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Tuesday, December 6, 2011
Flowering plants have evolved in many directions, becoming adapted to a wide range of habitats and acquiring many life styles. Plants growing on high mountains or on the northern tundra usually form mats or cushions close to the ground to escape the violent winds. The dwarf willow Salix herbacea, for example is a true willow, but it has underground stems and its aerial branches are rarely more than 3cm above the ground. The leaves and flowers of many mountain plants are darker than those of their low land relatives; dark colours absorb heat more efficiently in the cold climate.
Many plants manage to live in deserts despite the scarcity of water. Some of them are quick growing annuals that complete their life cycles in a short time and are not affected by the drought, but the typical desert plants store water through the dry season. Waxy or hairy coatings, and breathing pores sunk in deep grooves ensure that evaporation is cut to minimum. Many of the plants have very small leaves or none at all, although some species sprout leaves when the rains come and then drop them for the dry season.
Cacti nearly all grow in the American deserts and can survive prolonged drought. They have no proper leaves and their ribbed grooves store all their food. Wide spreading roots catch nearly all the rain that falls in the wet season and the stems swell up as they take up water. The stems shrink as they use up water in the dry season, and the grooves become deeper. The breathing pores in the grooves thus become better protected from the dry air. Spines protect the cacti from grazing animals seeking moisture.