Cross sections illustrating the lateral roots initiation.

Lateral roots or secondary roots develop from the main primary root or from secondary roots. They participate to expansion of the root system to help the plant to anchor in a more secure way. In addition, they store sugars and nutriments for future requirements of the plant growth.

Lateral roots are initiated perpendicular to the main root at a distance from apex varying from species. It’s a process of organogenesis as a new organ is formed. They originate from pericycle cells, situated in front of primary xylem poles (see figures below). The new primordium must break through the different layers of cortex to emerge outside as a new root. The process takes 3 to 4 days. Then the new emerging root will grow and organize in different regions as the main primary root.

Ci dessous microphotographies de deux coupes transversales dans des racines de Quercus montrant l’initiation de racines latérales.
A gauche coupe de 3 microns dans une racine enrobée dans le technovit et colorée au PAS. Réalisation : Audrey Berger (Master Université de Genève).  A droite coupe paraffine de 10 microns dans une racine enrobée dans la paraffine et colorée au bleu de toluidine.
Prises de vue : M.Crèvecoeur

Below, micrographs of two cross sections through roots of Quercus.
On the left a section 3 µm thick through a root embedded in Technovit and stained with PAS (From : Audrey Berger (Master University of Geneva).
On the right a section 10 µm thick through a root embedded in paraffin and stained with Toluidine Blue.

Red arrows indicate xylem poles in front of which the two lateral roots are initiated. They have crossed the cortex. On the longer root, number 1, there is continuity between xylem from the main root and xylem from the lateral root. Rh. Rhizoderm.

Three lateral roots a different level of initiation from pericycle, in front of xylem poles.