76 REACTION: THE INFLUENCE OF COMMUNITY ON HABITAT 



in the production of fine soil, especially when this accumulates in one 

 particular place. This is a property of all plants from unicellular 

 algae to the largest trees, but its importance depends largely upon 

 the size and extent of the root system. The initial conversion of rock 

 into soil is carried on by the pioneer lichens and their successors, 

 the mosses, in which the hairlike rhizoids assume the role of roots in 

 breaking down the surface into a fine dust. This process is exceedingly 

 slow in granites and lavas, but proceeds more rapidly with sandstones 

 and limestones. It is promoted by the action of frost, as well as wind 

 and water, in forming tiny cracks into which the corroded dust is 

 washed and ants and other animals find space and small herbs may 

 invade. By virtue of their root systems, these carry on the process of 

 weathering more effectually, each successive stage in the xerosere tak- 

 ing a larger part in the process. At the same time, the decay and 

 excrement of each generation add organic matter until a more or less 

 uniform soil is constituted. 



Reaction upon Wind-borne Material. The major reaction of the 

 plant body upon wind is to lessen its velocity and thus bring about the 

 dropping of its burden of sand or dust. A minor effect results from 

 its constituting a definite obstacle to the movement of particles, a fact 

 often recorded in the abrasion of stems or the etching of trunks. In 

 addition, the plant serves as windbreak for the accumulation at its 

 base and thus renders it difficult for the wind to pick up the grains 

 again. The life form is of the first importance in this reaction, tall 

 plants and especially those with single stems having little or no effect, 

 while mat, bunch, and bush forms attain the greatest success. Roots 

 and rhizomes exert a complementary reaction by binding the accumu- 

 lating material, fibrous ones being naturally the most effective. Of 

 even greater significance than form is the faculty of developing new 

 shoots as the crown is buried, thus permitting plant and community to 

 keep pace with the accretion of dune or ridge. The ability of grasses 

 to produce tillers is peculiarly advantageous in this process and hence 

 the grass form is probably the best adapted to dune formation and 

 stabilization. 



The action of shoots upon wind-borne snow is essentially identical, 

 but the accumulation is transient, and the reaction is primarily upon 

 the water content of the soil. 



Reaction upon Water-borne Detritus. The mechanical action of 

 plants upon currents of water is essentially similar to that already 

 noted for air currents. Movement is impeded, and the load is depos- 

 ited in whole or in part. Stems and leaves also make difficult the 

 removal of material once dropped, and root stocks and roots take 



