the exotic trees
In the footsteps of Marco Polo (visiting China by land) and the great navigators, Magellan and Cook, for example, the intrepid travelers have been continuously bringing new species in Europe from more and more distant origins. Animated by scientific curiosity, they have successfully acclimatized number of species that we forget today the remote origin. From exotic (that is foreign to our environment), these plants became "adapted ", "assimilated". The fascination of "Turqueries" (silks, spices, roses and culture described by Montesquieu in the Persian Letters) and "nitpicking" (still silks, spices and sublime flowers) fell again, as a fashion. There remain these endings "from India" which are found in the "Horsechestnut" or the "Lilac of India". There also remain all other young plants which do not support our climates, except in greenhous:
Tropical species, which are met for example in the the Caribbean, where the temperature is mild, never cold. In these regions too, transplant experiments have been tried, with flamboyant native to Madagascar which illuminate for example New Caledonia or palms which go around the globe or such pine species with long needles established in Madagascar by the colonists.
Species of tropical and equatorial forests, where there is more heat (25 ° C at least) and a constant high humidity. It falls more than 2 meters of rain per year (less than one meter in temperate climates). The leaves are leathery and smooth to allow water to flow into the trees of the lower stage. The fertile land supported by the red clay is only 10 centimeters deep. That is why the trees have few roots. They lean against each other, tangling their branches and sheltering each other from the wind. Many trees develop other methods of support: trunk base enlarged in buttress (eg Ceiba), stilt roots, roots pallets. The largest trees that grow in the upper part of the rainforest canopy, can reach 80 meters tall. In this greenery mass, three kinds of plants live:
- plants saprophytes which feed on decomposing material, decomposition speeded up by temperature and hydrometry conditions;
- plants epiphyteses which grow using other plants only as support. They are not parasites, with very few exceptions. They will absorb moisture from the air to find minerals, other particles and gases, as contained in the rainwater and dew. Many trees of the rainforest have a soft bark in order to discourage the growth of epiphytes.
- the autotrophes which feeds on the earth.
The economic role of this vegetation was first forestry development (rubber tree for the gum, cocoa for the chocolate, "exotic" wood for the furniture or the building). Then came the consciousness of the essential role played by forests in the upper atmosphere of the earth. Although it represents only 7% of the earth's surface, it produces more oxygen than CO2 and returns most of the moisture received. In addition, it contains active ingredients useful for health (Garcinia punctata, a tree of the Amazon rainforest, is used in the treatment of diarrhea, the Mexican yam in the production of cortisone and hydrocortisone).