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. The types of
ovule is given in Figure 1.8.
Orthotropous: In this type of ovule,
the micropyle is at the distal end and the micropyle, the funicle and
the chalaza lie in one straight verticalline. Examples: Piperaceae,
Anatropous: 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
Hemianatropous: In this, the body of the
ovule is placed transversely and at right angles to the funicle. Example:
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.
In addition to the above main types there are two more types of ovules they are, Amphitropous: The distance between hilum and chalaza is less. The curvature of the ovule leads to horse-shoe shaped nucellus. Example: some Alismataceae.
Circinotropous: Funiculus is very long
and surrounds the ovule. Example: Cactaceae