=321. Succession by exhausting the soil.= This is a reaction not at all understood as yet in nature. A number of phenomena, such as the “fairy rings” of mushrooms and other fungi, the peripheral growth and central decay of lichens, _Lecanora_, _Placodium_, _Parmelia_, and of matforming grasses, such as _Muhlenbergia_, and the circular advance of the rootstalk plants, indicate that certain plants at least withdraw much of the available supply of some essential soil element, and are forced to move away from the exhausted area. It is probable that the constant shifting of the individuals of a formation year after year, a phenomenon to be discussed under alternation, has some connection with this. It will be impossible to establish such a relation, however, until the facts are exactly determined by the method of quadrat statistics. So far as native formations are concerned, there can not be the slightest question that prairies and forests have existed over the same area for centuries without impoverishing the soil in the least degree, a conclusion which is even more certain for the open vegetation of deserts and plains. With culture formations, the case is quite different. The exhaustion of the soil by continuous or intensive cultivation is a matter of common experience in all lands settled for a long period. Calcium, phosphorus, and nitrogen compounds especially are used up by crops, and must be supplied artificially. The reason for this difference in reaction between native and culture formations seems evident. In harvesting, not merely the grain, but the stems and leaves, and in gardening often the root also, are removed, so that the plant makes little or no return to the soil. In nature, annual plants return to the ground every year all the solid matter of roots, stems, leaves, and fruits, with the exception of the relatively small number of seeds that germinate. Perennial herbs return everything but the persistent underground parts. Shrubs and trees replace annually an immense amount of material used in leaves and fruits, and sooner or later, by the gradual decay of the individuals or by the destruction of the whole formation, they restore all that they have taken from the soil. This balance is further maintained to an important degree by the activity of the roots, which take from the deep-seated layers of the soil the crude materials necessary for the formation of leaves and fruits. Upon the fall and decay of these, their materials are incorporated with the upper layers of the formation floor, from which they may be absorbed by the undergrowth, or find their way again into the layers permeated by the tree roots. From the universal occurrence of weeds in cultivated regions, the pioneers in impoverished or exhausted fields are uniformly ruderal plants. As is well known, the seed production and ecesis of these forms are such that they take possession quickly and completely, while their demands upon the soil are of such a nature that the most