Placentation in Plants
What is Placentation?
The pattern of ovule arrangement in the ovary is called placentation. The structure on which ovules are borne is known as place placentation. Generally the placenta is a cushion shaped structure situated along the line of fusion of carpellary margins. Whereas the placenta is a distinctive structure in families like Solanaceae. Cucurbitaceae, etc., in primitive families such as Winteraceae, Degeneriaceae, etc. there is little or no modification of the region where ovules are borne. Each placenta is generally regarded as composed of two half placentae borne on contiguous margins of the same or different carpels. Two halves of placenta are generally easily distinguishable by the presence of a narrow sterile area separating the two halves and orientation of the ovules in opposite directions on the two halves.
The various types of placentation are generally defined and distinguished primarily on the basis of number of loculi in the ovary, and the position of the ovules. These are not always reliable criteria. These may vary considerably in many cases and may not provide correct assessment.
Placentation is a useful character in taxonomy and thus the classification of the placentation should be based on more reliable criteria.
Besides the two topographical criteria mentioned above (number of loculi in the ovary and the position of the ovules), Professor V. Puri suggested the following three anatomical criteria for a scientific classification of placentation:-
- Composition of the vascular supply of a placenta-whether from one or two carpels.
- Orientation of the vascular supply of a placenta- normal of inversely oriented with reference to the floral axis.
- Location of the placenta-on the fused margins of the same or of different carpels.
Types of Placentation
Let us assess the different types of placentation in the light of the above mentioned anatomical criteria besides the traditional topographical criteria.
On the basis of both topographical and anatomical criteria the marginal placentation is defined as follows:
- The gynoecium is monocarpellary, apocarpous and unilocular.
- The placentae are borne on fused margins of the same carpel.
- The placentae derive their vascular supply from the ventral of the same carpel.
- The ventral bundles are inversely oriented with reference to the floral axis and lie almost on the same radii as the dorsals.
This type of placentation can be clearly distinguished from the one met in Pseudomonomerous gynoecia (in Poaceae and other) where placentae are essentially parietal as they occur at the junction of two margins belonging to different carpels and receive their vascular supply from either one margin or two margins belonging to different carpels.
The following criteria apply to axile placentation :
- The gynoecium is multicarpellary, syncarpous and multilocular.
- The placentae are borne on fused margins of the same carpel.
- The placentae derive their vascular supply from ventrals of the same carpel.
- The ventral bundles are inversely oriented almost on the same radii as the dorsals.
The term ‘axile’ should be clearly distinguished from ‘axial, as the latter means that the ovules are borne on the floral axis, which is not implied.
For parietal placentation the main criteria are as follows:
- The gynoecium is multicarpellary, syncarpous and unilocular.
- The placentae are borne on fused margins of different carpels.
- The placentae derive their vascular supply from the ventrals of different carpels.
- The ventral bundles are normally oriented with reference to the floral axis and lie on different radii from those of the dorsal.
Puri (1952) emphasizes on a slightly different conception of parietal placentation from the one commonly held. He does not give much emphasis to the number of locules. If the placenta are borne on fused margins of different carpels and derive the vascular supply from the ventral’s of different carpels, he consider the placentation as parietal though by cohering of placentae the ovary becomes multilocular. On such considerations in families like Cucurbitaceae, Passiflo-raceae, etc. the placentation is parietal.
The vascular supply of placental region in families like Brassicaceae, Capparaceae and families like Brassicaceae, Capparaceae and Papaveraceae the ventral (placental) bundles are inversely oriented as in axile placentation. Such type of parental placentation is considered to have been derived from axile placentation, which can be distinguished from simple parietal placentation.
Free Central Placentation
In free central placentation the gynoecium is multicarpellary, syncarpous and unilocular. The placentae derive their vascular supply from the ventrals of the sme carried and the ventrial bundles are inversely oriented and lie almost on the same radii as the dorsals. The the Caryophyllaceae or they are separate from the very begining as in the fused margins may break away during development of the gynoecium as in Primulaceae.
The term is used for the unilocular gynoecium with a single ovule which arises or appears to arise from the base of the ovary. A really basal ovule arises from axile placentation. The condition in the Poaceae and certain Ranunculaceae where single ovule has descended from latera position may better be called as sub-basal.
Superficial or Lamellar Placentation
When entire or most of the internal surface of the ovary wall is covered with ovules, the condition is described as superficial of lamellar. Such a placentation occurs in families like Butomaceae and Nymphaeaceae.
Evolutionary Trends in Placentation
In the evolution of placentation it appears that no uniform pattern is followed. Marginal has been generally accepted as a simple and primitive placentation. Free central placentation in all cases seems to have been derived from axile placentation.
The basal placentation as it exists in the families like Cyperaceae and Polygonaceae has also been derived from axile placentation. The condition in the Poaceae, Asteraceae and some Ranunculaceae though usually described as basal, is derived from parietal placentation.
Thus free central and basal placentations are considered as derived types.
It is difficult to decide whether axile or parietal placentation is more primitive. As mentioned earlier, in a number of families such as Cucurbitaceae, Passifloraceae, Brassicaceae, Capparaceae, Papaveraceae, etc. the parietal placentation is derived from axile placentation. It is also believed that evolution has progressed from ovary cells as many as carpels to less than the carpels. There are also many exponent of the view that parietal placentation is more primitive than axile placentation. However, Puri (1952) states that it is far from conclusive that parietal placentation is more primitive than axile.
The evolutionary history of laminar placentation is not quite clear. It is present in the families generally considered as primitive, both in dicotyledons and monocotyledons-Nymphaeaceae, Cabombaceae, Butomaceae, Hydrocharitaceae, etc. In this type, the ovules are distributed all along the inner walls of carpel except the dorsal suture. The families like Degeneriaceae and Winteraceae show evidence of transition from laminar to submarginal placentation.
Marginal placentation is considered as primitive. It gives rise to axile placentation. Parietal placentation is derived from axile placentation. There is also possibility of evolution from parietal to axile placentation and from axile to marginal placentation. Basal placentation may be derived from axile as well as free central placentation. Free central placentation is also derived from axile placentation. There is also possibility of derivation of free central placentation from basal placentation, and basal placentation from marginal as well as parietal placentation.
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