How do plants that evolved in different places end up being so similar?
Organisms evolve to fit the conditions they are in. There are only so many ways to minimize the use of water. Plants in arid climates evolve very similar structures to lessen water use. They also have structures to protect themselves from the sun. The cactus on the right is from South America. The cactus on the left is from North America. These plants are one many examples of organisms evolving under very similar conditions and turning out to be very similar.
Dry climates receive very little rainfall. They also have high rates of evaporation. This makes them even drier.
The driest climates are deserts. Most occur between about 15° and 30° latitude. This is where dry air sinks to the surface in the global circulation cells. Deserts receive less than 25 centimeters (10 inches) of rain per year. They may be covered with sand dunes or be home to sparse but hardy plants. With few clouds, many deserts have hot days and cool nights.
Some deserts receive a small amount of rain during the winter and again during the summer. This is enough to keep plants alive as in the left photo. Some deserts receive very little rain or rain only one season a year. These deserts have few plants as in the right photo.
Other dry climates get a little more precipitation. They are called steppes. These regions have short grasses and low bushes (see Figure below). Steppes occur at higher latitudes than deserts. They are dry because they are in continental interiors or rain shadows.
A steppe in Mongolia.
desert: A very dry climate that receives less than 25 centimeters (10 inches) of precipitation each year.
steppe: A semi-arid climate that receives up to 40 centimeters (16 inches) of precipitation each year.
- Deserts receive very little rainfall. They are often home to very unusual and well adapted plants.
- Steppes receive more rain than deserts. They are higher and have grasses and scrub.
- Plants adapt to local conditions. If conditions are the same but the plants are far apart they may still be very similar.
Use this resource to answer the questions that follow.
Assignment Discovery: Desert Biomes at http://videos.howstuffworks.com/discovery/28106-assignment-discovery-desert-biomes-video.htm (2:19)
- What defines a desert?
- What is the largest desert?
- What are the most familiar deserts?
- What are the most common desert plants?
- How have plants adapted to the desert?
- How have animals adapted to the desert?
Assignment Discovery: Temperate Grassland Biomes at http://videos.howstuffworks.com/discovery/28110-assignment-discovery-temperate-grassland-biomes-video.htm (1:20)
- What plants dominate the grassland?
- How does most precipitation fall?
- What are the other names for the temperate grasslands?
- What are animals are found on the grasslands?
- What is the difference between a desert and a steppe in temperature and precipitation?
- What is the difference between a desert and a steppe in vegetation?
- What adaptations do arid climate zone plants have to the dry conditions?