Secondary Growth in Roots, Practice problems and FAQs

Increase in the girth of a plant is due to the secondary growth of the stem. It is very visible, since the stem is outside the soil. But roots are not visible above the ground. Do you think they increase in girth like the stems do? Do they also undergo secondary growth? If yes, is the process similar to the secondary growth of a stem or does it differ much? 

Even though roots remain under the soil, they do undergo secondary growth because they need strength to support the plant body. Only if roots undergo secondary growth and increase in girth, they can absorb enough water and minerals to support the stem which continues to thicken due to secondary growth.

We know that there are differences in the anatomy of stem and root. Not just that, differences exist in the anatomy of monocot and dicot plants too. Hence the process of secondary growth in the stem will be different from that in the root. In fact, the process of secondary growth in a dicot plant will also be different from that in a monocot plant. In this article we are going to discuss the secondary growth in roots. Let's find out how the secondary growth in a root differs from the secondary growth in a stem. 

Table of contents:

• Secondary growth in dicot root
• Secondary growth in monocot root
• Significance of secondary growth
• Practice Problems
• FAQs

Secondary growth in dicot root

Secondary growth can be observed in dicot roots but there are exceptions such as some short lived herbs and submerged aquatics. Secondary growth occurs due to the activity of secondary meristematic tissues, that is, vascular cambium and cork cambium. It starts off as a complete and continuous wavy ring that later becomes circular. But from where does this vascular cambium originate?

The vascular cambium originates from the tissue between the phloem and the xylem from the portion of the pericycle tissue above the protoxylem. Secondary vascular tissues are formed by vascular cambium and periderm is formed by cork cambium.

Stages of secondary growth in dicot root

The process of secondary growth in dicot root can be divided into two stages- formation of secondary vascular tissues and formation of periderm. Now let’s discuss more about the stages.

Formation of secondary vascular tissues by vascular cambium

If we observe the primary structure of a dicot root, we cannot find the cambium in the vascular tissue system. But it appears later as a secondary meristem and initiates secondary growth. The parenchyma cells of the conjunctive tissue that lie beneath the phloem become meristematic. 

These cells can be called as cambial cells. As the cambial cells divide tangentially, they form strips of cambial tissue. Simultaneously, the cells of the pericycle that lie above the protoxylem also become meristematic and join with the previously formed cambial strips to form a wavy ring of vascular cambium which lies internal to the primary phloem and external to the primary xylem. 

                                          Fig: Formation of wavy cambial ring

The cells of the vascular cambium divide again and again to form secondary tissues. The part of the vascular cambium ring that lies internal to the phloem becomes active first and starts dividing to form cells both on the inner and the outer side. The cells formed on the inner side differentiate into secondary xylem while those on the outer side differentiate into secondary phloem. The first formed secondary tissue from the cambium is secondary xylem. 

Secondary xylem consists of vessels, xylem parenchyma and a few fibres. Tracheids are rarely found in secondary xylem. The vessels of the secondary xylem are broader but comparatively thin walled as compared to those in the metaxylem. Secondary phloem consists of sieve tube elements, companion cells, phloem parenchyma and a few phloem fibres. 

The vascular cambium being more active on the inner side, divides to add more secondary xylem compared to that of secondary phloem. This causes the cambial strips and phloem to be pushed outwards and the wavy ring of vascular cambium to become circular. 

                                Fig: Formation of secondary xylem and 

secondary phloem

The cambium is more active towards the inner side during the formation of secondary vascular tissues. As a result, the secondary xylem grows at a faster rate than the secondary phloem. Both the primary xylem as well as secondary xylem persist and continue to grow in girth. As more secondary xylem is added, the primary and secondary phloem get crushed due to increasing pressure from the growing xylem.

 Fig: Gradual increase in girth of the root due to activity of vascular cambium during secondary growth

Cambium cells generated from the pericycle lying above the protoxylem do not participate in formation of secondary vascular tissues. These give rise to the primary medullary rays. They cut off parenchyma cells on both the outer and inner sides, unlike the other cambium cells. Primary vascular rays or medullary rays are multiseriate radial bands of parenchyma cells. In later stages, the primary vascular rays become narrow and become uniseriate. Ray initials also develop from other parts of the vascular cambium which produce uniseriate secondary vascular rays. 

The portion of the medullary ray found in the xylem is known as xylem ray, whereas the portion found in the phloem is known as phloem ray. The radial conduction of water minerals and food materials is aided by the medullary rays. Ray cell formation is slower than secondary xylem formation on the inner side and secondary phloem formation on the outer side. This also causes the wavy cambium to become circular and the depressed portion of the vascular cambium pushes outward.

                             Fig: Formation of secondary tissues and medullary rays

We know that the activity of vascular cambium varies seasonally. It is more active during the spring season and less active during autumn and winter. This difference in activity is prominently visible in dicot stems due to the appearance of annual rings formed by thicker spring wood and narrower autumn wood. But in roots seasonal changes in the activity of the vascular cambium is not visible as the climate of the soil does not change much during different seasons.

Formation of periderm

The pericycle layer becomes meristematic to give rise to a secondary meristem called cork cambium or phellogen shortly after the initiation of secondary growth in the vascular region. It will be either directly or after a few divisions. Both the exterior and inner edges of the cork cambium cells divide to form new cells. Secondary cortex, also known as phelloderm, is the tissue developed on the inner side. The cells that form on the outer side die due to deposition of suberin on their walls and deposition of tannins inside the cell. Thus they form the cork tissue or phellem. The activity of the cork cambium is more on the outside than on the inside. The phellogen, phellem and phelloderm together form the periderm.

                                                              Fig: Periderm

Cork has lenticels for gas exchange in several spots. Water cannot penetrate the cork. It protects the underlying tissues against pathogens and mechanical damage. The secondary ground tissue, or periderm, is made up of the phellem, phellogen, and phelloderm. Outside the cork, all of the fundamental tissues die and produce a dead bark.

                                                                Fig: Lenticel

Tissues responsible for secondary growth

So we can conclude that in dicot plants the secondary growth is done by the two lateral meristems; vascular cambium and cork cambium.

                                                         Fig: Types of cambium

Secondary growth in monocot root

Monocots, in general, do not go through secondary growth. As monocots do not have a vascular cambium, they do not create secondary xylem and phloem when they grow in size (like palm trees and yucca plants).

Significance of secondary growth

• The tissues formed help in the long distance transport.
• It increases the girth of the plant.
• It provides mechanical support.
• Tissues formed protect the plant from abrasion, heat and attacks of pathogens. 

Practice Problems

Q 1. In a dicot root with considerable secondary growth, what happens to the primary xylem?

a. It is retained in the centre of the axis.
b. It gets crushed.
c. May or may not get crushed.
d. It gets surrounded by primary phloem.

Answer: Excessive secondary growth in a dicot root results in the constant addition of secondary xylem layers on the inner side and secondary phloem layers on the outer side. As new phloem develops, the main phloem, as well as some of the surrounding secondary phloem, is crushed. The primary xylem is more or less intact and is kept in the centre of the axis. The primary xylem, however, may stop growing due to the pressure of the newly created neighbouring tissues.

Hence the correct option is a.

Q 2. Which of the following statements concerning secondary growth in dicot root is incorrect?

a. It is very similar to secondary growth in the dicot stem.
b. Pericycle cells above metaxylem become meristematic.
c. During the process, a wavy ring of cambium is produced.
d. Cork is produced by the cork cambium on the outside and secondary cortex on the inside.

Answer: In terms of the processes and structures created, secondary growth in dicot roots is quite similar to secondary growth in dicot stems.The vascular cambium is secondary in origin in roots. It is formed when the cells of the conjunctive tissue that are found below the primary phloem and the cells of the pericycle that lie above the protoxylem become meristematic. As these cells start dividing, they form a complete and continuous wavy cambium ring. Because the roots are exarch, the metaxylem is oriented toward the pith.

Hence the correct option is b.

Q 3. In the case of vascular cambium, the origin is entirely secondary in which of the following?

a. Dicot stem 
b. Monocot stem
c. Dicot root
d. Monocot root

Answer: In dicot roots, the vascular cambium is entirely secondary in origin. This means that vascular cambium is not present at the start of secondary growth and develops later. Dedifferentiation of permanent tissues results in the formation of the vascular cambium. The characteristic wavy vascular cambium in dicot roots is formed by several tissues such as parenchyma cells below the primary phloem and cells of the pericycle.

Hence the correct option is c.

Q 4. The cork cells are dead due to deposition of

a. lignin
b. pectin 
c. suberin
d. cellulose

Answer: The cork cambium divides to add cells of secondary cortex or phelloderm on the inside and on the outside, it forms cells which die due to deposition of suberin on their walls and deposition of tannins inside the cell. These cells form the cork or phellem

Hence the correct option is c.

FAQs

Q 1. How does the secondary root help plants?
Answer: Secondary functions are performed by certain roots that are specifically modified for the purpose only in some plants, such as food storage, additional support, absorption of atmospheric moisture, assimilation/photosynthesis, sucking food from the host, reproduction, respiration, improved gaseous exchange, and mechanical functions such as floating and balancing.

Q 2. What is the difference between secondary and tertiary roots?
Answer: The primary root penetrates the soil horizontally. Smaller lateral roots, known as secondary roots, grow from the parent root. Secondary roots, in turn, create tertiary roots. These roots develop in a variety of orientations and aid in anchoring the plant in the soil.

Q 3. What causes lateral roots to grow?
Answer: Lateral roots form when cells in the pericycle, the layer of cells surrounding the central vascular cylinder, divide, forming extra cell layers that push through the primary root's outer cell layers, eventually forming a second root meristem.

Q 4. What are feeder roots, and what do they do?
Answer: Feeder roots emerge from massive woody roots and typically develop up to the soil surface. Feeder roots mingle with lawn and shrub roots near the surface, competing for the water, oxygen, and nutrients that are more available there
 

YOUTUBE LINK: https://youtu.be/pJAa2tm2Pqs 

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