Seed & Fruit Development

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson we'll be learning about the development of the seeds and fruits during plant reproduction. We'll also go over parthenocarpy and how this process produces seedless fruits.

What Are Fruits and Seeds?

Many of us enjoy a refreshing slice of watermelon on a hot day, or have gone apple picking in the fall. Fruits are a major part of many people's diet and have been for millions of years. But what are fruits, really? And where do they come from? Today, we're going to look at the answers to these questions.

Although it might seem gross, fruits are actually ripened plant ovaries. Plants are non-motile, yet they reproduce sexually just like humans do. Plants don't engage in physical sex however. Rather, plants release sperm in the form of pollen, which is carried by wind and animals to flowers, which contain the plant egg cells inside an ovary. Seeds, and in some cases, fruits, develop from this fertilization. Let's look at the steps to make each in detail next.

Seed Development

Seeds are the reproductive units of plants, and as such, most seeds start with fertilization. Pollen grains travel from the stamen, the male reproductive organ of plants, to receptive flowers. Pollen grains that land on the pistil of the female reproductive structure germinate and form pollen tubes that travel through the style into the ovary.

Reproductive structures in angiosperms
flower

In the ovary, one pollen nuclei fuses with the egg cell to form a zygote. This cell will develop into the embryo that will ultimately germinate to become a new plant. But, in order to form a new plant, that single cell must divide. The zygote gets messages from surrounding tissues in the form of hormones. These chemicals promote the development of the embryo from the zygote. For example, cytokinins cause the zygote to go through cell division and auxins produced by the embryo cause cells to expand.

In angiosperms, or flowering plants, an additional pollen nuclei from the pollen tube fuses with two polar nuclei inside the ovary. This forms a triploid cell that will become the endosperm. The endosperm serves as a food source for the developing embryo. It contains carbohydrates, lipids, and proteins needed for growth of the embryo while it waits for germination.

However, not all angiosperms develop an endosperm. In orchids, the endosperm formation is suppressed, and in many cases, it is reduced and replaced by other food sources for the embryo. For example, beans use the cotyledon as a nutrition source, which will ultimately become leaves on the new seedlings.

The embryo and endosperm, if present, is encased in a seed coat. The seed coat is a hard outer layer of the ovary composed of layers called integuments. The main function of the seed coat is to protect the embryo.

Like human embryos need to be safely protected in the mother's uterus while they develop, similarly, plant embryos exist in a vulnerable stage in development. The seed coat prevents the embryo from drying out, or conversely being flooded by heavy precipitation. It also helps to regulate the temperature of the seed, preventing it from freezing in cold temperatures, or overheating during the heat of the day.

Fruit Development

Fruits are ripened ovaries of plants. Prior to fertilization, the carpel of the flower protects the embryo sac and helps to guide the pollen tube. After fertilization and seed formation, the carpel wall switches function to develop into fruit in a process called fruit set.

Several hormones help this process take place, specifically auxin, gibberellins, cytokinin, and ethylene. Similarly to seed development, the production of auxin by the embryo also stimulates the expansion of cells needed to form fruit. In addition to starting the process of developing fruit from an ovary, auxin also stimulates fruit growth; ripening; and abscission, where fruits drop from the plant for dispersal.

During early fruit set, gibberellin hormones also increase and are necessary for fruit development. Like in the developing embryo, cytokinins cause fruit cells to go through cell division, increasing the mass of the ripening fruit.

After the fruit develops, it must ripen. Ripening fruits become more sweet and soft as well as change color to become appealing to animals for seed dispersion. A major hormone involved in the fruit ripening stage is ethylene. Ethylene is a gas hormone produced by plants that causes biochemical changes inside the plant. Enzymes are produced that cause starch to be broken down into sugar and acids to be broken down to decrease the tart taste of unripe fruit. Proteins holding fruit cells together are also broken down, causing fruit to become soft.

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