A fruit is the result of a fertilized flower in angiosperms that are the means by which these plants disseminate their seeds . In flowers, the ovary is the protective vessel in which ovules are nourished until they reach maturity. Within this vessel, the ovules remain attached to parent tissue on the placenta. These zones of placentation are known as carpels.

An ovary is a part of the female reproductive organ of a flower. It is the part of the pistil  which holds the ovule/s and is located above or below or at the point of connection with the base of the  petals  and sepals . The pistil may be made up of one carpel  or of several fused carpels and therefore the ovary can contain part of one carpel or parts of several fused carpels. Above the ovary is the style  and the stigma. Ovaries can be composed of one to many carpels. The number of ovules associated with each carpel varies from one to many. Some ovaries can be separated into several distinct chambers while others consist of only one chamber. These chambers are called locules. The number of locules is often (but not always) equal to the number of carpels.

Plant fruits are a major portion of the food eaten by animals. Fruits vary greatly in shape, size, sweetness, moisture content and texture.

Fruit Morphology:

* Pericarp – the fruit wall.

* Exocarp – outermost layer of the pericarp. 

* Mesocarp – middle layer of the pericarp. 

* Endocarp – inner layer of the pericarp.

* Placenta – region of attachment of seeds on the fruit wall. 

* Funiculus – stalk attaching the seed to the placenta. 

* Seed – mature ovule.

Types of Fruit – based on floral origin

Simple Fruit – Derived from a single pistil of one flower.  Peas, Tomato, Lilies, Apple.

* Dry Indehiscent Fruit Types– Fruits that do not split open at maturity.

* Dry Dehiscent Fruit Types – Fruits that split open at maturity.

* Capsule Types – Based on type of dehiscence. Grevillea, Hakea.

* Schizocarpic Fruit Types – Fruits derived from a compound ovary in which the locules separate at fruit maturity simulating fruits derived from the simple ovaries like those found on Aeschynomene brevifolia.

* Fleshy Fruit Types – Citrus, Curcubita.

* Aggregate Fruit – Derived from more than one pistil on one flower. Strawberry, Raspberry.

*Multiple Fruit – Derived from several inflorescences. Morinda citrifolia.

* Accessory Fruits – Composed of the ovary wall plus other flower parts. Apple, Cucumber, Strawberry.

Fruit Development:

The basic formation of fruit begins with fertilization then follows 4 major steps to seed dispersal. The fruit begins to grow as the cells rapidly multiply. Maturation is the process where the cells stop multiplying and the seeds begin to mature. The ripening stage is where the seeds have matured and the fruit takes on certain traits like the accumulation of glucose, soften or have aromas which make them attractive to their niche dispersal agents. The last stage is when the fruits reach Senescence and are susceptible to pathogens and fungal attack.

Fruit development occurs in 5 phases – fruit set, cell division, cell expansion, ripening maturation and senescence.

Fruit set 

involves the plant in aborting the ovary or proceeding with fruit development. Fruit set is usually dependent on pollination and the Hormones GA found within the pollen. Pollen therefore is the strongest influence for fruit development.

Fruit set involves the plant in aborting the ovary or proceeding with fruit development. Fruit set is usually dependent on pollination and the Hormones GA found within the pollen. Pollen therefore is the strongest influence for fruit development.

Experiments applying the hormone auxin or gibberellin to the ovaries of flowers can induce a fruit to develop without pollination or fertilization. These fruits are known as parthenocarpic fruits and are seedless. However some plant species are known to be GA deficient and are still able to produce fruit indicating that this is not the only hormone to induce fruit development. In the absence of these hormonal signals, the flowers abort. Auxin and Gibberellin production lag slightly behind the growing pollen tube. Continued fruit development usually relies on the continued presence of developing seeds. Seed abortion or removal usually causes fruit abortion.

Following Fruit set there is a period where the cells multiply rapidly, surprisingly this period is only about 12mm to 25mm of the time taken from fruit set to fruit maturation in soft body fruits. The phase of rapid cell division involves all sections of the fruit. The number of fertilized ovules in a fruit correlates with both the initial cell division rate and the final size of the fruit. Fruits with an uneven distribution of seeds are often lopsided. While there is a correlation between higher concentrations of cytokinin in the meristem of plants and in developing embryos, there is no direct evidence that cytokinin is active in or regulates fruit cell division.

Cell expansion phase is the gradual cessation of rapid cell division and the expansion of the cells in the fruit. This varies among fruits and also among different cell tissues within a fruit. Tissues made up of many small cells at maturity continue dividing while tissues composed of large cells begin to expand. Cell expansion accounts for the largest increase in the volume of fruits. Fruits often expand in excess of 100 fold in size.

Fruit ripening represents the shift from the protective function to the dispersal function of the fruit. Ripening occurs either immediately after seed and embryo maturation or synchronously with the seed and embryo maturation.

Dry fruits like those of grasses, nuts & Grevillea ripening consist of desiccation and are considered mature when the moisture levels drop to a given point suitable for dispersal.

Fleshy fruits are designed to make the fruit appealing to the dispersant. Ripening involves the softening, increased juiciness and sweetness, and color changes of the fruit. Fleshy fruits are either climacteric or nonclimacteric. Climacteric fruits produce a respirative burst with a concomitant burst in ethylene synthesis, as the fruits ripen. These include fruits with high degrees of flesh softening, like tomato, banana, avacado, peach.

Ripening can be induced prematurely by exposing fruit to air that contains only a few parts per million of the gaseous plant growth regulator, ethylene. Ripening can be delayed by storing fruits in air kept as free of ethylene as possible and supplemented with extra CO₂, which antagonizes the physiological action of ethylene.

Fruit softening involves a partial breakdown of cell walls. Several enzymes are known to be involved in this process. Polygalacturonase hydrolyzes bonds in pectins.

When its seeds are ready for dispersal, the fruit ripens. Some characteristics of ripening are familiar:

* Colours – These changes are usually from green to some more attractive, contrasting or appealing colour to the seed distributor. A fruit’s colour changes due to chlorophyll breakdown and the formation of other pigments in the chromoplasts usually to form various forms of carotenoid pigments.

* Sweetening – Sweetening is due to the accumulation of glucose which can reach concentrations of 20mm in some Syzigium and up to 25mm by weight in Vitaceae.

* Texture – The softening of fruit tissue during ripening involves the breakdown of cell wall components of which pectin is the most common.

* Aromas – The aromas of fruit tissue during ripening involves the breakdown of cell wall components of which pectin is the most common.

The seed is the diploid offspring of 2 haploid cells that reach maturity matured ovule.

Parts of a Seed: The Embryo is the young plant within the seed which consists of the following parts. See Glossary page 22 & 23.

* Epicotyl– Is the new leaf

* Hypocotyl – Is the new stem

* Radicle – Is the new root

* Endosperm – Is the food reserve derived from the fertilization of the polar nuclei.

* Cotyledons – Are the seed leaves which serve as the food source following germination until the plants roots develop to a stage that they support itself.

* Seed coat – Is the structure derived from the wall of the ovule to protect the inner vital parts of the seed. 

* Hilum – Is the the funicular scar on the seed coat.

* Micropyle – Is the hole through the seed coat in which water passes to stimulate germination.

Seed dispersal is the removal of the seeds preferably away from the parent plant to another location. Plants have adopted an amazing number of methods from the simple method of allowing the seeds to drop to the ground using Gravity, enticing an Animal to pick the seed up and place it elsewhere, myriads aviation structures so technically advanced that flight engineers are replicating the shapes and methods to utilize the power of the wind, the use of water flowing down a creek or river or tidal power to the physics of thermal bicellulose strips and sling shots in explosions equivalent to high explosives.

The term dispersal unit refers to any detached plant part serving as a vehicle for seed dispersal.

* Gravity: These seeds are usually round and have a tough outer shell. They often grow on slopes or have characteristics that rely on another form of dispersal once they hit the ground. They may roll some distance from the parent plant. The higher up the tree they are, and the larger the tree is, the further they can roll. The various Acacia species seeds are usually further dispersed by ants who collect the seeds for the sweet tasting funicle which is still attached. Red mangro ve Rhizophora stylosa uses gravity with a sharp point to anchor the seed firmly into its muddy terrain.

If they have a soft skin, they may break open when they hit the ground and the individual seeds may be scattered.

* Animals: As well as eating the fruits, some animals collect the fruits or seeds and bury them to eat later. Some are forgotten about while some are stored for the coatings or other parts then removed to other locations to germinate. Ants play the largest role in arid areas for seed dispersal using these methods.

The Mistletoe Bird eats the seeds; of the various Mistletoes, which are covered in a sweet mucous substance and pass the seed after the mucous is digested depositing the seed on a new branch on a new tree.

Many plants produce fruits or individual seeds covered in hooks or spines which attach the seed to the animal’s fur or feathers. The seeds are then carried a sufficient distance from the parent plant to give them space to grow. The seed may fall off or removed by the animal or bird often during preening of their mate. Themeda australis is a good example of seed adhering to animals.

* Water: All water plants obviously have their seeds dispersed by water like Nymphaea violacea while Avicennia marina seeds often germinate prior to being dropped floating on the tides to a new location to grow.

There are many other ways in which water plays a part in dispersing seeds. Plants which grow in riparian zones often rely on water to transport their seeds for them. They may produce light seeds which float like the seed pods of Castanospermum australe, or they may be fluffy like the seeds of many of the Lomandra genus.

Seeds of some tropical trees can even be carried along by ocean currents to land on shores half a continent away. The Sea Bean’s seeds of Entada rheedii, are carried from their homes beside rivers in Africa, Australia or South America to new coastlines or river estuaries.

*  Wind: Tall trees often produce seeds with stiff wings covering the seed that enable them to fly long distances. The wings are twisted and balanced so that the seed spins around as it is carried along by the wind. These natural adaptations for using the wind to transport the weight of the seed are amazingly technically accurate like the seeds of the Flindersia specie. These wings usually support one seed each, but may start off as a two-winged pod that later splits in two to release the seeds.

Sometimes seeds have thin wings as an extension of the seed that enable them to glide in the wind like Pandorea jasminoides. They don’t need so much wind as the seeds that fly, but they are not so heavy.

Some seeds have long, feathery tails which help them to fly, like the tail of a kite like those found on Clematis aristata.

Additional features like various forms of fluff which are almost weightless but increase the volume of the seeds, so that they can be picked up by the slightest breeze and carried over long distances like the seeds of Phragmites australis.

Many members of the Daisy family provide their seeds with a flat disk or ribs then add fine hairs to produce a parachute to keep the seed aloft. Here the Oleria species are well known for their pappilose hairs at one end to disperse their seeds in enclosed habitats.

*  Explosion: There are several methods plants use to fling their seeds out of the seedpod. All of them rely on different types of lignums or cellulose and the effect of evaporation of water in the seedpod. The lignum or cellulose dries at different rates causing a bending or twisting pressure on the fruits which in turn causes them to explode. This method of seed dispersal usually takes place in the sun and is common in arid areas. It is very common amongst members of the Papilionaceae.

Another method is where the side of the seedpod facing the sun dries out more quickly than the side in the shade, causing the pod to explode flicking the seeds out beyond the parent plant.

Pelargonium australe also uses the heat from the sun to disperse their seeds away from the parent plant. The seeds are in a ring at the base of the style, each one has a cover which is attached to the tip of the style by a thin woody strip. When the seeds are ripe, the seed covers split violently, and the strip rips up the style but is stopped suddenly because the strip remains attached to the style causing the seeds to be catapulted out like the aborigine with his throwing stick with a huge mechanical leverage.

Oxalis chnoodes and Telopea speciosus seeds are covered by an elastic coating similar to a loaded sling shot. When the pod dries out, it splits suddenly, shooting the individual seeds out of the pod.

Seed germination is when the seed embryo begins to grow.

Plants have evolved various methods to delay the onset of germination until the most advantageous environmental conditions prevail for germination to take place. All seeds must imbibe water to germinate and for a few this is the only requirement. Most however, contain growth inhibitors that must be removed from the seed prior to germination commencing. These take the form of chemical inhibitors or physical properties or a combination of the two.

Seeds that require chemical treatments often pass through the stomach of animals, need alkaline, saline or acid conditions to break down the inhibitors. Some require certain chemicals from the soil to accumulate in the seeds to help set up chemical reactions or stimulate enzymes while others rely on the fruits decomposition with the production of alcohols.

Physical properties include the destruction of the outer case by abrasion or thermal cold and heat cycles to crack the seed case.

Yet others have light or have photoperiod requirements where the seeds need to be buried first.

Aging of the seed is another method of physical imbibition. All these mechanisms ensure the seeds germinate in the correct season and when moisture is available.

After ripening is when a seed is shed and needs to go through a dormancy period. This can be a few weeks as is the case in many grasses to several years as is the case in many desert plants. Many rainforest plants on the other hand have no after ripening period and can be sown immediately after the seeds have been shed.

Some seeds are dormant merely because they possess tough seed coats impervious to water or oxygen or mechanically preventing growth of the embryo. After dispersal in nature, exposure to the elements gradually breaks down and weakens the seed coat, eventually permitting germination. This behaviour tends to space out seeds from the same year’s crop, allowing the species to take advantage of chance opportunities whenever they occur.

Other seeds show a self-imposed dormancy of the embryo itself, which can be overcome by a specific environmental signal.

*  Cold temperature. These seeds require extended exposures to cold temperatures near or below freezing before they will germinate. A rise in temperatures will indicate that conditions are favourable for growth. These seeds are usually found amongst alpine or temperate zone plants.

* Exposure to light. Many small seeds and rainforest plants seeds are photosensitive and require exposure to light or at times shade before they will germinate. This helps ensure that they will not germinate when buried too deeply for the seedling shoot to reach the surface of the ground or are not in competition with other plants shading them. When land is tilled, seeds buried in the soil are brought to the surface, exposing them to light. This enables weeds to reappear quickly. 

*  Dry heat. These seeds are adapted to colonizing ground that has been opened up by forest or bush fires. They germinate only after exposure to the heat from these fast moving fires. Banksia specie cones will only open after the death of the plant, a limb or fire. The seed immediately is reactivated from its dormancy on the parent plant to germinate as soon as rains moisten the soil.

* Heavy rainfall. Leaching dependent seeds of desert annuals ensure that germination will occur only after enough rain has fallen to permit seedlings to complete their life cycle, even if it doesn’t rain again that season.

Germination begins with the physical uptake of water by the dry seed, called imbibition (imbibe = to drink) followed by resumption of growth by the embryo.

Key Functions of Imbibition are the rehydration of the embryo cells and enzyme activation

Once germination requirements have been met, respiration begins in the rehydrated cells, new cell wall materials are produced, and the plant embryo begins to grow. This causes a combination of cell expansion and cell division within the seed and is responsible for the initial growth. Germination is considered to be complete when the first true leaves have completely emerged and developed.

Viviparous plants produce seeds that germinate before they detach from the parent. The mangroves are good examples of viviparity. This is where the seed germinates and grows under its own energy or rarely partially supported by the parent plant while still attached to its parent. When the sprouted seed is finally shed; usually near water or over water the taproot immediately penetrates the mud or sand thereby effectively planting the seedling. This is more common in plants that grow in hostile wet climates where competition is keen; giving the seedling a head start or environmental conditions dictate that the plant needs to root quickly before being washed away. Castonospermum australe maybe considered a plant that is in a transitional zone in evolution between being viviparous and non viviparous.

In some trees, like Syzygium australis and some Citrus specie, the seeds can be found already germinated while the fruit enters a period of over ripeness. This is in the strict term is not viviparous. This condition bought about by moisture, humidity and a decline in the acidity of the fruit due to the early or middle stages of decomposition. Viviparous seeds can germinate under moist soil conditions too.

Remusatia vivipara is a true viviparous plant where the seeds germinate while still attached to the parent plant before being carried away on the pelt of passing animals.

Seed Viability

Seed viability is extremely variable with some plants in the moist tropics mangroves and swamps remaining viable for only a few weeks. The green spores of ferns like Todea barbara also remain alive for just a few weeks. Desert plants and those or the arid interior on the other hand may remain viable for hundreds of years. The Acacia specie and Anigozanthos specie are well known for their seed viability. A noticeable exception to this is Nelumbo nucifera where seeds found in a dry lakebed in China’s Xingjian province were over 1300 years old.

Seed dormancy is responsible for seed banks building up in the soil. Many seeds shed each year do not germinate in that or the next growing season, and some may not germinate for many years, even though still viable. This builds up the seed bank in the soil where their numbers often greatly exceed the number of growing plants in the area.

Records show that 1000 to 5000 viable seeds per square meter of soil have been found under various kinds of vegetation and 20,000 to 80,000 weed seeds per square meter in horticultural soils. This seed accumulation in the seed banks ensures that you will have a continuous regrowth of plants despite all your best efforts to control them. The good news is the longer you use mulches and prevent seeds from germinating the fewer seeds will germinate and after 1300 years you will only have a few desirable species to contend with that is if you can prevent the 1000 to 5000 seeds accumulating in your soil over this period!!

Remember:

1. A fruit is the mature ovary in an angiosperm as a result of a fertilized flower in angiosperms.

2. Distinct chambers inside an ovary are known as locules.

3. The pericarp is the wall of the fruit.

4. The conduit between seeds and the placenta is known as the funiculus.

5. A mature ovule is called a seed.

6. A simple fruit is derived from single pistil.

7. Dry indehiscent fruits do not split open at maturity. 

8. Dry dehiscent fruits split open at maturity.

9. An aggregate fruit is derived from more than one pistil on one  flower.

10. Multiple fruit are derived from several flowers.

11. An accessory fruit is composed of flower parts in addition to the ovary wall.

12. Fruit development is under the influence of the plant hormones Auxin and gibberellin.

13. A parthenocarpic fruit contains no seeds and has not been fertilized.

14. The softening of fruit tissue during ripening involves breakdown of pectin in the cell walls.

15. The seed epicotyl becomes the plant leaves. 

16. The seed hypocotyl becomes the plant stem. 

17. The seed radicle becomes the plant root.

18. The dispersal unit of a plant is any detached part of the aiding in the dispersal of the seeds.

19. Seed germination is the beginning of growth of the embryo.

20. Germination begins with the physical uptake of water by the dry seed is known as imbibition.

21. Viability is the period of time the seeds can remain dormant and still germinate.

22. The soil seed bank enables the regeneration of plant populations  after any catastrophic event.

23. After Ripening is when a seed needs a dormancy period  following its shedding from the parent plant.

24. Viviparous plants, seeds germinate while still attached to the parent plant.

Further Comments from Members:

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