Question

What are cotyledons?

Answer 1

Cotyledons present in flowering plants are also known as seed leaves. A cotyledon is an integral part of the embryo within the seed of a plant. It is the first part which appears from a germinating seed.

Complete answer:
It is the structure that contains stored food that gives energy for the plant to grow in the embryonic stages. Some cotyledons upon formation will expand after the germination of seed , come out of the seed shell and rises above the ground whereas some other will remain inside the soil and wither.When these reserved food becomes used up, the cotyledons will turn green and will start photosynthesis or it will wither as the first leaves takes over food production through photosynthesis. It is also known as embryonic leaf in seed bearing plants or angiosperms. The number of cotyledons is the character that is used to classify the angiosperms . The plant with only one cotyledon is known as monocotyledonous or monocots. Example: coconut, areca nut, bamboo. Plants with 2 cotyledons are called dicotyledonous or dicot plants. Example: mango tree, jackfruit tree etc. The number of cotyledons present in the embryo of gymnosperms will vary from a range to 8 to 20 or more. Some cotyledons last a few days after birth whereas some others last a year.
In dicots the cotyledons are photosynthetic and they function in a very much similar to that of leaves. But cotyledons are formed prior to seed germination in the embryonic stage along with roots and shoot meristems. But true leaves are formed in the post-embryonic stage from the shoot of apical meristem which holds the responsibility of producing further aerial portions of plant.They have plumule and radicle which are precursors of shoot and root respectively.

Cotyledons present grasses and all other monocots is characterised as a highly modified leaf which consists of scutellum and coleoptiles. Scutellum is a tissue present inside the seed which is designed to absorb food and nutrients from the endosperm present adjacently. Plumule is the precursor of stem and leaves of the plant. In monocots it is covered by a protective cap called coleoptiles. Radicle is also present as the precursor of roots.
In gymnosperms there are a lot of cotyledons present varying from 8 to 20. They form a whorl around the hypocotyl, which is the embryonic stem that surrounds the plumule. The number of cotyledons vary from species to species also within the species. In Monterey pine the seeds have 5 to 9 cotyledons whereas in Jeffrey pine there 7 to 13 cotyledons. The plant having the highest number of cotyledons is the big-cone pinyon which has around 24 cotyledons.

Note: The cotyledons that come out of the soil on seed germination breaking of the seed shell are known as epigeal and they might turn photosynthetic. Those which remain inside the soil without expanding are called hypogeal. In this case the cotyledons become the storage organ such as in nuts and acorns.