Rain forests, temperate forests, grassland, and deserts--all are immense developments of plant societies depending upon climatic differences for their occurrence. There are some other plant communities which also depend for their development on still other differences of climate. Two such are the vegetation of mountain tops in the tropics, and that strange tundra vegetation near the poles which lives all its life on the ice, only the roots and soil in which it grows thawing out during the brief summer. Temperature rather than rainfall is the cause of these and some other plant societies of more local occurrence. But what of such well-known plant societies as bogs, in which peat is formed, or the plants growing along the sea beaches all over the world? These, and scores of other plant communities play their part in the distribution of plants, but nearly all of them depend not upon climate, but upon usually purely local conditions of soil. Sandy, actually nearly sterile soil, the acidity of cranberry bogs, the alkaline regions in our own West, the salt lakes and inland seas, regions below sea level, the serpentine outcrops, all the hundred and one differences which local conditions exhibit--all these have a very direct bearing upon plant distribution. It is impossible here to go into the details of the different sorts of plant societies which inhabit such specialized places, nor into the truly wonderful adaptations of certain species to peculiar conditions. But in looking at the vegetation of regions through which one travels it must never be forgotten that its general type, such as forest, or grassland, or desert, or what not, is the result of usually widely operating climatic forces, while many, often quite extensive, plant societies in the region are the result of the local environment. There is often an active struggle as to the dominance of the type dictated by the climate of the place, and the local conditions of soil that tend to nullify general response to it. On Long Island, New York, for instance, there are areas which climatically should produce dense woods of the summer forest type so general all through the Northeastern States. Actually the water-worn sands and gravels that covered the south side of the island in glacial times, are so poor in plant food, that many square miles of this region are now covered only by low scrub oaks and other plants suited to poor soils. A final word of caution is necessary to those who see in the foregoing brief account of some of the chief causes of plant distribution an answer to questions that many of us ask about why plants or vegetation are of such and such a kind in a particular locality. It has been convenient--nay, it has been necessary--to consider these various factors one by one, but the distribution of almost no individual, and certainly of no widely spread plant community, is the result of any one of these factors operating singly.