Angiosperms

The origin of flowers is a very important part of this lab, and then how flowers turn into fruit. Across Lubbock by the greenhouses is a cluster of bright orange flowers in the spring, you'll have to look for new ones in the fall. These flowers are in all stages...you can see the buds, surrounded by the green sepals. When the flowers open, you can see the petal and stamens very clearly. Of interest in these flowers is that the evolution of the stamen from a microsporophyll is much more obvious. The stamen is FLAT, like the leaf it evolved from, rather than having a filament and anther. The surface is covered with microsporangia, which release pollen.

Slightly older flowers have the stamens starting to wither and fall away, while the gynoecium in the center becomes more visible. It is composed of three fused carpels, with three stigmas, one style, one one ovary. As you look at fertilized flowers, you can see the ovary becoming longer, and longer, and longer...the sepals, petals, stamens, and the stigma and style wither and fall off. The ovary, which is full of ovules, elongates dramatically to several inches long. I would strongly recommend to the TA to go around campus and find a bush, tree, or patch of flowers that has all stages of fruit formation, from flower bud to mature fruit, visible.

This is a great lab. I usually have the students sit on tabletops, and we start to dissect stuff. I bring in a new potato with a thin red skin and an older potato with a thick brown skin, and show how it's a stem forming cork tissue (the peel) on exposure to air. I also bring in carrots, which have a stele in the root that is gold and can be peeled out of the dark orange cortex. I hand them razor blades and let them work at it until a student gets the golden cylinder out of the middle. I point out that the orange of the carrot is the epidermis, the cortex, and the endodermis.....the yellow stele in the center is the pericycle surrounding xylem and phloem. If they look, they can see that it's an actinostele, with a many-pointed star-shape of xylem in the center.

The next neat object to have them dissect is a citrus fruit. Grapefruit are the best because the skins of each section are very thick, but limes are much cheaper. Have the students peel a fruit and take out a section....this is a seed leaf, a sporophyll, that has evolved to form a fleshy bit. On the midvein of the leaf are the seeds...the spongy mesophyll forms the flesh. The papery skin is a very thin leaf that wraps around and seals together where it touches the skin.

The VERY BEST example to show the students how ovaries evolved is a snap-pea. These are flat pea pods with a few peas on the inside. If you take a snap-pea, and slice open the edge opposite the peas, you can unfold it....and IT'S A LEAF!!!! Specifically, it's a sporophyll with ovules down the center of it! Green beans are good, but don't work quite as well. For students still confused on how a sporophyll is a carpel after this, I can suggest one more demonstration. Take a piece of paper, and tell them to pretend it's a leaf/sporophyll. Put your hand at the bottom of the paper, as the "ovule." Roll the paper around your hand, and have someone tape it, so that you have a cylinder of paper pointing up from your hand. Scrunch it into a skinny "style" above the "ovary" with your hand, and leave the top of the paper unscrunched and slightly flattened to form a stigma.

Apples are the final thing to show them. Get a couple of apples, and slice them across their equator. This shows a star-shape on the inside. Grab some forceps, and pull out some of the tough, shiny tissue that surrounds a seed. That tough, shiny tissue of the core is the mature sporophyll. One of the students can pull out the entire sporophyll, which looks almost exactly like a small white pea pod. So what is the rest of the apple??? Explain how the five ovaries are sunk into and surrounded by the stem. When fertilized, the stem surrounding the base of the flower swells and becomes fleshy. The "peel" of an apple is cork! The small, thin, very old green book has some very good pictures of this. Have fun with this lab, but make sure the students sponge off the counters afterwards; the fruits can leave it very sticky.

There is also a large cardboard container with a clear top showing different sorts of fruits. It should be beside the shelf in the back of the classroom.

Hosted by www.Geocities.ws