Montag, 13. Februar 2012

Fruits of the angiosperms

In this Article, I want to write something about the fruits of the Angiosperms. My goal is to give you some information about the different types of fruits, their anatomy and their creation. This Topic will be divided into two sub-articles. The first one should clarify the basic definition of the term “fruit”, their anatomy and the different fruit-types.

1 Definition of a fruit & Basic anatomy

Fruits exists in many forms and variations. Some are big and beautiful, other ones are small and inconspicuous. They are smooth or hard. However, there is a clear definition of the term “Fruit” in botany; regardless of their shape and seize.

A fruit is a flower at the time of the maturation of its seed”

That means, that a fruit is the metamorphosis o the floral organs of a flower after the pollination. In the most cases, these organs are the female ones: the Carpels. After successful pollination, the carpels will become a new structure, which is called Pericarp. The Pericarp is subdivided in three layer: the Endocarp (central part around the seed), the Mesocarp (the largest part) and last but not least the Exocarp (the outer layer). In some cases, especially the citrus, the Mesokarp is called Albedo, because it has a white color 

 
A good example is the peach: Take one and cut in right through the middle. You will see the hard core (endocarp), the yellow fleshy layer (mesocarp) and a hairy skin (Exokarp). Unfortunately, I've no picture, because you cannot buy peaches in Germany during winter.

The Morphology of these three layers is essential for the type of fruit. There are the following basic types, based on the composition of their layers.

2. Types of fruit

Now, it's time to take a closer view on some types of fruits. First, we start with the simple ones. In this case, the flower had only one carpel.

Table 1: Basic Types of Simple fruits
Type
Endocarp
Mesocarp
Exocarp
Opening mechanism
Example
Follicle
Leathery, dry
Leathery, dry
Leathery dry
Along the ventral side
Genus: Consolida
(larkspur)
Legume
Leathery
Leathery
Leathery
Along the ventral and dorsal side
All Fabaceae (e. g. peas, beans)
Berry
fleshy
fleshy
fleshy
none
Genus: Actaea (baneberry)
Drupe
hard
fleshy
leathery
none
Genus: Prunus (peaches, cherries)

However, many plants have more than one carpel. In this case, the ripe fruit is a compound fruit and we need to divide the fruits into two subtypes: apocarp and coenocarp. Apocarp fruits are fruits with at least two, free carpels, while the carpels of coenocarp fruits are grown together.

Please note, that only the number of carpels is relevant, if a fruit is a compound fruit or not. A flower can have only on seed but two or more carpels.


Table 2: Coenocarp compound fruits
Type
Endocarp
Mesocarp
Exocarp
Opening mechanism
Example
Berry
Fleshy
Fleshy
Fleshy
none
Solanaceae (tomatoes, potatoes)
Nut
Hard (woody)
Hart (Woody)
Hard (Woody)
none
Hazel. walnut
Caryopsis 1)
Hard (woody)
Hart (Woody)
Hard (Woody)
none
All Poaceae (grasses)
Achene 2)
Hard (woody)
Hart (Woody)
Hard (Woody)
none
Asteraceae
Drupe
hard
fleshy
leathery
none
Sambucus
Capsule
dry
dry
dry
Variable
Many different families

1) The Caryopsis is the nut-fruit of the Poaceae (grasses). It's based on three carpels, but two of them are so heavy reduced, that the Caryposis seems to have only one carpel

2) The Achene is the nut-fruit of the Asteraceae (Dandelion, Daisy, Sunflowers). The Pericarp is connect with the embryo (but not fused), what makes it to a special form of the nut.

Table 3: Apocarp compound fruits
Type
Endocarp
Mesocarp
Exocarp
Opening mechanism
Example
Compound Nut
Hard (woody)
Hart (Woody)
Hard (Woody)
none
Genus Geum
(Aven)
Compound Follicle
Leathery, dry
Leathery, dry
Leathery dry
Along the ventral sides
Genus Paeonia
(Peony)
Compound Drupe
hard
fleshy
leathery
none
Genus: Rubus (Blackberry; Strawberry) 1)

1) It's a common mistake to call these fruits “Berries” but they are compound fruits, which consists of many small Drupes

3. Opening Mechanism

After ripening, the main function of a fruit is to spread the seed. There are many strategies of spreading, but this should not be a topic of this posts. For now, yo need to know, that a fruit can be opened or not and that this opening can happen in many different ways.

  • Indehiscent fruits: This type does not open. The ripe fruits remain as whole on the tree or fall down. Nuts, berries, Drupes and so on are indehiscent fruits

  • Dehiscent fruits: The ripe fruit opens along a seam. Examples are Follicle, Legume or pods. Another, special variant are capsules, which opens in many different ways like through a cap, pores or by a burst (e. g. Impatiens)
Another opening mechanism is the disintegration of the fruit (s. 4)

4. Shizocarp fruits

There are also other types of fruits, which don't fit into this scheme. These fruits form fake wall beside the carpels and disintegrate after ripening along these fake walls. This fruits are called Shizocarp. It's a fusion between the morphology of the fruit and it's opening mechanism.

The most important types are:

  • shizocarp fruits: these are compound fruits with two carpels, which disintegrate along their septa into to sub-fruits (Mericarps). Good Examples are the fruits of the Acer or the Apiaceae family. In the later, the fruits remain linked by a stalk, which is called Carpophor
  • Klausenfrucht: this is the special fruit of the Lamiaceae and Boraginaceae families. Unfortunately, I haven't found a English name. Both families have two fused Carpels and four seeds. They form fake septa within the carpels and at ripening, the fruit disintegrates into four mericarps.
  • Articulated legume: This is a special form of the legume. Fake septa separated the legume into smaller mericarps.
That should it be for today. In the next Article, we will take a closer view on the anatomy of a fruit and the structures within it.

5. Pictures and Examples

 Malus domestica as example for
follicle (please rember, that the pulp doesn't belong
to the fruit)

 Rubus laciniatus as example for compund drupe

 The fruits of Sorbus aucuparia are berries

The legumes of Robinia pseudoacacia

The nut of Quercus robur

The fruits of Aesculus hippocastanum are also nuts
(the spiky hull is an extra structure and not a part of the fruit)

 Acer pseudoplatanus with its winged, shizocarp fruits

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