Fruit ID #6

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Fruit Identification Photos #6

Multiple Fruits Of The Mulberry Family

Table Of Contents

  1. Unusual Fruits of the Mulberry Family
  2. The Remarkable Genus of Figs (Ficus)
  3. Which Figs Grew in the Holy Land?
  4. The Black Mulberry (Morus nigra)
  5. Does Black Mulberry Stain Clothing?
  6. The White Mulberry (Morus alba)
  7. The Pakistan Mulberry (Morus macroura)
  8. Silk From Moth That Eats Mulberry Leaves
  9. References From Economic Botany Page

1. Fruits of the Mulberry Family (Moraceae)

Multiple fruits of the mulberry family are composed of numerous, seed-bearing, ripened ovaries derived from numerous separate flowers. The multiple fruit of a mulberry (Morus) is composed of a cluster of drupelets superficially resembling a blackberry; however, unlike a blackberry, each drupelet arises from a separate minute flower. In the aggregate fruit of a blackberry, all the drupelets of the cluster come from a single flower. Multiple fruits of the Persian or black mulberry (M. nigra) become purplish-black at maturity. The red mulberry (M. rubra), native to fertile bottomlands of eastern North America, has red to dark purple fruits. The white mulberry of China is also grown for edible fruits, and its leaves are the primary food source for silkworm larvae and an ancient textile culture dating back more than 4,000 years. Male trees of the white mulberry are commonly planted in southern California. Known as "fruitless mulberry," they make a fast-growing shade tree without all the messy fruits of female trees that stain clothing and walkways with a purple juice. Because they produce large amounts of pollen during the spring, fruitless (male) mulberries can raise havoc with allergy (hay-fever) sufferers. [Note: "Cottonless cottonwoods" are male trees of the dioecious genus Populus that do not bear masses of fluffy seeds as in female cottonwood trees.]

Other interesting multiple fruits in the mulberry family are jackfruit (Artocarpus heterophyllus), breadfruit (Artocarpus altilis) and osage orange (Maclura pomifera). For more information about these fascinating fruits, click on the links below. Other multiple fruits include the pineapple and a large-leaved relative of the houseplant called philodendron. The pineapple is a multiple fruit in the bromelia family (Bromeliaceae), and the aroid Monstera deliciosa is a multiple fruit in the arum family (Araceae).

Three examples of multiple fruits: A. Jackfruit (Artocarpus heterophyllus); B. Pineapple (Ananas comosus); and C. Breadfruit (Artocarpus altilis). All three fruits are refered to as "multiple fruits" because they are derived from the coalescence of ovaries from many individual flowers plus a fleshy stem axis. The pineapple belongs to the bromelia family (Bromeliaceae).


Close-up view of a jackfruit (Artocarpus heterophyllus) showing the numerous, crowded pistils (carpels). Each carpel represents an individual flower and the entire cluster of fused carpels plus the fleshy sten axis is called a multiple fruit.

The jackfruit tree (Artocarpus heterophyllus) bears enormous fruits up to 75 pounds (34 kg), the largest fruits produced by a tree.

This percussion cricket was made in Thailand from the wood of jackfruit (Artocarpus heterophyllus) or from the wood of monkey pod (Samanea saman). The cricket is played by rubbing its toothed underside with the wooden mallet. The block of wood (right) is from the breadfruit tree (Artocarpus altilis).

Mulberry fruits are eaten raw and made into pies, tarts, jellies, puddings, syrups, sauces, drinks and wines. Dried fruits are used like dried figs and raisins for snacks and in puddings, cookies, muffins and confections. There are numerous cultivated varieties and hybrids of red, white and black mulberries. In fact, Stephen Facciola (Cornucopia II: A Source Book of Edible Plants, 1998) lists 28 cultivars of Morus alba, M. nigra and M. rubra.

Most genera in the Moraceae are wind-pollinated by male (staminate) flowers. Many species are dioecious with male and female flowers on separate male and female trees. In monoecious species with male and female flowers on the same tree, pollination typically requires pollen from a different tree because the maturation sequence of male and female flowers on the same tree is incompatible. The ripened ovary (technically a bicarpellate gynoecium) usually contains one seed within a single locule. Each minute fruit has an outer fleshy layer (exocarp and mesocarp) and a stony inner layer (endocarp) surrounding the seed. For this reason the individual fruits are often called drupelets, although some authors refer to them as achenes. Some species and varieties produce fleshy, seedless fruits without pollination. These fruits are called parthenocarpic because they mature without pollination and seed formation.

Many species of trees in the mulberry family have a milky latex sap and stringy bark that peels off into strips. The milky sap of the Central American milk tree (Brosimum utile) is actually edible. Known as "palo de vaca" to the locals, the fresh sap is sweet and tastes like cream. In fact, it has been used in coffee and to make "vegetable cheese" in British Guiana. The breadnut (B. alicastrum) is also used for food by local people and was an important food tree in the Maya civilization of Mexico and Central America. The seeds are eaten raw, boiled, or mashed like potatoes. Ground seeds are made into a meal that is mixed with corn for making tortillas. The ground seeds are also made into a coffee-like beverage and mixed with milk and sugar to make a drink similar to a milk shake.

See A Milk Tree (Brosimum utile) In Costa Rica
Cecropia Tree: Host Of Symbiotic Azteca Ants

The pliability and usefulness of the bark of mulberries and related trees of the Moraceae is due to clusters of bast fibers in the inner phloem region. For countless centuries, native people throughout the world have used the stringy bark to make twine, paper and clothing called "bark cloth." Mexican bark paper is still being made to this day, particularly in the state of Puebla. The fibrous inner phloem fibers are separated from the outer bark in strips and boiled for several hours in water containing lime. This procedure softens the fibers and makes them separate more easily. After rinsing, the strips are arranged in a grid pattern on a smooth board and then beaten with a flattened stone until the fibers mesh together. The sheets are left on the boards and allowed to dry in the sun. The bark of several tree species are used, including a fig (Ficus tecolutensis) and a mulberry (Morus celtidifolia). A yellow dye from mulberry wood chips is used by the Mayo people for dyeing wool and other fabrics. It is also blended with other natural dye stuffs.

A yellow dye from boiled mulberry wood chips is used by the Mayo people of Mexico. In order to make the dye colorfast, the cloth must be soaked in a dye bath containing a mordant such as alum.

A process similar to that of bark paper can also be used as a substitute for cloth. The best known bark cloth is called tapa cloth, which was a major source of clothing for native Polynesians. The bark was obtained from the paper mulberry (Broussonetia papyrifera), a member of the mulberry family (Moraceae). Native figs, such as the Polynesian banyan (Ficus prolixa) were also used locally on some islands for bark cloth. Strips of bark were peeled off the trunk, and the outer coating scraped off with a shell. After they were soaked in water and cleaned, the strips were placed on a hard wood surface and pounded with a mallet. Individual strips were fused together by overlapping the edges and beating them together. Depending on the thickness of the sheets, the finished tapa cloth varied in appearance from a muslin-like material to a tough, leather-like cloth.

Paper mulberry (Broussonetia papyrifera): A. Multiple fruit composed of numerous separate pistils from numerous separate female flowers; B. A small branch showing strips of stringy bark peeled away; C. Strips of stringy bark twisted into a crude twine. The pliability and usefulness of the bark is due to clusters of bast fibers.

A Mbuti ceremonial bark cloth shirt from the rain forest of N.E. Zaire. The Mbuti people fashion clothing from the bark of fig trees (Ficus) that grow in the local rain forest. The man makes two horizontal cuts around the trunk and then slices vertically between the cuts. He peels away the rough out bark, and then peels off the inner layer between the sapwood and the bark. Although this layer includes phloem tissue, it usually does not kill the tree. He wets this inner layer with water and hammers it with a mallet made of ivory or wood. After allowing it to dry, he repeats the wetting, pounding and drying process until the bark cloth is pliable and the correct thickness. When the bark cloth is ready, a women paints her unique designs using twig brushes and natural plant dyes from the forest.

Mulberry Family Hyperlinks:

Bark Cloth & Paper From Mulberry Family
A Good Definition Of Pathenocarpic Fruits
See Photos Of The Remarkable Jackfruit
See Photos Of The Polynesian Breadfruit
See The North American Osage Orange


2. Ficus: The Remarkable Genus Of Figs

The fig (Ficus) is a very unique genus in the mulberry family with a special kind of multiple fruit called a syconium. The pollen-bearing male and seed-bearing female flowers line the inside of a fleshy, flask-shaped structure (inflorescence) called a syconium. The tiny female flowers are pollinated by symbiotic female wasps who enter the syconium through a pore (ostiole) at one end which is lined with scales. Male flowers consist of one-several stamens, while female flowers consist of a single pistil (gynoecium). There are two types of female flowers, some with long-styles and some with short styles. The female wasp inserts her ovipositor into the short styles and oviposits inside the ovaries of these flowers. Since her ovipositor is too short, she is unable to deposit an egg in the long-style flowers. Consequently, the ovaries of long-style flowers contain a seed rather than a hungry wasp larva, and this is crucial for the perpetuation of the fig life cycle. There are up to 1,000 different species of Ficus worldwide, and virtually each one has its own symbiotic species of pollinator wasp. Since the female wasp must crawl into the floral cavity within the syconium to lay her eggs and pollinate the receptive female fig flowers inside, neither fig nor wasp can perpetuate itself without the other. Without the wasp there can be no fig seeds, and without the fig tree and its syconia there can be no reproduction for the wasp.

The large syconium of Ficus dammaropsis is about the size of a baseball. This unusual fig from the rain forests of New Guinea has large leaves up to two feet (0.6 m) wide and three feet (0.9 m) long. The floral cavity within the syconium contains unisexual male and female flowers. Seed-bearing female flowers are pollinated by a minute wasp (Ceratosolen abnormis) that enters the syconium through an opening at the upper end which is lined by a series of overlapping scales. Because this species is so unusual among figs, it was once classified as a separate genus and species Dammaropsis kingiana. For more information about the remarkable fig life cycle, see the references below.

Ficus dammaropsis, an unusual fig from the rain forests of New Guinea, has large leaves up to two feet (0.6 m) wide and three feet (0.9 m) long. The leaves are used by indigenous people of New Guinea for wrapping pork and for lining their cooking ovens. The bark is used in making string and head coverings. A softball-sized syconium (multiple fruit) is also shown in the photo (red arrow). According to A.B. Graf (Tropica, 1978), the syconia are harvested by native Melanesians of Kikiepa, New Guinea. Because this species is so unusual among figs, it was once classified as a separate genus and species Dammaropsis kingiana. For more information about the remarkable fig life cycle, see the references below.

The large syconia of Ficus dammaropsis are about the size of a baseball. This unusual fig from the rain forests of New Guinea has large leaves up to two feet (0.6 m) wide and three feet (0.9 m) long. The floral cavity within the syconium contains unisexual male and female flowers. Seed-bearing female flowers are pollinated by a minute wasp (Ceratosolen abnormis) that enters the syconium through an opening at the upper end which is lined by a series of overlapping scales.

One of the earliest records of any fruit eaten by people of the Middle East is the common fig (Ficus carica). Remnants of figs have been found in archeological excavations dating back 6,000 years ago. The fig is the first tree mentioned in the Bible in the story of Adam and Eve. Some biblical scholars think the fig, and not the apple, was the forbidden fruit picked by Eve in the Garden of Eden. Chauvinistic males also believe the penalty for this unauthorized fruit-picking was a sorrowful menstrual cycle. The scratchy leaves of this tree were reportedly used to cover the genitalia of the first humans. The fig is native to Caria--an ancient region of Asia Minor between the Mediterranean and Black Seas. It grows readily from seeds and cuttings, especially along water courses and rich, bottom lands, and was introduced by people throughout the Holy Land. Fig trees provided shade, fire wood and several crops of nourishing fruit a year. Dried figs could be squeezed into loaves or placed on strings and used as food during long, arduous journeys across the desert.

Kalamata string figs. In ancient times people carried strings of dried figs such as these on long arduous journeys across the desert. The figs provided them with a nutritious high protein, high carbohydrate food source in a region where food was scarce.

The common fig (Ficus carica) is a dioecious species with male and female trees. A tiny female wasp (Blastophaga psenes) enters an opening on the syconium to pollinate the flowers. In F. carica, only the syconia of female trees are edible, and without pollination they typically do not ripen and fall from the branches. [Note: There are many parthenocarpic varieties in which the seedless syconia ripen without pollination.] Male trees, called caprifigs, produce inedible syconia containing short-style female flowers bearing wasps and pollen-laden male flowers. [Capri refers to goat and the syconia were apparently fed to livestock.] During the late 1800s, Calimyrna fig growers in California were puzzled why their trees would not set fruit. They discovered that the female trees needed the fig wasp and male caprifigs from the Old World. Each June in California's hot Central Valley, bags of wasp-bearing caprifigs are placed in the Calimyrna orchards. This amazing pollination process is called caprification and the crunchy, seed-bearing syconia of female trees have a superior nutty flavor. [Premium fig newtons are made from caprified figs and contain numerous seed-bearing nutlets or endocarps.]

3. Which Figs Grew In The Ancient Holy Land?

An excellent article entitled "The History of the Fig in the Holy Land from Ancient Times to the Present" was written by Asaph Goor in Economic Botany 19: 124-135 (1965). The fig species discussed by Goor is the common edible fig (Ficus carica). This tree was cultivated for its fruit more than 5,000 years ago and is native to the region between the Mediterranean and Black Seas, sometimes referred to as the ancient region of Caria in Asia Minor. It is a dioecious species with separate male and female trees, and a symbiotic pollinator wasp (Blastophaga psenes) that is propagated inside the fruits (syconia) of male trees called caprifigs. It grows wild over a large area, including southern Europe and the Middle East. Goor (1965) stated that Ficus carica grew wild in the Holy Land thousands of years ago; however, this doesn't necessarily mean that it was truly native (indigenous) to the Holy Land. It may have been introduced by people to this region, either by seeds or cuttings. Ficus carica and its symbiotic wasp have even been introduced into California, including male and female trees that grow wild in San Diego County. In fact, the symbiotic wasps live at the Wayne's Word headquarters in a caprifig that produces three crops of inedible figs (syconia) each year, including a wasp-bearing, overwintering mamme crop that remains on the bare branches when the tree is devoid of leaves. There are several varieties of male caprifigs and hundreds of varieties of female Ficus carica trees, some of which develop delicious, seedless, parthenocarpic fruits that do not require pollination. There are also varieties in which the female trees will shed their entire crop if they are not pollinated by the symbiotic fig wasp. These varieties have been selected by people over countless centuries. The trees are readily propagated by cuttings and were transported and cultivated by people thousands of years ago. Apparently many ancient civilizations were aware of the fact that Ficus carica required pollination in order to produce edible, seed-bearing fruits, a process called caprification. In 350 B.C., Aristotle described fig wasps that came out of caprifigs and penetrated the unripe female fig fruits, thus fertilizing them. Theophrastus (372?-287? B.C.) discussed caprification in detail, and Pliny (23-79 A.D.) devoted an entire chapter to the practice of caprification in Italy. The subject of fig pollination and "gallflies" in ancient Babylonia is also mentioned by Herodotus (Book I, 485?-425? B.C.). Early horticulturists were undoubtedly aware that the seeds impart a superior, nutty flavor to the fruit, and in some varieties the fruit will not set if it is not pollinated by fig wasps. The fig referred to in ancient Babylonia was probably Ficus carica, but another species called the sycomore fig (Ficus sycomorus) was also used for food in the eastern Mediterranean region. According to Goor (1965): "The sycomore fruit is much inferior and cheaper... It is eaten by the poorer classes and by shepherds in plains where it grows alone." In addition it does not survive cold winters like Ficus carica, and Ficus carica has a much wider range, particularly in colder regions of Iraq and northward.

Another excellent article about ancient fig cultivation was written by J. Galil entitled "An Ancient Technique for Ripening Sycomore Fruit in East-Mediterranean Countries" (Economic Botany 22: 178-190, 1978). When the term "fig gashing" in the Near and Middle East is mentioned in various articles and books (including the Bible), it most likely refers to the sycomore fig (Ficus sycomorus), a species that is actually native to eastern Central Africa. Although the true East African pollinator wasp is not present in the Holy Land, an ovipositing, nonpollinator wasp does induce parthenocarpic fruits containing wasps instead of seeds. The ancient technique of gashing also induces edible, parthenocarpic fig fruits that enlarge and ripen rapidly before the wasps inside mature.

Fig & Fig Wasp Hyperlinks:

The Calimyrna Fig & Its Pollinator Wasp
Creeping Fig: One Source Of Grass Jelly
Sexuality and Political Correctness In Figs
A Summary Of The Ficus carica Life Cycle
The Amazing Fig & Fig Wasp Relationship
Pollination Patterns In Dioecious Fig Species
Cauliflory In Tropical Species Of Figs (Ficus)
Strangler Figs & Banyans: Remarkable Trees
Gall Controversy--Do Fig Wasps Induce Galls?
The Evolution Of Dioecious Fig (Ficus) Species
A Petrified Fig Syconium From The Cretaceous
Nonpollinator Fig Wasps With Long Ovipositors
Figs Of The Holy Land (Their Role In Religions)
Sex Determination In Common Fig (Ficus carica)


4. Black Mulberry (Morus nigra)

The black mulberry (Morus nigra) is a dioecious tree native to western Asia. The bumpy surface of the fruit is due to many tightly-packed, seed-bearing ovaries (drupelets), each with separate styles that appear like black hairs. It is technically a multiple fruit (called a syncarp) composed of a cluster of drupelets superficially resembling a blackberry (Rubus) of the rose family (Rosaceae); however, unlike a blackberry, each drupelet arises from a separate, minute, unisexual (female) flower. Individual flowers do not have petals, but have a calyx composed of four minute sepals. The inconspicuous sepals are closely appressed to the ovary, but are visible under 10x magnification. Mulberry flowers are produced in catkins, typically with male and female catkins on different trees. Male flowers have four stamens while female flowers have a single pistil (gynoecium). In the aggregate fruit of a blackberry, all the drupelets of the cluster (syncarp) come from a single bisexual flower with a ring of many stamens surrounded by five white petals.

See Aggregate Fruit Of The Blackberry (Rubus)

The black mulberry (Morus nigra), a dioecious tree native to western Asia. The bumpy surface of the fruit is due to many tightly-packed, seed-bearing ovaries (drupelets), each with separate styles that appear like black hairs. It is technically a multiple fruit (called a syncarp) composed of a cluster of drupelets superficially resembling a blackberry; however, unlike a blackberry, each drupelet arises from a separate, minute, unisexual (female) flower. Mulberry flowers are produced in catkins, with male and female catkins on different trees. Male flowers have four stamens while female flowers have a single pistil. In the aggregate fruit of a blackberry, all the drupelets of the cluster (syncarp) come from a single flower.


5. Does The Persian Black Mulberry (Morus nigra) Stain A White T-Shirt?

Place your cursor on following image of mulberries on a T-shirt:

  1.   Click to see mulberries removed from stained shirt.
  2.   Click again to see shirt washed in laundry detergent.
  3.   Click again to return to image with mulberries on shirt.  

Click on the photograph to see T-shirt with mulberries removed.

Left: An old, dingy T-shirt stained with black mulberries. Right: The same T-shirt after one washing in detergent with cold water. The red juice comes out in one washing, but it may take several washings to remove the faint dark stain. The stain might have been more resistant if it had dried on the shirt for a longer time period, or if a mordant was used.


6. White Mulberry (Morus alba)

Male (staminate) catkins from the white mulberry (Morus alba), a fruitless variety commonly planted as a shade tree in southern California. Individual male flowers contain four stamens, each with an anther and a filament (see the next photo). At the base of each filament is a fleshy green sepal. Male trees are known as "fruitless mulberry" because they do not produce messy fruits that stain clothing and walkways with reddish-purple juice. Since mulberries are wind-pollinated, male trees produce copious pollen which can raise havoc with hay-fever sufferers.

Microscopic view of a male (staminate) catkin from the white mulberry (Morus alba), a fruitless variety commonly planted as a shade tree in southern California. Individual male flowers contain four stamens, each with an anther and a filament. At the base of each filament is a fleshy green sepal. Male trees are known as "fruitless mulberry" because they do not produce messy fruits that stain clothing and walkways. Since mulberries are wind-pollinated, male trees produce copious pollen which can raise havoc with allergy sufferers.


7. 'Pakistan' Mulberry (Morus macroura)

The Elongate, multiple fruits of the 'Pakistan' mulberry.
This variety is originally from Islamabad, Pakistan.

A female catkin from the 'Pakistan' mulberry, a variety originally from Islamabad, Pakistan. Mulberry flowers are produced in a catkin, with male and female catkins on different trees. Male flowers have four stamens while female flowers consist of single pistil tightly enveloped by four inconspicuous sepals. Each carpel or pistil (also referred as a gynoecium) consists of a forked stigma, a short style and a spherical ovary. Each ovary (carpel) becomes a drupelet and the ripened cluster of drupelets (called a syncarp) is called a multiple fruit. In the aggregate fruit of a blackberry, all the drupelets of the cluster (syncarp) come from a single flower. Seedless, parthenocarpic fruits may be produced without pollination by male trees.

Microscopic view of the female catkin from a "Pakistan' mulberry, a variety originally from Islamabad, Pakistan. Mulberry flowers are produced in a catkin, with male and female catkins on different trees. Male flowers have four stamens while female flowers consist of single pistil tightly enveloped by four inconspicuous sepals. One of the sepals is shown by a red arrow. Each ovary (carpel) becomes a drupelet and the ripened cluster of drupelets (syncarp) is called a multiple fruit.

A. The 'King White Pakistan' mulberry turns light green, then white as it ripens. This cultivar has elongate, sweet fruits like the 'Pakistan' mulberry; however, the fruits do not have the purple juice that stains clothing and walkways. B. 'Pakistan' mulberry.

Mulberry Pie Made By Anita Marks

Delicious mulberry pie made from the elongate multiple fruits of 'Pakistan' mulberry.


8. Silk From A Larva That Eats Mulberry Leaves
 See Silkworms In Thailand 

Leaves of the Chinese white mulberry (Morus alba) and the black mulberry (M. nigra) provide the primary food source for larvae of the silkworm moth (Bombyx mori). The larva feeds on mulberry leaves for about 45 days until it reachs a length of two inches (50 mm). At maturity, the larva spins a cocoon around itself and changes into a pupa, a process known as metamorphosis. The proteinaceous silk filament used to spin the cocoon is secreted from modified salivary glands in the mouth. [Note: Butterflies typically do not spin a coccon and their mummy-like pupa is called a chrysalis.] Silk thread is unwound from silkworm cocoons placed in hot water. In fact, the discovery of silk dates back at least 4,000 years to the Chinese empress Si-ling. According to one story, the empress accidently dropped a cocoon into a cup of hot tea and discovered the strands of the cocoon became loose and unraveled between her fingers. She tried weaving this natural fiber into cloth, and her experimentation eventually lead to the commercial rearing of silkworms (called sericulture) and the silk industry. For centuries, merchants exported this luxurious and valuable textile across vast regions of Asia in silk caravans. Mulberry trees were gradually introduced into European countries by the time of the Roman Empire, but China and Japan continue to produce 75 percent of all the silk in the world today. Most of the silk produced in India is made into saris, while the kimono accounts for nearly 90 percent of all the silk used in Japan. Silk for the famous wedding dress worn by Diana, Princess of Wales was spun by silkworms raised at Britain's silk farm near Sherborne in Dorset.

In San Diego County, Mulberry street in San Marcos was once the site of a mulberry grove planted for silkworm culture during the 1920s. 45,000 mulberry trees were planted in 1926, and eventually the orchard contained more than 100,000 trees. The original San Diego County Silk Company (later called the American Silk Factories) was located along Mission Road, in a large building later occupied by the Teledyne Aero-Cal Company. The San Marcos silk industry collapsed with the stock market crash of 1929 and the perfection of a silk substitute called rayon by the DuPont Chemical Company.

Tussah silk is spun in India and Pakistan from the cocoons of wild silk-producing larvae which feed of "sal" trees (Shorea) in the Diptocarpaceae. The resinous sap of large rainforest trees of the genus Shorea are also the source of dammar resins. Dammars are East Indian and southeast Asian resins similar to copals. Like copals they are shiny and transparent when dry and are used extensively in the paint and varnish industry. Another creamy white or reddish silk called "Eri" is produced from larvae that feed on castor bean plants (Ricinus communis) of the Euphorbiaceae.

In Japan, genetically engineered silkworms containing the gene for human collagen spin their cocoons with silk thread containing this vital protein. Collagen is a component of fibrous connective tissue and is found in skin, tendons, ligaments and bones. In fact, the thickening agent or hydrogel called Gelatin® is made from denatured collagen by boiling animal connective tissues in water. Collagen can now be chemically extracted from silk, providing an economical source of this valuable protein. Pharmaceutical companies hope to harvest human collagen for many applications, including artificial skin and wound dressings.

A single silkworm cocoon may contain more than 3,000 feet (1 km) of continuous filament. Depending on the reference, other estimates range from 1,000 feet to 3,000 yards per cocoon. The exact length is probably somewhere in between these extremes. While most plant fibers are cellulose polymers composed of many glucose subunits linked together, silk is a polypeptide polymer composed of amino acid subunits. This fundamental chemical difference explains why plant and animal fibers react differently to heat, chemical dyes, water and predatory insects. Compared with cotton, silk fibers are less absorbent to water and are subject to denaturation by heat; however, silk fibers generally accept permanent dyes more readily and are more elastic. Raw silk actually consists of two proteins, fibroin and sericin. The fibers are very fine and lustrous, about 1/2500th of an inch in diameter. About 2,000 to 3,000 cocoons are required to make a pound of silk. Based on 2/3 of mile (1 km) per cocoon, ten unraveled cocoons could theoretically extend vertically to the height of Mount Everest, the world's highest mountain. It is estimated that at least 100 million pounds of raw silk are produced each year, requiring nearly 10 billion pounds of mulberry leaves. According to E. L. Palmer (Fieldbook Of Natural History, 1949), one pound of silk represents about 1,000 miles of filament. The annual world production represents 70 billion miles of silk filament, a distance well over 300 round trips to the sun!

Two silkworm cocoons, pieces of silk cloth and a fresh mulberry leaf. Through a remarkable process of assimilation and metamorphosis, the leaf tissue is digested by the larva as it grows in size. At maturity, the larva secretes a proteinaceous filament known as silk from special salivary glands to form a cocoon. Each cocoon is unwound into a continuous silk filament nearly a mile in length, which is twisted into silk thread.

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