In primary roots, lateral roots arise opposite the protoxylem poles. They develop from root apices which form in the pericycle. Each apex has a root cap. The new roots have to grow through the cortex to reach the outside, with enzymes dissolving ahead of the growth the material that bonds cells together (middle lamella), or by physically forcing through the cortex and epidermis. This type of development is described as endogenous. Because the origin of lateral roots is close to the central vascular material, new vas-cular connections can be made readily. Because the lateral roots arise oppo-site the protoxylem poles, the number of rows of lateral roots is indicative of the number of protoxylem poles in the parent root, so, for example, three rows of laterals indicates a triarch primary.
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