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 available, 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 sweeping 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 only 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 maintain that if only we would 

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



