PLANT MORPHOLOGY 77 



eventuated in the various appendages as we see them. Then may 

 follow definitions of the parts, which may or may not succeed in 

 assigning their strict limits. When this is accomplished, a termin- 

 ology may follow, which shall segregate parts which have had a 

 separate phyletic origin. Thus an evolutionary morphology of the 

 shoot would be built up. But it is useless to accept the thesis merely 

 in the abstract, that the basis of morphology must be in phylogeny: 

 the principle must also be put in practice, and be ultimately re- 

 flected in our methods, and in the definitions of our terms. 



A step in this direction will be the recognition that at present 

 the word "leaf" is loosely applied: it is, indeed, a temporary make- 

 shift borrowed from colloquial language, and used in a descriptive 

 rather than in a strictly scientific sense. It designates collectively 

 objects which have, it is true, formal and functional, and even 

 topographical features in common, but have not had the same 

 phyletic history. There is every probability that the word "leaf" 

 will continue to be used in this merely popular sense. 



This position, with its conservative use of terms fitting awkwardly 

 upon advancing phyletic ideas, can only be properly understood by 

 glancing back at the history which has produced it. So long as 

 species were regarded as the individual results of creative power, 

 the complexity and variety of their form was relegated to the arcana 

 of the Divine Mind, and organic nature presented the aspect of a 

 series of isolated pictures; any similarity which these might show 

 was to be regarded as indicative of the underlying divine plan. Now 

 that species have been threaded together by evolutionary theory 

 into developmental sequences, they, like the ribbon of a cynemeto- 

 graph, present phyletic history to the mind with all the vividness of a 

 living drama. While monophyletic views held the field, this seemed 

 comparatively simple: .but the conclusions thus arrived at in plant 

 morphology were often palpably improbable. Such difficulties, 

 together with the substantiation of examples of parallel develop- 

 ment on a sound comparative basis, led to the modification of mono- 

 phyletic views, and opened the way for less cramped conceptions. 

 It is now customary to contemplate the plural origin of such leading 

 features as sexual differentiation, foliar development, heterospory, 

 the seed-habit, as well as a host of minor characters. On such ex- 

 amples we base a general belief that similar structures may be ar- 

 rived at by divers evolutionary routes. It is this conception of 

 polyphyleticism that we must make clear in our descriptions, if not 

 even in our terminology. 



It will be objected that to carry through a method of designating 

 by the same term only such parts as are shown to be of common 

 descent would produce unwieldy results. Doubtless this is true. 

 But in the terminology of a science it is not so much convenience as 



