Placentation – Definition, Types & Examples

The placement of the placenta or placentae in a flower’s ovary is referred to as placentation. Have you ever seen the pea pod? The pea seeds are arranged so nicely on one side inside the peapod. Are the seeds of the tomato arranged the same as that of the pea? No! Do you know this arrangement of seeds (which are fertilised ovules)? This is the attachment of ovules inside the ovary.

In this article, let’s learn more about types of placentation and their formation with some examples.

What is Placentation?

The ovules remain attached to the inner wall of the ovary to a parenchymatous outgrowth or cushion called the placenta. The mode of arrangement of ovules within the ovary is called placentation.
Funiculi connect the ovules present in the flower’s ovary. The Placenta is the part of the ovary where the funiculus is attached.

Types of Placentation in Flower

The arrangement of seed in a fruit depends on the type of placentation. There are different types of placentation:

Fig: Types of Placentation

1. Marginal
a) In this type of placentation, the gynoecium is monocarpellary (consisting of one carpel) and unilocular (one-chambered).
b) This type of placentation is mostly found in the family Leguminosae or Fabaceae.
c) Examples of this placentation are: Pea, Gram, Mung, etc.

2. Parietal
a) In this type of placentation, the gynoecium is polycarpellary (consisting of two or more carpels) formed by fusing two or more carpels with their margins.
b) Carpels are united to form only one chamber.
c) Placentae are seen on the inner surface of the ovary, at the junctions of the carpels.
d) Sometimes, the one-chambered becomes two-chambered by the developing of a false septum or replum.
e) This type of placentation is found in mustard, Argemone and other members of the Brassicaceae family.

3. Axile
a) In this type of placentation, the gynoecium is polycarpellary, syncarpous (united).
b) The ovary is many chambered, and the number of chambers corresponds to the number of carpels.
c) The placentae arise from the central axis. So, it is called axile placentation.
d) This type of placentation is found in Petunia (bilocular), Asphodelus (trilocular), China-rose (pentalocular), Althaea (multilocular), tomato, lemon, etc. This is one of the most common types of placentations in plants.

4. Central
a) In this type of placentation, the gynoecium is polycarpellary.
b) The partition walls are found in the early stage of the ovary forming chambers which are later being dissolved, the ovary becomes unilocular.
c) Septa are absent.
d) Only one swollen placenta bearing a few ovules is borne at the centre of the ovary, which is quite separate from the ovary wall.
e) This type of placentation is found in Dianthus, Primula, etc.

5. Basal
a) In this type of placentation, the gynoecium is polycarpellary.
b) The ovary is unilocular.
c) A single ovule develops at the base of the ovary.
d) The placenta is placed on the tip of the thalamus at the floor of the ovary.
e) This type of placentation is found in the members of the family Asteraceae (sunflower family) and Poaceae (grass family).
f) Examples of this placentation: sunflower, rice, wheat, maize, etc.

6. Superficial
a) In this type of placentation, the gynoecium is polycarpellary, syncarpous and multilocular.
b) The placentae develop all around the inner surface of the partition walls and many ovules are borne on the placentae without any specific order.
c) This type of placentation is found in Nymphaea (Waterlily) and mainly in the Nymphaeaceae family.

Significance

The significance of placentation is:
a) It helps in the transfer of nutrients, respiratory gases, and water from maternal tissue to the developing embryo.
b) It protects the developing ovules.
c) It also helps in the elimination of waste from the embryo.

Summary

Placentation in flowering plants is the arrangement of the ovules in the ovary and considers the number and position of placentae, septa, and locules. The arrangement of seeds in fruits depends on placentation. The different types of Placentation found in angiosperms are marginal, axile, parietal, central, basal, and superficial; for example, pea shows marginal placentation, whereas tomato shows axile placentation. The primary significance is protecting the developing ovules. In addition, the placenta, which is arranged in the Placentation, provides nutrients to the developing embryo.

FAQs

Q.1. What is placentation?
Ans: The mode of arrangement of ovules within the ovary is called placentation. The placenta is a tissue that develops along the inner wall of the ovary. The ovule remains attached to the placenta.

Q.2. What is the significance of placentation?
Ans: Placentation provides nourishment to the developing ovule and protects them too.

Q.3. What is central placentation?
Ans: central placentation is a type in which the gynoecium is polycarpellary, and ovules borne at the center of the ovary, which is quite separate from the ovary wall by any septum. It is found in Primrose, Dianthus, etc.

Q.4. Which family has axile placentation?
Ans: The axile placentation is found in the families of Solanaceae and Liliaceae.

Q.5. How many types of Placentation are there?
Ans: The main types of placentation are marginal, parietal, axile,-central, basal, and superficial.

Q.6. Is Lemon axile placentation?
Ans: Yes, lemon is axile placentation.

Q.7. What is monocarpellary?
Ans: When only one carpel is present in the gynoecium of a flower, it is known as monocarpellary.

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