1906.] Wheeler, Relations of Anis to Plants. 4. 1 I 



from establishing any large general trails when they go forth to forage. 

 Where the base of the nest is completely surrounded by water, they 

 are either completely isolated or must climb from leaf to leaf along the 

 plants until they reach terra finna. In general, such hills undergo 

 no further additions to their bulk. 



"3. Plants are rather quick to gain a foothold on the ant-hills. 

 The first and main vegetation of the nests is Polytrichmn strictum, 

 which forms a dense carpet. The condition which leads this moss 

 to its invasion is the comparative dryness of the nest. The Poly- 

 trichum carpet gradually spreads until it completely covers the hill. 



"4. The remaining plants that creep up onto the hills usually 

 belong to species growing in the immediate neighborhood. Other 

 plants are rarely found and these grow on the umbilicus or summit 

 which they haA^e reached as seeds and where they have found a place 

 suitable for germination. 



"5. The outer form of the ant-hill depends on the carpet of 

 Polytriehum. If this moss advances onto the hill from the side, the 

 substance of the nest flows over, so to speak, onto the opposite side. 

 In other words, the ants continue to add to the mound on the unin- 

 vaded side. One observes, moreover, that the nest-substance always 

 spreads in the directions where the moss carpet is least developed. 

 In other words, the ants withdraw as the Polytriehum carpet 

 advances. 



"6. Owing to the conditions stated in the preceding paragraph, 

 the basal portions of the ant-hill are abandoned by the ants pari passu 

 with the advance of the carpet. This takes place step by step. The 

 ants do not leave the basal portions till the carpet has risen too far. 

 This explains why the inhabited portions of the nest extend down 

 deepest into the hill where the diameter of the umbilicus is greatest. 



"7. Another result of the invasion of the Polytrichmn is the 

 pronounced reduction in the number of ants in the hill due to emi- 

 gration, since they are unable to add to the mound, while its habitable 

 portion is continually growing smaller. These conditions account 

 for the greater number of ant-hills in the zone of Sphagnum hummocks. 



"8. When the Polytrichmn carpet has reached a certain height 

 the ants cease to build. This follows directly from the preceding 

 paragraph. 



"9. The Polytriehum carpet steadily advances till it displaces 

 the ants completely. 



"10. The apparent aversion shown by the ants for the inner 

 portions of the hill already covered by the Polytriehum is due to the 



