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PLANT PHYSIOLOGY AND ECOLOGY 



zones of growth do not increase in size until tliey make the zones 

 of formations, but they serve as examples of the action of growth 



in z on at ion. 



322. Zones due to migration and ecesis. The growth of run- 

 ners or rootstocks away from the plant or mass in all directions 

 is a very effective means of migration. The seeding of the plants 

 thus carried away from the central mass is most certain at the 

 edge of the newly occupied area. The circle thus becomes larger 



Fig. 121. A growth zone of Muhlenhergia gracilis, produced by the radial 

 growth of the original mat and the dying of the older parts. 



year by year. Sooner or later the younger and more vigorous 

 circumference becomes a more or less complete zone. This is 

 due to the reaction of the central individuals upon the habitat, 

 so that they are readily replaced by invaders, or to their increas- 

 ing age and dying out. 



323. Zones due to reaction. Certain reactions of plants upon 

 habitats produce zonation. The zones of fungi seem to be caused 

 by the exnaustion of the organic matter present. In the mats 

 of mosses and flowering plants it is probable that the continued 



