel types on the chalkboardis usually systematics.New York:Harper & Row. 362 THEAMERICAN BIOLOGY TEACHER, VOLUME 54, NO. 6, SEPTEMBER192 Ancestral gymnosperm Think Denver. Think Diversity. NABT's National Marginal Convention Downloaded from http://online.ucpress.edu/abt/article-pdf/54/6/362/370811/4449511.pdf by guest on 30 September 2021 & Exhibition Parietal November 11-15, 1992 Denver,Colorado 4~~~~~CC featuring... Axile * Top-notchSpeakers * Free central Hands-on Workshops * InformativeSessions Figure2. Diagramshowing the evolution of majortypes of compound pistils from a simple unicarpellatepistil. *A Dynamic Exhibition * Social Events * Fabulous Field Trips *And lots more! For more details, or your copy of Convention Highlights, call (703) 471-1134 Hurry!Preregistrations mustbe postmarkedby October9th. Allothers willhave to registeronsfte. Figure 3. Green pepper sliced crosswise to show four-loculateaxile placentation. ANGIOSPERMCARPEL 363 .