“Plant and Man”
Homo sapiens has observed, collected and used plants for food and medicine for tens of thousands of years. At the end of the last Ice Age, about 11,500 years ago, humans started to select the plants that most suited their needs. By doing so, farmers influenced the evolution of countless plant species over the millennia. Now is the time to tell this story from the plants’ point of view. Our food and drink come mainly from plants. We dress and preserve food with plants or plant derivatives. We smoke, poison ourselves and alter our state of mind with plants. We stimulate and calm ourselves with plants. We enjoy sugar and chocolate. Plants provide us with an endless array of substances and active principles to treat seasonal ills, aches and pains — not to mention serious diseases such as tumors. We use plants to make ourselves more beautiful, to smell nice, to wash ourselves and to color the world. Plants interact with insects, and both plants and insects with us. Many plants have used the human race as a means to spread across the world; in some respects, we can say that plants have domesticated us!
The Dizzying Depths of Time
Before we embark upon our journey through the coevolution of humans and plants, let us explore the vast tracts of time through our eyes, and those of our plant cousins. First, we need to step back millions of years. Amber is a fossil resin that can drop small insects and plant remains. Although extinct creatures cannot be cloned from the blood found inside preserved mosquitoes, today we know that amber maintains the exterior of the mosquito intact, and preserved its genetic material for almost 25 million years. Now let’s go back thousands of years. A piece of fossilized bread, found in Ledro (Trentino), is almost 4,000 years old! The first gatherers probably selected species of spelt (Triticum spelta) with tough ears, because they were easier to harvest. The crossing of these wild species with domestic varieties produced large quantities of bare seeds, which led to the creation of grain. And finally to the age of modern science: the great Swedish physician Carl Linnaeus (1707-1778) drew up the first systematic classification of plants in 1753; this key work distinguished plants by their specific traits, places of origin and reproductive systems.
A Long History of Coevolution
Around 11,500 years ago, the ice Age began to lessen its rigorous grip. Homo sapiens then began to cultivate some of the wild plants he had been gathering, and to raise some of the less dangerous animals he had been hunting. Humans modified plants, and animals, over the course of time, through selective cross breeding. The domestication of plants and animals did not only occur in the Middle East, but began independently in various parts of the world, perhaps six or seven times, in a period between 12,000 and 7,000 years ago. With human settlements came new, more complex social hierarchies. In every region, the local plants and animals were domesticated in the context of a diversity we still have to fully comprehend. From these early settlements, knowledge spread in two ways: neighboring groups learned the techniques through cultural transfer, or with demographic expansion; the agricultural colonists themselves migrated to new areas. The growth in population led to the diffusion of peoples, colonization, a mixing of races — and conflict. In virtue of the new relations between the human race and plants, the world was no longer the same place.
The Beginning of a Revolution
Agriculture was not something decided around a table and put into effect the next day. It was neither a discovery nor an invention. A prospective farmer did not just decide to follow a more sedentary life; after all, he had no model to follow. At the beginning, the choice would have been a gradual one, and would have included both lifestyles. It was just an alternative; but one that was to revolutionize human history. The domestication process differed from place to place, depending on the species available and the nature of the land, mountains and climate; more often than not, it was just an accidental process. There were some rather bizarre situations too, like the case of several Native Americans populations who became sedentary, but cultivated nothing. The Japanese and the Peruvians, instead, first chose a place to settle in, and only a lot later started to grow crops. On the contrary, some peoples became farmers but chose not to settle down — as in the case of the populations of New Guinea. So, initially, the proto-farmer of 11,000 years ago may have regarded his vegetable patch as something to fall back on in hard times.
Sculpting Plants Via Domestication
The domestication of a plant can be defined as the process whereby man selects those genetic mutations which will make it more useful or more suitable for consumption. Some cultivated varieties have characteristics that would be counter-productive in Nature, such as over-sized fruits: domestic apples, for instance, are three times the size of wild ones; peas are ten times bigger, and corn cobs have increased from 1 to 45 cm! In other cases, the wild species is harmful, and sometimes even lethal (e.g., almond) and in yet others, disadvantages in nature — such as seedless fruits — are perpetuated by selection (e.g. banana); therefore, mutant individuals, which under natural conditions would be eliminated by natural selection, have been (more or less deliberately) selected by man exactly on account of their mutations.
Eaters of Tubers … As Long As They’re Cooked!
Tubers were widespread in the dry African climate of two million years ago. However, they were impossible to digest raw. Thanks to sporadic fires in the savannah, at a certain point species belonging to the Homo genus started eating them cooked or semi-cooked. Tubers contain a type of starch which cannot be digested if eaten raw. The cooking process breaks down the chemical structure of this type of starch, so it can be assimilated by the gut and thus become a good source of nutrition. It is thought that this unattractive food played a crucial role in human history because, according to researchers from Harvard, along with animal proteins, it was co-responsible for the evolution of our great and hard-to-please brain. So, our ability to use fire to cook tubers has made us what we are today.
A Chamomile for Neanderthal
Did Neanderthal man drink lemonade? Well, maybe not as a steaming cup of herbal tea, but our Neanderthal cousins did chew it. Evidence of this comes from an extraordinary discovery that was made at an archaeological site at El Sidròn, in the north of Spain. Small grains of azulene, the active principle of chamomile, were found on remnants of dental plaque. As the plant tastes unpleasant, and has little nutritional value, Neanderthals were probably using it for therapeutic purposes, understanding its calming properties. So, Neanderthal used plants not only as food, but also as medicine. Vegetable compounds such as coumarins (from Achillea, a plant with analgesic and anti-inflammatory properties), traces of nuts, raw and roasted vegetables were also found in the teeth. Evidence of a very varied diet!
The Chimp’s Pharmacy
A primate’s size tells us a lot about what he eats: smaller primates mostly eat insects, while larger ones like the anthropomorphic monkeys are mostly vegetarian. Orangutans, gorillas, chimpanzees and bonobos mostly eat plants: leaves, flowers, fruit, seeds and bark. To a lesser degree, they integrate their diet with proteins derived from insects, small invertebrates, and bird’s eggs. Only species of the Pan genus; i.e., the chimpanzee and the bonobo, our closest cousins, also eat other mammals. But the plant world is not only a source of nutrition: when chimps feel unwell, they seek specific plants, which have demonstrated pharmacological properties. They sometimes integrate their diet with the tougher parts of a plant, such as the bark or the stem, which have little nutritional value, but are rich in chemical compounds that play a preventative role in diseases such as malaria, infections or other disturbances: a pharmacy within arm’s reach!
The Pacific Yew (Taxus brevifolia) was an essential tree for the Native Americans of the Pacific coast, as it yielded an excellent wood that was used to make hunting bows, and its leaves made an infusion that served as a tonic and medication. In 1958, the US National Cancer Institute launched a vast research program into the anti-tumor properties of plants. It analysed more than 35,000 species, and discovered that the bark of the Pacific yew held a substance, taxol, which proved extremely useful as an active principle in the formulation of anti-cancer medication — particularly against womb, breast and lung cancer. The results paved the way for further similar research into the periwinkle (Vinca rosea) and the Happy tree (Camptotheca acuminata). The latter, which is native to China and grows also in the Botanical Garden in Padua, is the main source of camptothecin — a highly effective drug used in the treatment of ovary, lung, colon and rectal cancer.
Chewing Coca Leaves
The Quechua, the descendants of the ancient Incas, live in the Andean areas of Peru, Bolivia and Ecuador. For this people, chewing Coca leaves has a meaning that transcends the psychoactive effects of cocaine. It is a ritual that plays a fundamental role in the preservation of shared cultural principles. The coca plant is an integral part of this people’s lives, and a powerful symbol of cultural identity. Chewing the leaves identifies a person as a member of the community. The Quechua use the coca plant as a balsam for treating the pain of life. The substances contained in the leaves of Erythroxylum coca can alleviate pain and lessen the perception of fatigue and hunger. But chewing coca leaves should not be confused with taking cocaine, as it has no long-term effects on the body and mind. It is simply how Quechua culture has adapted to life in the highlands of the Andes.
The Neem, Tree of the 21st Century
Azadirachta indica is part of the Meliaceae family, and is native to India and Myanmar, where it is known as Nim or Neem. For a number of years, this has been hailed as one of the most promising species for the future of humanity, to the extent that the UN has declared it “Tree of the 21st Century”. Knowledge of the tree’s medicinal properties is firmly rooted in the past. It is one of the main plants in traditional Indian medicine, where each part of the tree is used to treat a range of ailments. Consequently, it is nicknamed “the village pharmacy”. For over 2,000 years, millions of people have cleaned their teeth with Neem twigs, treated skin complaints with its leaves, and have used it in infusions as a tonic or to fight bacteria. All of these properties have reawakened the interest of Western medicine. Research continues to discover that this plant has new and promising medical properties, including ones that may help to fight tumors and malaria.
Shamanism and Medicine
In traditional cultures, medicine, magic and religion often mingle. The shaman is a prime example of someone who combines knowledge of nature and medicine, magical rituals and prayer. To gain entrance to the spirit world, shamans often seek to alter their state of mind, and hallucinogenic plants are essential for this. Tribal people use a range of species to achieve similar effects, exploiting the psychoactive principles that the plants share. This traditional knowledge has led to the spread of new types of recreational hallucinogens, such as the peyote, but it may also provide inspiration for new medicines that treat the nervous system and other parts of the body.
Humans have always tried to treat their ailments with remedies from their natural surroundings, and have devised a range of medicines based on their environment, plants, animals and climate — not to mention their traditions and culture. Ethno-medicine is the study of traditional remedies and looks at prevention, hygiene and treatment in magical, spiritual and empirical medicine. There is increasing interest in this field, even from the World Health Organization (WHO). In an increasing number of cases, the formulae for new pharmaceuticals, or the discovery of a new active principle, have been based on traditional knowledge of botany or herbs. The millions of dollars these discoveries create, however, do little to improve the quality of life of the peoples who have handed down their invaluable knowledge for centuries — if not millennia.
Drugs: Psychoactive Plants
Humans have tried to alter their mental state since the dawn of history. In every corner of the globe, peoples have found a way to use psychoactive substances (almost always plants) that induced euphoria, relieved fatigue and triggered mystical experiences. We have tasted, boiled, inhaled, smoked and distilled anything we could get our hands on. The plant kingdom — the world’s biggest chemical laboratory — is our main supplier of psychoactive substances, which plants normally produce for entirely different purposes, often as a defence against predators. Some of these plants have been known since pre-historic times (cannabis, opium, tobacco and vine), and they have exploited our needs to have themselves transported and grown across the planet. Although we are aware of the harm that taking narcotic substances causes, and despite the fight against illegal producers, the success of their domestication continues unabated.
Plants for Smoking: Tobacco
When Columbus landed in what is modern-day San Salvador on 1492, the indigenous population greeted him with gifts, including some dried, foul-smelling leaves. He accepted the gift, but not knowing what to do with it, returned to his ship and threw the leaves into the sea! Pre-Columbian peoples, however, had been using the leaves of Nicotiana tabacum for centuries. They gave strong symbolic value to smoking tobacco; it was a gesture of goodwill, and the nicotine created a feeling of well-being. Once tobacco had arrived in Europe, smoking became fashionable in the salons of the well-to-do; it was later to become a common ‘vice’ and one of history’s biggest businesses, as production continued to increase. It is only in recent decades, since research has shown the dangers of smoking, that the trend is in decline — at least in Europe and the United States.
Cocoa: "Food of the Gods"
The Swedish botanist Carl Linnaeus called the cocoa plant the “food of the gods” (Theobroma cacao). Its seeds — the cocoa bean — are the main ingredients for one of the world’s most delicious beverages: chocolate. Native to Central and South America, it was domesticated by the pre-Columbian peoples about 2,000 years ago. Its seeds were used to prepare prized foods — such as chocolate-coated chicken — and beverages, mainly for the nobility. Cocoa beans were also used as currency. Once Christopher Columbus and the Spanish conquistadores had brought it to Europe, the cocoa plant spread to nearby tropical zones, and today parts of Africa are the biggest cocoa producers. Unfortunately, as it often happens, the enormous amount of money in the cocoa business only trickles down to a small part of the population in producer countries.
Preservation and Taste: Spices Open Routes Across the Globe
Every peppercorn or cinnamon stick, if listened to carefully, has a tale to tell. In the mid-1400s, ships were leaving Europe on a quest to discover new trade routes, especially to India. Explorers went searching for gold, precious stones and exotic species, which were extremely valuable commodities, and the source of many a furious row in the markets of coastal cities — the crossroads between East and West. Spices are generally the seeds, fruits, bark and leaves of plants, which are then dried and used, whole or powdered, to flavor and preserve food. For centuries, their medicinal properties earned Italian apothecaries the epithet of ‘speziali’ (from the Italian word spezia, meaning ‘spice’). Each spice has its own way of tuning in to our senses. Consequently, spices appear on the tables of different peoples from distant countries today just as they did centuries ago.
The Friendship Drink: Coffee
When we offer a friend a coffee, few of us think of the cultural, social and economic importance of this drink. With origins in Ethiopia, coffee is second only to petroleum as the world’s most precious legal export, and is the major source of our most common psychoactive substance. At different times in history, coffee has been used as an aphrodisiac, a laxative, a nerve tonic or an elixir of long life. Some would even say that western society, with its frenetic lifestyle, would collapse without the regular consumption of caffeine. And yet, after all, this is just the seed of a berry that grew wild on the mountains of Ethiopia, to begin with.
“Plant and the Environment”
Geographical Distribution and Importance of Tropical Rainforest
High, constant temperatures throughout the year (24-26° C) and abundant rainfall (1,800-3,500 mm/year, and up to 10,000 mm), distributed evenly through the year, define an environment that knows no limiting factors on plant growth. The natural vegetation of such places is tropical rainforest — a dense, complex structure, characterized by evergreen species, tall trees (up to 40 m), and elevated biomass and biodiversity. Forests of this type can be found: — in the Americas, in particular in Amazonia and hilly areas at the foot of the Andes; — in Asia, especially in the Indo-Malesian area, with various sub-areas as far as Eastern Queensland; — in Africa (the Congo basin, the southern part of Nigeria, Benin and Togo, and Eastern Madagascar).
The “Princess of Plants”
The great naturalist Linnaeus defined it Principes plantarum for its majestic bearing and dense crown of leaves. On September 27, 1786, during his Viaggio in Italia (Italian Tour), Goethe visited the Botanical Garden in Padua, and — as we have seen — he was particularly impressed by the palm that later came to be known as Goethe’s Palm. Palms belong to the Arecaceae family, and are mainly found in the tropical and sub-tropical areas of the Old and New Worlds. Only a few can live in temperate regions. There are about 2,500 species of palms, the best known being the Date palm, the Coconut palm and the Oil palm. The ‘dwarf’ variety is the only spontaneous palm in the Mediterranean region. Palms play a vital role in the human economy, as they provide foodstuffs, building materials, cosmetic ingredients and handicraft materials; thus, the palm can rightly be called the “Princess of Plants” — and the Queen in our Garden of Biodiversity.
In this section, there is also a reproduction of the impressive specimen of the fossil palm Latanites maximilianii De Visioni (1867): (with) more than 3 meters long, this is the undoubted ‘star’ of the Palm Hall in the Geological and Paleontological Museum in Padua, and it comes from the Oligocene beds of Sostizzo (about 30 Million years), near Salcedo (Vicenza). This beautiful fossil was discovered in 1863, eleven years after the first report of (a) fossil palm (being) discovered in Italy. The species was dedicated to Maximilian I (1832-1857), monarch of the Second Mexican Empire, and a passionate botanist. Maximilian, (the) younger brother of the Austrian Emperor Franz Joseph, died executed by a firing squad some months before the publication of the De Visioni’s paper. The counterpart of Latanites maximilianii is presently housed in the Museo dei Fossili (Fossil Museum) at Villa Godi-Malinverni (Lugo, Vicenza), together with a remarkably complete 9 meters-long specimen of Latanites maximilianii Fiore (1932).
The Bromeliaceae are a large family of higher plants, and belong to the Monocotyledons. There are about 2,700 species, found mostly in the tropical belt of the American continent (New World). Many Bromeliaceae live as epiphytes at the tree level of tropical rainforests. The Botanical garden in Padua has a rich collection of about fifty species belonging to ten different genera (Aechmea, Billbergia, Cryptanthus, Fosterella, Guzmania, Neoregelia, Nidularium, Pitcairnia, Tillandsia, Vriesea), some of which are exhibited in the new glasshouses, in the section dedicated to neo-tropical rainforests. The pineapple (Ananas comosus) is a versatile plant which also belongs to the Bromeliaceae family. Grown for its fruit, it has curative properties, and is used locally for its fibres too.
Bromeliaceae Get Water and Nutrients Without Contact with the Ground
The Bromeliaceae have evolved ecologically and morphologically in a fairly uniform way. Many are epiphytes, which wind their roots around the branches of trees in the rainforest. They have a rather short stalk with a crown of leaves in a rose-like shape. At the center of the crown, is a cavity where they can gather water, and the nutrients they need. These nutrients come from rainwater, which trickles down the branches, and also from organic and vegetable matter from organisms which live in the cavity itself, a sort of micro-habitat in its own right: mono-cellular algae, insects, worms, tree frogs, salamanders, small snakes, crustaceans, etc.
The Araceae are a family of Monocotyledons with 107 genera and over 3,700 species, found in the tropical rainforests of the New World but also present in the temperate regions of the northern hemisphere and in the tropical belt of the Old World. In many Araceae, pollination occurs in a strange way: the female flowers open first, attracting insect pollinators with a smell of rotting meat and excrement. When they get to the bottom of the inflorescence, the insects are trapped by hairs and, in their struggles to escape, shed the pollen they have picked up from previously visited flowers. As the male flowers bloom later, there is no danger of self-pollination. Finally the hairs become limp, so the insects escape to pollinate other flowers. Many plants of the Araceae family are thermogenic, i.e. they produce heat. The heat enables the smelly chemical compounds to evaporate and create a warm refuge for the insects. The size of the inflorescences can vary considerably: those of Amorphophallus titanium can reach 3 meters, even though they only last a few days; curiously, the family also comprises aquatic plants of less than a millimetre.
Philodendron Move in the Forest
There are 900 species in the genus Philodendron, but diversity within the genus is yet to be fully explored. Only a few varieties grow directly from the soil; the majority are epiphytes of hemi-epiphytes. Many play an important role in the ecosystems of tropical rainforests, both on account of their sheer numbers and also their great capacity to climb on plants and trees thanks to their adventitious aerial roots, and thus move from layer to layer in the forest. Philodendra are involved in a complex network of relations with animal organisms too, including ants. Some species of hemi-epiphytic Philodendron begin their lives as seeds transported by animals in the branches of the forest’s upper layers. As soon as the new plant has reached sufficient dimensions, it starts to produce temporary aerial roots which reach the ground, where it can directly access the soil’s mineral salts. The plant ’s first priority is to seek a spot in a well-lit area, and then it starts to look for nutrients. Other species of Philodendron behave in an opposite manner: the seed germinates in the ground, the plant grows, and then starts to climb and become an epiphyte. The plant does not need to germinate near a tree, because it will move towards one anyway, since initially it will seek out shade — like that cast by a tree, precisely. Once the plant finds a tree, it will move upward in search of light, thus shortening and strengthening its internodes.
The Adaptation of Herbivores
Plants have developed effective systems to prevent or reduce attacks by herbivores. Herbivores, however, have also evolved with a series of adaptations designed to neutralise plant defences. They have developed generic, widespread adaptations — such as highly efficient grinding teeth that make short work of a plant’s tight fibres, rumination, as well as prehensile lips and tongue to tear off leaves and grass more easily. Some herbivores have even adapted to deal with a plant’s toxic secondary metabolites. The black Rhinoceros, for example, is immune to the poisonous alkaloids contained in the latex of some Euphorbia species. Others have learned to chew the thorny parts of a plant without particular difficulty thanks to a mucus membrane that protects the mouth.
Protection Against Being Eaten in Deciduous Tropical Rainforest
Animals first started eating plants in the remote Paleozoic period. In terms of herbivore insects, tropical rainforests, however, have a much more complex network of herbivore-plant relations than forests in temperate climates; therefore, plants are also more specialized in defending their most vulnerable parts; i.e. the young leaves. Defence mechanisms include rapidly expanding leaves and earlier maturity, as well as a higher content of potentially toxic secondary metabolites — such as alkaloids.
The Orchidacae are part of the Monocotyledon angiosperms, and embrace a vast number of species (more than 21,000), found in nearly all terrestrial ecosystems, but more common in tropical and, to a lesser extent, temperate regions. The reproductive biology of Orchidacae is highly specialized, but not as far as pollination and seed germination. Although there is a great variety of species, the flower’s morphology and the way it is pollinated are very similar for all of them. Differences arise in the survival strategies, some being rooted in the soil, while others are epiphytes, climbers or lianas. (e.g. Vanilla planifolia); there are also heterotrophic parasites. The Botanical Garden in Padua houses a rich collection of orchids, with 140 Neotropical, African and Asian species, mainly epiphytes.
In the Biodiversity Garden the plants have a common name and an official scientific (binomial) name. The latter is still written in Latin today, and it has the genus name followed by the species’ name (i.e., Homo sapiens are the sapiens species of the Homo genus). This simple method is called binomial nomenclature, and was established by Carl Nilsson Linnaeus in the first half of the 18th C. The first methodical plant classification system — a key work from 1753, listing specific differences, place of origin, and above all the reproductive organs as distinctive criterion — are also owed to this great Swedish physician and botanist. The detailed description of the reproductive organs and sexual systems of plants, however, caused him some trouble. They were too realistic, and he was accused of ribaldry.
Ecological Importance of Humid Tropical Areas
Humid tropical areas — and temperate ones as well — are environments of great productivity and biodiversity, with vital ecological functions, such as carbon stockage, water management and flood protection, and removal of pollutants through phyto-depuration. Humid tropical areas are home to species which have been fundamental to the evolution of human society: Asiatic rice (Oryza sativa) and African rice (Oryza glaberrima) provided food, while papyrus (Cyperus papyrus), used by the Egyptians as paper, led to its use as support for writing, and the development of culture. However, humid areas are also fragile habitats, more sensitive to environmental threats than other ecosystems, and their conservation is guaranteed by international agreements, such as the Ramsar Convention of 1971.
Giovanni Marsili, Botanist and Book-lover
Before coming to Padua to head the Botanical garden in 1760, Giovanni Marsili had been on a long journey of discovery in Europe’s main cities and academies. In London, he had even been admitted to the prestigious Royal Society. Doctor, botanist, literary man and book-lover, Marsili was particularly interested in medicinal plants. He knew some of the period's most notable scientists, such as the doctors Giovanni Battista Morgagni and Lazzaro Spallanzani, and today, a species of aquatic ferns — Marsilea — bears his name. He was responsible for the creation of the Botanical garden’s luxuriant arboretum, which at the time contained 165 exotic and mountain trees, amongst which were the Plane tree (Platanus orientalis), Lawson Cypress (Chamaecyparis lawsoniana), Cedar of Lebanon (Cedrus libani) and Shagbark Hickory (Carya ovata). Marsili’s collection of 2,417 volumes was later purchased by his successor, Giuseppe Antonio Bonato, who in 1834 donated it — together with his own books — to the gardens, thus laying the foundations for the library. A bust of Marsili is still on display among the plants in today’s gardens.
The genus Acacia: Geography, Ecology and Uses
The Acacia genus — from the Greek akis, meaning ‘thorn’ — belongs to the Fabaceae, or Leguminosae, family. It comprises about 1,300 woody tree and/or shrub species, which, as the name suggests, often have thorns. The Acacia genus is the most diverse on the Australian continent, to which it is native. Australia is still home to the majority of these species (just under 1,000), with only a few hundred populating the tropical and warm-temperate zones of Europe, Africa, southern Asia and the Americas. Acacias are typically found in the savannah environments, where they are central to a series of relationships with the animal kingdom. The Acacia genus has myriad uses, and it features heavily in the cultures and traditions of many peoples worldwide. These trees are both a direct (seeds and legumes) and an indirect (honey) source of food, rubber, wood, medicine, perfumes, tannins and dyes. Some species are also used to make incense.
Ecological Adaptation of Tropical Aquatic Species
The tropical tank contains a significant variety of plants from the aquatic and marshy tropical areas of the Americas, Africa, Asia and Oceania. To adapt to life in these environments, they require special ecological features, such as: special tissues — called aerenchyma tissues — rich in aerifying gaps and canals, which allow gas to circulate efficiently. These are common in marsh plants and are, for example, so developed in the leaf petiole of the Water Hyacinth (Eichhornia crassipes) that the entire plant can float; other adaptations include: — reduced mechanical tissues and conducting elements; — often there is little root development because the plants can absorb nutrients from the water easily; — wide, floating water-repellent leaves, as in Victoria cruziana and Euryale ferox; — lack of stomata on submerged leaves and on the underside of floating ones; — ribbon-like or frayed submerged leaves to withstand damage caused by turbulent water; — various survival strategies (e.g. species that are mobile, or anchored to the bed, or submerged/semi-submerged).
Mangroves are trees which grow in tropical coastal areas, where water levels rise and fall due to the tides. They belong to a variety of families and genera (Rhizophora, Avicenna, Sonneratia, Conocarpus, etc.), which can live in soils constantly affected by salty water and with insufficient oxygen levels. Gaseous exchanges are assured by adventitious roots or special roots, known as pneumatophores. These depart from a network of horizontal roots, which provide anchorage but not gaseous exchanges for the plant, then grow upwards and emerge from the mud, particularly during low tide, and thanks to lenticellate apertures they take oxygen directly from the air. Excess salt is stored in cellular vacuoles or excreted trough the leaves. The close tangle of stems, pneumatophores and aerial roots create an efficient barrier which holds sediments, reduces erosion and breaks the intensity of even the largest waves (such as the tsunami of Dec. 2004). Mangrove habitats are rich in animal species, acting also as a nursery for many fish and crustaceans.
Distribution and Characteristics of Sub-Humid Tropical Forests
Sub-humid tropical forests, also known as monsoon forests, are found in regions with a warm climate all year round. They have two seasons: a wet season with heavy, even violent rainfall, followed by a dry season that severely restricts plant growth. This biome covers two parallel strips between 10° and 20° latitude north and south of the Equator. Sub-humid tropical rainforests are particularly well-developed in South-east Asia, where tall teak and bamboo trees typically grow. Although these areas are less biodiverse than humid tropical forests, they still teem with life, including numerous lianas and epiphyte orchids. The sub-humid tropical forest becomes less compact and continuous as the degree of aridity rises, gradually dissipating into an open area where trees are scattered and the ground is covered by a thick, continuos layer of grass, typical of the savannah biome. In geographical terms, the savannah is a transitional biome between a forest and a desert or a prairie.
Ecological Adaptation of Sub-Humid Tropical Species
The leaves are the organs through which a plant returns into the atmosphere most of the water it absorbs from the ground. This process is called ‘transpiration’. Leaf adaptation is thus particularly important if a plant is to save water. Plants may therefore do one or more the following: — develop a thick waxy coating; — reduce the overall surface of their foliage system and the number of compound leaves (as in Acacia sp.); — turn leaves into thorns (e.g. Acacia horrida); — shed leaves during the dry season (i.e. Baobab, Adansonia digitata); — develop a thick, coarse bark with a high cork-content that insulates the inner tissues of the trunk against the high seasonal temperatures; — the underground parts of the plant develop more than the subaerial parts (e.g. the Andira, Andira humilis, or the Brazilian Palm, Attalea exigua), and the roots burrow into the soil to reach the aquifer.
Ecological Adaptation of Mediterranean Biome Species
The morphology and anatomy of plant leaves afford a fascinating insight into how the Mediterranean biome species have adapted to their environment. Mediterranean biome species often have small leaves as they reduce their surface area to prevent excessive loss of water through transpiration. Many plants have become evergreen, which enables them to photosynthesize during the winter. Sclerophylls — from the Greek, meaning “hard leaves” — are also common, and they have developed a thick, leathery waterproof coating on their epidermis, which can be either single- or multi-layered. Sclerophyllous plants also contain a considerable amount of mechanical tissue, which prevents their leaves from shriveling, even when water is in short supply. They may shed most or all of their leaves, and let their stem take care of the photosynthesis. To regulate transpiration, plants open and close their stomata. These microscopic pores govern gas exchange with the surrounding environment, and are mostly located on the underside of the leaves, in cavities coated with protective hairs.
Burning as an Ecological Factor in Mediterranean Biomes
The plant species and communities in a Mediterranean biome are the result of a lengthy evolutionary interaction with climatic conditions, natural fires and agricultural burning. Today, land is still deliberately burned in the Mediterranean, as it is in savannah ecosystems, but the hot, dry Mediterranean climate also exposes its ecosystems to the risk of natural fires. Mediterranean plants, however, have adapted their growth and reproduction in order to tolerate fire, creating both passive pyrophytes and active pyrophytes. Examples of passive adaptations to fire may include a thick bark, such as that on the cork oak (Quercus suber), which insulates the trunk’s interior, or the low flammability of wood in trees such as those in the Erica genus or the Tamarisk (Tamarix sp.). Examples of active adaptations include plants growing copious offshoots after a fire, or the stimulation of seed germination.
Distribution and Characteristics of the Mediterranean Biome
The climate of a Mediterranean biome has a marked distribution of seasonal rainfall, which mainly coincides with the equinoxes; it also includes a warm season (average summer temperatures of 25°), with little or no rainfall, and a cold season with mild temperatures (minimum rare fall below 10°C). These conditions occur in five regions of the planet (California, Chile, the Mediterranean basin itself, the Cape province in South Africa, and southern/south-western Australia), distributed over an area between 30° and 40° of latitude. Mediterranean biomes cover only about 5% of the planet’s landmass, but are home to about 20% of the world’s vascular plants and a wealth of other flora, including numerous native species.
The Ecological and Functional Adaptation of Mediterranean Biome Species
Mediterranean biome species have adapted their functions in order to survive the dry season and to reproduce themselves. Many perennial herbaceous species live through hot summers by entering a dormant state and protecting their seeds underground on their tubers, roots and inside bulbs, thus enabling the plants to recommence activity at a later date. In particularly arid conditions, a plant’s life cycle shortens and becomes annual. The plant flowers in spring, produces seeds in the early summer, then withers and dies. The seeds it produces, however, enable it to remain in the environment and spawn new plants. During hot, dry periods, the activity of perennial woody plants slows down, and very often they even suspend their seed-formation process until the autumn, when the temperatures are milder, and more water is available.
In 1767, the great Swedish botanist Carl Nilsson Linnaeus named the genus of this flowering plant Protea after the Greek God Proteus, who symbolized water and the changing tides, and in 1789 the French botanist Antoine Laurent de Jussieu named the family Proteaceae. The eight or so genera in this family of Eudicots comprise 1,600 species of mainly woody plants found principally in South Africa and Australia, two major centers of diversification. The origin of this family reportedly dates back about 90 million years, when the supercontinent Gondwana still existed. Its large, showy flowers and the pollination mechanisms are two of this family’s hallmark traits. The Protea and Banksia genera are characteristic of the Mediterranean plant communities found in South Africa and Australia respectively.