a. apical meristems
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Primary growth causes roots and shoots to become longer. Apical meristems at the tips of shoots and roots are the origin of this lengthening process, which involves the lengthening of differentiating cells under the apical meristem. Lateral meristems allow the thickening, or secondary growth, of older shoots and roots. These lateral meristems consist of the cork cambium and the vascular cambium.