In most angiosperms, a tissue is present inside the seeds appearing post fertilization known as Endosperm. It is basically responsible for providing nutrition to the embryo.
It is basically of 3 types:
Nuclear
Cellular
Helobial
Nuclear type:
Most common.
Primary endosperm nucleus is responsible for the formation of numerous free nuclei.
The free nuclei however remain as a peripheral layer and in the embryo sac a large central vacuole appears.
Formation of cell wall occurs later following which there is the formation of cellular endosperm.
Example- Coconut
Cellular type:
This endosperm is cellular as the first division of the primary endosperm nucleus is followed by the formation of a cell wall.
Example- Datura, Petunia etc.
Helobial type
It is an intermediate between the above mentioned 2 types, i.e. Nuclear and cellular types as the first division is similar to cellular endosperm and results in a large micropylar cell along with a small chalazal cell and a further division of micropylar cells which is similar to nuclear endosperm.
Most commonly seen in Monocotyledons.
Example- Eremurus, Drimya etc.
Final Answer
Hence, endosperm is a tissue which provides nutrition to the embryo and is present inside the seeds of most angiosperms and is basically of 3 types: Nuclear type, Cellular type and Helobial type