Development of the Vegetation of New York State 131 
the aggressiveness of Sphagnum in leading a bog invasion 
to the ultimate extinction and burying of forest may 
be found e. g., in Warming’ where its role as an agent of 
invasion and peat forming is more fully described.” 
(2) The Characteristics and Role of Heath Shrubs. 
The word heath is used in rather the popular sense. Per- 
haps the expression heath family will be generally under- 
stood to apply in the sense in which it is used in Gray’s 
Manual. The heath kinship circle, under the order name 
Ericales* embraces the following families: (1) white alder 
family (2) wintergreen family (3) indian pipe family, (4) 
heath family (5) huckleberry family (6) diapensia family. 
The entire order of Hricales is of special interest not only 
in the more restricted ecological relation but equally in the 
broader aspects (historical and other) of plant geography. 
We have noted their occurrence among the arctic species of 
Mt. Marcy, in bogs of the extreme type, in the filling of 
glacial “kettle holes”, in sand barrens, and on arid moun- 
tain ridge habitats (Shawungunk Mts.). Perhaps the chief 
immediate interest lies in the occupation by certain heath 
shrubs of these “ difficult ” or unfavorable soils — substrata 
—of peat and sand. 
The unfavorable qualities of such soils are associated with 
factors which make the absorption of soil water or soil nu- 
trients or both difficult. They have been thought to consti- 
tute “ physiologically dry”? habitats and to support there- 
fore a xerophytic vegetation. Certain characteristics of 
heath shrub structure are interpreted in this connection, 
e. g., the ericoid type of leaf. We have already made refer- 
ence to factors of toxicity, acidity, lack of available nutri- 
ents, ete. Certainly so far as peat and sand soils, from 
1 Warming, /. c., p. 201 and following. Again, p. 361. 
2 As to the structure and life history of Sphagnum, see college text 
books on morphological botany. 
3 Britton, N. L., and Brown, Addison, Illustr. Flora Northern U. S. 
and Canada. Second edition, 3 vols., 1913. 
