ALTERNATION AND ZONATION 



295 



321. Zones due to growth. The causes that prochice zones 

 are either biological or physical. The first are connected with 

 some feature of the plant, the second with the physical factors 

 of the habitat. Biological causes arise from the method of growth, 

 the manner of migration, or the reaction of the species upon the 

 habitat. The formation of circles as a result of radial growth 

 is a very common occurrence. It is well known in the case of 

 the "fairy-rings" of certain mushrooms. It is also found in a 

 large number of molds, mildews, and other fungi, especially the 



Fig. 120. Bog, meadow, thicket, and forest zones at the upper end of 



Lake Moraine. 



foliose lichens. The thalloid liverworts show a similar radial 

 growth. Flowering plants, and many mosses also, furnish excel- 

 lent examples in those species that form mats, turfs, or carpets. 

 Growth results in zonation only when the older central por- 

 tions of the individual or mass die away, leaving a gradually 

 widening belt of younger plants or parts. This seems to be due 

 in part to the greater age of the central portion, and in part to 

 the successful competition of the young, actively growing parts 

 for the water and nutrient material of the soil. Such miniature 



