PAPERS PRESENTED AT GENERAL SESSIONS 61 



plants are plentiful. The clump habit of these plants 

 causes hummocks (what Alaskans call " nigger-heads") 

 and reticulate ridges with puddles between where water 

 often remains and Xostoc sp. and hydrophytic mosses 

 are found. 



The question of a climax is a difficult one. In the opin- 

 ion of the writer it seems probable that the climax is the 

 Sphagnum-Ericad tundra, (1) because it seems from the 

 observations to be most extensive in conditions not made 

 unusual by peculiar drainage conditions, and (2) because 

 it was an association found in areas of greatest age, for 

 example on the Arctic Divide. This association has 

 Sphagnum abundant but not very conspicuous because 

 of the larger size of the shrubs, the most important of 

 which are Ledum palustris, Vaccinium ulignosum, V. 

 Vitis-idaea, Betula glandulosa, and especially Eubus 

 Chamaemorus. On the drier hummocks, which are still 

 characteristic, are sedges and Polytrichum. Deep layers 

 of peat are usually found beneath this association, and 

 are frequently exposed by erosion of streams which cut 

 thru it and expose the frozen, rock-like layers below. 



So far as the writer observed the grasses have place 

 only on Hood plains of alluvial character, or of peat char- 

 acter, if the former tundra vegetation of the peat has 

 not survived transplantation. It is a brief stage and ap- 

 parently gives way to either the willows or to the sphag- 

 num and heaths. The willows are particularly interest- 

 ing in that they are distributed in regions where the 

 ground is thawed and wet. This condition is brought 

 about usually by seepage thru the peat of water draining 

 from higher levels. The drainage lines are usually very 

 conspicuous, and the line of extent of the willows coin- 

 cides with these drainage lines. These shrubs are of the 

 espalier habit described by Warming ; they grow about 

 five to six feet high and attain an inch or so in diamter 

 in fifty to sixty years of growth. Growing beneath the 

 willows are mesophytic mosses, sphagnum, and many 

 herbaceous plants. 



It is evident from this brief discussion that any change 

 causing new drainage lines will cause changes in the plant 

 associations: and because the peat is eroded easily this 



