We call the poles of the earth to the circles of land situated at the north and the south of the globe. The Artic Circle including the north pole and the Antartic Circle including the south pole. Midway between the poles of the earth is the equator, an imaginary line that circles the globe all around. The earth spins about its axis which includes the north and south poles, but it bulges to its greatest extent at the equator.

We refer to climate as the simply weather averaged over a time period of one year or more. At the earths' poles the climate is termed polar, and at the equator it is termed tropical. Each of these climate zones is characterized by a distinctive temperature range, rainfall amount, and type of vegetation.

At the poles the number of days without sunset, and days without sunrise is maximum. Considering the low sun angles at each pole, the solar gain is minimal so winters are long and quite severe, while summers are mild and brief. The soil near the poles thaws for just a short time each year at the surface, beneath this the ground is permanently frozen (permafrost). This of course limits root growth and, ultimately, plant size. Plants that can survive hard frosts or freezes are called Hardy Plants. These plants can handle varying levels and duration of freezing, explaining their distributions up into the high latitudes toward the Polar Circles.

Where permafrost exists the thin layer of soil that thaws briefly in summer supports the distinct vegetation known as "tundra". These are among the toughest plants on earth. They go through their life cycle in just a few weeks of summer and then tolerate extreme cold for the rest of the year. They tend to be short in stature.

At the equator, there are the tropical plants. These plants generally cannot survive winters, and so are restricted to their homelands. Among the popular plants that we can find at the equator are the mango and the banana trees.