1922] WATERMAN—PLANT COMMUNITIES : 23 
may be held by humus in the soil, or by a high moisture content in 
sand relatively free from humus, and it seeems possible that this 
may also be true for Tsuga, but not for Fagus. Acer saccharum 
seemed to hold an intermediate position between the two, and its 
apparent absence from any advanced positions on the sand ridge 
habitat was a distinct surprise. Morainic soil equally with humus 
seems to afford a suitable substratum for the establishment of all 
the species mentioned, either because of the supply of necessary 
mineral elements, or because these minerals make possible a better 
utilization of the water present. For these reasons the advance- 
ment of the deciduous formation seems to be the result of increased 
humus content and mesophytism, and also a pushing forward by 
sheer force of numbers. The parent seed trees being so near at 
hand and supplying so many seeds, it follows that in time a fair 
number of seedlings have been able to find conditions favorable to 
growth and so become established. In the other case, the morainic 
substratum affords a soil peculiarly favorable to the deciduous trees, 
and while a much smaller number of seeds have lodged on it, a 
relatively larger proportion have become established. In view of 
the fact that the shade and moisture conditions vary greatly 
on different parts of this ridge, it would seem to be the chemical 
constituents of the soil which give to it its favorable character- 
istics. 
The question may be raised as to whether this condition may 
not be the result of the prehistoric fires, previous to which the 
beech-maple-hemlock elements may have been more widely dis- 
tributed, and the present isolated groups may be relics preserved 
because of the protection of bodies of water. Against this view may 
be set the evidence of tradition and the entire absence of stumps in 
other moist habitats of the region which seem to have been un- 
touched by fires. Unquestionably even hemlock stumps and logs 
do not last as long as pine, but it might be expected that some traces 
would remain if they had been at all widespread in comparatively 
recent times. This, however, would not account for the appearance 
of hemlocks on the edges of swamps at some distance from these 
telic patches. It is also reasonable to suppose that reforestation 
after a fire would proceed along general lines similar to those of 
