The basis for all study of the response of plants to the conditions under which they grow must rest upon the response of their different organs to those factors, just as our general movements are dictated by sufficient food or air or water to keep ordinary bodily functions going in the ordinary way. But the study of such plant response has shown that certain kinds of environmental conditions have resulted in quite similar response nearly throughout the world. Often totally unrelated plants assume characteristically similar growth forms where the conditions in widely separated areas are climatically or otherwise similar. In our own Southwest we have the dominant cactus vegetation, matched in parts of South Africa by giant cactuslike spurges. In Mexico we find the wealth of century plants, which are confined to the New World, matched in the Old World dry regions by the aloe, a group of succulent plants nearly as well suited to such areas. The _species_ of plants characterizing peculiar regions may well be the result of geographic distribution that rests on more widely operating factors such as we saw in the previous section of this chapter, but the _type_ of plants growing in a particular place hardly ever fails to be dictated by the local condition. With this in mind, a vast amount of time has been spent in studying the various factors of the environment, such as climate, soils, altitude, light, etc. And an equally valuable study has been the response of individual plants or their organs to such conditions. From this great body of information, obviously impossible to include here, we all recognize certain well-marked societies or groupings of plants which, wherever they occur, exhibit similarity of general response to the different conditions responsible for their occurrence. Once these typical plant societies or groupings are understood we can recognize them wherever they may occur, and we shall see that they are as widespread as are plants themselves. Just as societies or races of men have often obscure beginnings, reach a climax, and afterward die, so these plant societies may be considered as exhibiting a similar progression. What these plant societies are, at least the more important of them, will be considered in the next section of this chapter. It should never be forgotten that the _species_ of plants making up the dominant plant societies in different parts of the world are dictated by quite other conditions than those that result in the dominance of the society itself. Perhaps as good an illustration as any is the aristocratic type of mankind, recognizable throughout the world by the possession of finer qualities than the common run, but differing in individuals as much as the best type of Americans, the British peerage, and the samurai of Japan differ one from the other.