Grassland and Tree Vegetation. All over the world there is a contest between grassland and tree vegetation for dominance wherever both occur. In this thorn veld in Natal the struggle is particularly keen. (After Bews. Courtesy of Brooklyn Botanic Garden.)
northern South America, and in North America it has dwindled to a handful of species confined to the region south of the Great Lakes and generally east of the Mississippi. In Mexico and south of it the genus is better represented. But with the persimmon, as in so many other types, our existing species are the remnants of preexisting ones, without a knowledge of which their present dispersal would be impossible to explain.
It is easy to reason from the foregoing that widely separated but related species of plants are all either very ancient, or directly descended from ancient ancestors, and that other things being equal a widely dispersed species is older than one with a restricted distribution. It is most certainly true that relics are unquestionably very ancient, and do actually represent the last outposts of a preexisting condition. But many isolated plants are relatively very new, so far as their immediate origin is concerned, as witness the hosts of young species of the daisy family, some of which have spread scarcely at all from their obvious point of origin. In judging of the distribution of even the commonest tree or shrub of our woodlands there are these links with the past, as well as response to present conditions, to be weighed if we are to understand the story aright.
Once a plant reaches a new and for it a strange country it is remarkable how quickly it will often capture the new territory. In the United States over six hundred of our commonest weeds have come from Europe and Asia. The daisy, dandelion, wild carrot, many hawkweeds, dozens of wild mustards, and many others are among the somewhat undesirable immigrants that now reach over a great part of the country—all brought over by the early settlers of America. From their home in subtropical Asia the lemon, lime, and orange have invaded every part of the tropical world. Once, in the most remote part of the Sierra Maestra Mountains in eastern Cuba, where only by the most arduous cutting could a passage through the dense tropical forest be forced, the writer found within a few square rods an orange and a tree from tropical India. Such cases could be multiplied, and all over the world we see this endless struggle of plants to conquer new territory, often at the expense of existing vegetation. On Long Island, New York, the introduced locust tree, brought from the southeastern United States about a hundred years ago, has completely routed the native trees in many places along the north shore of the island. And in Hawaii, some seeds of a screw pine, washed up among refuse along the beach or brought in by early aborigines, have made this Malayan plant a common tree thousands of miles from its home.
3. Home Economy of Plants, or Ecology
The geographic distribution of species of plants may be, as we have seen, the result of the geological changes of the past, of bird migrations, of more or less fortuitously water-borne seeds, or more usually of the slow spreading by invasion of those species apparently not so well supplied with external helps to dispersal. But no matter where plants grow, nor how they got there, they must fit the particular environment in which they find themselves or perish. This home economy of plants, or how they meet the environment and each other, is called Plant Ecology, a phase of botany now much studied, for it tells us more directly than most other plant research what the actual response to various factors of the environment may be. Just as plant distribution is the reflection of many, usually widely operating forces, so ecology narrows down to individual plants or groups of them the impact of the immediately surrounding conditions upon vegetation.
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.