Desert plants have evolved ways of conserving and efficiently using the water
available to them. Some flowering desert plants are ephemeral; they live for a few
days at most. Their seeds lie dormant in the soil, sometimes for years, until a
soaking rain enables them to germinate and quickly bloom. Woody desert plants
either have long root systems that reach deep water sources or have spreading
shallow roots that are able to take up surface moisture quickly from heavy dews
and occasional rains. Desert plants usually have small leaves. This conserves water
by reducing surface area from which transpiration can take place. Other plants drop
their leaves during the dry period. The process of photosynthesis—by which
sunlight is converted to energy and usually conducted primarily in leaves—is taken
over in the desert by the stems. A number of desert plants are succulents, storing
water in leaves, stems, and roots. Thorns, which are modified leaves, serve to
guard the water from animal invaders. These plants may take in and store carbon
dioxide only at night; during the day their stomata, or pores, are closed to prevent
evaporation. Desert plants growing on saline soils may concentrate salt in their sap
and then secrete the salt through their leaves.
Rain Forest, tropical woodland, characterized by lush vegetation and great
biological diversity. There are more species of plants and animals in tropical rain
forests than in all the rest of the world’s ecosystems combined. About 70 percent of
all plant species in these forests are trees. Tropical rain forests are vertically
stratified, having three to five layers of plant life: visible emergent trees protruding
partly or entirely above the upper canopy; one to three closed canopies; and an
understory. The upper canopy of trees reaches 30 to 50 m (100 to 165 ft) above
the ground. Woody vines called lianas that can exceed 20 cm (8 in) ...