[Illustration: FIG. 43.—TWO KINDS OF SOIL THAT HAVE BEEN WET AND THEN DRIED. The loamy soil above remains loose and capable of growing plants; the clay soil below has baked and cracked. ] =Roots require moisture= in order to serve the plant. The soil water that is valuable to the plant is not the free water, but the _thin film of moisture which adheres to each little particle of soil_. The finer the soil, the greater the number of particles, and therefore the greater is the quantity of film moisture that it can hold. This moisture surrounding the grains may not be perceptible, yet the plant can use it. _Root absorption may continue in a soil which seems to be dust dry._ Soils that are very hard and “baked” (Fig. 43) contain very little moisture or air,—not so much as similar soils that are granular or mellow. =Proper Temperature for Root Action.=—_The root must be warm in order to perform its functions._ Should the soil of fields or greenhouses be much colder than the air, the plant suffers. When in a warm atmosphere, or in a dry atmosphere, plants need to absorb much water from the soil, and the roots must be warm if the root-hairs are to supply the water as rapidly as it is needed. _If the roots are chilled, the plant may wilt or die._ [Illustration: FIG. 44.—ROOT-HAIRS OF THE RADISH. ] =Roots need Air.=—Corn on land that has been flooded by heavy rains loses its green colour and turns yellow. _Besides diluting plant-food, the water drives the air from the soil, and this suffocation of the roots is very soon apparent in the general ill health of the plant._ Stirring or tilling the soil aërates it. Water plants and bog plants have adapted themselves to their particular conditions. They get their air either by special surface roots, or from the water through stems and leaves. =Rootlets.=—_Roots divide into the thinnest and finest fibrils: there are roots and there are rootlets._ The smallest rootlets are so slender and delicate that they break off even when the plant is very carefully lifted from the soil. _The rootlets, or fine divisions, are clothed with the_ =root-hairs= (Figs. 44, 45, 46). _These root-hairs attach to the soil particles, and a great amount of soil is thus brought into actual contact with the plant._ These are very _delicate prolonged surface cells of the roots_. They are borne for a short distance just back of the tip of the root. [Illustration: FIG. 45.—CROSS-SECTION OF ROOT, enlarged, showing root-hairs.]