The origin of arboreal vegetation in subtropical and temperate climate goes back to great antiquity. Their appearance is connected with the rise of Angiosperms in the Lower Crestaceons age. Arboreal vegetation of the Upper Crestaceons age belonged in its nature to the so-called mixed flora, which included races of the present tropical, subtropical and temperate floras. As the tropical arboreal plants, with some exceptions, cannot with stand negative temperatures, they could not have existed under the conditions both of subtropical and temperature - climate characterized by negative temperatures. As to the arboreal plants of subtropical and temperate climate, they are able to grow under the conditions of tropical climate, therefore it is to be presumed that the mixed cretaceous flora was entirely tropical, while the arboreal species of subtropical and temperate climates belonging to its composition, must have also been endowed with properties of tropical arboreal species.
