Like stems, roots grow in length only at the tips. Although they grow where moisture occurs, moisture is not the only requirement for their growth. Sugar produced in the leaves by photosynthesis is also necessary, as are hormones, which are also produced in the leaves. Hormones must be transported from the leaves to the roots in order to influence root growth. Temperature; the presence of minerals and fungi; and acidity or alkalinity are other important factors in both root and plant growth. In addition to moisture, air in the soil is also significant. Interestingly, the roots of a pear tree can spread to nine or more times the diameter of the aboveground portion of the tree yet not reach much depth.
Figure 33-1 The root system may be ten times the diameter of the aboveground portion of a tree.
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