Secondary Growth

Secondary Growth

Secondary growth is an increase in diameter/girth/circumference due to the addition of secondary tissue(secondary cortex, secondary phloem, and secondary xylem) formed by the activity of vascular cambium in stelar region(region inside pericycle) and cork cambium (phellogen) in extrastelar region (the region outside pericycle i.e., cortex). It is found in the dicot stem, dicot roots, and gymnosperms. Herbs, shrubs, and hydrophytes do not show secondary growth. Monocots rarely show secondary growth(e.g., Aloe, Dracaena, Agave)....

Secondary Growth in dicot Root

It occurs behind root hair zone. In the root, primary cambium is absent. First of all, the cells of conjunctive tissue below phloem region and then, cells of pericycle lying opposite the protoxylem and cells of conjunctive tissue between phloem and xylem regions become meristematic by dedifferentiation and form strips of cambia. Thus, cambium in roots is secondary meristem. The number of such strips depends upon the number of phloem/xylem bundles. These strips extend both ways in between xylem and phloem bundles and finally unite to form a wavy  band of vascular cambium. The cambium strip below phloem region becomes active first and divides earlier. Its activity is much faster on the inner side to form secondary xylem. Due to this, the phloem and cambium strips below it are pushed outward making wavy band of cambium a circular one. Now the whole of the cambium becomes active and divides to form secondary xylem. Due to this, the phloem and cambium strips below it are pushed outward making wavy band of cambium a circular one. Now the whole of the cambium becomes active and divides to form 8-10 times more secondary xylem on inner side than secondary phloem on outer side in steler region. It is due to more divisions on the inner side. This causes the pushing of the primary xylem towards the pith and primary phloem towards the periphery. The pith, cortex, and endodermis are fully lost and the primary and old secondary phloem get crushed as the new secondary phloem becomes functional; primary and old secondary xylem persist in old roots. The primary xylem is distinguished by its position. The cells of cambium strips above protoxylem act ad ray initials and divide to form parenchyma on both outer and inner side forming broad, multiseriate parenchymatous primary medullary rays for lateral conduction of water and food. These ray initials are secondary in origin. Secondary medullary rays are more prominent in the root than in the stem. Annual rings are not formed because in the soil, temperature is almost uniform throughout the year....

Anomalous Secondary Growth

Any deviation from the normal type of secondary growth is called anomalous secondary growth. It is common in plants of tropical regions for example:...

FAQs on Secondary Growth

Question 1: What do you mean by Secondary Growth?...