Fruits (page 1)

   

Fruit formation is stimulated by the fertilization process described in a prior lesson. In the two photographs above you can see the beginning stage and ending stage of the angiosperm reproductive cycle. The Bunchberry flower appears early in the spring and by mid-summer parts of the flower have been transformed into a bright red fruit cluster. An individual fruit, of a flowering plant, always contains at least one mature ovary. Maturation includes enlargement and modification of the ovary wall and ovary contents. Completion of this developmental process forms a simple fruit derived from a single ovary. Other plants produce fruits formed by the fusion of several ovaries found on the same flower. This second process, of ovary fusion and maturation, forms an aggregate fruit. A third kind of fruit results from the maturation and fusion of ovaries of separate flowers of the same plant. This process forms a multiple fruit. It is also common to find simple, aggregate and multiple fruits to contain not only modified ovaries but also modified floral (perianth) structures including parts of the calyx, corolla and receptacle. These are called accessory fruits. Fruits without accessory floral parts are known as true fruits. As this lesson continues we will learn the names of the simple, aggregate and multiple fruits and how their individual structure is related to the flower from which they originated.

I. The Simple Fruits (page 2)

A simple fruit always develops from a single ovary containing one or more carpels and may or may not include additional modified accessory floral (perianth) structures. In addition, a simple fruit is either fleshy or dry. Fleshy fruits are often edible and are seen in the fresh fruit and vegetable section of your local super market. Fleshy fruits include the berry, drupe, pome, pepo, and hesperidium.

A. Simple Fleshy Fruits 1. The Berry

 Grapes and tomatoes are classified as berries because the ovary wall of the carpel becomes almost completely fleshy at maturity. The number of carpels in each species varies from one to several and their skins can be thin and tender or thin and tough. The number of seeds also varies from one per carpel to many per carpel. The seeds of all berries, are embedded in the fleshy tissue of the carpel.

 2. The Hesperidium

 

  Hesperidium type fruits are always covered with a leathery rind and the partitions separating their carpels are tough and fibrous. The orange, lemon and grapefruit, all members of the citrus family, are good examples of the hespiridium type of fruit.

    3. The Pepo

    The pepo is covered by a rind that is hard and thick. The cucumber, pumpkin and watermelon are good examples of the pepo type of fruit. Below the rind, the the rest of the ovary wall is soft and fleshy. In the photographs above seeds fill the locule of each carpel.

 4. The Drupe

 

   If a fruit is fleshy and it has a hard stony pit containing a seed it is classified as a drupe. Drupes are covered by a thin skin derived from the outer tissue layer of the ovary. The soft fleshy tissue below the skin is derived from the middle layer of the ovary and the hard stony pit is derived from the inner tissue layer of the ovary. Cracking the pit open reveals a single seed formed from an ovule contained within the ovary of the flower. Coconuts have fibrous walls instead of the fleshy walls found in most drupes.

5. The Pome

The pome consists of tissue derived from the ovary and from the

perianth. For this reason it is often called an accessory fruit. In the example (apple) the core is composed of modified tissue from the ovary wall while the pulp is composed of tissue from the base of the floral tube and receptacle. The seeds were once, early in development, ovules containing fertilized eggs and endosperm. The pear is another pome type of fruit.

I. The Simple Fruits (page 3)

A fruit, classified as simple, always develops from a single ovary containing one or more carpels and may or may not include additional modified accessory floral structures. Simple fruits are either fleshy or dry. Dry fruits are classified as either dehiscent or indehiscent. Dry dehiscent fruits crack open along two seams and shed their seeds into the environment when the fruit is ripe. We will examine three common kinds of dry fruit.

  B. Simple Dry Dehiscent Fruits1. The Legume

The legume splits along two lines of dehiscence following maturation and drying. The legume type fruit is derived from a simple ovary (one carpel) with two rows of ovules. This type of fruit structure is characteristic of peas, beans and peanuts. The peanut is one of the few legumes that does not split open when ripe. This is probably because the fruit of the peanut develops in the soil rather than in the air.

2. The Capsule

    The capsule is another type of dry dehiscent fruit. Unlike the legume, the capsule is composed of more than one carpel. Fruits like the lily split length-wise into sections corresponding to the number of carpels. The Sweet Gum fruit, being a cluster of capsules, releases winged seeds as each ovary cracks open at maturity.

3. The Follicle

    Columbine and milkweed plants produce fruit that is known as a follicle. Fruit of the Follicle type develop from a single ripened ovary and split once to release their seeds. The split is always lengthwise, along one edge of the carpel. Legumes you remember split along two edges of the carpel. Follicles may occur singly (milkweed) or in clusters (columbine). When the fruit opens it resembles a dry leaf and reminds us that carpels are modified leaves that first produce spores, then gametes and finally seeds.

The Simple Fruits (page 4)Dry indehiscent fruits retain their seeds and do not crack open after ripening. We will

examine four different types.

C. Simple Dry Indehiscent Fruit1. The Achene

The achene consists of a single seed that is attached to the wall of the ovary at only one point. The wall of the mature ovary is also thin and undeveloped so when it dries out the fruit formed has a seed-like appearance. Examples of achenes include sunflowers, dandelions and buckwheat. Don't be confused to learn that the sunflower "seed" is actually a fruit. Remove the dried up wall of the fruit and the sunflower seed is found underneath.

  2. The Grain

 If the wall of the dry indehiscent fruit is thin, transparent and firmly attached to all points of the seed coat the fruit is a grain. Grain type fruits are produced by members of the grass family which include important food crops such as rice, corn and wheat.

  3. The Samara

   

 The Samara is a wind borne fruit containing a single seed. It is much like an achene except for the paper-like wing which develops from the ovary wall of the flower. Winged Samara fruits are characteristic of elms, maples and ashes.

4. The Nut

   

 Except for its larger size, the nut is very similar in structure to the achene. In nuts such as the acorn and chestnut the shell is the coat of the fruit. It, the coat, develops from the ovary wall after fertilization. Some nuts have a husk covering the hard shell. In this case the husk is formed from the outer layer of the ovary wall and the hard coat of the nut is formed from the inner layer of the ovary wall. Walnuts and pecans are examples of the later type of maturation.

II. The Aggregate Fruits (page 5) Aggregate Fruit Examples

 An aggregate fruit results from the joining together of several ovaries of the same flower. This process starts with a flower that contains several carpels with an equal number of ovaries. Each ovary contains a single ovule that develops into a seed following fertilization. As the ovaries develop and increase in size they become attached together. Aggregate fruits are also either true, without accessory parts, or accessory, with additional floral parts. The raspberry is an aggregate of true fruits. Each fruit in the aggregate is a drupe containing a stony pit. The strawberry is an accessory aggregate fruit. The many ovaries of the strawberry flower develop into achenes which become embedded on the surface of the enlarged flower receptacle.

III. The Multiple Fruits (page 6)  Multiple Fruit Examples

 All of the fruit types we have studied so far arise from a single flower. Multiple fruits, unlike the others, develop from a cluster of flowers. Individual fruits of the multiple cluster develop from one ovary and some accessory parts of the flower. The pineapple is a good example of a multiple fruit. It is cluster of mature ovaries each with accessory parts including the receptacle and portions of the sepals and bracts. The fruit's surface tissue you see in the photograph covers the ovary. This tissue arises from three overlapping sepals and the tip of a bract. Green bracts stick out of the top of the whole pineapple. Flowers were not borne in the axils of these bracts. Seeds are usually absent from commercially grown plants.