PLANTS IN THE HOME

House plants have become an almost routine addition to the home, providing city dwellers, in particular, with a bit of natural beauty. Bringing greenery into their homes helps to compensate for the absence of wild flowers and plants in a built-up area. People choose plants according to their own individual tastes, provide them with the necessary care, and delight in their blossoms or fruits. So house plants fulfil one purpose – the aesthetic – that is of prime importance to Man. However, plants have other important effects. They increase the oxygen in the air, capture and reduce the amount of dust in the atmosphere, and through the process of transpiration, act as humidifiers. Because their electric charge is opposite to that of Man, they also counteract the effects of static electricity. Furthermore, many plants secrete substances that destroy a variety of micro-organisms, thereby making the atmosphere healthier. Finally, the aromatic substances or essential oils produced by some plants, such as eucalyptus and conifers, are beneficial to people with respiratory problems.

The history of house plants

Growing decorative house plants has a history that goes far back into the past. The ancient inhabitants of Mesopotamia – the Hittites, Assyrians and Babylonians – grew flowers for healing, for use in religious rituals and also for decoration. Ancient Egyptian wall paintings and the remains of plants found in tombs provide evidence of their flower cult. Plants accompanied men to their graves so that they could enjoy them even in the afterlife. Besides making animal sacrifices to the gods, the ancients also burned ornamental plants, especially fragrant woods. The ancient Greeks and Romans, renowned for their admiration of beauty, decorated both their homes and temples with flowers. Animals offered as sacrifice were adorned with flowers to please the gods. The Roman deities included the goddess Flora, in whose honour a festival was held in Rome each spring. To this day, flowers and plants remain important decorative objects in Japan and China, and the traditional art of arranging them in beautiful designs continues to be popular. In Japan, Bonsai are handed down from one generation to the next and are even given as imperial gifts.

The first cultivated house plants were of local origin, but with global exploration and commercial expansion, these were soon augmented by foreign species. The choice of plants has always been determined by the conditions of the home in which they are grown. Consequently, in earlier times, popular house plants were ones that required cool conditions during the winter months. However, in the past few decades, houses have changed quite markedly. Small windows have been replaced by picture windows and patio doors, while central heating has virtually done away with open fireplaces. Modern thermostats can now guarantee a temperature that is both constant and much higher than in the winters of the past. These changes have vastly increased the range of plant species that can be grown indoors and have also made alternative methods of cultivation possible. Besides the traditional method of growing plants in a compost-filled flowerpot, many are now grown in nutrient solutions (hydroponics) which eliminates the need for regular watering and feeding. Easy methods of heating and lighting glass cases make it possible to grow tender tropical plants, moisture-loving Nepenthes or orchids, as well as thermophilous succulents.

Vegetational zones

The commonest house plants, whether grown in living rooms, halls, verandas or conservatories, are usually different species from the native flora and often from a different genus. They are generally plants native to the warmer zones of both hemispheres, primarily the sub-tropical and tropical zones, less often the meridional zone or warm regions of the austral zone. This is because decorative plants of the temperate zone generally have a dormant period in winter when the top parts die back – an unwelcome characteristic in plants grown for room decoration. The exceptions are woody evergreens and perennials that have green foliage even in winter.

Because decorative house plants have their origins in different climatic zones, they also have different cultivation requirements. However, even plants from the same zone may have different requirements. For example, cold-loving plants are found even in the tropics, growing above the tree line at heights over 4,000 m (13,000 ft), in mountainous regions like the Andes. The micro-climates of the individual zones are determined both by the altitude and the moisture conditions (oceanic or continental). The total amount and the distribution of rainfall during the growing season, plus precise ecological conditions including soil type, the amount of sunlight and the nature of the plant community, are distinctive and characteristic for each individual species.

Before they could be grown as house plants on a large scale, these exotic species had to become acclimatized to the changed conditions. Then they generally underwent the process of selective breeding. Acclimatization was the factor that made cultivation possible. Original wild species require far greater care than acclimatized species, so the results would not always be commensurate with the effort of cultivating them. Thanks to selective breeding, there is now a wide range of cultivated forms and hybrids that are relatively easy to grow. These often differ from the original in shape or colour. They may, for example, be small and bushy, have bizarre leaves or double flowers or they may have spotted or variegated foliage. Horticulturalists constantly produce new cultivars and hybrids and so the selection of decorative plants is continually changing and increasing.

ORIGIN OF DECORATIVE PLANTS

At first sight, it may seem surprising that even the most familiar house plants, which often grow side by side, come from such widely different parts of the world. However, it is not so strange when one remembers the changes that the continents underwent in past geological periods. As a result, not only individual species and genera, but even whole plant families, evolved in their own separate ways.

The earth is divided into six floristic regions, unequal in size, but bearing living testimony to this geological history . These six regions are further divided into nine zones . Each zone has its own typical vegetation, primarily determined by the prevailing climate. The two most influential aspects of the climate are temperature and moisture.

The Capetown region in South Africa, although the smallest of the floristic regions, has had an enormous influence on the selection of indoor plants. All species of the family Aizoaceae, some of which, such as the genus Lithops, resemble pebbles in appearance, are from this region. It is home to about 450 species of heaths, and the natural habitat of all pelargoniums, clivias, amaryllises, stapelias, dracaenas, sansevierias and many species of the genus Chlorophytum.

By far the largest number of indoor plants originated in the sub-tropical and tropical zones of two similar but separate floristic regions – the tropics of the Old and New Worlds. These two regions have plants with similar formations and which resemble each other externally. The American cacti and the succulent spurges of the African savannas look similar, for example. Nevertheless, more than half the species that make up the flora of these zones are entirely different. Plant families typical of America include Cactaceae, Agavaceae, Bromeliaceae, Cannaceae and Tropaeolaceae. In addition, many species of the Araceae family, such as the genera Anthurium, Dieffenba-chia, Monstera, Philodendron, and the genera Tradescantia, Zebrina, Bougainvillea and Passiflora are characteristic American plants. Typical of the Old World are species of the Nepenthaceae, Musaceae and Pandanaceae families and many genera of the family Orchidaceae. Popular tropical African plants, for example, are species of the genera Saintpaulia, Strelitzia and Ceropegia and tropical Asian genera include Piper, Hoya and Coleus.

Almost all the indoor plants that originated in the holarctic region are from the southernmost vegetational zone, known as the meridional zone, which circumscribes the world. Of course, the requirements of plants from the maritime regions of this zone are quite different from those of plants from the steppe or desert regions in the middle of the continents. Even within the zone, different species and genera are found in the two maritime regions of the Americas (different plants grow on the Pacific coast from those on the Atlantic seaboard), in the Mediterranean region and in the Far East. The meridional zone from the Mediterranean to Central Asia, for instance, is characterized by low summer rainfall. On the other hand, rainfall in the southeast Asian part of the meridional zone is high throughout the year. The most commonly grown house plants from this zone are from the Mediterranean region. They can be transferred outdoors in their containers in summer and do not have to be watered. Typical Mediterranean plants are Nerium oleander and Laurus nobilis. Plants from the Far East are also popular. These, too, can be transferred outdoors in summer but require regular watering. Common examples are Aucuba japonica, Fatsia japonica, Hoya carnosa and Camellia japonica. A great many of the shrubby evergreen indoor plants are native to the maritime regions of the meridional zone.

The flora of Australia is distinctive and different from that of the other regions. Eucalpyti are among the most familiar Australian plants and Callistemons are also popular and widely cultivated. No decorative plants from the arctic region or from the cold zones of the holarctic region are grown as house plants. They are only planted outdoors in parks and gardens.

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