Plants reproduce using eggs and sperm, just like people. However, they don’t have physical sex! Male plants release pollen, which fertilize ovaries. The fertilized ovaries develop into seeds, which can then be distributed to make new plants. As the fertilized ovaries develop, a surplus of carbohydrates and proteins are stored inside the seed. This will serve as food for the plant until it can develop leaves. Next, the fertilized plant ovaries develop the hardened coat we see as the outside of seeds to protect them against harsh environmental conditions. The seeds then can be carried away by wind or animals to create new plants.
A seed consists of three main parts: an embryo, an endosperm and a seed coat. The embryo is a tiny plant that will grow into a full-sized plant once the seed germinates. A large part of the interior of a seed is taken up by the endosperm, which is made up of proteins and carbohydrates that nourish the embryo for the first few days after germination. These two structures are enclosed by the seed coat, which protects the seed until it germinates.
Seed Coat Dormancy
Hard seed coats that are impermeable to air and water induce a type of dormancy, called seed-coat dormancy, by restricting the embryo’s access to air and water. Embryos can’t grow without air and water, so the seed remains dormant until the seed coat is penetrated. This protects the embryo by preventing germination before the time is right. In nature, a seed coat can be broken down by microorganisms, weathering, fire or being partially digested by animals.
Breaking Through Seed Coats
Nature has a plan for breaking through seed coats, but when a gardener plants seeds in the garden he doesn’t want to wait for the seed to complete the seed-coat dormancy cycle naturally. Manually abrading or softening the seed coat is called scarification. You can scarify seeds by nicking them with nail clippers or a sharp knife, abrading them with sandpaper. Take care that you don’t damage the internal embryo when nicking or abrading seed coats. However the most popular method is simply soaking them in warm water for 4 hours or overnight.
Plumeria Seed Germination
Germination is the process of seeds developing into new plants. First, environmental conditions must trigger the seed to grow. Usually, this is determined by how deep the seed is planted, water availability, and temperature. When water is plentiful, the seed fills with water in a process called imbibition. The water activates special proteins, called enzymes, that begin the process of seed growth.
First the seed grows a root to access water underground. Next, the shoots, or growth above ground, begin to appear. The seed sends a shoot towards the surface, where it will grow leaves to harvest energy from the sun. The leaves continue to grow towards the light source in a process called photomorphogenesis. Plumeria seeds begin the germination process when the seed coat is interrupted and the internal parts of the seed begin to take up water. As the seed swells and the embryo gains access to oxygen, it begins to metabolize the available store of carbohydrates and make proteins that will become radicles, the lower part of the plant that becomes roots, and the plumule, the shoot eventually becomes the upper foliage. As the roots form they descend into the soil in search of moisture and nutrients, while the shoot pushes its way above the soil.
Several factors influence if, and how, seeds germinate. The most important factors are water availability and temperature.
Water is crucial to seed germination. The seed must go through imbibition to activate root growth. However, too much water can be a bad thing, as most gardeners know. When a plant is still growing underground, during root formation, it cannot use the sun to make food like most grown plants do. It must rely on the stored food inside the seed, and oxygen from the environment to make energy. If the soil is too soggy, there will not be enough oxygen and the plant will not thrive.
Temperature is also an important factor. Plumeria seeds only germinate when the weather reaches spring temperatures, which is why we see so much plant growth in the spring in temperate climates.
There are three basic parts of a plumeria seed: an embryo, food storage or nutritive tissue, and the seed covering.
A mature plumeria seed has a diploid embryo which develops from a fertilized egg or zygote. It results from the union of a sperm, from a germinated pollen, with a female egg in the embryo sac. The embryo can be distinguished from the other major parts of a seed based on component parts and function. It consists of the epicotyl, hypocotyl, radicle, and two cotyledons. It is the one which develops into a plant with an upward growing shoot and a downward growing root system.
The epicotyl is a tiny shoot from which the entire plant shoot system develops. The growing tip of the epicotyl is the plumule. The hypocotyl is the transition zone between the rudimentary root and shoot; the radicle is a small embryonic root. Cotyledons are specialized seed leaves which develop from the plumule and occur in pairs in dicot seeds. They are the most prominent part of a fully developed embryo. Monocot means one cotyledon while dicot means two cotyledons.
The stored food is present in plumeria seeds in the form of carbohydrates, fats and proteins. This stored food is found in the cotyledons of the Plumeria seed. The stored food is used to support the embryo during seed germination. In the Plumeria the cotyledons serve as the food-storage tissue. When the embryo stores its own food reserve, within the cotyledons, the seed is called exalbuminous.
When you look at a hard seed such as a bean, corn or nasturtium seed, you’re not seeing the actual seed but the seed coat. A hard seed coat protects the internal parts from drying out and prevents water and insects from accessing the tender embryo inside. It also prevents premature germination by forcing the seed to remain dormant until the time is right.
The seed coat is developed from the outer covering of the ovule, or integument. But it is not immediately apparent in the angiosperms because the seed is encased in a fruit wall or pericarp. `
There are usually two layers of the seed coat. The outer layer, known as the testa, is thicker. The inner one is more delicate, known as tegmen.
Externally, some parts of a seed are obvious. On some seed coats, the opening in the integuments of the ovule, called micropyle, is visible. The hilum is usually visible also, the scar left by the stalk which attached the seed to the placenta. The hilum is equivalent to the navel in humans to which the umbilical cord is attached. It appears dark in color when the seed becomes physiologically mature and is thus used as an indicator of seed maturity.
From the outside, seeds may be smooth, wrinkled, or hairy as in cotton, or winged. In the castor bean (Ricinus communis), there is wart-like growth at the hilum, called the caruncle. In mangosteen, the seeds are enveloped by a white fleshy aril which is edible.
Reproductive Parts of A Plumeria
Leaves, stems, roots, flowers – you know the basic anatomy of plants. But if you’re growing plumeria from seed and you should know the parts important to forming plumeria seeds, it helps to get familiar with the reproductive parts of a plumeria. Plumeria have all of the vital parts in one flower. Here are the key players and what you need to know about them.
The male parts of a flower consist of an elongated cluster of sacs, called an anther, which emerges atop a thin filament when the flower opens. Inside the sacs are particles of pollen, which sit on the outside of the anther until wind, bees or other pollinators transfer it to the female parts. Together, the anther and filament are known as the stamen (pronounced “stay-men”). If you want your plants to produce seeds or fruit in an indoor garden, you can help by using a small thin paint brush to transfer the pollen grains from the male stamen to the female stigma. If your goal is to get unfertilized flowers with no seeds, you want to snip off the stamen as soon as they appear so none of the pollen is transferred.
On the female side, the pollen needs to land on one of two upright stigmas, one-quarter to one-half inch long, usually pale-colored, that come up from a little green pod called the floral bract. Though it’s only about ⅛ inch across and ¼ inch long, the bract is like the womb: when pollen reaches it through the stigma, a seed develops within each bract and causes it to swell.
The stigma and bract show up first inside a tight cluster of tiny leaf-like sepals. The cluster, called a calyx (say it “kay-licks”), may also contain glands that secrete sticky substances which help pollen grains stick to the stigma.
To further ensure successful pollination, some species produce pistils from the calyx. These thin strands often appear as short, red-orange, “hairs.” The pistils gather pollen from the air and move it down into the plant’s ovary, where it stimulates the production of the seed.
Flower buds typically form on top of a cluster of leaves at the end of a limb, the spots that get the most light. The terminal spots are known as colas (like the soda). Experienced growers use plant-training techniques to increase the number of colas on a plant, which leads to increases in volume and size of the flower buds.