REACTIONS 131 



stems, leaves and fruits, with the exception of the relatively 

 small number of seeds that germinate. Perennial herbs 

 return everything but the persistent underground parts: 

 shrubs and trees replace annually an immense amount of 

 material used in leaves and fruits, arid sooner or later by 

 the gradual decay of the individuals, or by the destruction 

 of the whole 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 

 necessary 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. Prom 

 the universal occurrence of weeds in cultivated regions, the 

 pioneers in impoverished or exhausted fields are uniformly 

 ruderal plants. As is well-known, the seed 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 rapidlv 

 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. 



(7) Succession by the accumulation of 

 h u m us . This is the characteristic reaction of peat bogs, 

 and cypress swamps (oocyilia), in which the accumulation of 

 vegetable matter is enormous. The plant remains decom- 

 pose slowly and incompletely under the water, giving rise 

 to the various humic acids. These possess remarkable 

 antiseptic qualities, and have an injurious effect upon pro- 

 toplasm. Like alkalies, they hinder the absorption of 

 •water by the root hairs, though this is also due in part to poor 

 aeration, and in consequence call forth xerophytic modifi- 

 cations. The same tendency exists in practically all inland 



