The simpler the animal, the more we know about how it works: what makes it behave at all and what selects one behavior over others. This chapter illustrates the processes of variation and selection of individual behavior in some very simple cases – orientation mechanisms in plants and protists. The next chapter describes elementary processes such as habituation, adaptation, and the “laws” of reflex action, which are some of the ingredients of adaptive behavior in higher animals. Chapter 4 introduces the notion of feedback in connection with direct (taxic) orientation.

Simple orienting mechanisms

Finding the proper habitat, a place not too hot or cold, nor too dry or wet, safe from predators, and with a supply of food, is a major behavioral problem for all organisms. For simple, single-celled animals such as protists and primitive invertebrates, it is the main problem. Since these organisms possess either no or only the most rudimentary nervous system, they must get nourishment and stay safe using very simple means. Their simple orientation mechanisms exhibit the properties of adaptive behavior in its clearest form.

The movement of climbing plants provides a good illustration of the orientation problem, and of a simple solution to it. An important objective for any green plant is to gain maximum access to sunlight. Obstacles are presented by other plants with similar requirements. Where the need for light is not outweighed by other considerations, such as avoidance of predation or extremes of temperature or the effects of wind, plants therefore grow vertically and seek the highest, best-lighted point.

Darwin, an experimenter as well as a theorist, identified rotation of the growing plant tip (he called it circumnutation) as a key element in directed plant growth. He describes circumnutation in a hop plant: