HARSHBERGER: PHYTO-GEOGRAPHIC SKETCH 149 
The mutual aid which plants show is quite as important in the 
grouping of associations, as the struggle for light and soil room. 
The beech-drop for example is dependent on the beech, and 
hence it is only found where the beech is an element of the forest, 
The Indian-pipe occurs only in woods where there is abundant 
humus, as also the majority of ericaceous plants provided with 
mycorhiza. Certain funguses are dependent upon the material 
formed by the decay of certain other plants and they, therefore, 
abound only where this material is found. The fungous flora of a 
particular forest area comes and goes in an inexplicable manner. 
One season there will be an abundance of a particular species and 
during another season that species cannot be found, but will be 
replaced by some other form. The character of the rainfall, 
whether light, heavy or frequent is a determining factor in the 
appearance of plants. With a heavy downpour most of the water 
which has fallen runs off the surface and does very little good. 
On the other hand, gentle rains which come frequently soften the 
ground, and thus furnish some water to the lower strata, or at 
least preserve the supply which is already there. The writer be- 
lieves that there is a delicate balance of some kind established be- 
tween fungi and the climatic and edaphic conditions of any neigh- 
borhood. It is only when the climatic and edaphic conditions are 
Suitable that the fungus species again appears.* The conditions, 
therefore, which control the character of the vegetal covering of 
a mesophytic forest are most complex and intricate. 
HEMLOCK FORMATION. — This formation occurs on the sloping 
hillsides and precipitous banks of streams and is developed notably 
on Wissahickon and Crum creeks. It is found generally along 
the Wissahickon creek and at the ox-bow of Crum creek below 
the Springfield water works on the north and west banks of the 
stream at this point. The forest of hemlocks consists in a few 
Places of a pure growth without the admixture of any other tree 
Species, but usually associated with the hemlock, 7suga Canaden- 
sis (L.) Carr. the botanist finds the beech, Fagus Americana and 
ted maple, Acer rubrum. These trees are tolerant of the dense 
shade of the hemlocks. Where the forest floor has not been dis- 
* G. Whitney, M. & Cameron, F. K. The chemistry of the soil as related to 
‘top production, U.S. Dep. Agr. Bureau Soils Bull. 22 : 47-55- 
