The process of lateral root deveolopment takes place in three steps.  These steps are initiation, organization and emergence.  Often it is very hard to differentiate these steps.  The formation of lateral roots are stimulated by auxin and other growth regulators.  This is a longitutudinal section of rice root that is stained with toluidine blue.
The lateral root arises at the exterior of the vascular cycliner.  Since rice has polyarch root arrangement the lateral root arises opposite of the phloem.  It begins with unequal, periclinal division of the the pericycle intial cells.  Following these divisions are more periclinal and anticlinal divisions.  As a result a lateral root promordium is formed.  Meanwhile the endodermis is also dividing, which contributes some cell layers to the root primordium.  These layers will become the protoderm, which is the future epidermis of the root.
The vascular tissue differentiates at the basal part of the lateral root and develops towards the tip.  The vascular tissue develops in the same direction as the vascular tissue of the parent root.  Once the phloem and xylem of the lateral root begin to differentiate they will become connected with the same tissues in the parent root.  
At the time when the tip of the lateral root primordium reaches the epidermis of the parent root, the cortex and vascular cylinder are formed within in primordium.  Rice root is unique in that the emerging lateral root does not crush the cortical parenchuma cells.  Similarly to root hairs, formation of lateral roots create new absorbing surfaces, thus bringing these surfaces in contact with more areas of soil to take-up nutrients and water.  
Introduction | Flowers&Fruit | Roots | Stems | Leaves

© Thomas L. Rost 1997
Section of Plant Biology Division of Biological Sciences