Seeds are one of the wonders of evolution, a package of food surrounding the embryo and enclosed in a thick, protective seed coat. It is one of the characteristics of flowering plants that has allowed them to move into almost every environment on this planet. After sprouting, every stage of the new plant's life is focused on survival, on growing large enough to flower and set seed itself.
Each seed has one or two embryonic seed leaves, called cotyledons, depending on whether it is a monocot or a dicot, single or double cotyledon plant. It also contains the first root, or radicle, the growing point for a shoot just above the cotyledons and the food storage area, the endosperm. All of these structures are contained within the seed coat. On the outside of many seeds, you can see a point of attachment to the seed pod. There may also be a lens or weak point in the seed coat that allows it to split open during germination.
A seed gets ready to germinate by absorbing water. When moist enough, an enzyme is activated that converts stored food into a form the embryo can use. Cells within the embryo start multiplying and respiration increases. The growing embryo soon becomes so large that it splits the seed coat.
The first thing to emerge is the single root, the radicle. Once the new plant is firmly fixed in the soil, the base of the cotyledons starts to grow, forming a hook, lifting them up to the surface. Once they reach the light, it straightens and the growing point of the seedling now begins to form the first true leaves. The cotyledons remain for a few days, then wither and drop off.
The radicle may continue to grow into a taproot, stretching downward as much as 5 or 6 feet in the first year in alfalfa plants, or it may branch out into a network of roots that stretch horizontally. Each root develops tiny root hairs that have thin walls and are able to absorb water and nutrients.
The new shoot may also grow straight upwards or form a network of branches. Each shoot or branch has a terminal bud and a number of axillary buds that may be dormant until the terminal bud is removed. Leaves sprout from nodes along the shoot, each one having an axillary bud at its base.
A Strong Framework
As the plant grows, it may develop woody tissue that support the branches and allow the framework of the plant to grow much taller than simple stem tissue would allow. A thin green layer of growing tissue called the cambium develops beneath the bark. This layer gives rise to both the bark and the vascular tissue, the hollow tubes inside the branch or trunk that transport water and nutrients through the plant. These are of two types. The xylem conducts water and dissolved minerals and supports the plant with its stiff fibers. The phloem conducts organic compounds made in the leaves by photosynthesis.