Placenta and placentation(types)- digieduco

Placenta and Placentation:

A. PLACENTA is the term applied to a zone of marked protuberance of the carpellary  leaf in the ovary to which ovule or ovules are attached.


Placentas may develop either at the base or apex of the carpel, or along the fused margins of carpel or carpels known as ventral sutures.

the following types of placentation are noted-

1. Marginal-When placenta develops along the fused margins of the carpel in the ovary i.e. along the ventral suture and ovules are borne in a double row along this suture, it is termed marginal. Here ovary is one-chambered. This type is found in the members of the family Leguminosae.

2. Parietal-In this type the ovary is one-chambered though two or more carpels unite along the ventral suture-placentas develop on the ventral sutures of their carpellary leaf and placenta bearing ovules remain attached at the confluent margins of the respective carpels in the ovary-this is seen in families Violaceae, Moringaeeae, Caricaceae (Carica papaya), Cruciferae etc.

3. Axile-When the pistil is formed of several united carpels and the ovary is divided into as many chambers as the number of carpels, the ventral sutures of respective carpels meet at the centre. The placenta bearing ovules ultimately arise from the central axis or from the inner angle where the respective ventral sutures of carpels meet, this placentation is known as axile. Axile placentation is common in Malvaceae, Solanaceae, Rutaceae,
Liliaceae, Amaryllidaceae etc. e.g. species of Asparagus, Allium, Polyanthes, etc.

4. Free central or Central-This type arises from axile placenta in the initial stage of development. At maturity in the ovary the partition walls are broken and disappear, the placenta bearing ovules appear to be situated at the central axis arising from the remnants of ventral sutures of carpels free from the ovary wall, the ovary apparently becomes one chambered. This type is found in the members of families Caryophyllaceae, Portulacca sp. of Portulaccaceae etc.

Free central placentation may also occur by peg-like protrusion of the thalamus inside the unilocular ovary. Placenta bearing ovules develop on it, e.g., Primula sp. (Primulaceae). According to some botanists organographically this type of placentation is the true central placentation.

5. Basal- In Helianthus annus , Tagetes patula(compositae) the placentation is basal. In this type placenta bearing single ovule sometimes a few arises on the thalamus at the base of the one chambered ovary.

6. Superficial or Laminal-In Nymphaea sp. (Nymphaeaceae) ovary is manychambered and carpels are many, placenta bearing ovules are situated throughout the whole inner surface of the ovary ; the placentas are not restricted to localised spot as in axile or parietal.