The Essence of 'Plantness'

W HAT iS a plant? In the most basic and fundamental terms, what is it about a plant that makes it a plant and not an animal? Kenneth Wells of the University of California at Davis asked a similar question on his mycology final exam more than 25 years ago, and I found answering it to be an unexpectedly thought-provoking exercise. After attempting to stimulate undergraduates in my introductory botany course with this type of question and receiving disappointing answers, I began to collect the major points I felt should be included, first in outline form and, more recently, in the essay which follows. I believe that the principal factor shaping the evolution of plants and animals is the fundamental difference in their nutritional modes. This has resulted in the differences we now observe between them (Whittaker 1969).