742 METASPERMAE OP THE MINNESOTA VALLEY. 



ceding chapter, where the original presence of palms 

 and allied forms was discussed. On the other hand, 

 the Metachlamydeae with their 1.8 per cent, of trees and 

 7.1 percent, of shrubs have as yet failed to develop many 

 shrubby or arboreal plants in this latitude. The antiquity and 

 lower organisation of the Monocotyledones are therefore seen 

 to have been reflected in the physiognomy of the valley to-day 

 in a manner similar to that in which the recentness and higher 

 organisation of the Metachlamydeae have been reflected. The 

 absence of monocotyledonous trees and shrubs is due to their 

 having been obliterated ages ago by the stronger archichlamy- 

 deous forms, together with the geological progression of climatic 

 and topographical changes. The absence of the metachlamy- 

 deous trees and shrubs, compared with the abundance of the 

 Archichlamydeae,is doubtless owing to entirely different causes. 

 Not obliteration but failure to reach the valley is the explana- 

 tion of their absence. Palseontological remains do not indicate 

 that metachlamydeous trees or shrubs were ever before so 

 abundant in the Minnesota valley as they are to-day. The same 

 evidence shows that in the Cretaceous and Tertiary periods 

 there were palms in the valley. Thus by direct evidence is 

 corroborated what might a priori be derived from the study of 

 modern distribution. Together with the predominant herbace- 

 ousness of the Monocotyledones must be read their antiquity 

 and their preponderantly extra and intra-continental width of 

 range. Together with the only less predominant herbaceous- 

 ness of the Metachlamydeae must be read their recentness and 

 their preponderantly endemic and limited range. Thus the 

 character of the other taxonomic group may be stated in 

 terms as follows: 



The Archichlamydeae, forming 86. 7 per cent, of the arboreal 

 element and 72. 1 per cent of the shrubby element have on the 

 one hand had sufficient time to develop their habit and to reach 

 the Minnesota valley, while on the other hand they are not to 

 such an extent a lower group, nor so ancient that they have 

 been t erased from the distinctively extratropical regions. 

 From both sides they have been favored in the development of 

 arboreal characters, and for ages will doubtless maintain them- 

 selves in extratropical regions as the characteristic trees and 

 shrubs, although ultimately, the logic of history would seem 

 to destine them for extinction under the attack of arborescent 

 Senecios,Helianthi or Solidagos.or of other composite or composite- 

 like forms that had attained the arboreal habit. 



