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Chapter: Plant Biology - Human uses of plants

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Uses of Plants: Uses of secondary compounds, Plants as symbols, Horticulture

Rubber, derived from a Brazilian tree, has been highly valued and is used today in tyres. Resins and other aromatic plant products are much used in ceremonies, e.g. as incense, and many dyes come from plants.

PLANTS FOR OTHER USES

Key Notes

Uses of secondary compounds

Rubber, derived from a Brazilian tree, has been highly valued and is used today in tyres. Resins and other aromatic plant products are much used in ceremonies, e.g. as incense, and many dyes come from plants. Plant oils may be used in soap and tannins from trees are used to treat leather.

Drugs

The two most important commercial drugs from plants are caffeine from coffee and tea, and nicotine from tobacco. Coffee and tea are both large cash crops from tropical countries, much prized in Europe and North America. Tobacco depletes soil nutrients quickly and was traditionally grown in new soil as the southern USA was colonized. Other drug plants are grown in scattered plantations and can fetch high prices.

Plants as symbols

Many plants have symbolic associations, often those with other useful properties. Trees that are worshipped usually have an unusual feature. Plants accompany all festivals, and some symbols, such as the olive for peace, are universal. Painting and writing about plants has influenced the conservation ethic.

Horticulture

Plants have been grown in gardens for millennia, for medicinal use, shade or recreation. Some plants have become rare from over-collecting.

Ancient garden plants such as roses are much modified, with petals replacing stamens and multiple hybridizations producing a huge range of cultivars. They are propagated by cuttings and grafting onto roots of a different rose. Most plants can hybridize with related species and this has been widely used. Much propagation is by cloning from cuttings or root division. The growth form can be modified, e.g. the bonsai.

 

Uses of secondary compounds

Secondary compounds within certain groups of plants (Topics J5 and M3) are among the most commercially valuable of plant products. Rubber, the solidified latex of the Brazilian tree Hevea brasiliensis (and occasionally other species)

makes a very strong flexible solid material, particularly after treatment, and remains the main component of tyres. For many years it was the exclusive preserve of Brazil despite attempts to plant it elsewhere and fetched a high price, until in the 1870s when about 70 000 seeds were, in effect, smuggled to Kew in London under the guise of botanical specimens. Just 22 seedlings from these reached Malaysia to form the basis of the millions of hectares of rubber

plantation. Rubber is tapped from the tree without doing any permanent damage and a tree can produce latex for over 20 years. Ecologically, this has the great advantage that an area remains forested, with associated benefits of soil stabilization, and it has helped to stop some Amazonian forests from being exploited.

    Resins and similar aromatic compounds come from many flowering plants, especially trees and shrubs, and conifers. Traditionally, these have been used extensively in perfumes and were believed to have healing and magical properties. They became used for religious and other ceremonies, e.g. frankincense (Boswellia spp.), myrrh (Commiphora spp.), and other members of the Burseraceae all used as incense. Other resins made fine varnishand lacquer and are the base for paints, glues and some cosmetics and organic solvents such as turpentine. Their value has sometimes led to their preservation along with associated flora and fauna. Similarly, many dyes come from plants. These may derive from leaves, roots, flowers, fruits or seeds and include our most ancient dyes such as henna, derived from the leaves of the Asian Lawsonia inermis, and woad from Isatis tinctoria. Many of these became associated with ceremonies. Many soaps and detergents are made partially from plant oils, particularly the oil palm Elaeis guineensis, and coconut, mixed with animal tallow.

    Tannins are dark phenolic compounds deriving from bark, leaves or other parts of many trees such as oaks and are traditionally used to treat leather rendering it more waterproof and keeping it flexible. Tannins in tea leaves give tea its flavor.

Drugs

Some plants contain substances that affect the nervous system, either as stimulants or depressants and these have been used for centuries, often in traditional ceremonies. As they were introduced to temperate regions, by their nature they became enormously desirable among wealthy people and at times commanded high prices. The most important of these in commercial production today are caffeine from tea and coffee, nicotine from tobacco and various medicinal drugs  as well as alcohol considered. The most commercially valuable is coffee deriving from species of Coffea, mainly Coffea arabica which comes from Ethiopia originally, but is now planted throughout the tropics. It requires adequate rainfall and is often grown in hilly districts. The markets were controlled by Arab traders until the 17th century when it was planted elsewhere and it is one of the most important export crops from South America. Tea, Camellia sinensis, originally from China, was also much desired. Tea plantations in India became a foundation of British occupation and high taxes on tea set by Britain on exports to North America were one of the triggers for the American war of independence.

     Tobacco, Nicotiana tabacum, originates in the American tropics and is planted mainly in America. It is a plant requiring a fertile soil which is depleted rapidly, and tobacco production requires high inputs. It thrived on the newly cultivated soil in southern North America. Medicinal drugs such as opiates from poppies and some, mainly illegal, psychoactive drugs such as cannabis and coca derive from plants.

 

Plants as symbols

Plants have been used ceremonially since paleolithic times. Although they may have had uses as food, timber, medicine or narcotic drugs, certain individuals were frequently singled out for different uses. This has had an effect on the ecology of many regions of the world since certain plants have been preserved at the expense of others and some owe their survival to such a role. The ginkgo tree  has an edible seed but may owe its survival to its ceremonial role as it is unknown outside cultivation. On every continent certain plants, often trees, are traditionally worshipped, leading to the preservation of certain places as sacred groves or certain strategically positioned individual trees as sacred. Often, trees with an unusual shape are revered, such as the baobab with its swollen trunk in Africa, or the sacred banyan of India with its numerous aerial roots spreading the tree (a fig) over large areas. The properties or associations of some plants have led to their symbolic uses, for example the soothing action and antiseptic properties of rose oil probably led its importance as a symbol of all aspects of love. All festivals involve plants, and flowers are strongly associated with birth, marriage and death. The olive branch is a global symbol of peace, coming from early Mediterranean civilizations when olive oil was one of the most valuable of commodities and used as a kind of currency.

    Plants feature widely in numerous paintings and writings, these having a marked influence on the way in which we perceive our landscape and to some extent have shaped the environmental conservation ethic of today.

 

Horticulture

For millennia in Roman, Chinese, Arabic and other civilizations, plants have been planted in pleasure gardens. Some had herbal and medicinal use or were planted for shade but some were planted purely for aesthetic reasons and certain species gained a kind of ‘cult’ significance, such as the plum blossom in China. During the 19th century with the huge increase in knowledge and travel, the demand for exotic plants in Europe and, to an extent, North America, became intense, leading to many plant-collecting expeditions. Some plants were over-collected, even in Europe, and became rare  as a result. The interest in the rarer ferns of Britain at this time resulted in local extinction and the continuing rarity of a few species. Many orchid species are prized for their exotic blooms, but some have restricted natural ranges and are often scarce. A black market with high prices for spectacular rarities has grown, making them rarer and some in danger of extinction.

     Enormous numbers of plant species are now cultivated purely for their ornamental value. Most plants are capable of hybridizing with related species, sometimes leading to sterility but more frequently partial sterility, so further hybridization is possible. In some groups, such as the orchids, species from several genera can be fully interfertile, making hybrids particularly easy to generate. In nature hybrids are usually at a selective disadvantage. Much reproduction for horticulture is done by cloning either through taking cuttings or dividing roots. Most roses and many other garden plants have double flowers, with many of the stamens replaced by petals; in many plants, petals probably derived initially from sterile stamens . Cultivated roses come from a wide range of species distributed across Eurasia from extensive breeding and hybridization through the centuries, andcultivars, those cultivated varieties which are marketed commercially, are mostly sterile or partially sterile multiple hybrids.

   Many woody plants are reproduced by cuttings, small pieces of a side stem or more rarely a leaf that is separated and rooted in soil, making all plants of one cultivar genetically identical and part of one clone. Some, such as roses and fruit trees, are grafted onto the rootstock of a wild or vigorous related species because these roots are stronger and the specimen grows more vigorously. Grafting involves detaching the stem, or scion, around ground level and fixing it to the rootstock that has been similarly detached, following which the plantsestablish vascular connections. These roots may produce other stems directly, known as suckers, which are genetically the root not the cultivar. Occasionally, cells from the rootstock of a graft occur above ground, mixed with cells from the scion, making a chimera, e.g. Laburno-cytisus, which has brownish flowers deriving from a mix of yellow flower cells from the Laburnum scion and purple flower cells from the Cytisus stock.

    A plant is flexible in its growth form and can be trained into shapes quite unlike the wild plant. Hedge trimming stimulates axillary side shoots to grow and this can be refined further to topiary, making a tree a recognizable shape, or training fruit trees to espaliers or fans. The most extreme example is the miniaturizing of trees to bonsai, developed in Japan, through elaborate root trimming and other techniques applied over a long period.

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