=333. Growth.= The causes that produce zones are either biological or physical: the first have to do with some characteristic of the plant, the second with the physical features of the habitat. Biological causes arise from the method of growth, from the manner of dissemination, or from the reaction of the species upon the habitat. The formation of circles as a result of radial growth is a well-known occurrence with certain plants, but it is much more common than is supposed. In the case of agarics, this phenomenon has long been known under the name of “fairy-rings.” It is found in a large number of moulds, and is characteristic of early stages of the mycelium of the powdery mildews. It occurs in nearly all maculicole fungi, and is exhibited by certain xylogenous fungi, such as _Hysterographium_. Among the foliose lichens, it is a common occurrence with the rock forms of _Parmelia_, _Placodium_, _Physcia_, and _Lecanora_, and with the earth forms of _Parmelia_ and _Peltigera_. The thalloid liverworts show a similar radial growth. The flowering plants, and many mosses also, furnish good examples of this sort of growth in those species which simulate the form of the mycelium or thallus. These are the species that form mats, turfs, or carpets. Alpine mat formers, such as _Silene acaulis_, _Paronychia pulvinata_, _Arenaria sajanesis_, etc., are typical examples. Xerophytic, turf-forming species of _Muhlenbergia_, _Sporobolus_, _Bouteloua_, _Festuca_, _Poa_, and other grasses form striking ring-like mats, while creeping species of _Euphorbia_, _Portulaca_, _Amarantus_, etc., produce circular areas. Rosettes, bunch-grasses, and many ordinary rootstalk plants spread rapidly by runners and rhizomes. The direction of growth is often indeterminate in these also, and is in consequence more or less bilateral or unilateral. Growth results in zonation only when the older central portions of the individual or mass die away, leaving an ever-widening belt of younger plants or parts. This phenomenon is doubtless due in part to the greater age of the central portion, but seems to arise chiefly from the demands made by the young and actively growing parts upon the water of the soil. There may possibly be an exhaustion of nutritive content, as in the case of the fungi, but this seems improbable for the reason that young plants of the same and other species thrive in these areas. It must not be inferred that these miniature growth zones increase in size until they pass into zones of formations. Growth contributes its share to the production of these, but there is no genetic connection between a tiny plant zone and a zone of vegetation.