then two million visitors yearly assures that the plants do not thrive. It is in the large open areas closed to the public, where the soil is thicker and attention minimal that plants grow with the most freedom. Some of the houses with restricted access have been transformed in virtual greenhouses, with walls and roofs keeping in moisture and heat.Many of the paved roads are not affected by vegetation, some plants being unable to even settle between the stones. But roots from nearby trees have been known to affect the alignment of the pavement stones. The problem is more serious when considering the effect even the smallest plants can have on mosaic. If a mosaic is intact, no plant will be able to take hold on it, especially if it is a wall mosaic. But if a piece falls of, say from pressure from an outside root of a tree, plants will grow in the dirt that will be wind-blown in the hole and will detach even more pieces. This can happen in a matter of hours; just think of how fast dandelions grow on your lawn!Plants can also be found growing on walls, taking root in cracks or between bricks, and in the form of ivy. (See annex 2 for a representation of an affected wall.)D) Water:Water, not only has a nourishing agent to the vegetation, will cause damage on its own. It causes water stains on wall-paintings. It makes deposits of minerals on walls. There is also the repetitive action of the rain drops on the walls which first erodes the coats of pigments, and then of the mortar that holds the wall together. Has if that wasn't enough, when combined with the atmospheric pollution, it can become a powerful acid that damages marble and stone. When the mortar contains argyle, the material absorbs water, expends and can cause cracks and even collapsing of walls. Even if the houses are protected by a roof, water can seep in thru the base of a wall, which explains why most paintings are damaged on the lower portion. E) Sun:The sun caus...
