Plant modifications to extreme conditions see also p

A survey of plants worldwide shows what impressive structural and physiological modifications they possess to survive in demanding habitats. A few examples of British species are described here.

Marram grass (Ammophila arenaria) living on sand dunes controls water-loss by means of leaf lamina which in cross-section is shown to be rolled up. It also possesses extremely long roots (see page 78).

The yellow water lily (Nupar lutea) shows the following modifications: leaves with a thin cuticle (but with numerous stomata), large flat leaves and a stem with air sacs.

The coastal habitats provide many examples of species which must conserve water in salty, windy coastal conditions. Glasswort (Salicornia stricta) is a succulent with greatly reduced leaves which have a thick cuticle, and its stomata remain closed most of the day. The species is able to extract water from the seawater and is tolerant to internal salt concentrations that would kill most plant species (see plasmolysis, p123). An interesting halophyte is the sugar beet plant which was bred in France from a native coastal species. It is the only crop grown in Europe that receives salt (sodium chloride) as part of its fertilizer requirements.

Conservation

From the content of preceding paragraphs it can be seen that the provision of as extensive a system of varied habitats, each with its complex foodweb, in as many locations as possible, is increasingly being considered desirable in a nation's environment provision. In this way, a wide variety of species numbers (biodiversity) is maintained, habitats are more attractive and species of potential use to mankind are preserved. In addition, a society that bequeaths its natural habitats and ecosystems to future generations in an acceptably varied, useful and pleasant condition is contributing to the sustainable development of that nation.

The ecological aspects of natural habitats and horticulture have been highlighted in recent years by the conservation movement. One aim is to promote the growing of crops and maintain wildlife areas in such a way that the natural diversity of wild species of both plants and animals is maintained alongside crop production, with a minimum input of fertilizers and pesticides. Major public concern has focused on the effects of intensive production (monoculture) and the indiscriminate use by horticulturists and farmers of pesticides and quick-release fertilizers.

An example of wildlife conservation is the conversion of an area of regularly mown and 'weedkilled' grass into a wild flower meadow, providing an attractive display during several months of the year. The conversion of productive land into a wild flower meadow requires lowered soil fertility (in order to favour wild species establishment and competition), a choice of grass seed species with low opportunistic

A xerophyte is a plant adapted to living in a dry arid habitat.

A hydrophyte is a plant adapted to growing in water.

A halophyte is a plant adapted to living in a saline environment.

properties and a mixture of selected wild flower seed. The maintenance of the wild flower meadow may involve harvesting the area in July, having allowed time for natural flower seed dispersal. After a few years, butterflies and other insects become established as part of the wild flower habitat.

The horticulturist has three notable aspects of conservation to consider. Firstly, there must be no willful abuse of the environment in horticultural practice. Nitrogen fertilizer used to excess has been shown, especially in porous soil areas, to be washed into streams, since the soil has little ability to hold on to this nutrient (see p367). The presence of nitrogen in watercourses encourages abnormal multiplication of micro-organisms (mainly algae). On decaying these remove oxygen sources needed by other stream life, particularly fish (a process called eutrophication).

Secondly, another aspect of good practice increasingly expected of horticulturists is the intelligent use of pesticides. This involves a selection of those materials least toxic to man and beneficial to animals, and particularly excludes those materials that increase in concentration along a food chain. Lessons are still being learned from the widespread use of DDT in the 1950s. Three of DDT's properties should be noted. Firstly, it is long-lived (residual) in the soil. Secondly, it is absorbed in the bodies of most organisms with which it comes into contact, being retained in the fatty storage tissues. Thirdly, it increases in concentration approximately ten times as it passes to the next member of the food chain. As a consequence of its chemical properties, DDT was seen to achieve high concentrations in the bodies of secondary (and tertiary) consumers such as hawks, influencing the reproductive rate and hence causing a rapid decline in their numbers in the 1960s. This experience rang alarm bells for society in general, and DDT was eventually banned in most of Europe. The irresponsible action of allowing pesticide spray to drift onto adjacent crops, woodland or rivers has decreased considerably in recent years. This has in part been due to the Food and Environment Protection Act (FEPA) 1985, which has helped raise the horticulturist's awareness of conservation (see page 289).

A third aspect of conservation to consider is the deliberate selection of trees, features and areas which promote a wider range of appropriate species in a controlled manner. A golf course manager may set aside special areas with wild flowers adjacent to the fairway, preserve wet areas and plant native trees. Planting bush species such as hawthorn, field maple and spindle together in a hedgerow provides variety and supports a mixed population of insects for cultural control of pests. Tit and bat boxes in private gardens, an increasingly common sight, provide attractive homes for species that help in pest control. Continuous hedgerows will provide safe passage for mammals. Strips of grassland maintained around the edges of fields form a habitat for small mammal species as food for predatory birds such as owls. Gardeners can select plants for the deliberate encouragement of desirable species (nettles and Buddleia for butterflies; Rugosa roses and Cotoneaster for winter feeding of seed-eating birds; poached-egg plants for hoverflies).

Figure 3.6 Pyracanthas have good winter food for birds

It is emphasized that the development and maintenance of conservation areas requires continuous management and consistent effort to maintain the desired balance of species and required appearance of the area. As with gardens and orchards, any lapse in attention will result in invasion by unwanted weeds and trees. In a wider sense, the conservation movement is addressing itself to the loss of certain habitats and the consequent disappearance of endangered species such as orchids from their native areas. Horticulturists are involved indirectly because some of the peat used in growing media is taken from lowland bogs much valued for their rich variety of vegetation. Considerable efforts have been made to find alternatives to peat in horticulture (see p387) and protect the wetland habits of the British Isles.

Conservationists also draw attention to the thoughtless neglect and eradication of wild-ancestor strains of present-day crops; the genebank on which future plant breeders can draw for further improvement of plant species. There is also concern about the extinction of plants, especially those on the margins of deserts that are particularly vulnerable if global warming leads to reduced water supplies. In situ conservation mainly applies to wild species related to crop plants and involves the creation of natural reserves to protect habitats such as wild apple orchards and there is particular interest in preserving species with different ecological adaptions. Ex situ conservation includes whole plant collections in botanic gardens, arboreta, pineta and gene-banks where seeds, vegetative material and tissue cultures are maintained. The botanic gardens are coordinated by the Botanic Gardens Conservation International (BGCI), which is based at Kew Gardens, London, and are primarily concerned with the conservation of wild species.

Large national collections include the National Fruit Collection at Brogdale, Kent (administered by Wye College) and the Horticultural Research International at Wellesbourne, Birmingham, holds vegetables. The Henry Doubleday Heritage Seed Scheme conserves old varieties of vegetables which were once commercially available but which have been dropped from the National List (and so become illegal to sell). They encourage the exchange of seed. The National Council for the Conservation of Plants and Gardens (NCCPG) was set up by the Royal Horticultural Society at Wisley in 1978 and is an excellent example of professionals and amateurs working together to conserve stocks of extinction threatened garden plants, to ensure the availability of a wider range of plants and to stimulate scientific, taxonomic, horticultural, historical and artistic studies of garden plants. There are over 600 collections of ornamental plants encompassing 400 genera and some 5000 plants. A third of these are maintained in private gardens, but many are held in publicly funded institutions such as colleges, e.g. Sarcococca at Capel Manor College in North London, Escallonia at the Duchy College in Cornwall, Penstemon and Philadelphus at Pershore College and Papaver orientale at the Scottish Agricultural College, Auchincruive. Rare plants are identified and classified as ' pink sheet ' plants.

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