440 



THE AMERICAN NATURALIST 



[Vol. XLI 



a{)proxi 



matelv inferred from the variations m mature leave 



udy of the mature leaves is therefore sif,niificant, but it ought 

 to 1)6 confirmed by embryological examinations. 



Evolutionary Significance — A knowledge of leaf development 

 is of great importance to the paleontologist, and in a paj)er entitled 

 "Localized Stages in Development in Plants and Animals" Jack- 

 son presents a study of mature leaves from a geological and evolu- 

 tionary point of view.^ His conclusions are stated to be m the 

 direct line and the natural outcome of Hyatt's principles of develop- 

 ment. Professor Jackson's work upon leaves was continued by 

 Cushman who published three papers on localized stages m this 

 journal.^ The leaves at either end oi a biandi whidi tnmin.itis 

 in a flower are 'well known to be simj)l('r than those alon- Us mid- 

 dle portion. Simple leaves are expected luai iht bud MaUs, 

 the sepals, and the cotyledons. M. llihnre ((itioted by liecul) 

 sought to explain this arrangement by iiutini\e conditions, the 

 simple leaves in the young plant or near tlie (lower are (hie to ucak- 



The eaiK kaNts aie said to upi 

 forms; and the su((essi\e lea\e^ 

 most complex forms winch the 



similar to those of MHing plants, ti.eii uiiiix aiciits au to .u souo 

 in the adults of ancestral groups. 



^I^hp nature of ancestral forms is for the geologist to decide; 



