ZONATION 275 



nental climate, however, often results in the interruption of these, with the 

 consequence that these belts of vegetation are not always continuous. 



CAUSES OF ZONATION 



Z3i. 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 re- 

 action 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 phenom- 

 enon 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 mycel- 

 ium 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, Placodiiim, 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 my- 

 celium 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 

 Mnhlenbergia, Sporoholns, Boutcloiia, 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 doubt- 

 less 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. 



