- After double fertilization
– ovule develops into a seed
– ovary develops into a fruit enclosing the seed
– As the embryo develops, the seed stockpiles proteins, oils and starch.
- These nutrients are stored in the endosperm, but later in seed development in many species, the storage function is taken over by the swelling storage leaves (cotyledons) of the embryo itself.
- The endosperm is rich in nutrients, which it provides to the developing embryo.
In most monocots and some dicots, the endosperm also stores nutrients that can be used by the seedling after germination.
- In many dicots, the food reserves of the endosperm are completely exported to the cotyledons before the seed completes its development, and consequently the mature seed lacks endosperm.
- The first mitotic division of the zygote is transverse, splitting the fertilized egg into a basal cell, and a terminal cell which gives rise to most of the embryo.
- The terminal cell divides several time and forms a spherical proembryo attached to the suspensor.
- Cotyledons begin to form as bump on the proembryo.
- A dicot with its two cotyledons, is heart-shaped at this stage.
- Only one cotyledon develops in monocots.
- After the cotyledons appear, the embryo elongates.
- During the last stages of maturation, a seed dehydrates until its water content is only about 5-15% of its weight.
- In the seed of a common bean, the embryo consists of an elongate structure, the embryonic axis, attached to the fleshy cotyledons.
- Below the point at which the fleshy cotyledons are attached the embryonic axis is called the hypocotyls and above it is the epicotyl.
- At the tip of the epicotyl is the plumule, consisting of the shoot tip with a pair of miniature leaves.
- The hypocotyls terminates in the radiacle, or embryonic root.
The embryo of a grass seed is enclosed by two sheath, a coleorhiza, which covers the young root and a coleoptile, which cover the young shoot.