7l8 THE AMERICAX XATL-RAI.IS r. XXXVIII. 



ment which in its final form is best cxpi-cs>cel by a liiological 

 tree. A figure of this sort is difficult to construct, and there is 

 no agreement among investigators as to the particular form it 

 should take. While the figures in connnon use indicate a cer- 

 tain relationship in descent, they complctcl\- fail ti_) convey any 

 impression of the way in which the succession arises, and they 

 furnish no indication of possible gaps. They therefore consti- 

 tute a very poor working basis. 



I have long been accustomed in teaching to compare the vari- 

 ous lines of descent among plants with the branchings of a deli- 

 quescent tree, since it has always seemed reasonable to suppose 

 that the laws which govern the branching of a limb, which give 

 rise to all the varying forms of arrested development, and which 

 thereby determine a particular modification of the figure which 

 would otherwise result from unmodified growth, must be equally 

 applicable to the general evolution of the higlier forms of plants 

 from a common ancestral type. In endeavoring to secure a 

 natural growth which would best express idl the conditions 

 involved, a sympodium was first of all suggested, inasmuch •>s>> it 

 conveys the idea of succession through lateral meml)ers in such 

 a way as to indicate the direct line of descent. Hut synii)odia 

 as we usually know them fail to adeciuately exiiress the idea of 

 arrested development and su[)[)ressi<)n in their various forms. 

 In the branching of Acer f^latanouics all the conditions appear 

 to be met in a very satisfactorv manner. The branch of the 

 Norway maple, when of vigorous growtli. is a monopodium, 

 and it is obvious that such would not answei" tlie object in view, 

 since its most prominent feature would suggest the idea of a 

 continuous series of coterminous members from which lateral 

 members would arise at intervals. There is no evidence that 

 any phylum represents silch a series ; on the contrary there is 

 every reason to believe that such relations do not exist among 

 the various groups of plants. 



But in those branches of the Norway mai)le which exhibit 

 slow growth various forms of arrested de\e]opment are mani- 

 fested. These take the form of atrophied buds, or of branches 

 in all stages of development, and there thus arises a modified 

 monopodium which eventually becomes, in many cases at least,, 



