748 



METASPERMAE OF THE MINNESOTA VALLEY. 



discussed. It will be observed that the marsh and swamp ele- 

 ment forms a percentage of the Metachlamydeae exactly 

 equivalent to the similar percentage of the Archichlamydeae. 

 In this case the explanation may be that the younger group of 

 the Metachlamydeae furnishes so large a percentage >of this 

 element, comparatively, because the influence of the tensions 

 is such that in a younger group many weaker plants will be 

 forced into the morassic habitat. It is apparent that either the 

 plants of new and variable type or the plants of an older and 

 less plastic type will be less competent to struggle for the op- 

 timum habitat than a group of plants neither too modern nor 

 too ancient. In the marsh and swamp percentages of the 

 modern valley may therefore be read a word or two concerning 

 those long- past ages when the Archichlamydeae in turn were 

 similarly conditioned in their relations with the conifers of the 

 old Cretaceous flora. 



It will be interesting to examine the ranges in the continent 

 of these two groups of physiognomic elements. Tabulations 

 are readily compiled from Tables G, H, J and K. The follow- 

 ing tabulation will serve to indicate in a general way the range 

 peculiarities of the habit- elements: 



