228 BOTANICAL GAZETTE [MARCH 
crevices in all directions, and weaving among the cladonias and 
plants of the rock pools bring about the formation of a firm mat. 
The crevice vegetation is of extreme importance because of its 
rapidity of development and its preeminent part in the formation 
of the heath mat. Forest establishment is accomplished much 
sooner where crevices are abundant in the rock than where they are 
scarce. 
The rock pool subsuccession goes through its development in 
depressions where water stands at least a part of the time. These 
become gradually filled after the manner of the bog succession, and 
the vegetation later becomes an element in the formation of the 
heath mat. 
The heath mat results from the coalescence of the vegetation 
developed through the instrumentality of the three subsuccessions 
named above. 
The climax forest often follows immediately after the formation 
of the heath mat, the trees obtaining their first foothold in the 
crevices. A relatively xerophytic forest stage characterized by 
Pinus Banksiana and Picea mariana sometimes intervenes, in 
which Pinus is the pioneer and Picea remains for some time after 
the climax trees have attained dominance. 
Telescoping of stages is pronounced throughout the series, so 
that pioneer and climax forms, with those of all intermediate stages, 
are frequently found occupying a single limited area. 
The effect of special conditions upon the rock shore succession 
is expressed in the two following laws: 
t. The lower limit of possible forest extension is determined 
approximately by the upper limit of effective wave and ice work, 
the lake level remaining constant. 
2. The extent to which the forestable territory has been occupied 
at the present day depends upon the rapidity of invasion, which is 
governed by the character of the rock, the angle of slope, and the 
degree of exposure to winds. 
The operation of these laws results in the production of three 
phases of rock shore vegetation, characterized respectively by 
(A) climax forest to the water’s edge, (B) a zone of incomplete 
invasion, (C) abrupt transition from bare rock shore to climax 
forest. 
