1914] GATES—XEROPHILY 4409 
snow, but they exhibit these changes of position and color irre- 
spective of that fact. 
DECIDUOUS TREES AND SHRUBS 
The principal trees are Larix laricina and Acer rubrum, and the 
commonest shrubs are Aronia melanocarpa, Salix pedicellaris, S. 
discolor, S. sericea, Spiraea salicifolia, Betula pumila, Nemopanthes 
mucronata, Ilex verticillata, Gaylussacia baccata, Cornus paniculata, 
Cephalanthus occidentalis, Sambucus canadensis, and Rosa carolina. 
During the winter, bog trees and shrubs are leafless, which 
greatly reduces the transpiration. The snow that is present during 
the winter protects the root system and lower part of the stem 
from danger from excessively low temperature, but the upper parts 
of the trees and shrubs are not so protected. They must be able 
to resist water loss through their own modification. This is suffi- 
cient for the severest winters in southern Michigan. During the 
winter of 1911-1912, Spiraea was the only deciduous shrub to be 
killed down to the snow line. 
With the opening of spring the buds swell and develop into 
branches bearing leaves which carry on the work of the season. 
A separation layer is formed upon the approach of winter at the 
base of the petiole, and by the time winter has set in the leaves 
have fallen. 
HERBACEOUS PLANTS 
The seasonal history of the herbaceous bog plants follows two 
general lines: the plant which has developed during the growing 
season may die down completely before winter, leaving seeds to 
reproduce it the following year, or it may die down to the ground 
and be vegetatively reproduced the following year from under- 
ground stems, bulbs, rootstocks, or buds. Any of these ways is 
an absolute xerophytic adaptation on account of winter conditions, 
but does not interfere with summer development. 
Structure of certain peat bog plants 
ROOT SYSTEM 
Without exception, all of the forms dealt with had a very shallow 
root system, which was usually very poorly developed. It is in 
direct contrast to that of the xerophytes of the desert (CANNON 4). 
