Plants are exposed to unusual environmental conditions, daily and seasonally. Away from the equator, perennials such as trees and shrubs can be subjected to extreme cold in winter; plants growing at high altitude may experience, in addition to cold all year round (at least at night), drying winds and high levels of harmful ultraviolet radiation; desert plants must often suffer through long difficult periods of extreme high or low temperatures; in many locations, extended periods of drought or flooding may have to be endured; tolerance to increasingly saline soils may become necessary as we continue to abuse our arable lands; and soil, water, and air pollutants as a result of human activity may be encountered. Stress and how plants cope with it is the subject of the first chapter in Part IV.

Plants, both wild and cultivated, are surrounded by bacteria, fungi, nematodes, mites, insects, mammals, and other living hazards to their wellbeing, all hungry, many potentially harmful. Plants cannot easily avoid these enemies by moving away or hiding. The first part of the second chapter in Part IV focuses on the strategies plants use to combat the enemies in their environment, first and foremost, the ever-evolving chemical warfare that they wage against constantly adapting foes.

However, plants live in communities, as do other organisms, within which they compete with one another for moisture, light, and soil nutrients. Plants have evolved a variety of ways to create Lebensraum in their generally overcrowded world.