evolution of seed and structure??
Answers
The seed, unlike a spore, is a diploid embryo surrounded by storage tissue and protective layers. It is equipped to delay germination until growth conditions are optimal. Angiosperms bear both flowers and fruit. The structures protect the gametes and the embryo during its development.
The process of seed development begins within the flower, the plant's reproductive structure. The flower is a modified leaf structure and can be both male and female. The female part is the pistil, and the male part is the stamen. One flower may contain the pistil and stamen, as in beans, or they may occur in different flowers, as in corn.
A typical flower and its parts are illustrated in Figure 1. The pistil has a top portion (stigma), a middle portion (style), and a lower portion (ovary). The ovary may contain one or more ovules. The surface of the stigma produces a sweet, sticky solution as it becomes receptive to pollen fertilization.
Pollen is produced in the anthers at the ends of the stamen. Pollination occurs when pollen grains come into contact with the stigma. Wind and insects are largely responsible for the transfer of pollen from the anthers to the stigmas. Methods of transfer differ from one species to another.
If conditions are favorable, pollen grains begin to grow on the stigma surface and form pollen tubes. The pollen tube grows down the style and into the ovary, where it comes into contact with the ovule. Male gametes are transferred through the pollen tube into the ovule. Fertilization occurs when the male gametes unite with the female egg in the ovule. After pollination and fertilization, ovules develop into seeds.
In self-pollinated plants, pollen produced within each flower pollinates the stigma of the same flower. Cotton, barley, wheat, oats, tobacco, soybeans, okra, peanuts, and peppers are examples of self-pollinated crops. In cross-pollinated plants, the pollen grains pollinate flowers other than the one from which they originated. Examples include corn, rye, tall fescue, alfalfa, carrots, cucumbers, squash, and onions.
Fertilization occurs shortly after pollination and begins the process of seed development. Early stages in the development of strong plants depend on favorable growing conditions. Seed quality, on the other hand, is largely dependent upon environmental conditions and the promptness with which the seed is harvested after it matures.
Seed Structures
Knowledge of seed structure can help in understanding how seeds respond during harvesting, conditioning, germination, and seedling emergence.
Seed can be divided into two major classifications, monocots (monocotyledons) and dicots (dicotyledons), based on the number of cotyledons (seed leaves) in a seed. Monocots contain one cotyledon, whereas dicots have two.
Examples of plant species having monocot seeds are grasses–such as small grains, corn, or turfgrasses–and other crops such as onions. Plants with dicot seeds include legumes–such as peas, peanuts, soybeans, and clover–and other crops such as cotton and tobacco.
Seeds are composed of three basic structures: (1) the seed covering (seed coat or testa); (2) the embryonic axis (embryonic root or radicle and shoot or plumule); and (3) supporting tissues (the cotyledons and endosperm). The structures for monocots and dicots are illustrated in Figure 2.
If unbroken, the seed coat regulates water uptake by mature seeds. Variations in seed covering characteristics, especially in dicots, often affect the quality of seed when exposed to adverse weather. Some seeds, such as peanuts, have an extremely soft and delicate seed covering. This covering can easily break or slip and expose the embryo, making it susceptible to injury, deterioration, and pathogen attack.
Other seeds have extremely hard coverings that protect the seed from almost everything. Common weeds such as puncturevine, dock, knotweed, and pigweed are examples of these tough seeds. It is no wonder that most common weeds survive so long scattered across the land or buried in the soil. Lotus seeds have survived for many hundreds of years because of their tough, hard covering.
The embryonic axis normally includes the miniature plant, consisting of the root and shoot. Cotyledons and endosperm are usually considered supporting tissues. They are useful to the developing plant as a reserve food8 source through the course of germination and emergence until the plant can make its own food through photosynthesis. Supporting tissues in monocots are composed mostly of nonliving, starchy materials, whereas such tissues in dicots are composed of mostly fats and oils.
The location of seed structures plays an important role in determining the seeds’ susceptibility to mechanical injuries and weather damage. The embryonic axis is often just below the seed covering. Impacts to the embryonic axis can cause severe damage, resulting in abnormal seedlings or death of the seed. Peanut and soybean seeds can be damaged easily.