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Laboratory Three
Fruit structure and its link to the mechanism of seed
dispersal
Introduction
So far this semester in class we have studied and discussed:
- The life cycle of Gymnosperms and Angiosperms.
- The structure of flowers.
- The ABC genetic model, including the MADS box control system.
- The biochemistry of Germination.
- Meristem development and function.
- The genetic development of floral meristems. So it is only fitting to use this lab to consider the structure, function, and dispersal of fruit bodies. To recap: The ovules are contained in the ovaries of flowers. After pollination and fertilization of the egg inside the ovule, the ovule develops into a seed and the surrounding ovary develops into a fruit.
Figure 1: Generalized depiction of the structures produced during fruit development.
In some cases, ovaries develop into fruits without fertilization of ovules. This kind of fruit development which does not require fertilization is called
parthenocarpy and the resulting fruits are called parthenocarpic fruits.
As fertilization is required in order to produce a viable seed, parthenocarpic fruits do not have seeds. Many of the "seedless" varieties of fruits found in the supermarket (e.g., watermelons, grapes, bananas, cucumbers, etc.. .) are the result of parthenocarpy.
Fruit Structure
Fruits are classified according to the arrangement of the carpels from which the
fruit develops:
Multiple fruits consist of gynoecia of more than one flower. Pineapple and mulberry
are good examples of multiple fruits.
Aggregate fruits are formed from separate carpels of a single gynoecium.
Individual parts of aggregate fruits are known as fruitlets. Examples include raspberry, strawberry, and magnolias.
Simple fruits develop from one carpel or several united carpels. This is the most
common type of fruit, of which there are several categories:
1) Dry fruits are simple fruits that are dry, woody, or papery at maturity.
Dehiscent fruits are dry fruits that break open at maturity to release the seeds. Dehiscent fruits are classified by the way the ovary wall breaks apart:
- Follicles - the fruit wall breaks open along 1 seam (milkweeds)
- Legumes - the fruit wall breaks open along 2 seams (beans, peas, lentils)
- Siliques - the ovary wall breaks open with seeds intact on the central portion of the fruit (canola, members of the Brassicaceae (cabbage family)
- Capsules - split open longitudinally or have holes through which the seeds are released (okra, cotton and poppy)
2) Indehiscent fruits are dry fruits in which the seeds remain within the fruit
and are dispersed with the fruit wall intact.
- Achenes - small single-seeded fruits with the seed attached to ovary wall only at one point (buttercup and buckwheat)
- Samaras - achenes with "wings" modified for wind dispersal (ashes and elms)
- Caryopsis - seed and fruit wall are totally fused (rice, wheat and other cereals)
- Cypsela - the fruit of Asteraceae (dandelion and marigold) family that has an achene-like fruit but is attached to pappus, which helps in its dispersal.
shatter as the fruit flesh dehydrates, throwing the seeds a distance from the parent plant. For example, in the parasitic eudicot mistletoe, a very high hydrostatic pressure builds up in the fruit and the seeds are ejected as far as 15 meters from the parent!
Lab Activity:
You will be provided with several different kinds of fruits and it is your goal to classify each one. Examine your chosen samples using a dissecting microscope to achieve a greater detail of the internal structure. In the space provided draw and label, in detail, the structure of a:
- Multiple fruit
- Aggregate fruit As for the Simple fruits, draw, label, and identify three examples from the selection of:
- Dry fruits
- Indehiscent fruits
- Fleshy fruits An example of what to label is shown below.
Figure2: The internal structure of a tomato fruit.
In each case consider within your group about the possible method of dispersal of each fruit. Base these on the structure of each fruit examined, and FULLY explain your reasoning under each of your drawings.
Multiple fruit
Aggregate fruit
Simple Fruits Dry fruit (3)
Simple Fruits Indehiscent fruit (1)
Simple Fruits Indehiscent fruit (2)
Simple Fruits Indehiscent fruit (3)
Simple Fruits Fleshy fruit (3)
Post-lab Questions: (1). Immature fruits are generally green in color but change to bright, attractive colors when ripe. What is the potential evolutionary advantage of fruits changing color with respect to their potential for dispersal? (i.e., why aren’t immature strawberries red?).
(2) In seedless varieties of fruits (e.g., grapes, watermelons, bananas) fruits develop even though there has been no fertilization. Why are there no seeds in those fruits? How are those plants propagated if there are no seeds?
