We have seen in Chapter 1 how a flower looked like. The fruit is made after the pollination of the flower and the fertilization of the ovule. The fruit originated from the ovary, while the fertilized ovules will become the seeds. Let’s see it in more detail.
Once the pollen grain lands on the stigma and is compatible with the flower, it will germinate. A pollen tube is formed. It carries the sperm nuclei to the egg cell in the ovule. The pollen tube grows through the style, then in the ovary, and is attracted to the ovule. There is always only one pollen tube for one ovule. Some flowers have only one ovule, like stone fruit (apricot, peach, plum, …). Some others have multiple ovules in the ovary, like apples, pears, paprika, maize, wheat. The number of ovules in the ovary will determine the number of seeds produced by the fruit.
Once there, one sperm nucleus fertilizes the egg cell giving rise to the zygote and the embryo, later developing into a new plantlet when the seed germinates. The second sperm nucleus fertilizes the central cell developing into the endosperm, a triploid tissue. The endosperm is a transient and nutritive tissue that supports the growth of the embryo. It works a bit like the placenta in mammals. In cereals and crops like maize, wheat, rice (they are called monocotyledons), the endosperm is what we eat!
Because there are two fertilizations in flowering plants, we speak of double fertilization.
After the fertilization, the embryo is developing into the seed (fertilized ovule). And the ovary is growing into a fruit.
Inside the seed, the embryo and the endosperm grow and develop.
The embryo takes the shape of the future plantlet. There are different structures: the radicle, the epicotyl (embryonic stem), the hypocotyl (junction between the root and shoot). It also has one or two embryonic leaves, called cotyledons, that will store the energy that will be necessary during seed germination.
Arabidopsis thaliana (thale cress) seed and embryo (drawing on the left and microscopic picture on the right)
Plants with embryos developing one cotyledon are monocotyledons or monocots. Those are grass and grass-like flowering plants. It also includes cereal grains and crops like rice, maize, wheat, onions, garlic. When the seed is germinating, only one leaf will appear.
Plants with embryos developing two cotyledons are dicotyledons or dicots. Those are flowering plants like tomatoes, paprika, peas, beans, avocados, sunflowers, and many others, including the Magnolids. When the seed is germinating, two leaves will appear.
The endosperm surrounds the embryo, mediating the transfer of nutrients between the mother plant and the embryo. In some species like the Fabaceae, the endosperm will disappear, absorbed by the embryo. In these species, the cotyledons serve a storage tissue. In other species, like the monocots and the Poaceae, the endosperm persists when the seed matures. The endosperm is the storage tissue (fat, starch…). In cereal grains like wheat and rice, the endosperm is used for food and transformed into flour.
Germinate seeds from the two categories and compare the germinating seedlings
Take a few seeds from dry pea, bean or lentil, maize that you can find in the supermarket or the gardening shop. Put them in a small container or a plate on a humid cotton pad or paper tissue. This will need to be kept wet. Wait for a few days and observe.
The ovary, which contains the developing seeds, will grow into a fruit. The other parts of the flower (sepals, petals, stigma, style, stamens) die after the fertilization of the ovule. The fruit contains threes structures: the exocarp (outer layer), the mesocarp (fleshy part), and the endocarp (the innermost layer). The fruit wall is called the pericarp.
Here are four types of fruits from the Rosaceae family and their corresponding fruits
In peach, a drupe fruit, the ovary wall develops into the fruit flesh with a stone derived from the innermost layer (endocarp) of the ovary wall. Inside the stone is a seed. The hypanthium, a cup-like base resulting from the fusion of the lower half of the sepals, petals (and sometimes stamens), does not become part of the fruit (look at the pome fruit for a contrasting example).
In raspberry, of the drupetum fruit type, the receptacle (raised stem tip) of the flower is connected to the base of numerous carpels (apocarpous gynoecium) with long styles. Each carpel (enlarged and in green), upon fertilization, develops into a fleshy drupe-like fruit (enlarged and in red), termed a drupelet. A drupelet is like a mini-peach with a fleshy ovary wall and a stone encasing a seed inside. The receptacle subtending these drupelets is not fleshy.
There are many types of fruits, different in size, shape, color, toughness, thickness. The structure of the fruit is connected to the method of seed dispersal. The purpose of the fruit is to protect and nourish the seed during its development and enable their dispersal after maturity.
After the seed is finally develop, there is a mechanism to help disperse the fruit and seeds away from the plant. Remember, the plants are fixed in the soil. They cannot move. So to avoid competition between the mother plant and the new daughter plants, the seeds are dispersed away. This is also to colonize new aeras. Seed dispersal can use an external carrier to transport away the seeds. This can be an animal, a bird, insects like ants.
Like for the pollinators, edible fruits evolved to attract animals to eat them. Once the fruits eaten, the undamaged seeds go through the animal’s digestive system and are dispersed away from the mother plant. Other seeds have developed structures such as hooks or burs, get tangled in the fur or feathers of animals and carried away to new places.