CAUSES OF ZONATION 151 



result of radial f.riowth 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 Rysterographium. 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 -form- 

 ing 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 de- 

 mands 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 the zones of formations. Growth 

 contributes its share to the production of these, but there is 



