excessive, and plays an important role in the building up of the swamp bed. [Illustration: Fig. 67. Pine forest formation (_Pinus-xerohylium_), stage VI of the talus succession.] =320. Succession by enriching the soil.= This reaction occurs to some degree in the great majority of all successions. The relatively insignificant lichens and mosses produce this result upon the most barren rocks, while the higher forms of later stages, grasses, herbs, shrubs, and trees, exhibit it in marked progression. The reaction consists chiefly in the incorporation of the decomposed remains of each generation and each stage in the soil. A very important part is played by the mechanical and chemical action of the roots in breaking up the soil particles, and in changing them into soluble substances. Mycorrhizae, bacterial nodules, and especially soil bacteria play a large part in increasing the nutrition-content of the soil, but the extent to which they are effective in succession is completely unknown. The changes in the color, texture, and food value of the soil in passing from the initial to ultimate stages of a normal succession are well known, and have led many to think them the efficient reactions of such successions. It seems almost certain, however, that this is merely a concomitant, and that, even in anomalous successions where facies replace each other without obvious reasons, the reactions are concerned more with water-content, light, and humidity than with the food-content of the soil.