Seeds are the primary means of reproduction for flowering plants. They contain a miniature undeveloped plant and food reserves enclosed in protective coats.
Scientists are beginning to understand the genetic controls that determine seed development by studying a model plant, Arabidopsis thaliana. They hope that knowledge of this process will help them understand how seeds evolved from simpler ovules and seeds in gymnosperms to the diversity found in angiosperms.
Definition
A seed is a fertilized ovule that will grow into a new plant given proper growth conditions. It contains an embryo and food reserves enclosed in a protective outer covering or coat. The embryo is surrounded by the seed coat and in some seeds, a tissue called endosperm which serves as the source of nutrients for the embryo during germination.
Seeds are the primary source of many foods, including wheat, rice, soybeans and peanuts. They are also used in medicine, dyes and fuel. Seeds are classified as monocotyledonous and dicotyledonous based on the number of seed leaves, known as cotyledons. Monocots have one cotyledon while dicots have two.
In computer science, a seed is a value that determines a portion of the initial internal state of a DRBG mechanism. The seed must have enough entropy to support the security strength of the DRBG. The term is also used in sports to refer to the ranking and placement of teams or players in a tournament.
Functions
Seeds have several important functions, including storing the embryo of a new plant in a protective outer covering that can sprout under favorable conditions. They also store nutrients for germination.
The inner seed coat is sometimes hard, as in a coconut shell, but it can be soft like a pea or spongy like cotton. This outer layer is known as the testa or tegmen.
A key function of a seed is its ability to survive periods of unfavorable environmental conditions, such as drought or frost. Seeds have a natural dormancy response to ensure this, called physiological dormancy.
This dormancy is triggered by the need for water, oxygen and temperature to be in the right balance for germination. During the process of water uptake, or imbibition, metabolic processes that were suspended or greatly reduced during dormancy resume. This is caused by the action of protein kinases and phosphatases, which are part of a complex system of cell cycle regulation.
Origin
Seeds are one of the most significant innovations in plant evolution. They appear in the fossil record as early as the Devonian. They are the distinguishing feature of a clade of plants called gymnosperms, which are divided into fernlike plants such as cycads and palms and conifers such as pines. A recurrent feature of the gymnosperms is the production of wood from secondary xylem. Another important feature is a system of reproduction called heterospory, whereby each ovary produces two sets of spores.
Seeds come in a wide range of shapes and sizes. Some seeds are large, like those of the coco de mer tree, weighing up to 40 pounds and dispersing by floating in water; others are tiny, such as epiphytic orchid seeds. Seeds also come in a wide variety of colors. Most seeds germinate after a period of dormancy, either physiological (senescence) or photodormancy. Physiological dormancy is broken by cool wet conditions, while photodormancy is broken by light or enzymatic damage caused by ultraviolet rays.
Variation
Seed characteristics, such as seed size and seed number, vary significantly among genotypes within a population. These within-plant variations may reflect diversifying bet-hedging strategies in response to environmental uncertainty (figure 3a). Indeed, several studies have found that plant species with a greater long-term mean seed crop also exhibit higher year-to-year variability in their total seed production.
However, seed traits may also respond to nongenetic factors, such as the condition of mother plants during the vegetative phase of growth. For example, differences in 1000-seed weight and germination percentage (GP) between and within the eight cowpea and 54 mungbean accessions evaluated here suggest that maternal effects play an important role in variation in these seed traits.
Similarly, changes in seed size variation have been correlated with variables other than dispersal syndromes, such as climate, forest structure and life history (figure 4). These relationships are consistent with the hypothesis that shifts in fleshy PTs are more likely to be responses to underlying shifts in resource availability than direct consequences of the evolution of dispersal syndrome.
