The Various Ways in Which Plants Appeal 9 



of human faces are built up from exposures of many actual faces 

 upon the sensitive photographic plate. This is precisely what our- 

 Text-books are doing when they devote chapters to ''The Leaf," 

 "The Stem," and the like. These titles do not represent things, 

 but ideas; there are leaves in Nature but no such thing as the leaf. 

 But the analogy of these composite conceptions to composite 

 photographs goes yet a step farther, for, just as a real face is oc- 

 casionally seen which resembles the composite face of the photo- 

 graph, so an actual structure or phenomenon is sometimes found 

 which is like our mental composite of its kind. Such a real thing is 

 then said to be typical, and that is what is actually meant by this 

 word in science. When, however, no typical representative of the 

 composite is a^^ailable, we are still not without resources; for it is 

 possible to give exact and clear definition to the dim and elusive 

 outlines of the composite itself by drawing firm sw^eeping lines 

 through its more prominent places, — a process which constitutes 

 generalization, or conventionalization. When the data concerned 

 are expressed in figures, then the result is a round-number aver- 

 age, or conventional constant; when they are expressed in pictures, 

 the results are generalized drawings, or, if simplified to mere struc- 

 tural aids to the imagination, diagrams; when they are expressed 

 in words, the results are generalizations, or verities, the "aphor- 

 isms" of Bacon. Throughout this book, in accordance with its 

 aim to interpret plant life in the large, I have made great use 

 of composite conceptions, typical things, conventional constants, 

 generalized drawings, diagrams and verities, — to a degree which 

 will meet with much disapprobation from my scientific colleagues. 

 But I maintain that such generalized knowledge of plants is not 

 only infinitely better than no knowledge at all, but is actually 

 the most useful kind, as it is the onl}^ practicable kind, for the 

 non-technical learner, whose knowledge in other departments 

 of learning, — in geography, history, and so forth, — is largely of 

 this character. And I further mamtain that if only we would 

 make greater use of it, along with its logically-correlated methods, 



