VI 



PREFACE 



factors in the environment of plants if he is to attain any real insight into 

 the mechanism of their influence upon plant processes. Hence several of the 

 earlier chapters in this book have been devoted to a review^ and extension of 

 physico-chemical principles with vv^hich the student of physiology should be 

 familiar, and additional discussions of pertinent facts and principles of physics 

 and chemistry have been introduced as needed in the interpretation of plant 

 processes. Likewise the structure of organs, tissues, and cells has been de- 

 scribed at appropriate places as necessary background information. An 

 integrated picture of structure and process is usually necessary for a clear 

 concept of any phase of the physiological activity of plants. 



When conflicting evidence is presented some attempt at evaluation has 

 usually been made. We feel that the student is entitled to the considered 

 views of the authors when contradictory data are cited, as well as a chance 

 to acquire some feeling for the weighing of evidence which is an important 

 procedure in the methodolog}^ of science. Probably none of our colleagues 

 will agree with every single one of our evaluations, and it is likely that some 

 of the viewpoints which have been espoused will ultimately be shown to be 

 erroneous. 



A short list of textbooks and monographs suitable for collateral reading 

 has been appended to each chapter. There is also a selected bibliography 

 of journal references at the end of most of the chapters. Every paper cited 

 in the text or from which data in tabular or graphical form have been taken 

 is listed in the appropriate bibliography. No pretense is made of including a 

 comprehensive list of papers on any subject. We have merely attempted to 

 blaze a few trails into the original literature for the occasional student who 

 wishes to follow them on his own initiative or for those teachers who wish to 

 make a definite point of training students to look to the journal literature as 

 the original source of information. Citation of any paper is not necessarily 

 meant to imply that it is one of the outstanding papers in its field, nor is it 

 ever intended to indicate priority for that contribution. Other criteria have 

 guided us in the selection of many of the papers listed. An impartial review, 

 for example, can often be read by students with greater profit than the most 

 meritorious original contribution. Papers published in widely circulated 

 journals have usually been given preference over those which have appeared 

 in journals which are not likely to be available in many institutions. While 

 we have not hesitated to include some foreign language citations, in general 

 preference has been given to original literature appearing in the English lan- 

 guage. 



A representative list of questions has been appended to most of the chapters. 

 These are principally of the "problem" type and are especially valuable for 



