ECOLOGY 89 



plants differ greatly in regard to their ability to thrive on soil 

 containing little organic matter, and it is probable that this is 

 really the determining cause of this apparent soil relation. 

 Soils of unlike mineral constitution do not retain the organic 

 matter to the same degree and it is therefore probably the 

 amount of humus present in the soil that determines its adapta- 

 bility to any given plant. 



210. Occasionally soils are too unstable to permit vegetation 

 to secure a foothold. This is notably true of the shifting sand 

 dunes of many windward coasts and in sandy deserts. There 

 are a few plants, however, which are enabled to maintain their 

 position in such soil by virtue of rapid and deep rooting or, 

 better still, by means of stolons or runners which enable the 

 individual stocks to cling to each other and finally, forming a 

 felted carpet, protect the sand from the wind and hold it in 

 place. 



Relation of Plants to Each Other 



211. We have heretofore spoken of the green plants as being 

 independent in the sense of deriving their sustenance directly 

 from inorganic matter. This might be regarded as quite 

 generally true wherever the plant is provided with soluble 

 nitrogen compounds. However, these salts are by no means 

 everywhere present in the soil, and under such circumstances 

 green plants become dependent upon certain fungi, as we shall 

 presently see. 



212. Plants may be dependent upon other plants in a great 

 variety of ways, and in varying degrees. The climbers, for 

 example, get only mechanical support from their stouter neigh- 

 bors. Some cHng to their support by means of aerial rootlets, 

 which penetrate the outer layers of the bark of the host. Others 

 spread their long slender tendrils, which, on contact with a solid 

 object, coil around it and then draw the stem of the plant close 



