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 b}' 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. G8. Spruce forest formation (Picca-Pscudolsiiga-hylium), stage 

 VII, the ultimate stage of the talus succession. 



production and ecesis of these forms are such that they take possession 

 quickly and completely, 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 



