SUCCESSION 263 
formation, they restore all that they have taken from the soil. This balance 
is further maintained to an important degree by the activity of the roots, 
which take from the deep-seated layers of the soil the crude materials neces- 
sary for the formation of leaves and fruits. Upon the fall and decay of these, 
their materials are incorporated with the upper layers of the formation floor, 
from which they may be absorbed by the undergrowth, or find their way 
again into the layers permeated by the tree roots. From the universal oc- 
currence of weeds in cultivated regions, the pioneers in impoverished or 
exhausted fields are uniformly ruderal plants. As is well known, the seed 
Fig. 68. Spruce forest formation (Picea-Pseudotsuga-hylium), stage 
VII, the ultimate stage of the talus succession. 
production and ecesis of these forms are such that they take possession 
quickly and completeiy, while their demands upon the soil are of such a 
nature that the most sterile field can rapidly be covered by a vigorous growth 
of weeds. As indicated elsewhere, ruderal formations ultimately yield to 
the native vegetation, though in regions so completely given over to culture 
that native formations are lacking or remote, it is probable that successions 
reach their final stage within the group of ruderal plants. 
322. Succession by the accumulation of humus. This is the character- 
istic reaction of peat bogs and cypress swamps (oxodia), in which the 
