Lesson 9 of 12 · 7 min
Endosperm and embryo
NCERT §1.4.1, §1.4.2
Anu cuts open a tender coconut and drinks the water, then scoops the soft kernel from an older one. She has just tasted two stages of the same tissue. Which tissue, and why does it come before the embryo?
The lesson in notes
In short
Post-fertilisation events are the development of endosperm and embryo, the maturing of ovules into seeds and of the ovary into a fruit.
Endosperm develops before the embryo. The PEC divides repeatedly to form triploid endosperm tissue whose cells store food for the developing embryo.
In the commonest type, the PEN undergoes repeated nuclear divisions without walls, giving free-nuclear endosperm; walls form later and it becomes cellular. The number of free nuclei before cellularisation varies widely.
Tender coconut water is free-nuclear endosperm with thousands of nuclei; the white kernel around it is cellular endosperm.
Endosperm may be used up completely by the embryo before the seed matures (pea, groundnut, beans) or persist in the mature seed and be used at germination (castor, coconut).
The zygote sits near the micropyle, so that is where the embryo grows. In most plants the zygote waits to divide until some endosperm exists, an adaptation that guarantees food for the young embryo.
Early embryogeny is similar in monocots and dicots: the zygote gives a proembryo, then globular, heart-shaped and mature embryo stages.
A dicot embryo has an embryonal axis and two cotyledons. Above the cotyledons is the epicotyl, ending in the plumule (stem tip); below is the hypocotyl, ending in the radicle (root tip), which is covered by a root cap.
A monocot embryo has one cotyledon; in grasses it is the scutellum, set to one side of the axis. The radicle and root cap are enclosed in a sheath, the coleorrhiza. Above the scutellum's attachment is the epicotyl, whose shoot apex and a few leaf primordia are enclosed in a hollow foliar sheath, the coleoptile.