In angiosperms, after fertilization, at the same time that the ovule develops into the seed, the walls of the ovary are transformed into the fruit. The fruit may be regarded as the organ that contains the seed. It consists of a mature ovary, with some parts added from the receptacle, calyx and bracts. The fruit protects the seed and may also help in seed dispersal, either by active or passive mechanisms. For example, some seeds cannot germinate unless they have passed through the digestive tract of an animal.
In previous pages, it was mentioned that the histological organization of the ovary was similar to that of a leaf with an inner and an outer epidermal layer, and a parenchyma with vascular bundles in between. The development of these layers becomes the pericarp, which is actually the fruit without the seed. The pericarp consists of the exocarp, mesocarp and endocarp. The exocarp is the outer layer of the fruit, and the endocarp covers the seed. The tissue between these two layers is the mesocarp, made up of storage parenchyma or sclerenchyma. A fruit formed from a carpel or several fusioned carpels of the same flower is called simple, such as the peach and tomato. These fruits are developed from superior ovaries (see the flower page ). An aggregated fruit is formed from several separated carpels of the same flower, such as strawberries and blueberries. When a fruit involves several flowers, it is called a multiple or compound fruit, such as pineapples. There are plant species where other parts of the flower, besides the ovary, contribute to the formation of the fruit. They are known as complex fruits, like apples and cucumbers, where the pericarp does not make the whole fruit.
Dehiscent fruits are those that need to be opened to release the seed. In indehiscent fruits, the pericarp is strongly attached to the seed, and it degenerates during or after seed germination. Dehiscent fruit types are legumes, follicles and capsules. Legumes and follicles develop from only one carpel, while capsules form from 3 to 5 carpels. The pericarp of dehiscent fruits consists of one or two outer layers of parenchymatic cells with thick cell walls, sometimes lignified, and an inner layer of sclerenchyma may be present. Examples of dehiscent fruit types are achenes and caryopsis. The pericarp of these fruits may be fusioned with the seed, which is single. The seed coat is thin and parenchymatous, and it usually desintegrates during fruit maturation.
Depending on the plant species, the three layers of the pericarp may show different structures and dimensions, but fruits are generally divided into two groups regarding pericarp consistence: dry fruits and fleshy fruits.
In nature, dry fruits are more abundant than fleshy fruits. The pericarp of dry fruits is membranous or hard, non-well developed and has little hydration. There are three types of dry fruits (Figure 1). 1. Dehiscent fruits that develop from only one carpel, like leguminous fruits. 2. Dehiscent fruits formed from several carpels, for example, capsule fruits such as those in some species of the genus Hypericum. 3. Indehiscent fruits form compact fruit, for example, caryopsis fruits, such as most grasses.
The pericarp of fleshy fruits shows succulent (fleshy) tissues (Figure 2). Commonly, the exocarp and endocarp are monostratified cell layers and mesocarp is highly hydrated and succulent parenchyma. There are four main types of fleshy fruits. 1. Berries, such as grape and tomato, shows a cunitinized exocarp. 2. Drupes, such as peach, has a very hard, bone-like endocarp that becomes the hard coat surrounding the seed. 3. Pomes, such as apples, have and endocarp that shows a gelatinous consistence. In apples and peachs, most of the fruit is developed from the receptacle of the flower (accessory fruits) 4. Hesperidium, like citrus fruits, has a collenchymatic exocarp with glands, and a very thin mesocarp, while the endocarp is made up of juice sacs and forms most of the fruit. The exocarp and part of the mesocarp form a kind of coat, where the endocarp is the fleshy component of the fruit. Sometimes, it is difficult to discern the limits of the three layers.
The vascular bundles that irrigate the fleshy fruits reflect the disposition of these vessels in the ovary carpels of the flower, although new branches are frequent. The cell walls of the xylem vessels are softened, and they do not apparently conduct sap when the fruit is ripening.
Fruits have to be attractive for the animals that eat them, but not before the seeds are well-developed. Maturation is the process by which fruits are transformed into edible food by animals. During this process, the mechanical properties of the parenchymal cells of the fleshy fruits are modified, and they reduce cell adhesion through the cell wall (the middle lamella is dissolved). Degradation enzymes expressed during maturation are involved in these changes of the cell wall, which lead to a glycan depolymerization, and pectin solubilization and degradation. In addition, chlorophyll is degraded and substituted by other pigments that provide the fruit colors (red, brownish, orange, and many others). Starch and organic acids are transformed into other carbohydrates. In some species, the maturation is very fast and enormously increases the respiration rate, which can be measured by the consumption of oxygen. These fruits are called climacteric, and it is said that they show a climacteric maturation process. Tomato, pear, apple and advocate are climacteric fruits. Non-climacteric fruits undergo a progressive and longer maturation process, such as oranges, grapes, and strawberries. After maturation, fruits become mature, and then a senescence process begins. The maturation process is economically very significant because it determinse how fruits are processed and sold in markets.
