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.
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.
Not the least interesting feature of these plant societies is that we must view them as associations of plants, often of widely differing origin due to the vicissitudes of plant distribution, but all taking their part in the society to which they belong and often, as individuals, losing their life that the society may live. Upon such a conception a wood or prairie, or river bank, or salt marsh or alpine garden upon a mountain summit are, with many other plant societies, places of intense conflict. More cruel than any human society, these plant communities exist under conditions where only the individually strong survive, and only those societies are destined to reach their climax which can take advantage of every aid, quite without regard for severe losses or even death to the individual members of it. It is as if we poured into a crucible molten metals from many different sources, and after the incredible and relentless forces of manufacture had worked their magic upon them there resulted a product, purified and cleared of all dross. So the inexorable and relentless processes of nature work over the materials found in these plant communities, the results of which are the dominant types of vegetation in the world to-day. With this understanding of the part they play in plant distribution we may now consider a few of the most widely recognized plant societies and see how they have affected the vegetation, sometimes even the history of the regions in which they are found.