To begin we have to know what a
meristem is. The dictionary definition of a meristem is a region of plant tissue, found chiefly at the growing tips of roots and shoots and in the cambium, consisting of actively dividing cells forming new tissue. Basically meristems are what cause plants to grow in height and width, The Mesquite has both types, Apical and Lateral. The Apical Meristem is tissue found in the buds and growing tips of roots in plants. Its main function is to begin growth of new cells in young seedlings at the tips of roots and shoots. The Lateral Meristem, also called the secondary meristem, which is found in all woody plants, consist of the vascular cambium and the cork cambium. It produces secondary tissues from a ring of vascular cambium in stems and roots. Secondary phloem forms along the outer edge of the cambium ring, and secondary xylem (wood/bark) forms along the inner edge of the cambium ring. The cork cambium produces a secondary dermal tissue that replaces the epidermis along older stems and roots.
Auxin is a hormone that promotes elongation of cells in the stem. Auxin is synthesized by the tip of the stem, and is carried to the part of stem which receives less light. This will cause the elongation of that side, thus bending towards the bright side.