The embryo and the seed · Biology Class 11–12 | StemPy Lab
Course 19.3·Lesson 2 of 3
The embryo and the seed
The zygote grows into an embryo through the proembryo, globular and heart-shaped stages. A dicot embryo has two cotyledons. A monocot embryo has one, which in grasses is the scutellum. The ovule becomes the seed: the integuments become the seed coat, and the micropyle stays as a small pore. A seed may be albuminous or non-albuminous, and it dries to 10 to 15 per cent water.
about 30 min1 simulation1 practice question
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From a zygote to an embryo
The embryo grows at the micropylar end of the embryo sac, where the zygote lies. The early stages are almost the same in a monocot and a dicot. The zygote gives rise to the proembryo, and then to the globular embryo, the heart-shaped embryo and the mature embryo.
Five drawings in a row joined by arrows, showing the development of a dicot embryo. The zygote is a single oval cell with a nucleus. The proembryo is a small group of cells on a short chain of cells. The globular embryo is a round ball of cells on a chain of cells. The heart-shaped embryo has two lobes at the top. The mature embryo has two curved cotyledons, an embryonal axis between them and a root tip at the bottom.
This part is not in the NEET syllabus text, but A-Level and IB diagrams show it. The short chain of cells at the base of the young embryo is the suspensor. It pushes the embryo into the endosperm and passes food to it.
The embryo of a dicot
A typical dicot embryo has an embryonal axis and two cotyledons.
The part of the axis above the level of the cotyledons is the epicotyl. It ends in the plumule, which is the stem tip.
The part below the level of the cotyledons is the hypocotyl. It ends in the radicle, which is the root tip, covered by a root cap.
The embryo of a monocot
A monocot embryo has one cotyledon. In the grass family, the cotyledon is a shield-shaped scutellum, which lies on one side of the embryonal axis.
Two drawings, one above the other. At the top, a dicot embryo has two large cotyledons with the embryonal axis between them. The part of the axis above the level of the cotyledons is the epicotyl, which ends in the plumule. The part below is the hypocotyl, which ends in the radicle with its root cap. At the bottom, a monocot embryo of a grass is shown beside the endosperm. A large shield-shaped scutellum lies on one side of the axis. The shoot apex and leaf primordia in the epicotyl are covered by the coleoptile. The radicle and root cap are covered by the coleorhiza.
At its lower end, the axis has the radicle and the root cap, enclosed in a sheath called the coleorhiza.
The part of the axis above the scutellum is the epicotyl. It has a shoot apex and a few leaf primordia, enclosed in a hollow sheath, the coleoptile.
From ovule to seed
After fertilisation, the ovule becomes the seed. The seed is the final product of sexual reproduction. It is often described as a fertilised ovule. A seed has a seed coat, one or two cotyledons and an embryonal axis. Seeds form inside fruits.
A chart of eight arrows, from the left column, in the flower, to the right column, after fertilisation. Zygote, 2n, becomes the embryo, 2n. Primary endosperm nucleus, 3n, becomes the endosperm, 3n. Integuments, 2n, become the seed coat, 2n. The micropyle remains as a small pore in the seed coat. Nucellus, 2n, becomes the perisperm, 2n, in a few seeds such as black pepper and beet. The ovule becomes the seed. The ovary wall, 2n, becomes the pericarp, the fruit wall, 2n. The ovary becomes the fruit.
The integuments harden and become tough seed coats.
The micropyle stays as a small pore in the seed coat. It lets oxygen and water into the seed when it germinates.
The zygote becomes the embryo, and the PEN becomes the endosperm.
The cotyledons are thick and swollen in many seeds, such as the legumes, because they store food.
Two kinds of seeds
Non-albuminous seeds have no endosperm left, because the embryo has used it all. Examples are pea and groundnut.
Albuminous seeds keep a part of the endosperm. Examples are wheat, maize, barley, castor and sunflower.
In some seeds, such as black pepper and beet, some of the nucellus also stays. This left-over nucellus is the perisperm.
A seed that waits
As a seed matures, it dries. The water content falls to about 10 to 15 per cent of its mass, and the activity of the embryo slows down. The embryo may enter a state of rest called dormancy. When there is enough moisture and oxygen and the temperature is right, the seed germinates.
Seeds can wait for a long time. This part is not in the NEET syllabus text, but boards ask it. NCERT prints that a seed of Lupinus arcticus found in the Arctic tundra, about 10,000 years old, germinated, and so did a date palm seed about 2000 years old. These are records of unusual seeds, and the exact ages are not a part of any exam.
You should find that the curve rises steeply for a wet seed. The dry matter is the same, but the total mass changes a great deal as the water goes.
Try it in Python
The program uses the same idea to find the mass of a seed before and after it dries.
Practise with new numbers
Common mistakes
Mixing up epicotyl and hypocotyl. The epicotyl is above the cotyledons, and the hypocotyl is below.
Saying that every seed has endosperm. Pea and groundnut seeds do not.
Saying that the scutellum is the sheath of the shoot. The scutellum is the cotyledon. The coleoptile covers the shoot apex.
Saying that the seed coat comes from the nucellus. It comes from the integuments.
Saying that a seed is dead when it is dry. It is only dormant.
Remember these
Stages of the embryo: zygote, proembryo, globular, heart-shaped, mature.
Dicot embryo: two cotyledons, epicotyl with plumule, hypocotyl with radicle and root cap.
Monocot embryo of a grass: one scutellum, coleoptile around the shoot apex, coleorhiza around the radicle.
Integuments become the seed coat. The micropyle stays as a pore. The zygote becomes the embryo, and the PEN becomes the endosperm.