ROOTS. 411 



ward, while the geotropic character of the stem is shown in its 

 upward growth. 



The medium which the root has to penetrate offers consider- 

 able resistance, and the form of the root as well as its manner of 

 growth is adapted to overcome this difficulty. The slender, 

 conical, penetrating root-tip wedges its way between the minute 

 particles of soil or into the minute crevices of the rock, while 

 the nutation of the root enables it to search for the points of least 

 resistance. The root-tips having penetrated the soil, the older 

 portions of the root continue this wedge action by growth in 

 diameter, though, of course, elongation of the old parts of the 

 root does not take place. It is the widening growth of the taper- 

 ing root that produces the wedge-like action. The crevices of 

 the rock are sometimes broadened, but the resistance here is so 

 great, the root is often greatly flattened out. 



786. (2) Grappling the substratum. The mere penetration 

 of a single root into the soil gives it some hold on the soil and it 

 offers some resistance to a "pull" since it has wedged its way in 

 and the contact of soil particles offers resistance. The root-hairs 

 formed on the first entering root growing laterally in great num- 

 bers and applying themselves very closely to the soil particles, 

 increase greatly the hold of the plant on the soil, as one can 

 readily see by pulling up a young seedling. Lateral roots are 

 soon formed, and as these continue to extend and ramify in all 

 directions, the hold is increased until in the case of some of the 

 larger plants the resistance their hold would offer would equal 

 many tons. Even in some of the smaller shrubs and herbs the 

 resistance is considerable, as one can easily test by pulling with 

 the hand. To obtain some idea of the amount of resistance the 

 roots of these smaller plants offer, they can be tested by pulling 

 with the ordinary spring scales. 



787. (3) A congenial moisture, or water relation. In gen- 

 eral, the roots seek those portions of the soil provided with a modi- 

 cum of moisture. Usually a suitable moisture condition is present 

 in those portions of the soil containing the plant food. But if por- 

 tions of the soil are too dry and very nearby other portions con- 



