Right after fertilization, the zygote is mostly inactive, but the primary endosperm divides rapidly to form the endosperm tissue. This tissue becomes the food the young plant will consume until the roots have developed after germination.
The seed coat forms from the two integuments or outer layers of cells of the ovule, which derive from tissue from the mother plant: the inner integument forms the tegmen and the outer forms the testa.
When the seed coat forms from only one layer, it is also called the testa, though not all such testae are homologous from one species to the next.
In gymnosperms, the two sperm cells transferred from the pollen do not develop seed by double fertilization, but one sperm nucleus unites with the egg nucleus and the other sperm is not used. Sometimes each sperm fertilizes an egg cell and one zygote is then aborted or absorbed during early development. The seed is composed of the embryo and tissue from the mother plant, which also form a cone around the seed in coniferous plants such as pine and spruce.
The ovules after fertilization develop into the seeds. The storage of food reserves in angiosperm seeds differs between monocots and dicots. In monocots, the single cotyledon is called a scutellum; it is connected directly to the embryo via vascular tissue. Food reserves are stored in the large endosperm. Upon germination, enzymes are secreted by the aleurone, a single layer of cells just inside the seed coat that surrounds the endosperm and embryo.
The enzymes degrade the stored carbohydrates, proteins, and lipids. These products are absorbed by the scutellum and transported via a vasculature strand to the developing embryo. Monocots and dicots : The structures of dicot and monocot seeds are shown. Dicots left have two cotyledons.
Monocots, such as corn right , have one cotyledon, called the scutellum, which channels nutrition to the growing embryo. Both monocot and dicot embryos have a plumule that forms the leaves, a hypocotyl that forms the stem, and a radicle that forms the root. The embryonic axis comprises everything between the plumule and the radicle, not including the cotyledon s.
In endospermic dicots, the food reserves are stored in the endosperm. During germination, the two cotyledons act as absorptive organs to take up the enzymatically-released food reserves, similar to the process in monocots. In non-endospermic dicots, the triploid endosperm develops normally following double fertilization, but the endosperm food reserves are quickly remobilized, moving into the developing cotyledon for storage.
Upon germination in dicot seeds, the epicotyl is shaped like a hook with the plumule pointing downwards; this plumule hook persists as long as germination proceeds in the dark. Therefore, as the epicotyl pushes through the tough and abrasive soil, the plumule is protected from damage. Upon exposure to light, the hypocotyl hook straightens out, the young foliage leaves face the sun and expand, and the epicotyl continues to elongate. During this time, the radicle is also growing and producing the primary root.
As it grows downward to form the tap root, lateral roots branch off to all sides, producing the typical dicot tap root system. Monocot seeds : As this monocot grass seed germinates, the primary root, or radicle, emerges first, followed by the primary shoot, or coleoptile, and the adventitious roots. In monocot seeds, the testa and tegmen of the seed coat are fused. As the seed germinates, the primary root emerges, protected by the root-tip covering: the coleorhiza.
Next, the primary shoot emerges, protected by the coleoptile: the covering of the shoot tip. Upon exposure to light, elongation of the coleoptile ceases and the leaves expand and unfold. At the other end of the embryonic axis, the primary root soon dies, while other, adventitious roots emerge from the base of the stem. This produces the fibrous root system of the monocot. Depending on seed size, the time it takes a seedling to emerge may vary.
However, many mature seeds enter a period of dormancy marked by inactivity or extremely-low metabolic activity. This period may last for months, years, or even centuries. Dormancy helps keep seeds viable during unfavorable conditions.
Upon a return to optimal conditions, seed germination takes place. These conditions may be as diverse as moisture, light, cold, fire, or chemical treatments. Fruits are categorized based on the part of the flower they developed from and how they release their seeds. After fertilization, the ovary of the flower usually develops into the fruit. Fruits are generally associated with having a sweet taste; however, not all fruits are sweet.
In most cases, flowers in which fertilization has taken place will develop into fruits, while unfertilized flowers will not. The fruit encloses the seeds and the developing embryo, thereby providing it with protection. Fruits are diverse in their origin and texture. The sweet tissue of the blackberry, the red flesh of the tomato, the shell of the peanut, and the hull of corn the tough, thin part that gets stuck in your teeth when you eat popcorn are all fruits.
As the fruit matures, the seeds also mature. Fruits may be classified as simple, aggregate, multiple, or accessory, depending on their origin. If the fruit develops from a single carpel or fused carpels of a single ovary, it is known as a simple fruit, as seen in nuts and beans. An aggregate fruit is one that develops from numerous carpels that are all in the same flower; the mature carpels fuse together to form the entire fruit, as seen in the raspberry.
A multiple fruit develops from an inflorescence or a cluster of flowers. An example is the pineapple where the flowers fuse together to form the fruit. Accessory fruits sometimes called false fruits are not derived from the ovary, but from another part of the flower, such as the receptacle strawberry or the hypanthium apples and pears. Types of fruit : There are four main types of fruits.
Simple fruits, such as these nuts, are derived from a single ovary. Aggregate fruits, like raspberries, form from many carpels that fuse together. Multiple fruits, such as pineapple, form from a cluster of flowers called an inflorescence. Accessory fruits, like apples, are formed from a part of the plant other than the ovary.
Fruits generally have three parts: the exocarp the outermost skin or covering , the mesocarp middle part of the fruit , and the endocarp the inner part of the fruit. Together, all three are known as the pericarp. The mesocarp is usually the fleshy, edible part of the fruit; however, in some fruits, such as the almond, the seed is the edible part the pit in this case is the endocarp.
In many fruits, two, or all three of the layers are fused, and are indistinguishable at maturity. Fruits can be dry or fleshy. Furthermore, fruits can be divided into dehiscent or indehiscent types.
Dehiscent fruits, such as peas, readily release their seeds, while indehiscent fruits, like peaches, rely on decay to release their seeds. Some fruits can disperse seeds on their own, while others require assistance from wind, water, or animals.
In addition to protecting the embryo, the fruit plays an important role in seed dispersal. Seeds contained within fruits need to be dispersed far from the mother plant so that they may find favorable and less-competitive conditions in which to germinate and grow. Some fruits have built-in mechanisms that allow them to disperse by themselves, whereas others require the help of agents such as wind, water, and animals.
Modifications in seed structure, composition, and size aid in dispersal. Wind-dispersed fruit are lightweight and may have wing-like appendages that allow them to be carried by the wind. Some have a parachute-like structure to keep them afloat. Some fruits, such as the dandelion, have hairy, weightless structures that are suited to dispersal by wind. Wind dispersal : Wind is used as a form of dispersal by lightweight seeds, such as those found on dandelions.
Seeds dispersed by water are contained in light and buoyant fruit, giving them the ability to float. Coconuts are well known for their ability to float on water to reach land where they can germinate. Similarly, willow and silver birches produce lightweight fruit that can float on water.
Animals and birds eat fruits; seeds that are not digested are excreted in their droppings some distance away. Some animals, such as squirrels, bury seed-containing fruits for later use; if the squirrel does not find its stash of fruit, and if conditions are favorable, the seeds germinate. Humans also play a major role in dispersing seeds when they carry fruits to new places, throwing away the inedible part that contains the seeds.
Seed dormancy allows plants to disperse their progeny through time: something animals cannot do. Dormant seeds can wait months, years, or even decades for the proper conditions for germination and propagation of the species.
Privacy Policy. Skip to main content. Plant Reproduction. Search for:. Pollination and Fertilization. Pollination and Fertilization Plants can transfer pollen through self-pollination; however, the preferred method is cross-pollination, which maintains genetic diversity. Learning Objectives Determine the differences between self-pollination and cross-pollination, and describe how plants have developed ways to avoid self-pollination. Key Takeaways Key Points Pollination, the transfer of pollen from flower-to-flower in angiosperms or cone -to-cone in gymnosperms, takes place through self-pollination or cross-pollination.
Cross-pollination is the most advantageous of the two types of pollination since it provides species with greater genetic diversity. Maturation of pollen and ovaries at different times and heterostyly are methods plants have developed to avoid self-pollination. The placement of male and female flowers on separate plants or different parts of the plant are also barriers to self-pollination.
The three main structures of the pistil include the stigma, style and ovary. The stigma is at the top of the pistil, and it's connected to the ovary at the base of the pistil by the tube-shaped style. The male flower structure, which is called the stamen, consists of two parts: the filament and the anther.
The anther is the part of the flower that produces and releases pollen, while the filament is the structure that supports the anther. In flowers that have both male and female structures, the pistil is in the center surrounded by the stamen.
Pollination occurs when the gamete from the pollen fertilizes the ovule in the ovary. While complete flowers can self-pollinate, typically, pollinators such as bees transport the pollen from the anther to the stigma.
The stigma has a sticky surface that collects the pollen. Each grain of pollen consists of a pollen tube cell and a generative cell, advises Georgia Tech Biological Sciences.
When it reaches the stigma, the pollen tube cell guides the generative cell through the style into the ovary. The generative cell divides into two male gametes, or sperm cells, that fertilize the ovules inside the ovary of the plant. Fertilized ovules become the seeds that can go on to grow into new plants. When the pollen grains in the sacs become ripe, the anthers burst open, releasing the sticky pollen onto the stamen.
This bursting of the pollen sacs, called dehiscence, sometimes happens with enough force that the pollen propells forcefully out of the flower and onto the wind, which carries it to other flowers. The bright colors of the petals lure insects, where they alight and partake of the flower's nectar.
To reach the nectar, insects must climb all the way into the flower itself. During the process, pollen grains located on the anthers either become stuck to the insect's feet where they are transferred to another flower, in a process called cross-pollination or moved from the stamen to the pistil of the same flower self-pollination. This occurs in many instances, because of the pistil's location in the center of the flower surrounded by the stamen.
The pistil sometimes called a carpal houses the female organs of the flower and comprises the stigma a thin hollow tube and the ovary. At the base of the tube lies the ovary, which houses the ovule. Many flowers have more than one carpal, and some have more than one ovule egg in each ovary.
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