Part 81. Useful and harmful plants in general. From our study of some of the more important groups of economic plants we have learned not only that the very existence of the human race depends upon the vegetable kingdom but also that the progress of humanity at every stage has been profoundly influenced by the properties of plants and by man's knowledge of them. The needs of primitive man must have been met largely by wild plants. Through the cultivation of plants, as we have seen, civilizations were developed in those regions where the most useful plants grew most abundantly. The desire for spices and similar luxuries led to the discovery of America. The vegetable products of the New World are now revolutionizing human life to the remotest ends of the earth.

Our brief study of vegetable foods, food-adjuncts, medicines, and raw products has shown that what we take from plants for our own use has often a similar use for the plants themselves, though sometimes the use is quite different; and in some cases, so far as we can see, the product is of no use whatever in the plant's economy. In other cases it has been found that substances poisonous to us are also poisonous to the plants which produce them, just as the venom of certain animals may be fatal to themselves. Since, however, some of these plant poisons are among the most valuable of medicines, it is plain that no dividing line exists between harmful and useful plants. Judged in its relation to our welfare the same plant may be either useful or harmful according to what we do with it. Obviously, the more we know about their properties the less likely are we to suffer harm from plants, and the more likely are we to benefit by them.

The student should understand clearly that in this book the aim is only to introduce beginners to the study of plants. Our purpose is merely to lay a good foundation for future studies which shall further advance general culture. There has been no intention of giving here a complete outline of economic botany. Accordingly, a great many plants of high economic importance have not been mentioned; and some of the chief uses of plants, and some of the most serious ways of their working harm, have been passed over with bare mention, or have been ignored. Thus, in regard to the food of domestic animals but little has been said of the fodder raised for them, and nothing at all of pasture plants upon which some of the principal industries of the world depend. The many plants which afford bees the material for making honey and wax, and those which serve as food for silkworms or other insects of economic value have also been neglected. So also have we omitted reference to the plants which do great service in binding shifting sands that but for these sand-binders would devastate extensive areas: to those plants similarly used to prevent the washing away of soils; to trees set out as wind-breaks for protecting tender vegetation, as drainers of swamp land, or for shade and beauty; and to the innumerable flowers and foliage plants cultivated or collected for ornament. Likewise, among harmful plants neither weeds nor destructive parasites have been included.

Not only has our study neglected these groups of plants which especially affect the welfare of mankind but it has been forced to leave out of account some most extensive influences which vitally concern animals in general. For example, there is the influence of forests upon water-supply, by which is meant their action as reservoirs feeding the streams gradually in spring, thereby avoiding floods, and at the same time keeping back plenty for the dry season. Then, too, there is the important action of plants in soil-making, and the purifying influence of vegetation upon air and water whereby they are made to serve better the needs of animal life.

All these various relations of plants to the life of the world, and to our own lives in particular, are as profitable and attractive matters of study as any that have claimed our attention; and the student will do well to learn all he can regarding them. It should be said, however, that many of these relations are best understood in the light of vegetable biology. Moreover, the student's pursuit of economic botany cannot well proceed much farther than we have here attempted to go, without his first acquiring such an elementary knowledge of systematic botany as the following chapters may help him to gain.