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Anatropous Ovule vs. Orthotropous Ovule: What's the Difference?

Edited by Aimie Carlson || By Janet White || Published on February 6, 2024
Anatropous ovule is inverted during development, with a curved embryo sac, while orthotropous ovule remains upright and straight.

Key Differences

Anatropous ovules are the most common type in flowering plants, characterized by a complete 180-degree turn during development. This results in the micropyle (the opening to the ovule) being near the placenta. In contrast, orthotropous ovules are considered the primitive form and maintain their original orientation. Their micropyle, chalaza (base), and funiculus (stalk) are aligned in a straight line.
The curvature of the anatropous ovule facilitates efficient pollen tube entry and fertilization. It's a result of evolutionary adaptation for successful reproduction in angiosperms. Orthotropous ovules, however, lack this curvature. They represent a more straightforward, less evolved form where the structures are in linear alignment, potentially making fertilization less efficient.
In anatropous ovules, the integuments (protective layers) grow around the funiculus, enclosing the ovule except for the micropyle. This distinctive feature aids in the protection and nourishment of the developing embryo. Orthotropous ovules, on the other hand, have integuments growing parallel to the funiculus, resulting in a simpler structure.
The position of the embryo sac within anatropous and orthotropous ovules also differs significantly. In anatropous ovules, the embryo sac is curved, which is a consequence of the ovule's inversion. In contrast, the embryo sac of orthotropous ovules remains straight, consistent with the ovule's overall linear structure.
Anatropous ovules are often associated with a higher degree of specialization and are more common in advanced flowering plants. Orthotropous ovules, being less specialized, are often found in more primitive plants, indicating an evolutionary transition in ovule development.

Comparison Chart


Inverted, with a 180-degree turn
Upright, maintains original position

Embryo Sac

Curved due to inversion
Straight, aligned with the ovule axis

Position of Micropyle

Near the placenta, opposite the funiculus
Directly aligned with the funiculus

Structure Complexity

More complex, indicative of evolutionary adaptation
Simpler, more primitive structure


More common in advanced flowering plants
Typically found in primitive plants

Anatropous Ovule and Orthotropous Ovule Definitions

Anatropous Ovule

Anatropous ovule has a micropyle positioned near its attachment point.
The anatropous ovule in an apple flower allows for successful seed formation due to its structure.

Orthotropous Ovule

Orthotropous ovule has a linear arrangement of micropyle, chalaza, and funiculus.
The orthotropous ovule in cycads showcases the linear alignment typical of less advanced plants.

Anatropous Ovule

Anatropous ovule is enveloped by integuments, leaving only the micropyle exposed.
The protective layering of the anatropous ovule in an orchid safeguards the embryo.

Orthotropous Ovule

Orthotropous ovule is a feature of many primitive angiosperms and gymnosperms.
The magnolia, with its orthotropous ovule, reveals characteristics of early flowering plants.

Anatropous Ovule

Anatropous ovule represents an evolutionary advancement in seed plants.
The presence of anatropous ovule in sunflowers demonstrates evolutionary progress in plant reproduction.

Orthotropous Ovule

Orthotropous ovule is simpler in structure, lacking curvature.
The straightforward structure of the orthotropous ovule in mosses aids in basic reproduction.

Anatropous Ovule

Anatropous ovule, common in angiosperms, is characterized by its 180-degree turn during development.
The anatropous ovule in the rose bush is a key feature distinguishing it from more primitive plants.

Orthotropous Ovule

Orthotropous ovule is a straight, unaltered ovule with aligned structures.
In ferns, the orthotropous ovule remains straight, reflecting its primitive nature.

Anatropous Ovule

Anatropous ovule is an inverted ovule, curved around its stalk.
In the pea plant, the anatropous ovule ensures efficient fertilization through its unique orientation.

Orthotropous Ovule

Orthotropous ovule, seen in some gymnosperms, retains its original upright position.
The pine tree's orthotropous ovule illustrates a more basic form of seed development.


How does an orthotropous ovule differ in orientation?

An orthotropous ovule remains upright, with a straight alignment of its components.

What defines an anatropous ovule?

An anatropous ovule is inverted, with a 180-degree turn, making the micropyle near the placenta.

Can orthotropous ovules be found in angiosperms?

While less common, some primitive angiosperms do have orthotropous ovules.

What is the significance of the curvature in anatropous ovules?

The curvature in anatropous ovules facilitates efficient pollen tube entry and fertilization.

Are anatropous ovules common in flowering plants?

Yes, anatropous ovules are the most common type in flowering plants.

Do orthotropous ovules indicate a primitive plant form?

Yes, orthotropous ovules are considered more primitive and are often found in less advanced plants.

Is the embryo sac in orthotropous ovules curved?

No, the embryo sac in orthotropous ovules is straight, aligning with the ovule’s linear structure.

How do integuments grow in anatropous ovules?

In anatropous ovules, integuments grow around the funiculus, enclosing the ovule except for the micropyle.

How does the anatropous ovule aid in seed protection?

The anatropous ovule’s integuments and curved structure provide enhanced protection for the developing seed.

Are anatropous ovules a result of evolutionary adaptation?

Yes, the development of anatropous ovules is a result of evolutionary adaptation for improved reproductive success.

What is the evolutionary significance of anatropous ovules?

Anatropous ovules represent evolutionary adaptation for successful reproduction in angiosperms.

What plants typically have anatropous ovules?

Most advanced flowering plants, like roses and sunflowers, have anatropous ovules.

Is the orthotropous ovule’s position beneficial for fertilization?

The straight position of orthotropous ovules is less efficient for fertilization compared to the curved anatropous ovule.

Can the structure of ovules aid in plant identification?

Yes, the type of ovule can provide clues about a plant’s evolutionary status and classification.

Do all angiosperms have anatropous ovules?

While most angiosperms have anatropous ovules, there are exceptions with orthotropous or other types of ovules.

Do orthotropous ovules indicate a specific evolutionary stage?

Yes, orthotropous ovules indicate an earlier evolutionary stage in seed plant development.

What is the significance of the anatropous ovule in plant evolution?

The anatropous ovule is significant in plant evolution as it represents a sophisticated mechanism for ensuring successful fertilization and seed development in flowering plants.

Is the orthotropous ovule more common in gymnosperms?

Orthotropous and other primitive ovule types are more commonly found in gymnosperms.

What is the primary difference in micropyle placement between the two ovule types?

In anatropous ovules, the micropyle is near the placenta, while in orthotropous ovules, it is aligned with the funiculus.

Are orthotropous ovules simpler in structure?

Yes, orthotropous ovules have a simpler, more linear structure compared to anatropous ovules.
About Author
Written by
Janet White
Janet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.
Edited by
Aimie Carlson
Aimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.

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