1903] Blankinship,— Plant formations of E. Massachusetts 125 
found occupying a given habitat is called a “plant-formation” from 
its analogy to geological formations. 
There is often a considerable difference in the prevailing plant- 
formations of adjacent districts within the same biological region, due 
to difference in physiography and in the underlying rock formations, 
upon which the nature of the soil depends. In the district here con- 
sidered the limestone areas with their characteristic plant-formations 
appear to be wholly absent, while more inland the saline formations 
are wanting. Another important element in the study of the ecology 
of a region is the variation in altitude above sea-level. The variation 
here scarcely exceeds 600 feet and may be left out of consideration, 
but in more mountainous districts this altitudinal variation is often 
sufficient to affect materially the distribution of species, which tend 
to group themselves at the different altitudes into zones correspond- 
ing in large part to the larger latitudinal zones of the continent, each 
having its own system of species and plant-formations. 
In every district there is usually some dominant plant-formation, 
throughout which the other formations occur as lines and patches, 
without materially affecting the general physiognomy of the region, 
determined by the flora of this dominant formation, which is here the 
upland forest composed of the Sand Plain and Hilly Upland Forma- 
tions, dependent upon the nature of the soil. 
The classification followed differs in several important particulars 
from those usually given and is based (1) upon the soil, (2) the 
amount of contained moisture and (3) upon the relative amount of 
sunlight, which by their several combinations form the plant-habitats 
of this region, to which must be added the three biological forma- 
tions dependent upon other factors. The table below represents the 
possible combinations of these factors with the resulting plant-forma. 
tions as they appear in our flora. 
