xviii INTRODUCTION 



honeysuckles, the petals have become united, and finally, in most of 

 the asters, some or all of the flowers of the characteristic heads be- 

 come irregular as to their petals. In the mustards, pinks and gerani- 

 ums, the flower parts are regularly reduced to the number plan of 

 4 or 5, and the pistils are united. In the primroses, the petals become 

 united, and in the snapdragons and mints, the corolla as a rule is highly 

 irregular. 



Many trees and shrubs have the flowers typical of the family 

 to which they belong. The locust and redbud are readily seen to be- 

 long to the pea family, the apple, cherry and plum to the rose family, 

 the bamboo to the grass family and the matrimony vine to the potato 

 family. On the other hand, the flowers of the lilac and the ash seem 

 wholly unlike, though they are placed in the same family. The maple 

 and the boxelder appear to be very different, though they are usually 

 put in the same genus, while the flowers of willows and cottonwoods 

 seem to have nothing in common with the pinks and tamarisks from 

 which they have descended. In a sense however, these striking differ- 

 ences are superficial, and are due to the excessive reduction or com- 

 plete loss of calyx and corolla in consequence of lack of use in wind 

 pollination. The absence of a corolla renders many of our tree 

 flowers inconspicuous, especially those that bloom before the leaves 

 appear, and has led to the popular impression that many trees do not 

 have flowers. Trees, like all flowering plants, produce flowers every 

 year as a rule, and it is possible to determine their real relationships 

 even when the flowers are reduced to stamens or pistils alone. The 

 simple appearance of the flowers of many trees and shrubs has fre- 

 quently led to their being regarded as the lowest of flowering plants. 

 A comparison of the flowers of maples, elms, oaks, ashes, etc., with 

 those of the pine, spruce and fir will readily prove that this is not the 

 case, and that the majority of our tree flowers belong fairly high in the 

 scale of evolution. 



VEGETATION REGIONS. 



From the standpoint of vegetation, Minnesota may be divided 

 into three great regions, occupied originally by the evergreen forest, 

 the deciduous forest, and the prairies, respectively. Each of these 

 regions has its characteristic plants, while comparatively few of the 

 tiees and shrubs of the state are equally at home in all three of them. 

 The general outline of these regions is indicated in the map on page 

 xx. 



The evergreen forest originally covered the northeastern one-third 

 of the state, extending south to a line about half way between Duluth 



