128 BOTANICAL GAZETTE [FEBRUARY 
pine-black spruce forest that is pure; that is, where there is not 
present a more or less important element representing the climax 
stage. 
The penultimate stage in the succession is quite frequently seen. 
The jack pine has disappeared and the black spruces are present 
mainly as relicts. Calliergon Schreberi has come to share its posi- 
tion of dominance with the more mesophytic mosses Hypnum crista- 
castrensis and Hylocomium proliferum; and the climax trees have 
assumed their characteristic relations. 
b) Effect of special conditions upon the rock shore succession 
The shore vegetation as we see it today has not everywhere the 
same aspect, and the differences that appear are explained by the 
operation of the two following laws. 
1. 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.—The limit varies from the water level 
along the sheltered coves and harbors to a height of several meters 
upon the shores that face the lake. It is here far above the reach 
of the highest waves of the growing season, since the most severe 
storms take place during late fall and early winter (see ADAMS 4, 
p. 46). While depending in the main upon the size of the waves 
that break upon it, the position of the limit of forest extension 15 
modified by the character of the shore. If it be rough or broken by 
ledges, neither waves nor ice will be projected so far as if the slope 
were smooth and unbroken, and the forest in the former case may 
thus establish itself lower. The effects of such differences may be 
seen by comparing figs. 15 and 27. If by reason of a gentle sub- 
marine slope or a submerged reef the waves break far from shore, 
the limit of forest extension may be greatly lowered. 
2. The extent to which the forestable territory has been occupied at 
the present day depends upon the rapidity of invasion, which 1s 
governed by the character of the rock, the angle of slope, and the degree 
of exposure to winds——Abundance of irregularities and crevices 12 
the rock surface and the presence of large quantities of the products 
of disintegration tend to facilitate invasion, while smoothness © 
- surface, paucity of crevices, and freedom from disintegration prod- 
