I92I. No. 7. STUDIES ON THE LICHEN FLORA OF NORWAY. 5 



with the eyes of past times. Other species are so variable that a uniform 

 limitation is hardly possible. In the genus Cladouia it is in many cases 

 impossible to select one 'decisive character', and only a broad knowledge 

 of the whole complex of formae, their morphology, anatomy, chemical 

 substances, and last but not least their distribution and their 'behaviour' 

 as living plants in nature can decide the difficult questions. Intelligent 

 'collectors' often have a better founded opinion regarding such things than 

 herborists. 



Chemical characters may be of great value for purposes of determination. 

 But they are not always distinct, and they sometimes split up species or 

 even insignificant formae into chemical species which I have been unable 

 to recognize. 



Our knowledge of lichen physiology is still very slight. No other part 

 of lichenology affords problems of more general bearing than this, and 

 systematists might profit enormously by the works of physiologically trained 

 chemists on such problems. 



For systematism is the general denominator of all our knowledge of 

 plants. 



I am glad to express my gratitude to all those who have assisted me 

 in my work. I am specially indebted to J. J. Havaas for all his valuable 

 advice and information, to the Swedish lichenologists G. Einar du Rietz and 

 G. O. Malme and to m}' German colleage H. Sandstede who has deter- 

 mined many doubtful Cladoniac for me. I have also been in constant 

 touch with our phanerogamists Ove Dahl and Jens Holmboe who have 

 given many valuable suggestions. 



Kristiania, The Botanical Museum, 3. March 1921. 



