The ovule is part of the structure of a plant that develops into a seed in fertilization. It produces a complete adult plant after the female reproductive cells are made and contained. Afterward, it will develop into a seed and ripen.
After being fertilized, the ovule will start to swell and its walls will toughen up before becoming into a seed. Some plants have a single ovary, while others have many, which will develop into many seeds in the fruit. (Single ovary like Avocado, Multiple ovaries like Kiwifruit)
A mature ovule consists of food tissue that is covered by one or two future seed coats. It is known as the integuments. The small opening in the integuments will allow the pollen tube to enter and discharge its sperms nuclei into the embryo sac. Each ovule will be attached by its base to the funiculus that bears it.
There are two types of seed plants: gymnosperms and angiosperms (see the picture of Gymnosperm and Angiosperm). Gymnosperms are mostly conifers or cone-bearing (e.g. pine trees). Here, the ovules lie uncovered on the scales of the female cones. On the other hand, angiosperms are flowering plants that can have one or more ovules enclosed to its ovary within the carpel, which will develop into a fruit.
In a plant classification, variation in the form and position of the ovule is significant. These two also sets the line to identify the type of ovules.
Here are the six types of ovules:
1. Orthotropous ovule (Atropous)
Orthotropous, or also known as Atropous, is the body of the ovules that are erect or straight. It is when the chalaza (where nucellus and integuments merge), the funicle (which attached the ovule to the placenta), and the micropyle are all aligned in straight. ( e.g. Polygonum)
2. Anatropous ovule
Anatropous is the body of the ovule that becomes completely inverted during the development to make the micropyle close to the hilum. The hilum is the scar on the seeds that marks the attachment of the funicle to the fruit wall.
3. Hemi-anatropous or hemotropous ovule
Hemi-anatropous, also known as hemotropous ovule, is the body of the ovule that is placed diagonally at a right angle to the funicle. It looks like the ovule is lying on its side. Meanwhile, the micropyle and chalaza rest in one straight line. (e.g. Ranunculus)
4. Campylotropous ovule
Campylotropous is the type of ovule that is curved or bent so that the alignment between the chalaza and the micropyle is disintegrated. However, here, the embryo sac is only slightly curved. ( e.g. Leguminosae)
5. Amphitropous ovule
The curvature in this type of ovule is very much visible that the embryo sac also becomes curved like it. It can be compared to the shape of a horseshoe. (e.g. Allismaceae, and Butomaceae)
6. Circinotropous ovule
The nucellus and the axis are aligned in the beginning, but because of the rapid growth on one side, the ovule becomes anatropous. The funicle, in this case, is unusually long that it creates an almost full circle around a micropyle pointing upwards. (e.g. Opuntia)
These six categories of an ovule are based on their shapes and how they are projected in posture.
The 3 Components of Ovules
The ovules of the female reproductive part of the plants are made up of the nucellus, integuments, and the female gametophyte, which are found at the center.
Here are their definition and functions:
1. The Nucellus
The nucellus is the largest part of the ovule. It provides a house for the embryo sac, as well as the nutritive tissue. In some flowering plants, it remains present even after fertilization as a source of nutrients for the embryo.
The integuments form a tough outer layer to protect the ovule. Plants can either have one or two of this part. Gymnosperms, like pine trees and spruce trees, usually have a single integument in their ovule. It is called unitegmic. On the other hand, angiosperms, like maples and daisies, normally have two integuments. It is called a bitegmic. The integument surrounds the nucleus, except for the micropyle, which is a small opening in the surface of the ovule.
3. The Female Gametophyte
The female gametophyte is the part of the ovule that contains gamete-producing sex organs, which is necessary for sexual reproduction. The female gametophyte holds a single set of unpaired chromosomes (haploids). It is commonly called the embryo sac of the megagametophyte.
What Are The Functions of Ovules
The ovule plays a vital role in sexual reproduction. When the pollen grain lands on a stigma of a flower in the same species, it sends out a pollen tube down through the style. When this goes down, it will enter the ovary and reach the ovule of the plant to start the fertilization process. The merge of the nucleus and embryo sac through the pollen grains allows the plant to start fertilizing. Remember that for a male reproductive part, pollen is the alternative for an ovule, which contains the male gametophytes.
Just like how human beings reproduce their offsprings, other living things also undergo this same experience to propagate, including plants. As you can notice, the terms used in the process of plant replication are almost the same as the human female fertilization.
Just like human females, plants also need to be fertilized so it can multiply itself and produce more of its kind. However, the process discussed can only happen for seed plants like gymnosperms and angiosperms. It does not apply for vascular and nonvascular plants.
Meanwhile, vascular plants can reproduce themselves asexually either by budding, branching, or tillering (vegetative reproduction). They can also do it by producing seeds. On the other hand, nonvascular plants can propagate asexually or sexually. In the asexual process, fragments from the plant will be separated to grow genetically identical plants. While, for sexual reproduction, nonvascular plants need water or a moist habitat to undergo fertilization.
In conclusion, no matter what type of plant you have, you can make them reproduced naturally or alternatively. If you are managing a garden at home, you might want to see them blooming in groups. Then, just wait until they fertilized. And if you happen to have seedless plants, you can do other alternative processes to propagate them.