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Lateral roots arise from the pericycle. The pericycle is not easily identifiable in the fluorescent photographs that we have, but its position can be deduced by locating its neighboring tissues. The pericycle sits just inside the endodermis and surrounds the primary xylem and phloem. Periclinal divisions in the pericycle lead to lateral root formation. Below is a photograph of a lateral root emerging from the taproot. The taproot appears in cross section while the lateral root appears in longitudinal section. If you look close at the lateral root you can see where the primary xylem has already differentiated into long cells with spiral cell walls.
Note also in this photograph, that in the taproot secondary growth has started to occur.
Below is another photograph of a lateral root which shows considerable secondary growth. Some of the tissues have been labeled. The vascular cambium and the secondary phloem that have been identified belong the root that gave rise to the lateral root. The lateral root, viewed here in longitudinal section has also been identified. 
To learn about secondary growth in the cotton root... |
Introduction Flowers&Fruit Roots Stems Leaves |