Where Are Seeds Developed In Figure 8.1

Structure

The concave side of an ordinary kidney bean (a dicot) has a small white scar called the hilum. The hilum marks the point at which the ovule was attached to the ovary wall. A tiny pore called the micropyle is located right next to the hilum. If this bean is placed in water for an hour or two, it may swell enough to split the seed coat. Once the seed coat is removed, the two halves, called cotyledons, can be distinguished (Fig. 8.28). The cotyledons, which have a tiny immature plantlet along one edge between them, are food-storage organs that also function as the first "seed leaves" of the seedling plant. The cotyledons and the tiny, rudimentary bean plant to which they are attached constitute the embryo. Some seeds (e.g., those of grasses and all other monocots) have only one cotyledon.

The tiny embryo plantlet has undeveloped leaves and a meristem at the upper end of the embryo axis. This embryo shoot is called a plumule. The cotyledons are attached just

Coleorhiza
Figure 8.28 A common garden bean. A. Seed structure. B. Germination and development of the seedling.

Chapter 8

Rhizophora Mangle Hypocotyl

pericarp endosperm cotyledon (scutellum) coleoptile plumule pericarp endosperm cotyledon (scutellum) coleoptile plumule

Coleoptile And Cotyledon

prop roots (adventitious roots)

'primary root

Figure 8.2Q Corn. A. Grain structure. B. Germination and development of the seedling.

prop roots (adventitious roots)

'primary root

Figure 8.2Q Corn. A. Grain structure. B. Germination and development of the seedling.

below the plumule. The very short part of the stem above the cotyledons is called the epicotyl, while the stem below the attachment point is the hypocotyl. In an embryo, it is often difficult to tell where the stem ends and the root begins, but the tip that will develop into a root is called a radicle. When a kidney bean germinates, the hypocotyl lengthens and bends, becoming hook-shaped. The top of the hook emerges from the ground, pulling the cotyledons above the ground. Once the cotyledons have emerged, the hook straightens out. In lima beans and peas, however, the hypocotyl remains short so that the cotyledons do not emerge above the surface (see Chapter 11).

In other seeds, the cotyledon(s) may not play a significant role in food storage. In corn, for example, the bulk of the food-storage tissue is endosperm (see Chapter 23). Corn "seeds" (Fig. 8.29) also display other features not seen in beans. The plumule and the radicle are enclosed in tubular, sheathing structures called the coleoptile and the coleorhiza, respectively. These protect the delicate tissues within as the seeds germinate. After the coleoptile and coleorhiza have become several millimeters long, their development ceases, and the plumule and radicle burst through the tips.

Germination

Germination, which is the beginning or resumption of growth of a seed, depends on the interplay of a number of factors, both internal and external. In order to germinate, a seed must first be viable (capable of germinating). Many seeds for various reasons (e.g., death of the embryo within) are not viable, and all lose their viability after varying periods of time. Many seeds also require a period of dormancy (see page 217) before they will germinate. Dormancy is brought about by either mechanical or physiological circumstances or both. In the Legume Family (Fabaceae) and others, the seeds may have seed coats so thick or tough that they prevent the absorption of water or oxygen. Some seeds even have a one-way valve that lets moisture out but prevents its uptake. Dormancy in such seeds may sometimes be broken artificially by scarification, which involves nicking or slightly cracking the seed coats or dipping the seeds in a concentrated acid for a few seconds to a few minutes. In nature, such seeds may remain dormant until cracks in the seed coat are brought about by the mechanical abrasion of rock particles in the soil, alternate thawing and freezing, or in some cases, bacterial action.

Dormancy may also be brought about by growth-inhibiting substances present in the seed coat, the interior of the seed, or tissues of the fruit surrounding it. Many desert plants have inhibitors in the seed coat. These have to be washed away by soaking rains before germination will occur. The inhibitors function in survival of the species by preventing germination unless there has been sufficient rainfall for a seedling to become established.

Apples, pears, citrus fruits, tomatoes, and other fleshy fruits contain inhibitors that prevent germination of the seeds within the fruits. Once the seeds are removed and washed,

Seed Coat Caruncle
Figure 8.30 Castor bean (Ricinus) seeds. Note the small water-absorbing appendage (caruncle) at the end of each seed.

they germinate readily. The embryos of some seeds, such as those of the American holly, consist of only a few unspecial-ized cells when the fruit ripens. The seeds will not germinate after the fruit has dropped until the embryo has developed fully with the aid of food materials stored in its endosperm. Such a process of development is called after-ripening.

In many woody plants of temperate areas, germination stimulators need to be present to initiate growth. These normally do not develop unless the seeds encounter a wet period accompanied by cold temperatures. Usually this period needs to be a minimum of 4 to 6 weeks. The dormancy of such seeds can be broken artificially by placing them in a refrigerator, preferably in damp sand, for a few weeks.

Even when mechanical and physiological barriers to germination are not present, a seed will not normally germinate unless environmental factors are favorable. Water and oxygen are essential to the completion of germination, and light or its absence also plays a role. Many seeds imbibe 10 times or more their total weight in water before the radicle emerges. Some seeds, such as those of castor beans (Fig. 8.30) and certain spurges, have appendages that function in water absorption and thereby speed up the germination process.

After water has been imbibed, enzymes begin to function in the cytoplasm, which has now been rehydrated. Some enzymes convert stored proteins to amino acids, others convert fats and oils to soluble compounds, and still other enzymes aid in the conversion of starch to sugar. The soluble substances can then be conveyed to the embryo, and respiration, which in a dormant seed is almost imperceptible, can be greatly accelerated. In a few seeds such as rice and barnyard grass, anaerobic respiration initially furnishes the energy for embryo growth, but in most seeds, the energy is released through aerobic respiration. A new plant begins to develop as mitosis and cell elongation take place. Both forms of respiration are discussed in Chapter 10.

If seeds are kept waterlogged after planting, oxygen available to them is greatly reduced and germination then may fail to be completed. Most seeds require temperatures within certain ranges to germinate. These usually need to be above freezing but below 45°C (113°F). Germination percentages tend to be low approaching either extreme, however. Most crop plants have an optimum (ideal) germination temperature of between 20°C and 30°C (68°F to 86°F).

The role of light in germination varies with the kinds of plants concerned. Seeds of some varieties of lettuce will not germinate in the dark (see the discussion of phytochrome in Chapter 11), while those of other seeds, such as the California poppy, germinate only in the dark. In lettuce seeds, the light apparently inactivates germination inhibitors, while in the California poppy, it stimulates inhibitor formation. (See the additional discussion of dormancy in Chapter 11.)

Longevity

From time to time, one reads or hears of seeds of wheat or other edible plants germinating after lying dormant in Egyptian pyramids or Native American tombs and caves for thousands of years, but none of these reports has been confirmed. In fact, there is evidence in a few instances that rats or rodents in recent times carried the seeds concerned to their nests. However, reports of seeds of the aquatic lotus plant germinating after a little more than 1,000 years and another documenting the germination of Arctic tundra lupine seeds that were frozen for an estimated 10,000 years have been confirmed.

Seeds remain viable (retain the capacity to germinate) for periods that vary greatly, depending on the species and the conditions of storage. Some seeds, such as those of certain willows, cottonwoods, orchids, and tea, remain viable for only a few days or weeks, regardless of how they are stored, but the period of viability of most seeds is extended by months or even years when they are stored at low temperatures and kept dry.

By law, packets of vegetable and flower seeds sold in stores are dated, giving the buyer a rough idea of how long a significant number of the seeds might be expected to remain viable. Generally, seeds of Pumpkin Family (Cucurbitaceae) members (e.g., squash, cantaloupe, cucumber) retain a relatively high percentage of viability for several years, while those of members of the Lily Family (Liliaceae) (e.g., onion, leek, chives) retain a good percentage of viability for only 2 or 3 years. Properly stored wheat seeds have been reported to retain better than 30% viability for more than 30 years, and some weed seeds stored under conditions of low oxygen, high humidity, and cool temperatures have remained viable for even longer periods.

In 1879, William J. Beal, a botanist who pioneered in the development of hybrid corn, buried 20 pint-sized bottles of weed seeds on the campus of what is now Michigan State University in East Lansing, Michigan. Each bottle contained 1,000 seeds of 20 different species of weeds.

9 St

9 St

Healthy Seedlings Steps

Box Figure 8.1 A lotus pod with 20 fruits.

Rip Van Winkle would appreciate this. An Oriental Sacred Lotus (Nelumbo nucifera) seed collected from the sediment of a dry lake bottom near a small village in northeastern China has germinated after being dormant for over 1,200 years. It is one of the oldest living seeds ever found. The Beijing Institute of Botany donated Sacred Lotus seeds to a team of UCLA scientists who dated the seeds with a non-destructive method called accelerator mass spectroscopy. Small amounts of tissue (less than 10 milligrams) were sampled for radiocarbon dating prior to germination studies. Before the use of this newer dating method, whole seeds had to be destroyed in the process of dating, thereby eliminating the possibility of testing the seeds for viability. After lying dormant in a bed of black clay at depths of 0.5 to 2.8 meters (1.5 to 9 feet), the germinated 1,200-year-old seed (1,288 ± 271 years) was the oldest, but not the only survivor from the subterranean tomb. Three other ancient lotus seeds found at various depths germinated, and the UCLA team determined that one was more than 600 years old and another was more than 300.

While reports have claimed seed germination of more ancient seeds recovered from dry archaeological sites in Egypt (such as King Tut's tomb in the pyramids of Giza and from the tombs of other Pharaohs), experts now agree that these reports are unreliable. Apart from these lotus seeds, the oldest documented viable seeds are lupine seeds that were frozen in Arctic tundra and from Professor Beal's seed germination study (see text section entitled "Longevity" for discussion).

These Sacred Lotus seeds have managed to ward off the ravages of time. Existing in an impenetrable seed coat and mired in an oxygen-deficient mud, the seeds have intact genetic and enzymatic systems that reactivated when split open and soaked in water. (Enzymes are proteins that speed up chemical reactions in the cell.) A key enzyme that repairs proteins was present during germination; this enzyme has been found in similar quantities in modern-day Sacred Lotus seeds. The repair enzyme functions in converting damaged amino acids back to their naturally occurring functional form. This is especially important in "repairing" the proteins of the cell membrane. Without intact membranes, cells are not able to function, and such damage will lead to the death of the cell and eventually the organism.

The architecture of the fruit no doubt plays a key role in the longevity of these seeds. The fruits of the Sacred Lotus are round to oblong, 10 to 13 centimeters (4 to 5 inches) long, 8 to 10 centimeters (3 to 4 inches) in diameter, and each contains a single seed. The fruit wall, or pericarp, which is impervious to water and is also airtight, is initially green and turns purplish brown and becomes dry and notably hard. Chinese botanists who first investigated similar ancient lotus

Box Figure 8.1 A lotus pod with 20 fruits.

seeds collected from the same deposits were unable to get them to germinate, even after 20 months of soaking in water. It wasn't until they scarified (filed open to permit water absorption) the seeds, as prescribed in a 1,400-year-old Chinese manuscript, Ch'i Min Yao Su (Important Technology for People in Harmony), that they were ultimately successful in germinating the seeds. The hard, airtight fruit walls are the most significant of the structural features that contribute to the exceptional longevity of the seeds.

The Sacred Lotus was introduced to China following the introduction of Buddhism from India in the 1st century B.C. It is regarded as a symbol of purity and strength, emerging from the mire of lake waters and opening its crimson flowers to the heavens. The earliest dated depiction of Buddha (in a.d. 240) shows him surrounded with a halo and seated cross-legged on a lotus throne. From old Chinese manuscripts, we learn that lotus has been cultivated as a crop plant (most parts are edible) for the past 4,000 years and traditional Chinese herbal medicine considered the Sacred Lotus a mainstay of their pharmaceutical collections. The large number of lotus seeds collected from the site suggests that the plant was under cultivation in this now dried-up lotus lake. It is likely that the 1,200-year-old seed was derived from a plant cultivated by Buddhists at this site in northeastern China.

Unlike Rip Van Winkle, who, following a drink of liquor, slept for only 20 years, these Sacred Lotus seeds are remarkable in their capacity to revive after more than 1,000 years. Lessons learned from these plants can shape our thinking about our own aging and the possibilities that may exist in the future for extending life at the margins.

D.C. Scheirer

Flowers, Fruits, and See ds 151

Red Mangrove Seed And Flower

Figure 8.31 Young seedlings of red mangrove (Rhizophora mangle) whose seeds have no dormant period and germinate while the fruit is still on the tree. The seedlings grow to lengths of up to 25 centimeters (10 inches) before falling and becoming planted in the mud below. Ocean currents and tides also distribute mangrove seeds throughout tropical tidal zones. The dispersal has been so effective that there are 60,000 square kilometers (23,000 square miles) of mangroves in Southeast Asia alone. Mangrove wood is harvested for fuel, and in the past 40 years, the groves in some localities have been reduced in area by more than 50%. Incidentally, the fruit is sweet and edible.

Figure 8.31 Young seedlings of red mangrove (Rhizophora mangle) whose seeds have no dormant period and germinate while the fruit is still on the tree. The seedlings grow to lengths of up to 25 centimeters (10 inches) before falling and becoming planted in the mud below. Ocean currents and tides also distribute mangrove seeds throughout tropical tidal zones. The dispersal has been so effective that there are 60,000 square kilometers (23,000 square miles) of mangroves in Southeast Asia alone. Mangrove wood is harvested for fuel, and in the past 40 years, the groves in some localities have been reduced in area by more than 50%. Incidentally, the fruit is sweet and edible.

Every 5 years, a bottle of seeds was dug up and the seeds were planted, until the schedule was changed in 1920 to every 10 years. When the first bottle was dug up in 1884, seeds of most of the weeds germinated; in 1960, seeds of evening primrose, curly dock, and moth mullein still germinated; and in 1980, 29 moth mullein seeds, 1 mullein seed, and 1 mallow seed germinated—101 years after they were placed in the bottles. It is of interest to note that a mallow seed previously had not germinated since 1899. Only six of the original bottles now remain; they are not scheduled to be unearthed until the year 2040. Recent evidence indicates that the timing of the digging up of the seed bottles did not take into account the fact that certain temperature patterns are critical to the germination of many weed seeds and that if Beal's experiment had been conducted in a different way, the germination results would have been quite different.

A few species of both dicots and monocots produce seeds that have no period of dormancy at all. In some instances, the embryo, which develops from the zygote, continues to grow without pause in a phenomenon known as vivipary. In the red mangrove, a tropical tree associated with coastal waters and estuaries, each fruit contains a single seed in which the embryo continues to grow while the fruit is still on the tree, reaching a length of 25 centimeters (10 inches) or more before the seedling becomes detached and essentially plants itself in the mud below (Fig. 8.31).

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Responses

  • Scott
    Where are seeds developed in figure 8.1?
    7 years ago
  • nebay nasih
    What are coleoptile and coleorhiza in germinating seed corn?
    7 years ago
  • SHELDON ALLEN
    What is the plumule and radicle of a kidney bean?
    7 years ago
  • armi
    Where is the micropyle on a garden bean?
    7 years ago
  • aaron
    Where is seed develope?
    7 years ago
  • Will Puddifoot
    What is epicotyl corn?
    7 years ago
  • bisrat efrem
    Which part of the seed develops into the seedling?
    7 years ago
  • Adalrico
    What is the food storage tissue in corn seed called?
    7 years ago
  • Maik Kunze
    What is cotyledon in lima bean?
    7 years ago
  • vito
    What is the Rhizophora epicotyl?
    7 years ago
  • Ottavia
    Where is the plumule of a bean embryo?
    7 years ago
  • Alexander
    What is a lima bean seedling?
    7 years ago
  • cathy
    What will plumule grow into seedling?
    7 years ago
  • dominik freitag
    Where are seeds developed in fiure 8.1?
    7 years ago
  • belinda
    When does the lima bean drop its cotyledon?
    6 years ago
  • tesmi
    What is the function of cotyledon in a maize seed?
    6 years ago
  • Giraldo
    Where are seads developed in figure8.1?
    5 years ago
  • primo
    How is vivipary depicted in some species of selaginella?
    4 years ago
  • Nina
    WHICH PARTS FORMS THE COTYLEDONS IN AMAIZE PLANT?
    4 years ago
  • bilcuzal
    Which is scutellum, coleorhiza?
    3 years ago
  • mehret
    Why is bean seed hooked shaped?
    3 years ago
  • margaret
    Which of the bean seed will become the developing?
    2 years ago
  • bertoldo
    How is the delicate tip of a maize plumule protected from being damaged?
    2 years ago
  • petra
    What is the bulk food storage tissue in corn?
    2 years ago
  • cristiana derose
    What is seed germination and its figure?
    2 years ago
  • torsten hoffmann
    How many cotyledons does a bora seed have?
    2 years ago
  • merimas
    What is present in germination of bean?
    2 years ago
  • Laura
    What does the scar of maize seed cointains in human beings?
    1 year ago
  • primula gawkroger
    Does the plumule start developing after radicle attaches itself to the ground?
    1 year ago
  • carl
    What is the function of coleorhiza and coleoptile during germination of cereal seed?
    1 year ago
  • berilac
    What is Coleorzia pic?
    1 year ago
  • Krystian Gibson
    What is the function of pericarp in maize seed?
    1 year ago
  • zewdi
    How is the made availlable that is stored in cotyledon for the process of germination?
    3 months ago

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