180 NO RJ HERN GLACIAL ZONE 
phytes or plants with mycorhiza on the humus formed by 
particular species, and so on. 
While in general a given species will be found only or 
chiefly in a certain association, this is not always the case. 
One ecological combination of factors may replace another ; 
thus many alpine plants also occur in associations confined 
to the seashore, others again as epiphytes in the lowland 
forests. 
The study of plant associations and their mutual com- 
petition, and the effects produced by modifications of 
conditions is already yielding much information of value 
in geographical distribution, and throws much light on the 
study of evolution, morphology, &c. 
To describe the many associations occurring in the 
different zones would far exceed the limits of this work; 
but an indication is given of some of the more important 
features of the vegetation of each zone. 
Drude enumerates six zones of vegetation (see Map III. 
of his Atlas), as follows : 
/. Northern glacial zone \ This consists of the 
arctic region (including the Tundras), and is limited on the 
south by the northern limit of trees (i.e. the limit beyond 
which they do not naturally occur). A corresponding region, 
counted as part of this zone, occurs above 15,000 feet in 
Thibet, and in other northern mountain regions at corre- 
spondingly lower elevations ; e.g. in Britain the limit of trees 
rarely lies above 2000 feet. The mountain zone above the 
tree-limit is termed the alpine zone, and its characteristic 
plants are termed alpine plants . The general habit and 
structural features of alpine plants are much the same all 
over the world and resemble those of arctic plants. 
In the arctic regions the conditions of life are peculiar ; 
the period during which growth can take place is very short 
(1 — 4 months) ; the average temperature is low ; the light is 
bright and long-continued, and the sun describes an almost 
complete circle every day, so that the light falls upon the 
plants from every side in turn at a comparatively low angle ; 
1 Schimper, Warming, op.cit. ; Jungner, Wiesner, etc. cited on p. 143 ; 
Jaccard, Gesetze der Pflanzenvertheilung in der alpine Region , Flora, 90, 
p.349; Muller, Alpenblumen, Leipzig, 1881 ; Willis and Burkill in Trans. 
Bot. Soc. Fdinb. LXVI, 1901, and Ann. of Bot. XVII, 1903. 
