Carpel Definition

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In this article, we will learn the carpel meaning. The flower is the reproductive part of the plant. A typical flower is divided into four whorls. They are calyx(sepals), corolla(petals), androecium(stamen) and gynoecium(carpel). The carpel in plants or the gynoecium is the last whorl of the flower. It is known as the female reproductive part of the flower. It can be composed of one or more carpels. The carpel can be fused and free. The condition is called apocarpous when the carpel is free and it is said to be syncarpous when the carpel is fused or attached to one another. It helps in producing the ovule and then ultimately develops into fruits and seeds. The pollen-producing organs such as the stamens usually surround the carpel because it will be easy for the pollen to go to the ovule and then fuse and form an embryo. 

Since we have learnt about the carpel definition, let us look at the parts of the carpel. 

Parts of Carpel

After understanding the carpel meaning in the above paragraph, we will now learn about different parts of stamens and carpels. The carpel plant is divided into three parts. They are:

  • Ovary: It is present at the base, therefore known as the basal part of the carpel. It is also the swollen part of the carpel. This part bears one or more ovules. These ovules then form seeds when they are fertilized. The placenta is a flattened and cushion-like structure to which the ovules are attached. The ovary may have one or more than one chamber. An ovary that contains one chamber is called a unilocular ovary. Ovary containing two chambers is called bilocular and which has three chambers is called a trilocular and so on. The pericarp is formed from the wall of the ovary. The pericarp is known as the fruit wall. The ovary has an ovarian cavity. Megasporangia arises from the placenta. The ovary can have a single ovule or multiple ovules. 

  • Style: It is a tube-like structure. It helps in connecting the stigma to the ovary. It is present above the ovary in a carpel. 

  • Stigma: They are present at the tip of the style. They act as a receptive organ. This means that they help in providing the base for the anthers at the time of pollination. After the process of fertilization, the ovary develops into a fruit and the ovules develop into seeds. 

In conduplicate carpel no stigma, style and ovary are present. The above paragraph gives us a rough description of the carpel flower definition and its various parts. 

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After understanding the carpel meaning, we will study the ovule and the types of carpels in this paragraph. The ovule is attached to the placenta by the funicle. The hilum is referred to as the junction between the ovule and funicle or the point of attachment of the funiculus to the body of the volume. Around the body of ovules, we have integuments that cover the body of the ovule. At the tip of the ovule, there is a pore or passage that is present and it is called a micropyle end. At this micropyle end, no integuments are present. The Chalazal end is present opposite the micropylar end. The body of the ovule is formed by the nucellus. It is a parenchymatous mass of tissues that are enclosed in the integuments. Two types of ovules are there that are based on the development of the nucellus. They are Crassinucellate ovule and Tenuinucellate ovule. In the nucellus, we have the embryo sac and it is the female gametophyte. There are six types of ovules. They are:

  • Orthotropous: In this type, the micropylar end and the chalazal end are present in a straight line. It is considered to be the most primitive type of ovule. This is seen in the rose carpel.

  • Anatropous: A 180-degree angle is formed by the ovule. It is known as an inverted ovule. The micropylar end is present closer to the hilum. It is the most common type of ovule and is found in almost 80% of angiosperms. 

  • Hemianatropous: Here, the ovule makes a 90-degree turn. This turn is made upon the funcile. 

  • Campylotropous: It is curved at a more or less at the right angle to the funcile. The micropylar end in this ovule is slightly bent.

  • Amphitropous: The ovule and the embryo sac forms a horseshoe shape. 

  • Circinotropous: In this condition, the ovule takes more than a 360-degree turn. When this happens, the funicle is coiled around the ovule. 

Carpel and the Female Gametophyte

From carpel definition, we learned that it is a female reproductive organ of the plant. The megaspore mother cells that are known as the female gametes are formed by the process of megasporogenesis. In this process, the ovules differentiate the megaspore mother cell that is present in the micropylar region of the nucellus. The microspore mother cell is a large cell that contains dense cytoplasm and nucleus. The microspore mother cell undergoes meiosis and it forms a linear tetrad of four haploid megaspores. Almost in all of the angiosperms only one megaspore mother cell remains functional, the rest three get degenerated. The female gametophyte is developed from this one functional megaspore. The method by which this embryo sac is formed from a single megaspore is termed monosporic development. The nucleus of the chalazal end divides by mitotic division and forms two nuclei. These nuclei are then moved to opposite poles. This results in the formation of two nucleate embryo sacs. After this two more mitotic divisions occur that results in the formation of 8-nuclei. One nucleus from each pole moves to the middle and then forms the polar nuclei. Egg apparatus is formed when three of the nuclei are organised at the micropylar end. In these three nuclei, one is the egg cell and the other two are the synergids. Three nuclei form antipodal cells at the chalazal end. And in the centre, two nuclei form the polar nuclei. 

Organization of the Embryo Sac

The embryo sac is made of four cells. They are:

  • Synergids: These are also known as helper cells or co-operative cells. They have a micropylar nucleus and a chalazal vacuole. At their chalazal sides, the synergids lack cell walls. Filiform apparatus is present at the micropylar end. It is in the form of finger-like projections. These projections are microfibrils. After pollination, one synergid starts to degenerate. 

  • Egg: The egg shows cytoplasmic polarity opposite to the synergid. At the micropylar end, its walls are thicker. Between the egg and the synergids, a plasmodesmata connection is present. 

  • Antipodal Cells: These are also known as antipodal cells. They are the vegetative cells of the embryo sac. Three antipodal cells are present in almost every plant. 

  • Central Cell: This is the largest cell of the embryo sac. Two polar nuclei are present initially, then they form secondary nuclei just before fertilization. It is also known as a definitive nucleus. 

FAQ (Frequently Asked Questions)

1. Who Classified the Embryo Sac?

Answer: The embryo sac was classified by P. Maheshwari. He classified them based on the number of megaspore nuclei that are participating in the formation of the embryo sac. They were classified into three types:

  • Monosporic Embryo Sac: It is formed from a single megaspore. This takes place in Polygonum. 

  • Bisporic Embryo Sac: In this two megaspore nuclei take part in the development of the embryo sac. This takes place in Allium. 

  • Tetrasporic Embryo Sac: In this, all the four megaspore mother cells take part in the development of the embryo sac. This takes place in Adoxa. 

2. What Do You Understand about the Fate of Endosperm?

Answer: The work of endosperm is to provide nourishment to the developing embryo. It has two possibilities:

  • The endosperm can be completely consumed during the development of the embryo before the maturation of the seed. These types of seeds are called non-endospermic seeds. 

  • The endosperm cannot be completely consumed in the seeds and it remains in the seeds. These are known as albuminous shoes.