Question

why is cotyledons called the seed leaves

Answer 1

Cotyledons are actually part of the seed. They are the first part of the plant to emerge and they look like little green leaves (hence "seed leaves"). The cotyledons store food reserves for the growing seedling. The plant relies on this stored food for early growth. The cotyledons also can photosynthesize, which supplies extra energy until the first true leaves emerge.

Answer 2

because they contain the stored food which is used by the growing seedling(baby  plant) . some plants have one or more cotylends.