The endosperm is the result of triple fusion. It was Maheshwari and Rangaswamy who supported the arguments that endosperm is a nutritive tissue.
Types of Endosperm Development
Based on their mode of development, endosperm is of three types.
- Nuclear endosperm
- Cellular endosperm
- Helobial endosperm
Nuclear Endosperm
Nuclear endosperm is the most common type of endosperm seen in the majority of angiosperms. Here, the central cell with a triploid nucleus undergoes division without cell wall formation. This leads to the enlargement of the cell with a large number of nuclei.
Now, the cell is called a coenocyte. While dividing, some nuclei divide quicker than others. Only the early divisions of the nuclei are synchronized. In the later stages, most nuclei migrate to the chalazal, and micropylar ends apart from the peripheral region.
In some plants such as Nomie, the nuclei at the chalazal end are larger than the ones at the micropylar end. Later, the wall formation process starts.
In many plants, there may be hundreds of nuclei formed before the cytokinesis starts. At the same time, some plants have their cytokinesis process starting at an earlier stage.
During cytokinesis, the cell wall formation starts from the micropylar region and in some plants, only at the peripheral region. Most often, the free nuclear part at the chalazal region acts as haustoria and absorbs food from neighboring tissues of the ovules. Eg., Macadamia.
- In Grevillea robusta, the free nuclear region at the chalazal end forms a worm-like structure that functions as haustorium.
- In Lomatia polymorpha, apart from the chalazal end functioning as haustorium, there also are vesicle-like projections all over the endosperm that have the same function.
The multiple divisions of the triploid nucleus of coconut results in a large number of nuclei that lie in the sweet liquid inside the embryo sac. The cell wall formation progresses from the periphery towards the center, only the central parts will have the sweet liquid but without any nuclei. The peripheral tissue consists of the endosperm tissues which will be called kernels.
Cellular Endosperm
In the cellular endosperm development, the nuclear division of the primary endosperm nucelus is followed by a cell wall formation. The subsequent divisions also form a cell wall.
Thus the divisions produce individual cells rather than just nuclei. This will develop an enosperm that have multiple cells without any free nucelate stage. This type of endosperm development is seen in about 25% of dicots.
Depending on the orientation of the cell wall formation,cellular endosperm could be divided into three.
- Vertical cell wall orientation creates a longitudinal embryo sac. The second wall formation occurs at right angles to the first one, followed by cell wall formation restricted only to the micropylar region. Eg., Adoxa and Centranthus.
- Transverse cell wall formation of the first nuclear division is followed by vertical division. Eg. Verbascum and Scutellaria.
- Transverse division for the first 2-3 divisions as seen in the members of Annonaceae and Ericaseae.
- Oblique first division of the endosperm. This may create an equal or unequal cells. Eg. Myosytis arvensis.
- Indefinite first division may be seen in Seneca, Gunnera., etc.
One of more cells of cellular endosperm form an endosperm haustoria. It may be formed at the micropylar or chalazzal end. The haustoria formed will penetrate deeper into the nucellar tissue to absorb nutrition.
There could also be a secondary haustorium. In Magnolia obovata, a multinucelated chalazal haustoria is seen.
Helobial Endosperm
Helobial endosperm is seen only in monocots and are common in order Helobiales. This type of endosperm development is an intermediate between the nuclear and cellular types. Here, the primary endosperm nucleus migrates towards the chalazal end.
- The first division and wall formation forms a larger chamber towards the micropylar region and a smaller chamber towards the chalazzal region.
- The nucleus at the chalzal end remains undivided while the nucleus of the micropylar region divides multiple times.
- This nuclear division is followed by cell wall formation.
- The micropylar tissues forms a haustoria.
- This haustoria is tubular and unicellular.
- It will have outgrowths that help penetrate the nucellus in the chalzal end.
In addition to these three types of endosperms, there are two more endosperms- mosaic endosperm and ruminate endosperm.
- Mosaic endosperm is a non-homogenous tissue having differently colored tissues. This may happen due to the failure of triple fusion where the polar nuclei and male gamete do not fuse. Instead, they divide independently. Initially, thet become part of the multinucelate stage but later on form walls to become individual cells. These cells will have differential colors.
- Ruminate endosperm has uneven surface which will have characteristics similar to that of a seed coat. This is usually seen in members of Annonaceae, Araliaceae, Arecaceae, and Myristicaceae.
- Composite endosperm is seen in those plants where the ovaries lack ovules. In these plants, the sprogenous tissue develop multiple embryo sacs. After fertilization the primary endosperm nucleus of each embryo sacs move to the basal part and develop cellular endosperm. Later, each of these cellular endopserm enlarge and fuse together to form a composite endopserm.
Functions of Endosperm
- Endosperm is the nutritive tissue of almost all angiosperm seeds.
- The cells of endosperm are rich in reserve food such as fats, proteins, carbohydrates, etc.
- The stored food in the endosperm is used during the germination of the seeds.
- The embryo utilizes this food while growing until the seedlings develop chlorophyll and become autotrophic.
- It is an exclusive nutritive tissue for the growing embryo and grows before the zygotic division. In other words, zygotic division occurs only after the endosperm reaches a particular stage.
- Apart from the endosperm where the endosperm nutrition is not sufficient for the embryo, some seeds develop certain organs called haustoria for additional nutrition.
- In incompatible crosses, the endosperm does not survive and perishes sooner. Embryos in such seeds do not grow and these seeds become non-viable.
- Young endosperm tissues are rich in growth hormones to support the growth of the embryo. These hormones may be extracted and used in vitro to grow embryos of other angiosperms.
It is observed that the removal of endosperm causes failure in embryo growth. This proves that the main function of endosperm is nourishing the embryo.
References
- Sukumaran O R. Pre-Degree Botany. Murali Publications.
- Abraham P C. Anatomy, Embryology & Microtechnique. 1999. St. Mary’s Books & Publications.
- Endosperm: Types And Function
- https://egyankosh.ac.in/bitstream/123456789/16388/1/Unit-4.pdf
