Secondary growth is observed in vascular cambium and phellogen of dicot roots. It is seen that vascular cambium can be formed secondarily from conjunctive parenchyma and part of pericycle just lying opposite to the protoxylem points. Initially vascular cambium is seems to be wavy but later on it becomes circular and vascular cambium is derived from pericycle, which is form primary vascular rays. At some other places it forms secondary xylem on the inner side and secondary phloem on the outside. Secondary vascular rays are observed in different places whereas primary vascular rays are multiseriate at the beginning and uniseriate on later.
Secondary phloem do not persist as a result when it is become operational the previous phloem get crushed before it becomes operational. Secondary xylem and on the other hand primary xylem both are persist.
Annual rings are not present, because the areas where the roots of the plants grow are, very less variations of temperature of soil (due to less variations of season) are observed.
Cork cambium or phellogen are formed from pericycle which produces phellem or cork on the outside and on the other hand secondary cortex or phelloderm on the inner side. These three are together called periderm. In this case cortex and epiblema peel off, cork or phellem is made of dead cells. Lenticular are observed in different places.
Anomalous secondary growth of roots-
In some plants storage roots are observed where accessory cambial rings are formed which cut out less or little secondary xylem and more storage parenchyma in secondary phloem. Example- Beet, sweet potato.
1. What do you mean by primary xylem and secondary xylem?
Primary xylem is derived from procambium during primary growth and is consists of protoxylem and metaxylem.
Secondary xylem is formed from vascular cambium during secondary growth.
2. What do you mean by primary phloem and secondary phloem?
Primary phloem is formed from procambium during primary growth. It may or may be not show differentiation into protophloem and metaphloem.
Secondary phloem is produced during the secondary growth of vascular cambium.