Essential Parts of Flower: Gynoecium - Types of Ovules

Types of Ovules:

The ovules are classified into six main types based on the orientation, form, and position of the micropyle with respect to funicle and chalaza. Most important ovule types are orthotropous, anatropous, hemianatropous, and campylotropous. 

1) Atropous or Orthotropous:

The body of the ovule is upright in position. In this type of ovule, the micropyle is at the distal end and the micropyle, the funicle, hilum, and the chalaza lie in one straight vertical line. It is the most primitive and simplest type of ovule. The raphe is absent.  Examples: Piperaceae, Polygonaceae, Betel, Piper, Polygonum, and Gymnosperms.

2) Anatropous:

The body of the ovule completely turned at 180° angle, due to unilateral growth of funiculus, so it is also called an inverted ovule. The body of the ovule becomes completely inverted so that the micropyle and funiculus come to lie very close to each other. This is the common type of ovules found in dicots and monocots. This type of ovule can be observed in 80% of families of Angiosperms but not in Capsella. It is considered a “typical ovule” of Angiosperms or resupinate ovule. E.g., Pea, Malvaceae.

3) Hemianatropous:

In this ovule, the body of the ovule is bent on a funicle at a 90° angle, i.e., body of ovule is present at right angle to the funiculus. This is an intermediate type between ortho and anatropous ovules. This ovule is called horizontal ovule because body of ovule is present in horizontal position on the funiculus. Micropyle and chalaza are present in the same line but micropyle is situated away from the hilum. E.g., Primulaceae.

4) Campylotropous:

The body of the ovule at the micropylar end is curved and more or less bean-shaped. The embryo sac is slightly curved. All the three, hilum, micropyle, and chalaza are adjacent to one another, with the micropyle oriented towards the placenta. The body of ovule becomes inverted and again turned into a straight position due to the growth of the funiculus so that body of ovule is present on funicle at 360° angle. The entire body of ovule is surrounded by funiculus. It is called a coiled ovule. Micropyle is situated away from hilum.  Example: Leguminosae, Cruciferae. 

5) Amphitropous:

The curvature is effective in the nucellus and due to this effect of nucellus, embryo sac becomes horse-shoe-shaped. Micropyle comes close to the hilum. It is called a transverse ovule. Example: some Alismataceae, Lemna, Poppy.

6) Circinotropous:

The body of ovule becomes inverted and again turned into a straight position due to the growth of funiculus so that body of ovule is present on funicle at 360° angle. The entire body of ovule is surrounded by funiculus. It is called a coiled ovule. Micropyle is situated away from hilum. Example: Cactaceae, Optunia.