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 
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 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, 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 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 tvpical 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 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. 
