It is difficult to precisely define what a plant is, there are so many types and variations that even simple definitions have many exceptions. No single criteria separates all plants from all animals. The more complex plants and animals are, the easier it is to discern them from one another, but simpler forms are not so readily distinguishable. The following characteristics refer primarily to more complex plants and animals:

1. Manufacture food—Most plants manufacture their own food through the process ofphotosynthesis. Green plants have the ability to synthesize complex food from simplesubstances such as carbon dioxide, water, and minerals in the presence of chlorophyll (agreen pigment) utilizing light energy. In contrast, animals require ready-made food inthe form of plants or other animals.

2. Cell walls—Most plants have cell walls made of cellulose. The rigid cell walls create asturdy framework which results in the lack of mobility. Animal cells, in general, lack rigidcell walls and are typically flexible.

3. Indeterminate growth—Most plants have unlimited (orindeterminate) growth. The meristematic tissue (tissuecontaining actively dividing cells) remains active as long as theplant lives and the environment is suitable. While plants cancontinue to grow, most plants will have some expectedmature size and form. The situation is very different in thecase of animals, after an animal attains a certain characteristicsize and form, growth often ceases.