In the embryos of a typical dicot and a grass, true homologous structures are: (a) Coleorhiza and coleoptile (

NCERT Exemplar · Q17

Q. In the embryos of a typical dicot and a grass, true homologous structures are:

(a) Coleorhiza and coleoptile

(b) Coleoptile and scutellum

(d) Hypocotyl and radicle

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Concept understanding — Seed Structure Terminology

Seed Structure Terminology – A First Look

Think of a seed as a tiny, self-contained survival kit. A plant makes a seed to send its offspring out into the world, equipped with everything needed to travel, wait, and eventually sprout into a new plant. The terminology around seed structure is simply the names we give to the parts of that kit.

The Three Essential Parts

Every seed, from a mustard seed to a mango seed, has three basic components. You can remember them as the baby, the lunchbox, and the coat.

Embryo – This is the baby plant itself. It is a miniature, dormant plant with tiny leaves (cotyledons or plumule), a tiny stem (hypocotyl), and a tiny root (radicle). When conditions are right, the embryo wakes up and starts growing.
Endosperm – This is the lunchbox. It is a tissue that stores food (starch, proteins, oils) for the embryo to use when it germinates. In some seeds, the endosperm is consumed by the embryo before the seed matures, and the food is stored directly in the cotyledons instead.
Seed Coat – This is the coat. It is a tough, protective outer layer that shields the embryo from injury, drying out, and disease. It often has a tiny scar called the hilum where the seed was attached to the mother plant, and a small pore called the micropyle that lets water enter during germination.

Note

The micropyle is a tiny opening in the seed coat. It is not just a scar — it is a functional pore. Water enters through the micropyle to trigger germination, and later, the radicle (the first root) emerges through it.

Why This Matters for Exams

In NCERT textbooks (Class 11 Biology, Chapter 5), seed structure is taught to help you understand how plants reproduce and how seeds are adapted for survival. The key distinction you must know is between two types of seeds based on where the food is stored:

Albuminous seeds – The endosperm remains present and stores food. Example: castor, coconut, maize.
Exalbuminous seeds – The endosperm is completely used up during development, and food is stored in the cotyledons. Example: pea, bean, groundnut.

Important

In exalbuminous seeds, the cotyledons become thick and fleshy because they have absorbed the endosperm. In albuminous seeds, the cotyledons remain thin and papery because the endosperm does the job of food storage.

A Simple Way to Visualise

Imagine a packed lunch in a lunchbox inside a backpack. The embryo is the child, the endosperm is the lunch, and the seed coat is the backpack. In an albuminous seed, the lunchbox is still full when the child opens it. In an exalbuminous seed, the child has already eaten the lunch and packed the food into its own pockets (the cotyledons) before the journey begins.

Common Exam Terms at a Glance

Term	Meaning
Radicle	The part of the embryo that becomes the root
Plumule	The part that becomes the shoot (stem and leaves)

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The cotyledons of a dicot embryo and the scutellum of a grass embryo are true homologous structures, sharing a common developmental origin as embryonic leaves.

To understand which structures are truly homologous between a typical dicot and a grass embryo, we first need to grasp the concept of homology itself. In biology, homologous structures are those that share a common evolutionary origin, even if they have evolved to perform different functions or appear morphologically distinct. This indicates a shared ancestry. Conversely, analogous structures perform similar functions but have different evolutionary origins.

Let's examine the key components of a typical dicot embryo and a grass (monocot) embryo as described in NCERT, and then compare them to identify homologous pairs.

The Dicot Embryo

A typical dicotyledonous embryo, such as that of a pea or bean, consists of an embryonal axis and two cotyledons.

Cotyledons: These are embryonic leaves that are often thick and swollen due to the storage of food reserves. They are attached to the embryonal axis.
Embryonal Axis: This central axis has two ends:
- Plumule: The part of the embryonal axis above the level of cotyledons, which gives rise to the shoot system.
- Radicle: The cylindrical portion below the level of cotyledons, which forms the primary root.
- Hypocotyl: The portion of the embryonal axis between the level of cotyledons and the radicle.
- Epicotyl: The portion of the embryonal axis between the level of cotyledons and the plumule.

The Grass (Monocot) Embryo

Monocotyledonous embryos, particularly those of grasses like maize, exhibit some distinct features, though their fundamental organization is similar to dicots.

Scutellum: This is the single cotyledon of the monocot embryo. It is typically shield-shaped and positioned towards one side of the embryonal axis. Its primary function is to absorb nourishment from the endosperm and transfer it to the developing embryo.
Embryonal Axis: Similar to dicots, it has a plumule and a radicle.
- Coleoptile: The plumule in a grass embryo is enclosed within a protective sheath called the coleoptile.
- Coleorhiza: The radicle and root cap are enclosed within an undifferentiated sheath called the coleorhiza.
Epiblast: Some grass embryos also possess a small, rudimentary structure called the epiblast, which is considered a remnant of the second cotyledon.

Important

The NCERT textbook explicitly states that "The embryo of monocotyledons possesses only one cotyledon. In the grass family the cotyledon is called scutellum." This statement is crucial for identifying the homologous structure.

Identifying Homologous Structures

Now, let's evaluate the given options based on the descriptions above:

(A) Coleorhiza and coleoptile: These are both protective sheaths found exclusively in monocot embryos, enclosing the radicle and plumule, respectively. While they are both monocot adaptations, the question asks for homologous structures between a dicot and a grass. Neither of these has a direct homologous counterpart in a typical dicot embryo.
(B) Coleoptile and scutellum: The coleoptile is a protective sheath for the plumule, while the scutellum is the single cotyledon. These structures have different developmental origins and functions, and are not homologous to each other. …

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Q.In the embryos of a typical dicot and a grass, true homologous structures are:

The Dicot Embryo

The Grass (Monocot) Embryo

Identifying Homologous Structures

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