236 CLIMAX FOBMATIONS OF NORTH AMERICA. 



occurs on the alluvial fans at the base of the mountains, composed of pervious 

 gravelly soil free from salt. Artemisia, which reseeds readily and grows 

 rapidly, probably entered directly on these fans as they were formed. Most 

 of the Arfemmo owes its origin to this type of development and only a relatively 

 small area originated in a saline lake bed. 



"The other line of development initiated by the Allenrolfea association is the 

 result of the pecuHar condition encountered in Tooele Valley and many similar 

 valleys, due to the drainage of a greater or less supply of fresh rain, spring, or 

 irrigation water on to the more or less level area just below the sloping section 

 of the valley, where the land does not slope sufficiently to afford rapid surface 

 drainage. 



"Under these conditions Distichlis gradually pushes into the Allenrolfea 

 areas, often between the hmnmocks, and finally may entirely replace AUen^ 

 rolfea. Very old plants of Allenrolfea are often found in such areas. Dis- 

 Uchlis areas show a lower percentage of alkali than Allenrolfea areas (usually 

 somewhat less than 1 per cent). Distichlis is displaced by Sporobolus airoides 

 in many parts of the valley. As is characteristic of the early stages in the 

 colonization of so many plants, Sporobolus first forms large mats which by 

 extension gradually dominate the area, forming a sod cover. Distichlis often 

 remains as a scattered plant of secondary importance. Sporobolus areas are 

 usually invaded by Chrysothamnus graveolens glabratus. Chrysothamnus occurs 

 scattered on Sporobolus sod and forms a rather permanent stage over a large 

 portion of the area. It is a rapid-growing form and the plants are usually 

 young as compared with Allenrolfea or even Salicomia mats. If increasing 

 quantities of fresh water are supplied to this type of land, it will first become 

 a wet meadow and later a marsh. 



" Fire or sudden drought or heavy grazing may bring about many inter- 

 ruptions in the successions here noted. Fire, which kills out the Artemisia, is 

 followed by Bromus tectorum and Erodium dcutarium, and this stage by Guti- 

 errezia sarothrae, and this in turn by Artemisia. A. similar succession follows 

 a fire in Atriplex, which is, however, much less likely to burn. A severe drought 

 which kills the Artemisia or Atriplex has the same effect as fire, except that the 

 appearance, especially of the Atriplex land, is quite different for several years, 

 due to the dead plants. 



" Breaking on Artemisia land results in the same succession as that following 

 fire, except that Solanum triflorum, Verbena bracteosa and other ruderals occur 

 in the early stage, followed by or accompanied by Erodium and Bromus." 



SOUTHEASTERN CLIMAXES. 



Webber (1898 : 658) has foimd that on the east coast of Florida the deposition 

 of sand carried by the waves forms shoals or banks 200 to 400 feet from shore : 



This piling-up goes on until the bank projects above the water at low tide, 

 when the wind and waves raise its level still higher. The sand then becomes 

 colonized by Sesuvium portulacastrum, Iva imbricata, Cakile maritima, Panicum 

 amarum, etc. These serve as dune formers in originating a low line of dunes. 

 Uniola paniculata is the main sand-binding grass to be found on the top and 

 seaward side of the dunes. With it are associated Spartina, Panicum, Ipo- 

 moea, and other plants to a small extent. At the base of the main line of 

 dunes grow a nimiber of plants which serve also as dime-builders. In addi- 

 tion to Panicum, Iva, and Cakile, the most important ones are Ipomoea pes- 

 caprae and Battatas littoralis. 



Millspavgh (1907 : 241) has studied the order of invasion of plants on the 

 sand keys or islets off the coast of Florida. The endeavor has been made to 



