Secondary Growth

Term tissue ( French word meaning woven ) was used by N.Grew (1682) the father of plant anatomy. Nageli is regarded as the father of Modern Anatomy. A cell is the functional and structural unit of life. A tissue is a group of cells having a common origin, the same method of development, and function. Anatomy deals with the internal organization of plants. In plants, the terms anatomy and histology have the same meaning. Different organs in plants show differences in their internal structure. Within angiosperms, the monocots and dicots are also seen to be anatomically different. The internal structures show adaptations to diverse environments.

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).

Vascular cambium and cork cambium are lateral meristems. Secondary tissues are formed in secondary growth to provide Protection (by cork), mechanical strength (by secondary xylem), conduction of water, and nutrients ( by secondary xylem and secondary phloem). Secondary growth is characterized by an increase in the thickness or girth of the plant. It is caused by cell division in the lateral meristem.

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.

The secondary tissues formed by the activity of vascular cambium in steler region, exert a pressure on the outer tissue and cause rupturing of epidermis and crushing of cortex and finally peel off. To withstand this pressure, the cells of pericycle become meristematic to form cork cambium(phellogen). It form phellem(cork) on the outside and secondary cortex on the inner side. Cork cambium and secondary cortex together are called periderm which is protective in nature. Lenticels are formed in the cork for aeration. Bark formation is very early in the roots than the stem.

Why Secondary growth is absent in the Monocot root?

In general, monocots do not undergo secondary growth. If they do increase in girth (like palm trees and yucca plants), it does not result in the development of a secondary xylem and phloem, since monocots don’t have vascular cambium. An increase in girth without secondary growth is referred to as anomalous thickening.

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:

• Secondary growth in monocots like Dracaena , Agave, Yucca, Aloe by accessory cambia. 
• Growth in palms is by the activity of primary thickening of apical meristem and persistent leaf bases.
• Formation of phloem pockets in the xylem region by the abnormal behaviour of normal cambium in Bignonia, a lianas(woody climber), also called Pyrostegia.
• Cambium forms vascular tissues only in the region of vascular bundles.

Significance of Secondary Growth

1. Secondary growth adds to the girth of the plant. It provides support to increase the weight of the aerial growth.
2. Secondary growth produces a corky bark around the tree trunk that protects the interior from abrasion, heat, cold and infection.
3. It adds new conducting tissues for replacing old non-functioning ones as well as for meeting increased demand for long distant transport of sap and organic nutrients.

FAQs on Secondary Growth

Question 1: What do you mean by Secondary Growth?

Answer:

Secondary growth is characterized by an increase in the thickness or girth of the plant. It is caused by cell division in the lateral meristem.

Question 2: Who is the father of Modern Anatomy?

Answer:

Nageli is regarded as the father of Modern Anatomy.

Question 3: Define Periderm in Secondary growth of roots.

Answer: 

Cork cambium and secondary cortex together are called periderm which is protective in nature.

Question 4: What is the significance of Secondary growth?

Answer:

Secondary growth adds to the girth of the plant. It provides support to increasing the weight of the aerial growth.

Question 5: Where does secondary growth in roots occur?

Answer:

It occurs behind the root hair zone. In the root, primary cambium is absent.