BIO 322: Fruits and seeds.

Plums, peaches, apricots (the "stone" fruits).

            Because carpels are leaf-like structures (at least that is the current theory), they have a lower epidermis, a mesophyll, and an upper epidermis. These can often develop into:

1. lower epidermis -- an epicarp (also called exocarp) that is the skin,

2. mesophyll -- the mesocarp or flesh,

3. upper epidermis -- the endocarp or any innermost layer that is distinct.

These three layers can be found in many fruits, but they are not obvious in many others. And when they are present, we cannot be absolutely certain that they developed from the epidermises or mesophyll. Stone fruits do have these three layers and they develop as just described. The skin and flesh are easy to see, and the "stone" or "pit" is the endocarp. We often think of it as the seed, but it is actually the innermost part of the fruit. If you would actually crack it open (it is a tough mass of sclereids) you would find the true seed inside, with its own seed coat and embryo.

            You may actually be more familiar with this than you realize -- many of the nuts that you crack open and eat at Christmas, such as almonds and pecans, are the endocarp and seed of the fruit. The epicarp and mesocarp have been removed before they were sent to market.

Avocado.

            The epicarp and mesocarp are easy to identify here, but the endocarp is just a thin film.

Peppers.

            These are true fruits; the stalk is the pedicel -- the flower's stalk -- which has become much larger as the fruit matured. You can imagine that a great deal of phloem conduction is needed for the development of a large fruit compared to a small flower (the fruit stalks of pumpkins have huge sieve tube members). The green "skirt" or disk of tissue between the stalk and the fruit is the remnant of the sepals, which had fused together. The opposite end of the fruit is where the style and stigma were located, but they usually wither and fall off of bell peppers.

            In green bell peppers, chloroplasts continue to be chloroplasts even when the fruit is mature, but in orange, yellow or red ones, chloroplasts change their development and convert themselves into chromoplasts.

            Longitudinal sections: These are hollow fruits -- as the carpel wall matures into the fruit wall, it grows much more than is needed to hold the few, small seeds. Because this is a food plant, it might be that this is not a natural feature but rather one that has been bred artificially. However, this type of hollow fruit does occur in many natural plants. The ridges that project into the fruit and which hold the seeds are technically known as placentas.

Tomatoes.

            Tomatoes on the vine. The long green structures where the stalks attach to the fruits are sepals. Sepals, petals, stigmas and style typically wither and die soon after the flower is pollinated, but for some reason, sepals persist in many fruits. The dimple on the opposite end of each tomato is the scar where the style abscised. Because the fruit is located between the sepals and the style scar, it must have developed from a superior ovary.

            Transverse section. Tomatoes are true fruits -- they do not consist of anything other than developed carpels. The side walls of tomatoes are carpel walls -- if carpels are really leaf-like structures as we believe, then the wall of the tomato is also a thick, developed leaf. The "skin" (the epicarp or exocarp) is the abaxial (lower) epidermis, the flesh is the leaf mesophyll, and the adaxial (upper) epidermis becomes indistinct during fruit ripening. For those of you who cook, many recipes call for removing the seeds and pulp from the center and using just the wall -- it is drier and less watery than the center. But on large tomatoes, the center tissue where the seeds are attached is the bulk of the fruit. When we eat slices of tomato on a sandwich or hamburger, most of what we eat is this central part. The central part is the tissue that hold the seeds -- it is the placenta.

Apples and pears.

            These have inferior ovaries. Sepals, petals, stamens and carpels are formed in order, but the bases of all the organs become fused together into one thick structure that develops into the fruit. Look at the apple stalk -- it was the flower stalk. At the opposite end of the apple is the set of sepals (there should be five) and you may even see some dead, withered stamens. If the apple had developed from a superior ovary, then the sepals and stamens would be at the other end of the fruit, next to the stalk, as in tomatoes. (There are never any petals left on the fruit -- they always fall off a day or two after the flower opens.)

            Transverse section: Notice that there are five locules (hollow spaces) where seeds occur; this indicates that the fruit has developed from five carpels that have fused together. Each locule is the center of a carpel: the tissue immediately around the locule is the developed carpel -- the true fruit, the "core" that we throw away. All the outer tissue -- the part that we eat -- is false fruit that developed from the bases of the sepals, petals and stamens.

Bell peppers also develop from fused carpels, but they fuse together in such a way as to have a common locule, not separate ones.

Tunas (fruits of Opuntia).

            These are mixtures of false fruits and true fruits, somewhat like apples. In the center, there are many seeds in a fleshy matrix -- the flesh mostly consists of the funiculi, the stalks that connect the ovules to the placentae. In most plants, funiculi are tiny and short, but in cacti funiculi can be several millimeters long, and they can branch such that each funiculus supports four or five ovules. The layer of the fruit wall closest to the seeds is the true fruit, developed from the carpel of the inferior ovary. In apple, the ovary is inferior because the sepals and petals are fused to it, but in cacti it is inferior because a big section of shoot has grown up and around the flower (this occurs in no plants other than cacti). Because this false fruit develops from a stem, it has nodes, internodes and axillary buds. In some cacti, the axillary buds on false fruits develop into flowers -- the plant has flowers on fruits, and when those flowers mature, there are fruits on fruits.

Peanuts.

            Peanuts are legumes like beans and peas. Have you noticed that peanuts break longitudinally into two "shells" just as pea pods do? Pea pods are fruits just like peanuts are, and are made up of two fused carpels (apples have five fused carpels). String beans also consist of two carpels.

            After you open the peanut shell, you find two or three peanuts. What are they? What is the "paper" around each peanut. What are the two big parts that each peanut can be broken into.

Pomegranate.

            This is a true fruit, and the carpel wall develops into the thin outer skin of the pomegranate. There is not enough flesh there to bother eating, and of course we just peel this true fruit off and throw it away. The fleshy coating around each seed is the part we eat -- what is it? It is the outermost part of each seed -- the seed coat. If these were beans, this would be like eating only the red or black or pinto part of the seed.

Strawberry.

            Each strawberry that you eat is a combination of true fruit (from a carpel) and false fruit (from something other than a carpel). Can you guess which is which? The tiny little tan colored "seeds" are the true fruits: each one is a dry, papery fruit with one seed, and each developed from a carpel. The big red sweet part is the false fruit, it develops from the stem: it is the whole set of nodes, internodes, cortex, vascular bundles and pith. Almost all the fruits you will see in the lab today develop from carpels that have fused together into a single structure -- that is extremely common in flowering plants. Strawberries belong to a very small minority where all the many carpels (each "seed") retains its own separate identity.

Corn.

            Corn on the cob is very similar to strawberry in that it consists of many fruits (the kernels) that each have just one seed. If this were completely like strawberry, the cob would be the soft, sweet part that we would eat. The "silks" on the corn, which we clean off and throw away, are the persistent styles (almost the entire length) and stigma (just the tip of each silk). The husks, the parts that look like big green leaves, really are big green leaves. They are similar to the bracts on artichoke, except that none of them ever has a fleshy base and no matter how much butter we might dip them in, they will not taste good.

Corn is monoecious -- it has separate staminate and carpellate flowers. The inflorescence of carpellate flowers develops into the corn on the cob. The staminate flowers are located at the top of the plant in the "tassels." Because staminate flowers do not have carpels, they cannot develop into fruits. Many parts of a plant can develop into false fruits, but such things must be close to real carpels with real seeds so that when animals eat the false fruits, they will help disperse the seeds. Staminate flowers never have seeds, so it would be of no selective advantage for the staminate flowers to develop into false fruits.

Broccoli.

            What kind of a fruit is this? None. These are the flowers of the plant, and they are basically only edible before they mature into fruits.

Artichoke.

            The parts of an artichoke that we eat are not fruits. This is a whole inflorescence, composed of many tiny flowers surrounded by large bracts. After cooking, we throw away the outermost coarse bracts (their vascular bundles have so many fibers that they are too tough to eat) until we get to the inner, softer bracts with fleshy, thick bases -- those are the parts that are eaten. The actual flowers and any potential fruits are thrown away.

Pineapple.

            Like artichokes, pineapples develop from inflorescences (very strange ones, because after the plant makes many flowers, it converts back to a regular shoot and starts making more leaves -- the green leaves at the top of the pineapple). When the pineapple is making its flowers, each flower has a scale and many other parts. Usually these would either be very tiny, or they would fall off before the fruit became very big. But in pineapple, all the parts persist and act as if they are part of the fruit -- they become fleshy, juicy and sweet. The mature pineapple that you eat is a mass of many true fruits separated by many false fruits. The core that you throw away is the stem axis -- the set of nodes and internodes, the cortex, vascular bundles and -- because this is a monocot -- conjunctive tissue.

Ginger.

            You might think of this as a spice. That is a term that only means the plant has a strong flavor. Although you may find this with the fruits in a grocery store, this is not a fruit, it is a rhizome. All the rings on it are leaf scars, the places where ensheathing, protective scale-like leaves were attached.

Garlic.

            This is often found in the fruit section of a grocery store, but these are bulbs, not fruits. Garlics are very closely related onions, but you may have noticed that garlic bulbs always occur in clusters (each bulb is often called a "clove" -- which is not the same as the spice correctly called "cloves") whereas onions occur as single bulbs (rarely with a small, enclosed bulb). Garlic is clustered because it branches -- each of the cloves is a lateral branch that has grown from an axillary bud (each bud is in the axil of one of the papery or the fleshy leaves). Onions branch only occasionally -- the small enclosed bulb you sometimes find.