Advantages & Disadvantages of Diabetic Diets

Almost 26 million people in the United States have diabetes, according to the National Diabetes Education Program. Diabetes is a chronic illness characterized by high blood sugars. If left uncontrolled, diabetes can lead to other diseases such as heart or kidney disease, as well as an early death. Diet plays an important role in the treatment of diabetes. While there are a number of advantages to following a diabetic diet, there may be a few disadvantages for some people. Consult your doctor or dietitian to help you with your diabetic meal plan.

A diabetic diet can help you maintain a healthy weight. Photo Credit Photodisc/Photodisc/Getty Images

Advantage: Help With Blood Sugar Control

The primary goal in diabetes management is getting your blood sugar as close to normal as possible. Your doctor can help you determine your blood sugar goals, but in general, those numbers range from 90 to 130 milligrams per deciliter before meals and less than 180 milligrams per deciliter two hours after a meal. The carbohydrates you eat affect your blood sugar. A diabetic diet helps you control the amount of carbohydrates you eat each day and at each meal for better blood sugar management. Good blood sugar control may reduce your risk of diabetes-related complications.

Advantage: Good for Weight Management

The diabetic diet is a healthy diet in general. The diet encourages you to eat a variety of foods from all the food groups, emphasizing fruits and vegetables, whole grains, lean sources of protein and low-fat dairy. The diet also encourages portion control and eating meals regularly. These healthy diet principles are the same recommendations given to someone who wants to lose weight. If you're overweight or obese and have diabetes, losing as little as 10 pounds can help improve blood sugar.

Disadvantage: Too Rigid

The diabetic diet recommends you eat the same amount of food around the same time every day. Being consistent with the amount and timing of your meals aids in blood sugar control. Some people may have a hard time sticking to a rigid meal schedule. For example, if you are an emergency room nurse, you may have a difficult time eating meals at specified times. Additionally, not being able to eat the right amount of food at specified times can affect how your medication works, causing high or low blood sugars.

Disadvantage: Too Complicated

While there is no one diabetic diet, there are two meal planning tools -- the exchange list and carbohydrate counting -- used to help people with diabetes eat better. The exchange list divides foods into groups based on similarities in nutritional content, and you are allowed a certain number of servings from each food group based on calorie needs and food preferences. Carbohydrate counting requires you count the number of carbs you eat at each meal and snack, sticking to a specific amount determined by your doctor or dietitian. Both plans require careful counting and measuring of what you eat. Some people may find either of these meal planning tools too complicated to follow.

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The Best Morning Protein

If you often grab a plain bagel on your way out the door only to find yourself starving a few hours later, you know that not all breakfasts have the staying power you need. A study published in a 2009 issue of the "British Journal of Nutrition" found that participants who ate a high-protein breakfast, compared to one high in carbohydrates, reported greater feelings of fullness and better regulated their food intake later in the day. A protein breakfast is also beneficial after a morning workout, as the amino acids assist with muscle repair and recovery. Go for the most nutritious breakfast protein options to experience the greatest benefit from adding this nutrient to your morning routine.

A poached egg with asparagus and a slice of whole grain toast. Photo Credit Anna Pustynnikova/iStock/Getty Images

Egg It On

An egg offers B vitamins, iron, choline and antioxidants such as lutein and zeaxanthin. The cholesterol in egg yolks has very little effect on your blood cholesterol, and moderate consumption has no impact on your risk of developing heart disease, unless you have high cholesterol or diabetes, reports the Harvard School of Public Health. An egg breakfast also helps with weight loss. A study published in the "International Journal of Obesity" in 2008 found that an egg breakfast enhanced weight-loss efforts in participants who were cutting calories to lose weight more than a bagel breakfast of equal calories. One large poached egg contains 6 grams of high-quality protein.

Delight in Dairy

Low-fat dairy is a quality source of complete protein that's easy to grab on the go. It also provides you with a dose of calcium, potassium and vitamin D. Plain Greek yogurt offers 17 grams per 6-ounce serving; 2 percent cottage cheese has 12 grams of protein per half 1/2cup; and part-skim ricotta cheese provides 14 grams of protein per 1/2 cup. Eat these with fruit or toast, use them to top whole-grain waffles or to stir into hot cereal. Whey protein, a derivative of milk, offers 15 to 25 grams of complete and highly digestible protein per scoop -- depending on the brand -- and mixes easily into a fresh fruit smoothie.

Get Nutty

A 1-ounce portion of nuts sprinkled over cereal or eaten out of hand adds 4 to 6 grams of protein to breakfast, depending on the type you choose. Almonds, cashews and pistachios are among the higher protein choices. Nut butter trumps other bagel and toast toppers, such as butter, jelly and cream cheese, when it comes to protein content -- offering 7 grams per 2 tablespoons of peanut butter or almond butter. Nuts contain heart-healthy fats and B vitamins as well. A breakfast that includes peanut butter stimulates the body to release an appetite-suppressing hormone, with the effects lasting eight to 12 hours, a study published in a 2013 edition of the "British Journal of Nutrition" found. Eating peanut butter at breakfast also kept blood sugar levels moderated, even after some participants indulged in a high-carbohydrate lunch.

Go Lean With Meat

Regularly eating processed meats any time of the day, including at breakfast, is associated with an increased risk of heart disease and cancer, reported a major study of almost 1/2 million people published in "BMC Med" in 2013. This includes typical breakfast proteins such as ham, sausage and bacon. You can still get a morning meat fix, though, by making "sausage" patties with fresh, ground turkey or chicken, minced onion, fennel seeds, oregano, salt and black pepper. A 4-ounce patty will provide about 22 grams of protein. Other unprocessed meats also offer a dose of protein at breakfast time. Try baked flounder -- with 13 grams of protein per 3 ounces -- as a complement to eggs; wrap 3 ounces of flank steak and salsa in flour tortillas for breakfast tacos with about 24 grams of protein; or make a hash with 3 ounces cooked salmon, potatoes and onions to add about 22 grams of protein to your meal. These high-protein options are lower in fat -- with the exception of salmon which is rich in heart-healthy fats -- and chemical preservatives than processed meats.

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SEED DISPERSAL

Seed dispersal is the movement or transport of seeds away from the parent plant. Plants have very limited mobility and consequently rely upon a variety of dispersal vectors to transport their propagules, including both abiotic and biotic, vectors. Seeds can be dispersed away from the parent plant individually or collectively, as well as dispersed in both space and time. The patterns of seed dispersal are determined in large part by the dispersal mechanism and this has important implications for the demographic and genetic structure of plant populations, as well as migration, patterns and species interactions. There are five main modes of seed dispersal: gravity, wind, ballistic, water, and by animals. Some plants are serotinous and only disperse their seeds in response to an environmental stimulus.

Epilobium hirsutum seed head dispersing seeds

Benefits

Seed dispersal is likely to have several benefits for plant species. First, seed survival is often higher away from the parent plant. This higher survival may result from the actions of density-dependent seed and seedling predators and pathogens, which often target the high concentrations of seeds beneath adults. Competition with adult plants may also be lower when seeds are transported away from their parent.

Seed dispersal also allows plants to reach specific habitats that are favorable for survival, a hypothesis known as directed dispersal. For example, Ocotea endresiana (Lauraceae) is a tree species from Latin America which is dispersed by several species of birds, including the three-wattled bellbird. Male bellbirds perch on dead trees in order to attract mates, and often defecate seeds beneath these perches where the seeds have a high chance of survival because of high light conditions and escape from fungal pathogens. In the case of fleshy-fruited plants, seed-dispersal in animal guts (endozoochory) often enhances the amount, the speed, and the asynchrony of germination, which can have important plant benefits.

Seeds dispersed by ants (myrmecochory) are not only dispersed short distances but are also buried underground by the ants. These seeds can thus avoid adverse environmental effects such as fire or drought, reach nutrient-rich microsites and survive longer than other seeds. These features are peculiar to myrmecochory, which may thus provide additional benefits not present in other dispersal modes.

Finally, at another scale, seed dispersal may allow plants to colonize vacant habitats and even new geographic regions.

Types

Seed dispersal is sometimes split into autochory (when dispersal is attained using the plant's own means) and allochory (when obtained through external means).

Autochory
Gravity

Barochory or the plant use of gravity for dispersal is a simple means of achieving seed dispersal. The effect of gravity, on heavier fruits causes them to fall from the plant when ripe. Fruits exhibiting this type of dispersal include apples, coconuts and passionfruit and those with harder shells (which often roll away from the plant to gain more distance). Gravity dispersal also allows for later transmission by water or animal.

Two other types of autochory are ballochory (the seed is forcefully ejected by dehiscence and squeezing) and herpochory (the seed crawls by means of trichomes and changes in humidity).

Allochory
Wind

Wind dispersal of dandelion seeds

Wind dispersal (anemochory) is one of the more primitive means of dispersal. Wind dispersal can take on one of two primary forms: seeds can float on the breeze or alternatively, they can flutter to the ground.  The classic examples of these dispersal mechanisms include dandelions, which have a feathery pappus attached to their seeds and can be dispersed long distances, and maples, which have winged seeds (samara) and flutter to the ground. An important constraint on wind dispersal is the need for abundant seed production to maximise the likelihood of a seed landing in a site suitable for germination. There are also strong evolutionary constraints on this dispersal mechanism. For instance, Cody and Overton (1996) found that species in the Asteraceae on islands tended to have reduced dispersal capabilities (i.e., larger seed mass and smaller pappus) relative to the same species on the mainland Also, species Helonias bullata, a perennial herb native to the United States, evolved to utilize wind dispersal as the primary seed dispersal mechanism; however, limited wind in its habitat prevents the seeds to successfully disperse away from its parents, resulting in clusters of population. Reliance on wind dispersal is common among many weedy or ruderal species. Unusual mechanisms of wind dispersal include tumbleweeds. Physalis fruits, when not fully ripe, may sometimes be dispersed by wind due to the space between the fruit and the covering calyx which acts as air bladder.

Entada phaseoloides – Hydrochory.

Water

Many aquatic (water ) and some terrestrial (ground) plant species use hydrochory, or seed dispersal through water. Seeds can travel for extremely long distances, depending on the specific mode of water dispersal.This is because some fruits are waterproof and can float.

The water lily is an example of such a plant. Water lilies' flowers make a fruit that floats in the water for a while and then drops down to the bottom to take root on the floor of the pond. The seeds of palm trees can also be dispersed by water. If they grow near oceans, the seeds can be transported by ocean currents over long distances, allowing the seeds to be dispersed as far as other continents.

Mangrove, trees live right in the water. Their seeds fall from the tree and grow roots as soon as they touch any kind of soil. During low tide, they might fall in soil instead of water and start growing right where they fell. If the water level is high, however, they can be carried far away from where they fell. Mangrove trees often make little islands as dirt and other things collect in their roots, making little bodies of land.

A special review for oceanic waters hydrochory can be seen at oceanic dispersal.

By Animals

The small hooks on the surface of a bur enable attachment to animal fur for dispersion.

Animals can disperse plant seeds in several ways, all named zoochory. Seeds can be transported on the outside of vertebrate animals (mostly mammals), a process known as epizoochory. Plant species transported externally by animals can have a variety of adaptations for dispersal, including adhesive mucus, and a variety of hooks, spines and barbs. A typical example of an epizoochorous plant is Trifolium angustifolium, a species of Old World clover which adheres to animal fur by means of stiff hairs covering the seed. Epizoochorous plants tend to be herbaceous plants, with many representative species in the families Apiaceae and Asteraceae. However, epizoochory is a relatively rare dispersal syndrome for plants as a whole; the percentage of plant species with seeds adapted for transport on the outside of animals is estimated to be below 5%. Nevertheless, epizoochorous transport can be highly effective if seeds attach to wide-ranging animals. This form of seed dispersal has been implicated in rapid plant migration and the spread of invasive species.

Seed dispersal via ingestion by vertebrate animals (mostly birds and mammals), or endozoochory, is the dispersal mechanism for most tree species. Endozoochory is generally a coevolved mutualistic relationship in which a plant surrounds seeds with an edible, nutritious fruit as a good food for animals that consume it. Birds and mammals are the most important seed dispersers, but a wide variety of other animals, including turtles and fish, can transport viable seeds. The exact percentage of tree species dispersed by endozoochory varies between habitats, but can range to over 90% in some tropical rainforests. Seed dispersal by animals in tropical rainforests has received much attention, and this interaction is considered an important force shaping the ecology and evolution of vertebrate and tree populations. In the tropics, large animal seed dispersers (such as tapirs, chimpanzees and hornbills) may disperse large seeds with few other seed dispersal agents. The extinction of these large frugivores from poaching and habitat loss may have negative effects on the tree populations that depend on them for seed dispersal.

The "bill" and seed dispersal mechanism of Geranium pratense

Seed dispersal by ants (myrmecochory) is a dispersal mechanism of many shrubs of the southern hemisphere or understorey herbs of the northern hemisphere. Seeds of myrmecochorous plants have a lipid-rich attachment called the elaiosome, which attracts ants. Ants carry such seeds into their colonies, feed the elaiosome to their larvae and discard the otherwise intact seed in an underground chamber. Myrmecochory is thus a coevolved mutualistic relationship between plants and seed-disperser ants. Myrmecochory has independently evolved at least 100 times in flowering plants and is estimated to be present in at least 11 000 species, but likely up to 23 000 or 9% of all species of flowering plants Myrmecochorous plants are most frequent in the fynbos vegetation of the Cape Floristic Region of South Africa, the kwongan vegetation and other dry habitat types of Australia, dry forests and grasslands of the Mediterranean region and northern temperate forests of western Eurasia and eastern North America, where up to 30–40% of understorey herbs are myrmecochorous.

Seed predators, which include many rodents (such as squirrels) and some birds (such as jays) may also disperse seeds by hoarding the seeds in hidden caches. The seeds in caches are usually well-protected from other seed predators and if left uneaten will grow into new plants. In addition, rodents may also disperse seeds via seed spitting due to the presence of secondary metabolites in ripe fruits. Finally, seeds may be secondarily dispersed from seeds deposited by primary animal dispersers. For example, dung beetles are known to disperse seeds from clumps of feces in the process of collecting dung to feed their larvae.

Other types of zoochory are chiropterochory (by bats), malacochory (by molluscs, mainly terrestrial snails), ornithochory (by birds) and saurochory (by non-bird sauropsids).

By Human

Seed dispersal by a car

Dispersal by humans (anthropochory) used to be seen as a form of dispersal by animals. Recent research points out that human dispersers differ from animal dispersers by a much higher mobility based on the technical means of human transport. Dispersal by humans on the one hand may act on large geographical scales and lead to invasive species. On the other hand, dispersal by humans also acts on smaller, regional scales and drives the dynamics of existing biological populations. Humans may disperse seeds by many various means and some surprisingly high distances have been repeatedly measured. Examples are: dispersal on human clothes (up to 250 m), on shoes (up to 5 km) or by cars (regularly ~ 250m, singles cases > 100 km).

Deliberate seed dispersal also occurs as seed bombing. This has risks as unsuitable provenance may introduce genetically unsuitable plants to new environments.

Epizoochory in Bidens tripartita; the seeds have attached to the clothes of a human.

Consequences 

Seed dispersal has many consequences for the ecology and evolution of plants. Dispersal is necessary for species migrations, and in recent times dispersal ability is an important factor in whether or not a species transported to a new habitat by humans will become an invasive species. Dispersal is also predicted to play a major role in the origin and maintenance of species diversity. For example, myrmecochory increased the rate of diversification more than twofold in plant groups in which it has evolved because myrmecochorous lineages contain more than twice as many species as their non-myrmecochorous sister groups. Dispersal of seeds away from the parent organism has a central role in two major theories for how biodiversity is maintained in natural ecosystems, the Janzen-Connell hypothesis and recruitment limitation. Seed dispersal is essential in allowing forest migration of flowering plants.

In addition, the speed and direction of wind are highly influential in the dispersal process and in turn the deposition patterns of floating seeds in the stagnant water bodies. The transportation of seeds is led by the wind direction. This effects colonization situated on the banks of a river or to wetlands adjacent to streams relative to the distinct wind directions. The wind dispersal process can also effect connections between water bodies. Essentially, wind plays a larger role in the dispersal of waterborne seeds in a short period of time, days and seasons, but the ecological process allows the process to become balanced throughout a time period of several years. The time period of which the dispersal occurs is essential when considering the consequences of wind on the ecological process.

References

1. ^ ^a b Harms, K; Wright, SJ; Calderon, O; Hernandez, A; Herre, EA (2000). "Pervasive density-dependent recruitment enhances seedling diversity in a tropical forest". Nature 404(6777): 493–495. doi:10.1038/35006630, PMID 10761916.
2. ^ Wenny, D.G. and Levey, D.J. (1998). "Directed seed dispersal by bellbirds in a tropical cloud forest". Proceedings of the National Academy of Sciences of the United States of America 95 (11): 6204–7. doi:10.1073/pnas.95.11.6204. PMC 27627. PMID 9600942.
3. ^ Fedriani, J. M., Delibes, M. (2009). "Functional diversity in fruit-frugivore interactions: A field experiment with Mediterranean mammals". Ecography 32 (6): 983. doi:10.1111/j.1600-0587.2009.05925.x.
4. ^ ^a b c d Lengyel, S.; et al. (2010). "Convergent evolution of seed dispersal by ants, and phylogeny and biogeography in flowering plants: a global survey". Perspectives in Plant Ecology, Evolution and Systematics 12 (1): 43–55. doi:10.1016/j.ppees.2009.08.001.
5. ^ Manzaneda, Antonio J.; Fedriani, Jose M. and Rey, Pedro J. (2005). "Adaptive advantages of myrmecochory: the predator-avoidance hypothesis tested over a wide geographic range".  (PDF). Ecography 28 (5): 583–592. doi:10.1111/j.2005.0906-7590.04309.x.
6. ^ ^a b c Manzano, Pablo; Malo, Juan E. (2006). "Extreme long-distance seed dispersal via sheep". Frontiers in Ecology and the Environment 4 (5): 244–248. doi:10.1890/1540-9295(2006)004[0244:ELSDVS]2.0.CO;2. JSTOR 3868790.
7. ^ "Dispersal of seeds by gravity". Retrieved 2009-05-08.
8. ^ Schulze, Ernst-Detlef; Beck, Erwin and Müller-Hohenstein, Klaus (2005). Plant Ecology. Springer. pp. 543–. ISBN 978-3-540-20833-4.
9. ^ Gurevitch, J., Scheiner, S.M., & G.A. Fox (2006). Plant Ecology, 2nd ed. Sinauer Associates, Inc., Massachusetts.
10. ^ Cody, M.L. and Overton, J.M. (1996). "Short-term evolution of reduced dispersal in island plant populations". Journal of Ecology 84: 53–61. doi:10.2307/2261699.  JSTOR 2261699.

Further Reading

• Ridley, Henry N (1930). The Dispersal of Plants Throughout the World. Ashford, Kent: L. Reeve & Co. ISBN 0-85393-004-X.

External Links

• Fruit and seed dispersal images at bioimages.vanderbilt.edu
• Interactive model of movement of plant species induced by climate change
• Secondary metabolites promote seed dispersal.

- Wikipedia 

Blood Type O Positive Diet Foods

The premise of Dr. Peter J. D'Adamo's Blood Type Diet is that you have a unique genetic makeup predisposing you to certain chronic conditions and diseases. By eating and exercising in a manner suitable to your blood type, D'Adamo maintains that people can live healthier lives. D'Adamo's diet for type O's focuses on lean, organic meats, fruits and vegetables while eschewing dairy, wheat, alcohol and caffeine. Always consult your doctor before beginning a new diet.

Large pile of sliced okra. Photo Credit vesmil/iStock/Getty Images

All About Type O's

Blood type O ancestors were aggressive predators. D'Adamo claims that today's type O's are characterized by being energetic extroverts and leaders with a keen ability to focus. But blood type O's, D'Adamo reports, are also prone to anger, hyperactivity and impulsiveness when under stress. When this leads to poor diet, lack of exercise and other unhealthy behaviors, type O's may experience negative metabolic effects, such as insulin resistance, hypothyroid and weight gain. Type O's are also predisposed to certain illnesses, including ulcers and thyroid problems.

Pack in the Protein

Type O's do well on a high-protein diet of lean, organic meats. The best sources are beef, lamb, mutton, veal and venison as well as cold-water fish such as cod, herring and mackerel. But blood type O's can eat any meat, except for bacon, ham, pork and goose, and any fish or seafood, except for barracuda, pickled herring, catfish, smoked salmon, caviar, octopus and conch. Blood type O's digest and metabolize meat easily, and vegetarian diets are not advised. Seafood, which is a rich source of iodine, is beneficial for type O's because iodine helps stabilize thyroid function.

Choose Vegetables Carefully

According to D'Adamo's diet guidelines, type O's should not eat certain vegetables, including mustard greens, cabbage, cauliflower and brussels sprouts because they interfere with thyroid function. Blood type O's should also avoid alfalfa sprouts, shiitake mushrooms and fermented olives, which can irritate the digestive tract. In addition, their mold content can worsen type O hypersensitivity problems. Eggplant and potatoes should be avoided because they can bring on arthritis in type O's. Type O's should not eat corn because it affects insulin production and can lead to obesity and diabetes for this blood type. Beneficial vegetables for type O's include kale, collard greens, broccoli, romaine lettuce and spinach, since they provide vitamin K. Vitamin K assists in blood clotting, and type O's are missing certain clotting factors. Type O's can also eat artichokes, chicory, dandelion, garlic, horseradish, leeks, okra, onions, parsley, parsnips, red peppers, sweet potatoes, pumpkin, seaweed and turnips.

Fruits to Enjoy

The best fruits for type O's are plums, prunes and figs, which help balance the acidity of the digestive tract to prevent ulcers. Type O's should avoid melons, such as cantaloupe and honeydew, which D'Adamo says can contain mold. Type O's should also avoid oranges, tangerines, strawberries, blackberries and rhubarb because they can further irritate a type O's naturally acidic stomach. Type O's should not eat coconut due to extreme sensitivity to the fruit. All other fruits are considered neutral for type O's.

Other Foods for Type O's

Type O's should strictly limit dairy and eggs, but they can have butter, farmer's cheese, feta, mozzarella, goat cheese and soy milk. Olive and flaxseed oils are beneficial for type O's, and canola and sesame oils are allowed, but corn, cottonseed, peanut and safflower oils should be avoided. Type O's should avoid all wheat products. Essene breads, made with sprouted grain, are beneficial for type O's, and amaranth, barley, rice, kamut, kasha, millet, rye and spelt are allowed. Pumpkin seeds and walnuts are most beneficial for type O's, but any nuts and seeds are allowed except Brazil nuts, cashews, peanuts, pistachios and poppy seeds.

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OVULE

In seed plants, the ovule ("small egg"), is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: The integument(s) forming its outer layer(s), the nucellus (or remnant of the megasporangium) and female gametophyte (formed from haploid megaspore) in its center. The female gametophyte—specifically termed a megagametophyte—is also called the embryo sac in angiosperms. The megagametophyte produces an egg cell (or several egg cells in some groups) for the purpose of fertilization. After fertilization, the ovule develops into a seed.

Location of ovules inside a Helleborus foetidus flower.

Location within the plant

In flowering plants the ovule is located inside the portion of the flower called the gynoecium. The ovary of the gynoecium produces one or more ovules and ultimately becomes the fruit wall. Ovules are attached to the placenta in the ovary through a stalk-like structure known as a funiculus (plural, funiculi). Different patterns of ovule attachment, or placentation, can be found among plant species, these include:

• Apical placentation: The placenta is at the apex (top) of the ovary. Simple or compound ovary.
• Axile placentation: The ovary is divided into radial segments, with placentas in separate locules. Ventral sutures of carpels meet at the centre of the ovary. Placentae are along fused margins of carpels. Two or more carpels. (e.g. Hibiscus, Citrus, Solanum).
• Basal placentation: The placenta is at the base (bottom) of the ovary on a protrusion of the thalamus (receptacle). Simple or compound carpel, unilocular ovary. (e.g. Sonchus, Helianthus,  Compositae)
• Free-central placentation: Derived from axile as partitions are absorbed, leaving ovules at the central axis. Compound unilocular ovary. (e.g. Stellaria,  Dianthus).
• Marginal placentation: Simplest type. There is only one elongated placenta on one side of the ovary, as ovules are attached at the fusion line of the carpel's margins . This is conspicuous in legumes. Simple carpel, unilocular ovary. (e.g. Pisum).
• Parietal placentation: Placentae on inner ovary wall within a non-sectioned ovary, corresponding to fused carpel margins. Two or more carpels, unilocular ovary. (e.g. Brassica).
• Superficial: Similar to axile, but placentae are on inner surfaces of multilocular ovary (e.g. Nymphaea).

Ovule orientation may be anatropous, such that when inverted the micropyle faces the placenta (this is the most common ovule orientation in flowering plants), amphitropous, campylotropous, or orthotropous.

Ovule structure (anatropous) 1: nucleus 2: chalaza 3: funiculus 4: raphe

In gymnosperms such as conifers, ovules are borne on the surface of an ovuliferous (ovule-bearing) scale, usually within an ovulate cone also called megastrobilus). In some extinct plants (e.g. Pteridosperms), megasporangia and perhaps ovules were borne on the surface of leaves. In other extinct taxa, a cupule (a modified leaf or part of a leaf) surrounds the ovule (e.g. Caytonia or Glossopteris).

Ovule Parts and Dévelopment

Plant ovules: Gymnosperm ovule on left, angiosperm ovule (inside ovary) on right.

The ovule appears to be a megasporangium with integuments surrounding it. Ovules are initially composed of diploid maternal tissue, which includes a megasporocyte (a cell that will undergo meiosis to produce megaspores). Megaspores remain inside the ovule and divide by mitosis to produce the haploid female gametophyte or megagametophyte, which also remains inside the ovule. The remnants of the megasporangium tissue (the nucellus) surround the megagametophyte. Megagametophytes produce archegonia (lost in some groups such as flowering plants), which produce egg cells. After fertilization, the ovule contains a diploid zygote and then, after cell division begins, an embryo of the next sporophyte generation. In flowering plants, a second sperm nucleus fuses with other nuclei in the megagametophyte forming a typically polyploid (often triploid) endosperm tissue, which serves as nourishment for the young sporophyte.

Integuments Micropyle and Chalaza

Models of different ovules, Botanical Museum Greifswald.

An integument is a protective cell layer surrounding the ovule. Gymnosperms typically have one integument (unitegmic) while angiosperms typically have two (bitegmic). The evolutionary origin of the inner integument (which is integral to the formation of ovules from megasporangia) has been proposed to be by enclosure of a megasporangium by sterile branches (telomes). Elkinsia, a preovulate taxon, has a lobed structure fused to the lower third of the megasporangium, with the lobes extending upwards in a ring around the megasporangium. This might, through fusion between lobes and between the structure and the megasporangium, have produced an integument.

The origin of the second or outer integument has been an area of active contention for some time. The cupules of some extinct taxa have been suggested as the origin of the outer integument. A few angiosperms produce vascular tissue in the outer integument, the orientation of which suggests that the outer surface is morphologically abaxial. This suggests that cupules of the kind produced by the Caytoniales or Glossopteridales may have evolved into the outer integument of angiosperms.

The integuments develop into the seed coat when the ovule matures after fertilization.

The integuments do not enclose the nucellus completely but retain an opening at the apex referred to as the micropyle. The micropyle opening allows the pollen (a male gametophyte), to enter the ovule for fertilization. In gymnosperms (e.g., conifers), the pollen is drawn into the ovule on a drop of fluid that exudes out of the micropyle, the so-called pollination drop mechanism. Subsequently, the micropyle closes. In angiosperms, only a pollen tube enters the micropyle. During germination, the seedling's radicle emerges through the micropyle.

Located opposite from the micropyle is the chalaza where the nucellus is joined to the integuments. Nutrients from the plant travel through the phloem of the vascular system to the funiculus and outer integument and from there apoplastically and symplastically through the chalaza to the nucellus inside the ovule. In chalazogamous plants, the pollen tubes enter the ovule through the chalaza instead of the micropyle opening.

Nucellus , Megaspore and Perisperm

The nucellus (plural: nucelli) is part of the inner structure of the ovule, forming a layer of diploid (sporophytic) cells immediately inside the integuments. It is structurally and functionally equivalent to the megasporangium. In immature ovules, the nucellus contains a megasporocyte (megaspore mother cell), which undergoes sporogenesis via meiosis. In gymnosperms, three of the four haploid spores produced in meiosis typically degenerate, leaving one surviving megaspore inside the nucellus. Among angiosperms, however, a wide range of variation exists in what happens next. The number (and position) of surviving megaspores, the total number of cell divisions, whether nuclear fusions occur, and the final number, position and ploidy of the cells or nuclei all vary. A common pattern of embryo sac development (the Polygonum type maturation pattern) includes a single functional megaspore followed by three rounds of mitosis. In some cases, however, two megaspores survive (for example, in Allium and Endymion). In some cases all four megaspores survive, for example in the Fritillaria type of development (illustrated by Lilium in the figure) there is no separation of the megaspores following meiosis, then the nuclei fuse to form a triploid nucleus and a haploid nucleus. The subsequent arrangement of cells is similar to the Polygonum pattern, but the ploidy of the nuclei is different.

After fertilization, the nucellus may develop into the perisperm that feeds the embryo. In some plants, the diploid tissue of the nucellus can give rise to the embryo within the seed through a mechanism of asexual reproduction called nucellar embryony.

Megagametophyte

Megagametophyte formation of the genera Polygonum and Lilium. Triploid nuclei are shown as ellipses with three white dots. The first three columns show the meiosis of the megaspore, followed by 1-2 mitoses.

The haploid megaspore inside the nucellus gives rise to the female gametophyte, called the megagametophyte.

In gymnosperms, the megagametophyte consists of around 2000 nuclei and forms archegonia, which produce egg cells for fertilization.

Ovule with megagametophyte: egg cell (yellow), synergids (orange), central cell with two polar nuclei (bright green), and antipodals (dark green).

In flowering plants, the megagametophyte (also referred to as the embryo sac) is much smaller and typically consists of only seven cells and eight nuclei. This type of megagametophyte develops from the megaspore through three rounds of mitotic divisions. The cell closest to the micropyle opening of the integuments differentiates into the egg cell, with two synergid cells by its side that are involved in the production of signals that guide the pollen tube. Three antipodal cells form on the opposite (chalazal) end of the ovule and later degenerate. The large central cell of the embryo sac contains two polar nuclei.

Zygote, Embryo and Endosperm

The pollen tube releases two sperm nuclei into the ovule. In gymnosperms, fertilization occurs within the archegonia produced by the female gametophyte. While it is possible that several egg cells are present and fertilized, typically only one zygote will develop into a mature embryo as the resources within the seed are limited.

In flowering plants, one sperm nucleus fuses with the egg cell to produce a zygote, the other fuses with the two polar nuclei of the central cell to give rise to the polyploid (typically triploid) endosperm. This double fertilization is unique to flowering plants, although in some other groups the second sperm cell does fuse with another cell in the megagametophyte to produce a second embryo. The plant stores nutrients such as starch,  proteins and oils in the endosperm as a food source for the developing embryo and seedling, serving a similar function to the yolk of animal eggs. The endosperm is also called the albumen of the seed.

Embryos may be described by a number of terms including Linear (embryos have axile placentation and are longer than broad), or rudimentary (embryos are basal in which the embryo is tiny in relation to the endosperm).

Types of  Gametophytes

Megagametophytes of flowering plants may be described according to the number of megaspores developing, as either monosporic, bisporic, or tetrasporic.

References

1. ^ Kotpal, Tyagi, Bendre, & Pande. Concepts of Biology XI. Rastogi Publications, 2nd ed. New Delhi 2007. ISBN 8171338968. Fig. 38 Types of placentation, page 2-127.
2. ^ Herr, J.M. Jr., 1995. The origin of the ovule. Am. J. Bot. 82(4):547-64
3. ^ ^a b Stewart, W.N.; Rothwell, G.W. (1993). Paleobotany and the evolution of plants. Cambridge University Press. ISBN 05213829477.
4. ^ Frohlich and Chase, 2007. After a dozen years of progress, the origin of angiosperms is still a great mystery. Nature 450:1184-1189 (20 December 2007) | doi:10.1038/nature06393;
5. ^ Gifford, E.M.; Foster, A.S. (1989), Morphology and evolution of vascular plants, New York: W. H. Freeman and Company
6. ^ The Seed Biology Place:Structural seed types based on comparative internal morphology.

Bibliography

• P.H. Raven, R.F. Evert, S.E. Eichhorn (2005): Biology of Plants, 7th Edition, W.H. Freeman and Company Publishers, New York, ISBN 0-7167-1007-2.
• Peter K. Endress.Angiosperm ovules: diversity, development, evolution. Ann Bot (2011) 107 (9): 1465-1489. doi: 10.1093/aob/mcr120.

External Links

• Morfología de Plantas Vasculares (Spanish)

- Wikipedia