20 PLANT PHYSIOLOGY. 



to explain important phenomena have much value as knowl- 

 edge to persons of culture, and they give a life and significance 

 to facts otherwise of little meaning. They stimulate interest 

 and mental activity, and are actually an invaluable tool of 

 scientific research. But no student is prepared to understand 

 the place and bearing of the leading theories unless he has tried 

 to develop for himself some interpretation for the facts those 

 theories explain. Students receive with but a languid interest 

 such a theory as that of the micellar basis of membranes, or 

 that which explains osmotic pressure, when these are formally 

 presented to them ; but they receive these theories with an 

 eager and intent interest when offered after they have tried 

 themselves to devise a theory to explain the facts they have 

 observed in their studies upon osmosis. All such theorizing, 

 however, must be kept in rigid control, subordinated to facts, 

 a means to an end, never an end in itself. The tentative and 

 insufficient nature of even the most widely accepted theories 

 should be illustrated by subjecting them to rigid criticism. It 

 must be made clear that theories are mostly attempts to explain 

 in a subjective form phenomena which may not be subjectively 

 comprehensible at all. In the exercises involving theorizing 

 in this course, it is not expected that students will be able to 

 solve the many theoretical problems proposed to them ; but it 

 is intended to impress upon them the fact that such problems 

 exist, and to lead them to exercise their minds and prepare 

 their attention through attempts to solve them. 



Lectures are of great value to correlate and extend topics 

 and to help to weld them into one harmonious system. They 

 should be studies in induction, proportion, apt illustration,* 

 suggestion, and stimulation of interest. Disconnected from 

 laboratory work they have little value, but closely following 



* Wall diagrams for illustration of lectures, etc., have their value. Two sets 

 have been published for physiological use an earlier, 60 in number, each 69 X 85 

 cm., by Frank and Tschirch (published by Paul Parey, Berlin, and costing 180 

 marks), and a later, 15 in number and somewhat larger, by Errera and Laurent 

 (published by E. Lamertin, Brussels, and costing 50 francs). They may be 

 imported through any dealer in foreign books. 



