Another characteristic of prairies that once they have started tends to keep out trees is their almost annual firing. Tree seedlings cannot survive this, and we know that the Indians fired huge tracts of prairie every year, not to mention fires started by lightning which may set fire to grasslands and actually does set fire to forests every year. The prairies in the United States--perhaps the most extensive in the world--are characterized chiefly by several grasses, buffalo grass (_Buchloë dactyloides_), gama grass (_Bouletoua oligostachya_), and several prairie grasses, such as _Sporobolus asperifolius_, _Koeleria cristata_, and some others. Among these, depending on the soil, are hundreds of prairie flowers which, during different parts of a single season, give quite different aspects to the region. Both the grasses and their associated herbs are well protected against too violent transpiration which their exposure to nearly continuous sunshine, high summer heat, and very considerable winds makes particularly active. In many places where the country is rolling, the lower and moister sites, besides developing more luxuriant growth of prairie plants, permits low shrubs, and in river bottoms even trees to flourish. But climatic conditions of small rainfall, high winds, and bitter winters make anything like a forest development out of the question. In some regions, both in America and central Europe, a rainfall that is high enough to permit trees to exist and low enough to favor at the same time a grassland formation has resulted in the parklike landscape that creates the most beautiful scenic features of the regions where it occurs. In such places there are irregular patches of forest and grassland, and the struggle for supremacy as between the different types depends, not upon the general climatic conditions to which they both respond, but to local conditions of available water supply and soil conditions and often upon fires. Naturally such regions are places of intense strife for dominance, and in them some remarkable collections of plants have been found.