A BOTANICAL CRITERION OF THE ANTIQUITY 

 OF THE ANGIOSPERMS 



EDMUND W. SINNOTT 

 Connecticut Agricultural College 



As to the origin of the angiosperms, that group of seed plants 

 which is now such a dominant element in the earth's vegetation, 

 we know almost nothing. They first appear as fossils in the deposits 

 of the lowest Cretaceous in eastern North America, Alaska, Green- 

 land, and Portugal, but just where they actually originated, and 

 how long ago, are still matters of great uncertainty. The aim of 

 the present paper is to throw a little light on the antiquity of this 

 great plant group by studying the rate of evolution displayed by its 

 members. 



Evolution has not been a uniformly rapid process. The fact 

 that plants recognized as "primitive" and others recognized as 

 "recent" exist together at the present time makes it evident that 

 certain vegetable types have changed but little throughout long 

 geological periods, whereas others have for one cause or another 

 become altered much faster. The degree of inherent "variability" 

 and the frequency of hybridization have doubtless been influential 

 in determining this rate of change, but a more important factor 

 perhaps than either seems to be the length of the generation or 

 period from seed to seed. A plant in which this cycle is completed 

 in a year or two is able to multiply its generations more rapidly, 

 and thus to accumulate heritable changes much faster, than one 

 which requires a longer time for the attainment of reproductive 

 maturity. This length of generation is definitely correlated with 

 the growth habit of the plant, being greatest in trees — which 

 usually reach an age of from fifteen to twenty years (in many 

 cases much more) before bearing fruit — less in shrubs, and shortest 

 of all in herbs, where one or two seasons from seed suffice to 

 produce a fruiting plant again. In a given length of time, therefore, 



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