Seeds contain an embryo and food reserves encapsulated in a protective outer shell. They develop from the fertilized ovule after sperm fertilize the egg in pollen.
Seeds require environmental conditions to grow into plants—including water, oxygen, and favorable temperatures. They can have a hard or thick seed coat (physical dormancy) that needs to be broken, such as by soaking or scarification.
Germination
The process by which a seed grows into a plant is called germination. Most seeds contain an embryo and a store of food reserves, wrapped in a hard shell. The germination process starts with uptake of water, known as imbibition. This causes the seed to swell and soften. The embryo then metabolizes its stored food reserves.
The seed also needs oxygen. The germination process is interrupted if a seed does not get enough oxygen. Some seeds have impermeable seed coats that prevent the uptake of oxygen. This is a form of physical dormancy that can be broken by wearing away the seed coat or exposing the seeds to abrasion.
Some seeds need cold temperatures to wake up from dormancy. This is why some farmers put their seeds in the refrigerator to make them grow faster. It’s also why passing through an emu’s gut can help a seed of the nodding geebung (Persoonia nutans) or snottygobble, in Australia, germinate.
Embryo
The term embryo is used for the very early stage of pregnancy, from fertilization to about week 11. At this time, most of a baby’s critical organs and body structures begin forming.
The single-celled fertilized egg, called a zygote, goes through a process of rapid cell division to produce many more cells. Then the zygote takes in fluid from the uterus to form a hollow sphere of cells called a morula or blastocyst.
At this stage, cells continue to divide and specialize in their functions, a process called differentiation. Cells that will become the embryo’s skeletal system start to develop, while blood cells are formed and circulate. The limb buds that will become the arms and legs grow longer, and a bump forms where the head will develop.
Root
Roots are the primary source of water and nutrients for a plant. They anchor the plant in the soil, transport water and dissolved minerals up through the xylem to the stem and store reserve foods.
The first root to form in seed plants is the radicle, which grows from the embryo after germination. As it grows, the radicle develops a thimble-shaped root cap to protect it in the soil. Behind the root cap is the apical meristem, a zone of actively growing cells that elongates as the root grows.
Most roots grow down into the ground, but some are aerating, rising above ground or above water (like those of mangroves) and having lenticels to allow for gas exchange. Many fungi attack these roots, killing or decaying them and thus reducing the plant’s water and mineral-absorbing capacity.
Leaf
The leaf is the plant’s primary photosynthetic organ, converting sunlight into food energy for the plant. It is typically thin and flat in shape to maximize sunlight penetration. Its surface is covered with a waxy cuticle to reduce water loss. A leaf’s outermost layer is called the epidermis, and it contains stomata (pores) for gas exchange. A pair of sausage-shaped cells, called guard cells, surround each stoma and regulate its opening and closing.
The inside of a leaf is lined with green vascular tissue, called mesophyll. Mesophyll cells are packed with chloroplasts that convert sunlight to sugar for the plant. They are connected to the phloem and xylem by a network of channels called veins.
Leaves are arranged differently in different plants. Some are simple leaves with a single leaflet attached to a long central rib, called a rachis. Other leaves are pinnately compound, with multiple leaflets arranged in pairs on either side of the rachis.
Flower
A flower is the reproductive structure of a plant. Flowers help plants make seeds that contain the genetic information for the next generation of a species. Flowers also attract pollinators to assist in the process of fertilization.
Flowers consist of an outer whorl called the calyx that contains green, leaf-like structures and a colorful inner whorl called the corolla. Flowers that have petals that are free or fused together are called polysepalous and those with petals that are separate but similar in size and shape are referred to as gamosepalous.
The male and female parts of the flower are called the androecium and the gynoecium respectively. The androecium produces stamens that produce pollen and the gynoecium contains the pistils that produce ovules. Flowers are heterosporous, producing two types of spores—microspores and megaspores.
