362 HOW CKOPS GROW. 



The Medium in which Boots Orow has a great influ- 

 ence on their extension. When they are situated in con- 

 centrated solutions, or in a Tery fertile soil, they are 

 short, and numerously branched. Where their food is 

 sparse, they are attenuated, and bear a comparatiyely 

 small number of rootlets. Illustrations of the former 

 condition are often seen ; moist bones and masses of 

 manure are not infrequently found, completely covered 

 and penetrated by a fleece of stout roots. On the other 

 hand, the roots -which grow in poor, dry soils are very 

 long and slender. 



Nobbe has described some experiments which com- 

 pletely establish the point under notice. {Vs. St., lY, 

 p. 212. ) He allowed maize to grow in a poor clay soil, 

 contained in glass cylinders, each vessel having in it a 

 quantity of a fertilizing mi;sture disposed in some pecu- 

 liar manner for the purpose of observing its influence on 

 the roots. When the plants had been nearly four months 

 in growth, the vessels were placed in water until the earth 

 was softened, so that by gentle agitation it could be com- 

 pletely removed from the roots. The latter, on .being 

 suspended in a glass vessel of water, assumed nearly the 

 position they had occupied in the soil, and it was ob- 

 served that, where the fertilizer had been thoroughly 

 mixed with the soil, the roots uniformly occupied its 

 entire mass. Where the fertilizer had been placed in a 

 horizontal layer at the depth of about one inch, the roots 

 at that depth formed a mat of the finest fibers. Where 

 the fertilizer was situated in a horizontal layer at half the 

 depth of the vessel, just there the root system was sphe- 

 roidally expanded. In the cylinders where the fertilizer 

 formed a vertical layer on the interior walls, the external 

 roots were developed in numberless ramifications, while 

 the interior roots were comparatively unbranched. In 

 pots, where the fertilizer was disposed as a central vertical 

 core, the inner roots were far more greatly developed 



