In this small book I have tried to confine myself to the absolute necessities in a field which requires a knowledge of both biology and physics. It is meant as a primer for biological undergraduates. I hope it will lead some of them to further, more advanced, study. It has not been easy to present the subject in so few pages, and I am aware of many omissions. I hope readers will agree that it is best to concentrate on a small number of topics, which together constitute an essay on plant-atmosphere relationships. Advanced students will be able to take the subject further if they look up some of the references. Text books that I particularly recommend are those by Monteith [38] and Campbell [lOO]. If the reader intends to carry out research investigations he should also consult Fritschen and Lloyd [105] for an introduction to instrumentation in environmental biophysics.