We usually call “ fruit ” any fleshy edible fruit , such as peaches or grapes. They are also fruits, in botanical terms, the tomato, the cucumber or the pepper , although we normally classify them as “vegetables”.
The structure of a fleshy fruit is made up of several layers. The outermost, which we call the shell , is the epicarp . Under it is the mesocarp , usually the fleshy part loaded with water and sugars, like the pulp of grapes or peaches. Next is an inner layer, the endocarp , which can be variable in appearance, depending on whether the fruit is a berry or a drupe.
Berries , such as grapes or tomatoes, usually have the endocarp with a fibrous appearance holding the seeds , usually very numerous. On the other hand, in a stone fruit , such as the peach or the olive, the endocarp is a kind of leathery shell that covers and protects the seed , generally there is only one.
But not everything we call fruit meets these general characteristics.
Hesperidium, the strange fruit of citrus
Citrus fruits such as lemon, orange or tangerine are one of those exceptions. Its fruit, in botany, is called a hesperidium , a highly modified kind of berry. In it is the skin, brightly colored and full of glands loaded with essential oils. However, the mesocarp , the middle layer, is not fleshy at all; it is reduced to a thin whitish layer, called the albedo .
In hesperidia, the fleshy part is made up of a series of juice-swollen hairs called trichomes , which hang from the endocarp, the inner layer of the fruit, which is the membrane of each segment. The seeds, which are naturally numerous, like in berries, are anchored to the endocarp, inside the segments.
Most of the oranges and mandarins that we find in the greengrocer, however, do not have seeds . It is wrongly assumed that this is due to their hybrid nature. It is true that all the citrus fruits that we consume have a hybrid origin, however, they maintain the ability to form seeds. The reason why they do not have them is that they are fruits that have matured without being pollinated . Fruits that develop without being fertilized by pollen grains are called parthenocarpic .
The nut, bone of a fleshy fruit
In the fruit of the walnut tree there is a confusing situation regarding botanical terminology. ” Nut ” in botany is a type of dry fruit that does not open naturally, also called nut . Among what we consider “nuts” in botanical terms, we find dried fruits such as hazelnuts or beechnuts —the fruit of the beech tree—. However, what we colloquially call a pecan, the fruit of the walnut tree, is not a botanical nut .
In fact, in botany, it is not even a dried fruit. It is the pit of a fleshy fruit of a type called trima , a variety of drupe . Although unlike normal drupes, such as peaches or olives, the fruit is dehiscent, that is, the epicarp and mesocarp —the fleshy part— dries up and opens naturally, falling off. The shell that we find is the endocarp , that internal layer of the fruit, coriaceous and hardened, that houses the seed, what we eat.
The pomegranate, a dry fruit
If the walnut is a fleshy fruit that we call “dry fruit”, the pomegranate is the opposite case. From the point of view of botany, it is a dry fruit of a particular type called balausta . In this case, the epicarp and mesocarp are leathery and form the shell. The endocarp, also fibrous, forms a branched structure inside the fruit and holds the seeds. If you look carefully all the layers of the fruit are dry , none is fleshy.
The juicy part of the pomegranate is not the fruit itself, but the seeds it contains inside, which have a layer, called sarcotesta , full of water and sugars.
The raspberry, the berry that is not a berry
When we talk about berries we think of the typical fruits of the forest , such as blueberries, blackberries or raspberries, and not of a tomato, an eggplant or a pepper —which are real berries, in botanical terms—. Of the fruits of the forest, blueberries or currants are berries; fleshy fruits with several seeds held by a barely perceptible filamentous endocarp. However, neither the raspberry nor the blackberry are true berries .
In both cases it is a fruit composed of multiple subunits, and each of them has a single seed covered by a hardened shell. If we look at it with a magnifying glass, each of the small spheres of a raspberry has the same structure as a peach . Each one is therefore a drupe. And the raspberry or the blackberry are polydrupes .
Strawberry, a stem with dried fruit
Of the strange types of fruit, one of the most amazing is the strawberry . Like the raspberry, it is a structure made up of multiple subunits with their own entity, but unlike them, in this case those subunits are not drupes; in fact, they are not even fleshy fruits. They are dried fruits , of a type that we call achene , the same type of fruit that presents anise or cannabis. Those “nuggets” that the strawberry has on its surface.
The fleshy part of the strawberry, large, red and tasty, is not, in fact, part of the flower, but rather the vegetative tissue of the plant. Specifically, it is the receptacle , the part of the stem on which the floral pieces are held, which has become fleshy when it matures, and which is called etherium . The strawberry is, therefore, a set of dried fruits arranged on a thickened stem.
REFERENCES:
Berendí González, C. 1997. XCIV. Punicaceae. In Iberian Flora: Vol. VIII . Royal Botanical Garden, CSIC.Muñoz Garmendia, F. et al. 1998. LXXXVII. Rosaceae. In Iberian Flora: Vol. VI . Royal Botanical Garden, CSIC.Muñoz Garmendia, F., et al. 2015. CXVIII. Rutaceae. In Iberian Flora: Vol. IX . Royal Botanical Garden, CSIC.Muñoz Garmendia, F., et al. 2015. Juglans. In Iberian Flora: Vol. IX . Royal Botanical Garden, CSIC.Pérez Morales, C. 1999. Morphology of spermatophytes . Ed. Celarayn.
