=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 numbers 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 general, the roots seek those portions of the soil provided with a modicum of moisture. Usually a suitable moisture condition is present in those portions of the soil containing the plant food. But if portions of the soil are too dry and very nearby other portions containing moisture, the roots grow mainly into the moist substratum (_hydrotropism_). If the soil is too wet, the roots grow away from it to soil with less water, or in some cases will grow to and upon the surface of the soil. The roots need _aeration_, and where the supply of water is too great, the air is shut out, and we know that corn, wheat, and many other plants become “sickly” in low and undrained soil in wet seasons. This can only be said in the case of our ordinary dry land plants, i.e., those that occupy an intermediate position between _water-loving_ plants and _dry-conditioned_ plants. This phase of the subject must be reserved for special treatment. (See Chapter XLVI.)