By contrast, the aerial roots of some orchids, in contact with neither soil nor water, are covered in a thick tissue called velamen. When dry, this is white and easily compressed. But, like blotting paper , it absorbs the slightest touch of damp, and in the rain it darkens and fills to capacity, channelling moisture to the aerial roots.
Monday, March 19, 2007
A manufactured suction pump can raise water no more than 33ft (10 m) before the weight of the water counteracts the atmospheric pressure, so breaking the lift. But a tree can raise water to ten times this height. The tree's 'pump' is powered by the sun - as water evaporates from the leaves, so more is drawn up to take its place through myriad tiny tubes in trunk and branches. These tubes are very thin, which prevents the rising column from breaking, and the chemicals taken up on the way adds to its cohesion. In this way, a medium-sized oak may draw up as much as 140 gallons (637 litres) of water in a day.