A seed is a plant part that can grow into a new plant. It contains three basic parts: the embryo, a supply of nutrients for the embryo, and the seed coat.
A seed also has a number of other features that allow it to spread and colonize new areas. Some seeds have fleshy appendages that entice animal dispersers to eat them; others have hooks, barbs, or sticky hairs that attach to animal fur or feathers.
The Ovule
The ovule is the reproductive organ of flowering plants that contains an egg cell that develops into a seed. It is found inside the ovary in angiosperms and attached to the inner surface of a female cone in gymnosperms.
Ovules consist of a megasporangium surrounded by one or two layers of tissue called integuments. The haploid female gametophyte or megagametophyte within the ovule produces spores that grow into embryo cells and eventually become a seed (Figure 1).
There are many different types of ovules, including orthotropous, anatropous, hemi-anatropous, campylotropous, amphitropous and circinotropous. In the orthotropous type, the chalaza, nucellus and micropyle are all aligned to the funicle.
In anatropous and hemi-anatropous ovules, the ovule body is straight while in the amphitropous and campylotropous ovules, the body is curved around. Finally, in the circinotropous ovule, the micropyle points upwards. This is the most common ovule orientation in flowers. It is also the only orientation that allows fertilization to occur, since sperm enter through the micropyle.
The Seed Coat
In higher plants, after fertilization, the ovule develops into a seed that is enclosed in a protective layer. This layer, called the seed coat, protects the embryo and endosperm.
During embryonic development, the two parts of the seed, the embryo and the endosperm, must communicate with each other. This coordination is essential for the success of a successful seed.
The development of the embryo involves one cotyledon in monocots and two cotyledons in dicots (see Chapter 2, Section 5). Both are positioned closely against the upper endosperm which provides food for the growing embryo.
The embryo and endosperm are surrounded by the seed coat which acts as a channel for transmitting environmental cues to the interior of the seed. This is an important function as an impenetrable seed coat would exclude the sensing of external environmental cues.
The Embryo
The embryo is a tiny, undeveloped part of an organism. It is distinct from the other parts of a body and has the genetic and epigenetic disposition to develop itself to the next mature stage along the path of human development.
The development of a human embryo starts at fertilization, when an egg is paired with sperm cells. The zygote divides several times to form a ball of cells that travels down the fallopian tubes into the uterus, where it eventually implants.
Once implanted, the embryo develops through a series of critical stages of growth and development: gastrulation, neurulation, and organogenesis.
Embryology is an important component of medical education. The content and presentation of this subject depends on the curricular objectives and goals of individual schools.
The Seedling
Seeds are the most common method of plant propagation and reproduction. They contain genetic material from the parents and are dispersed through a variety of mechanisms, such as wind, animals, insects, and water.
When a seed is germinated in the soil, it undergoes a series of developmental and growth events that lead to the development of a plant. These events include the germination of the seed coat, the initial opening of the cotyledons and the formation of photosynthesis.
Once the cotyledons open, they provide an opportunity for the first light-induced photosynthetic events. These events are regulated by complex molecular and biochemical processes, involving profound reprogramming of the genome.
During this early stage, the seedling uses the reserves stored within the seed to support its early growth. These reserves may be in the form of starch grains, protein granules, or lipid droplets.
