|Lab 14: Plants and their Interactions with the Environment||
Fruits and Dispersal
(A) Fruit Structure
Botanists have names for the different types of fruit found in flowering plants. “Fruits” in the botanical sense include much more than the kinds of things we think of as “fruit” at the supermarket lots of vegetables and grains are really fruits, too: the ripened ovaries of flowers. Here is a brief rundown of some of the main types of fruit, just to provide you with an idea of the diversity of form and function in fruits. Look over the examples of fruits displayed in lab, and try to determine what types they are.
I. Complex fruits: “Complex” fruits are derived from more than one ovary, and result in an aggregation of smaller fruit elements.
A. Aggregate fruits: Complexes of smaller fruits, which are derived from more than one ovary in one flower. Examples are raspberries and blackberries (in which each little member of the overall fruit is like a drupe, described below), and strawberries (in which each little fruitlet, the brown things on the surface of the fruit, is an achene, and the red part is derived from another part of the flower). Just to confuse you, note that raspberries, blackberries, and strawberries are NOT really berries at all, in the botanical sense!
B. Multiple fruits: Complexes of smaller fruits, which are each derived from a separate flower, but are fused together. Pineapples are multiple fruits, derived from a series of flowers that are packed together on a stem (V&C fig. 6.240).
II. Simple fruits: Simple fruits are derived from a single ovary, although that ovary can consist of more than one carpel, fused to other carpels. These fused carpels produce the internal divisions we see in some fruits (like tomatoes and oranges). Simple fruits are broadly divided into fleshy and dry fruits.
A. Fleshy fruits: Fleshy fruits have moist parts, and include many of the things we think of as fruits in the grocery store. There are three main types of fleshy fruits, and some of these have subdivisions:
1. Berry: Berries are fleshy fruits derived almost entirely from ovarian tissue, and the endocarp is fleshy (not hard). Within this general category are three subcategories:
a. Typical berries: The pericarp is usually fleshy throughout, and has a thin skin. Examples are tomatoes, grapes, peppers, and blueberries. See Rust, fig. 49 a-b; V & C fig. 6.241 for illustrations of tomatoes. In the cross section of the tomato, see the different carpels. Within each carpel are placentae, which bear the ovules.
b. Pepo: A berry with a hard rind. The receptacle of the flower contributes to the rind Examples are mainly members of the squash family, like watermelons, cantaloupes, gourds. Cucumbers and bananas are also pepos.
c. Hesperidium: A berry with a leathery, separable rind, and partitions between segments. Examples are oranges and other citrus fruits (V & C fig. 6.239).
2. Drupe: A drupe is a fleshy fruit in which the endocarp forms a stony pit, which encloses usually one seed. Cherries, peaches, plums, olives, and even walnuts and almonds are drupes (the shell you usually see is the endocarp, and the fleshy part has been removed). A coconut is actually a drupe, too.
3. Pome: In pomes, the endocarp is papery, or sometimes stony, and forms a core with several seeds. The outer part of the fruit is derived from the base of the flower (the “hypanthium”). Pomes occur in one division of the rose family, and pears and apples are two familiar examples. See Rust, fig. 49 c-d; V & C fig. 6.238 for illustrations of pomes.
B. Dry Fruits: In dry fruits, the entire pericarp is dry, not fleshy. There are two main divisions within dry fruits: dehiscent and indehiscent. Then there are several types within these categories; we will describe a few of these.
1. Dehiscent fruits. Dehiscent dry fruits split open when ripe, releasing their seeds.
a. Legume: Legumes have one carpel, which splits along two sutures. Peas, beans, and peanuts are legumes. See Rust, fig. 49 e-f; V & C fig. 6.233, 6.237 for illustrations of legumes.
b. Follicle: Follicles have one carpel, which splits along only one suture. Milkweed fruit is a follicle.
c. Capsule: A capsule is a dehiscent fruit with more than one carpel. The capsules of different types of plants may split in different ways, giving rise to several subdivisions. A number of flowers with small seeds produce capsules, including mustard and poppies.
2. Indehiscent fruits. Indehiscent fruits are dry fruits that do not split open when ripe to disperse seeds. The entire fruit disperses.
a. Achene: Achenes are dry, indehiscent fruits with a single seed, which is attached to the pericarp by a small stalk (a funiculus). In many achenes, the pericarp has hooks or fibers that permit dispersal in the wind. Example: dandelions and many others.
b. Samara: Samaras are similar to achenes internally, but the pericarp grows into one or two thin wings. Examples: maples, elms.
c. Nut: Nuts are also similar to achenes, but have a thick, stony pericarp and a cup at the base. Examples: acorns, hazelnuts.
d. Caryposis (grain): In caryposis fruits (or grains), the seed coat is fused to the pericarp, so the seed is not loose in the fruit. Examples: all grasses, including important grains like maize (corn), wheat, oats, and barley.e. Schizocarp: Schizocarps are dry, indehiscent fruits with more than one carpel. The carpels split apart from each other at maturity, but the seeds remain enclosed. Examples: carrot and parsley.
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