Atlas of plant and animal histology

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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 protects the seed and also may help in the seed dispersal either by active or passive mechanisms. For example, some seeds cannot germinate unless they have passed the digestive tract of an animal.

In previous pages, it was mentioned that the histological organization of the ovary was similar to 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 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 storing parenchyma or sclerenchyma. There are plant species where other parts of the flower, besides the ovary, contribute to form the fruit. They are known as complex fruits, like apples and cucumbers, where the pericarp does not make the whole fruit. Some fruits are actually aggregates of many small fruits, such as strawberries and blackberries.

Dehiscent fruits are those that get open to release the seed. In indehiscent fruits the pericarp is strongly attached to the seed, and it degenerates during or after seed germination. The three layers of pericarp show differences depending on the fruit type. Two types of fruits may be distinguished considering the pericarp consistence: dry and fleshy.

The pericarp of dry fruits is membranous or hard, non-well developed and with 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 coming from several carpels, for example the 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.

Dry fruits
Figure 1. Types of dry fruits. They show a strongly dry and hard pericarp. Sunflower pipe is an indehiscent fruit. Dehiscent fruits are beans, peanuts (follicle), and poppy (encapsulated).

The pericarp of fleshy fruits shows succulent (fleshy) tissues (Figure 2). Commonly, the exocarp and endocarp are mono stratified cell layers and mesocarp is highly hydrated and succulent parenchyma. There are four main types of fleshy fruits. 1. Berry, such as grape and tomato, which shows a cunitinized exocarp. 2. Drupe, such as peach, with a very hard bone-like endocarp that becomes the hard coat surronding the seed. 3. Pome, such as apple, where the endocarp shows a gelatinous consistence. 4. Hesperidium, like citrus fruits, which have a collenchymatic exocarp with glands, and a very thin mesocarp, while the endocarp is made up of juice sacs and form most of the fruit.

Fleshy fruits
Figure 2. Fleshy fruits. Different part of the pericarp are indicated. Drupes have a very hard endocarp protecting the seed. In apples, the pericarp is not all the fleshy part. The tissue surrounding the core is differentiated from the flower receptacle, an it is called hypanthium. The tomato pericarp and mesocarp show large stores of liquid. In oranges, it is difficult to discern between the exocarp and mesocarp. However, the endocarp is large, makes the majority of the fruit, and contains many cavities with liquid where seeds are found.
Hypanthium of an apple, superficial part.
Orange pericarp.

Fruits have to be attractive for the animals that eat on them, but not before the seeds are well-developed. Maturation is the process by which fruits are transformed into edible by animals. During this process, chlorophyll is degraded and substituted by other pigments that provide the fruit color (red, brownish, orange, and many others). At the same time the fleshy part of the fruit becomes softener by the action o enzymes that digest the cell wall pectin, mainly in the middle lamella. Starch is transformed into other carbohydrates. In some species, the maturation is very fast and enormously increases the respiration rate (it can be measured by the consume of oxygen). These fruits are called climacteric, an it is said that the show a climacteric maturation process. Tomato, pear, apple and advocate are climacteric fruits. Non-climacteric fruits undergo a progressive and longer maturation process, such a as orange, grape, and strawberry. After maturation, fruits become mature, and then a senescence process begins.

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