Furthermore, seeds remain in a state of dormancy induced by desiccation and the hormone abscisic acid until conditions for growth become favorable. Whether blown by the wind, floating on water, or carried away by animals, seeds are scattered in an expanding geographic range, thus avoiding competition with the parent plant.
Pollen grains are male gametophytes carried by wind, water, or a pollinator. The whole structure is protected from desiccation and can reach the female organs without dependence on water. Male gametes reach female gametophyte and the egg cell gamete though a pollen tube: an extension of a cell within the pollen grain. The sperm of modern gymnosperms lack flagella, but in cycads and the Gingko, the sperm still possess flagella that allow them to swim down the pollen tube to the female gamete; however, they are enclosed in a pollen grain.
Fossilized pollen grains : This fossilized pollen is from a Buckbean fen core found in Yellowstone National Park, Wyoming. The pollen is magnified 1, times. Seed ferns gave rise to the gymnosperms during the Devonian Period, allowing them to adapt to dry conditions. Seed ferns : This fossilized leaf is from Glossopteris , a seed fern that thrived during the Permian age — million years ago.
Seed ferns produced their seeds along their branches without specialized structures. What makes them the first true seed plants is that they developed structures called cupules to enclose and protect the ovule the female gametophyte and associated tissues which develops into a seed upon fertilization. Seed plants resembling modern tree ferns became more numerous and diverse in the coal swamps of the Carboniferous period.
This appears to have been the result of a whole genome duplication event around million years ago. Gymnosperms of the taiga : This boreal forest taiga has low-lying plants and conifer trees, as these plants are better suited to the colder, dryer conditions. Fossil records indicate the first gymnosperms progymnosperms most likely originated in the Paleozoic era, during the middle Devonian period about million years ago.
Following the wet Mississippian and Pennsylvanian periods, which were dominated by giant fern trees, the Permian period was dry. This gave a reproductive edge to seed plants, which are better adapted to survive dry spells. The Ginkgoales, a group of gymnosperms with only one surviving species, the Gingko biloba, were the first gymnosperms to appear during the lower Jurassic. Gymnosperms expanded in the Mesozoic era about million years ago , supplanting ferns in the landscape, and reaching their greatest diversity during this time.
It has been suggested that during the mid-Mesozoic era, pollination of some extinct groups of gymnosperms was performed by extinct species of scorpionflies that had a specialized proboscis for feeding on pollination drops. The scorpionflies probably engaged in pollination mutualisms with gymnosperms, long before the similar and independent coevolution of nectar-feeding insects on angiosperms. The Jurassic period was as much the age of the cycads palm-tree-like gymnosperms as the age of the dinosaurs.
Gingkoales and the more familiar conifers also dotted the landscape. Although angiosperms flowering plants are the major form of plant life in most biomes, gymnosperms still dominate some ecosystems, such as the taiga boreal forests and the alpine forests at higher mountain elevations because of their adaptation to cold and dry growth conditions.
Angiosperms, which evolved in the Cretaceous period, are a diverse group of plants which protect their seeds within an ovary called a fruit. Undisputed fossil records place the massive appearance and diversification of angiosperms in the middle to late Mesozoic era. Fossil evidence indicates that flowering plants first appeared in the Lower Cretaceous, about million years ago, and were rapidly diversifying by the Middle Cretaceous, about million years ago.
Earlier traces of angiosperms are scarce. Fossilized pollen recovered from Jurassic geological material has been attributed to angiosperms. A few early Cretaceous rocks show clear imprints of leaves resembling angiosperm leaves.
By the mid-Cretaceous, a staggering number of diverse, flowering plants crowd the fossil record. The same geological period is also marked by the appearance of many modern groups of insects, including pollinating insects that played a key role in ecology and the evolution of flowering plants.
Fossil evidence of angiosperms : This leaf imprint shows a Ficus speciosissima, an angiosperm that flourished during the Cretaceous period. Many authors have attributed the diversity of plants and insects to pollination and herbivory, which is the consumption of plants by insects and other animals. This is believed to have been as much a driving force as pollination. Coevolution of herbivores and plant defenses is observed in nature.
Unlike animals, most plants cannot outrun predators or use mimicry to hide from hungry animals. A sort of arms race exists between plants and herbivores. Other plants are protected by bark, although some animals have developed specialized mouth pieces to tear and chew vegetal material. Spines and thorns deter most animals, except for mammals with thick fur; some birds have specialized beaks to get past such defenses. Plant defenses from herbivory : a Spines and b thorns are examples of plant defenses.
Herbivory has been used by seed plants for their own benefit in a display of mutualistic relationships. The dispersal of fruit by animals is the most striking example.
An extreme example of collaboration between an animal and a plant is the case of acacia trees and ants. The trees support the insects with shelter and food. In return, ants discourage herbivores, both invertebrates and vertebrates, by stinging and attacking leaf-eating organisms. Grasses are a successful group of flowering plants that are wind pollinated.
They produce large amounts of powdery pollen carried over large distances by the wind. The flowers are small and wisp-like. Large trees such as oaks, maples, and birches are also wind pollinated. More than 80 percent of angiosperms depend on animals for pollination: the transfer of pollen from the anther to the stigma. Consequently, plants have developed many adaptations to attract pollinators. The specificity of specialized plant structures that target animals can be very surprising.
Many bird or insect-pollinated flowers secrete nectar, a sugary liquid. They also produce both fertile pollen for reproduction and sterile pollen rich in nutrients for birds and insects.
Butterflies and bees can detect ultraviolet light. Large, red flowers with little smell and a long funnel shape are preferred by hummingbirds who have good color perception, a poor sense of smell, and need a strong perch. White flowers that open at night attract moths. Other animals such as bats, lemurs, and lizards can also act as pollinating agents. Any disruption to these interactions, such as the disappearance of bees as a consequence of colony collapse disorders, can lead to disaster for agricultural industries that depend heavily on pollinated crops.
Animal-aided pollination : As a bee collects nectar from a flower, it is dusted by pollen, which it then disperses to other flowers. Human life has become dependent on plants for the qualities and developments that they provide, which include medicine and food production. Seed plants are cultivated for their beauty and smells, as well as their importance in the development of medicines.
Plants are also the foundation of human diets across the world. Many societies eat, almost exclusively, vegetarian fare and depend solely on seed plants for their nutritional needs. A few crops rice, wheat, and potatoes dominate the agricultural landscape. Many crops were developed during the agricultural revolution when human societies made the transition from nomadic hunter—gatherers to horticulture and agriculture.
Cereals, rich in carbohydrates, provide the staple of many human diets. In addition, beans and nuts supply proteins. Fats are derived from crushed seeds, as is the case for peanut and rapeseed canola oils, or fruits such as olives.
Animal husbandry also requires large amounts of crops. Importance of plants to humans : Humans rely on plants for a variety of reasons. Combined with sugar, another plant product, chocolate is a popular food.
Staple crops are not the only food derived from seed plants. Fruits and vegetables provide nutrients, vitamins, and fiber. A protein rich aleurone layer lies on the outside of endosperm.
Embryo lies on one side towards the upper pointed part. A single large cotyledon lies lateral and parallel to the embryo axis. It is called scutellum. Scutellum is attached to the middle part of embryo axis. Its outer layer in contact with endosperm is called epithelial layer. The layer secretes GA for formation of amylase during germination.
Embryo axis ends in plumule towards broader side and radicle towards pointed side. Radicle has a root cap. Plumule bears a few small leaves. Sheaths derived from scutellum cover the two ends of embryo axis, undifferentiated coleorhiza over the radicle root cap region and hollow folial coleoptile over the plumule. Area of embryo axis is between plumule and cotyledonary node is epicotyl while the area between cotyledonary node and radicle is called hypocotyl.
It is a small blackish endospermic monocotyledonous seed with wrinkled surface. Seed coat is quite tough.
It is coloured. Endosperm or food storage tissue is also tough. It is semitransparent. Embryo is curved. It is embedded in the endosperm. Embryo axis is small as compared to single cotyledon called scutellum. Epicotyl is inconspicuous. Plumule is not distinguishable. Instead shoot apical meristem is present. A notch occurs in the area of origin of single cotyledon. Hypocotyl is larger. It bears radicle or root tip.
The ability of seeds to retain the power of germination over a period of time is called viability of seeds. A viable seed is, therefore, that seed which is capable of germination under suitable environmental conditions after the completion of dormancy, if it is present.
Viability may range from a few weeks to several years. It is also influenced by conditions during storage and non-germination. Excessive dry or damp weather and high temperature are known to reduce viability of all seeds. Viability of several hundred years has been recently found out.
About years old seeds of Lupinus arcticus taken out from arctic tundra have germinated and produced plants that flowered and bore fruits. Viability of seeds can be known by two methods: i Ability to germinate, ii Testing their ability to respire. All viable seeds respire. This can be tested by immersing a section of seed containing the embryo in 0. The viable embryo will turn pink due to conversion of colourless triphenyl tetrazolium chloride into insoluble coloured dye called triphenyl formazan due to reduction.
Seed formation is, therefore, more dependable. It is most suitable for perennation through unfavourable periods. Seeds have reserve food for nourishing the young seedlings till they become nutritionally independent. As seeds are formed through sexual reproduction they carry a number of variations. Variations are essential for adaptability to diverse environmental conditions.
Seeds can be stored for later use. This is helpful for supply of food throughout the year and to overcome drought and famine conditions. Seed is the basis of agriculture. Agriculture originated when humans learnt to eat, store and sow seeds. Agriculture proved to be turning point for evolution of human civilisation, industrialisation, science and technology. Developing fruits protect the developing seeds from mechanical injury, insects and unfavourable climatic conditions.
Fleshy fruits provide food to animals who also act as dispersal agents of their seeds. Fleshy fruits generally have hard seeds e. Plants , Reproduction in Plants. You must be logged in to post a comment.
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