Plant vascular system, Types of vascular bundles, Practice problems and FAQs

What happens if you get an injury? You will get bleeding on your skin. Right? Likewise have you ever seen some liquid flowing out of a cut or injured part of a plant? Why do you think it happens? 

Just like the veins and arteries carry blood in our human body, plants also have similar types of vessels. Xylem and phloem are the major vessels present in the plant body. Together they form a vascular system in plants to perform the transportation of substances in plants. Do you know the functions of these vessels in the plant body? Xylem helps in the transport of water and minerals from roots to various parts of the plant whereas phloem helps in the transport of prepared food from the leaves to the various parts of the plant. But do you think that the arrangement of vascular bundles is the same for all plants? It definitely is not. In fact, the arrangement of vascular bundles also varies in different parts of a single part. Based on how xylem and phloem are arranged in the vascular bundles, they can be of different types. In this system we will discuss about the plant vascular system and the different types of vascular bundles.

Table of contents:

• The vascular tissue system
• Types of vascular bundles
• Difference between open vascular bundle and closed vascular bundle
• Practice problems
• FAQs

The Vascular Tissue System 

The complex tissues like phloem and xylem form the vascular tissue system in plants. The vascular tissue is derived from the procambium (meristematic tissue which divides and differentiates to give rise to the vascular tissues). In higher plants, these are composed of long strands of xylem and phloem arranged in bundles known as the vascular bundles.

                                           Fig: Vascular tissue system

Elements of the vascular bundle

The elements which make up the vascular bundles in dicot stems are xylem, phloem and cambium (intrafascicular meristematic tissue that helps in secondary growth). In monocot stems they are made up of xylem and phloem, without the presence of cambium. In roots the xylem and phloem tissues form separate bundles and lie on separate radii.

Xylem

Xylem is a complex permanent tissue that is composed of four different elements - tracheids, xylem vessels, xylem parenchyma and xylem fibres. Xylem is not only associated with the conduction of water and minerals but also helps in providing strength and support to the plant. 

Based on its origin, xylem can be of two types - 

• Primary xylem, which arises from the procambium and helps in primary growth of the plant. Primary xylem is of two types - protoxylem is the early formed xylem and metaxylem is the primary xylem formed after the protoxylem. Based on the arrangement of protoxylem and metaxylem, xylem can be classified as -

1. Exarch - Protoxylem lies towards the periphery and metaxylem towards the centre.

                                             Fig: Exarch xylem

1. Endarch - Protoxylem lies towards the centre and metaxylem towards the periphery.

                           Fig: Endarch xylem

• Secondary xylem, which arises from the vascular cambium and contributes to secondary growth (growth in girth) of the plant.

Phloem

Phloem is also a complex permanent tissue composed of four different tissues - sieve tubes, companion cells, phloem parenchyma and phloem fibres. Phloem is primarily responsible for the conduction of food. Companion cells are lacking in the phloem of pteridophytes and gymnosperms and primary phloem of all angiosperms lack phloem fibres.

Similar to that of the xylem, phloem can also be of two types based on its origin -

• Primary phloem, which arises from the procambium and helps in primary growth of the plant.
• Secondary phloem, which arises from the vascular cambium and contributes to secondary growth of the plant.

Cambium

Cambium is a layer of meristematic tissue which is found in the area between xylem and phloem. The vascular cambium divides and differentiates to form secondary xylem and secondary phloem during secondary growth of the plant. 

Types of vascular bundles

Vascular bundles are of different types. They can be classified on the basis of - 

• the arrangement of vascular bundles
• the presence of cambium

Based on the arrangement of vascular tissues

Vascular bundles are of different types based on the arrangement of xylem and phloem. They can be radial, conjoint, collateral, bicollateral, and concentric.

Radial 

In a radial vascular bundle the xylem and phloem form different bundles and lie in different radii alternating with each other. It is the most primitive type of arrangement of vascular bundles. It is present in roots. 

                                                    Fig: Radial vascular bundle

Conjoint

In conjoint arrangement vascular bundles contain both xylem and phloem which are arranged on the same radius. It is of two types - collateral and bicollateral. 

                               Fig: Conjoint vascular bundle

Collateral 

In a conjoint bundle, xylem is present inside the pith and phloem is present outside, facing the cortex. It can be of two types - if cambium is present in the collateral vascular bundle, then it is called collateral open and if cambium is absent then it is called collateral closed.

    Fig: Collateral vascular bundles with xylem and phloem in the same radius

Bicollateral

In a conjoint bundle, if phloem is present both on the outer and inner side of the xylem, then it is called a bicollateral vascular bundle. Usually it possesses a strip of cambium on both outer and inner sides of the xylem. 

                                  Fig: Bicollateral

Concentric

In this arrangement one type of vascular tissue encircles the other type of vascular tissue.

It is of two types as follows: Amphivasal and Amphicribral.

Amphivasal 

In an amphivasal type of vascular bundle arrangement, the phloem lies in the centre and is surrounded by xylem. It is also called lepto centric arrangement.

                            Fig: Amphivasal

Amphicribral

In an amphicribral type of vascular bundle arrangement, xylem forms the central core and is surrounded by phloem. It is also called hadrocentric arrangement. 

                        Fig: Amphicribral 

Based on the presence of cambium

Based on the presence or absence of cambium, collateral vascular bundles are of two types as follows: 

Open vascular bundles

In an open vascular bundle the cambium is present in between the phloem and xylem. It is normally present in dicot stems. 

                                   Fig: Open vascular bundle

Closed vascular bundles

In closed vascular bundles, the cambium is absent between the xylem and phloem. These cannot form secondary tissues and are usually present in monocots.

                                        Fig: Closed vascular bundle

Difference between open vascular bundle and closed vascular bundle

Practice Problems

Q 1. Fill in the blanks: 

In the …1… stems, the vascular bundles have no cambium present in them. Since they do not form secondary tissue, they are referred to as …2... vascular bundles.

a. 1—monocotyledonous, 2—open
b. 1—dicotyledonous, 2—open
c. 1—monocotyledonous, 2—closed 
d. 1—dicotyledonous, 2—closed 

Answer: In monocotyledonous stems, the xylem and the phloem are conjoint. The vascular bundles in monocots have no cambium tissue present between the xylem and phloem and hence there is no secondary growth due to formation of secondary xylem or phloem. Thus, these vascular bundles are said to be closed.

Hence the correct option is c.

Q 2. Which of the following statements is/are true?

I. Protoxylem and metaxylem is found in secondary xylem
II. In exarch condition, the protoxylem is towards the periphery
III. Protoxylem is formed before the metaxylem.
IV. In endarch condition, the metaxylem lies towards the centre.

a. I and IV only
b. II and III only
c. III and IV only
d. II and IV only

Answer: Endarch and exarch conditions are based on the position of protoxylem and metaxylem. Protoxylem and metaxylem are the two types of primary xylem. They are not seen in the secondary xylem. The primary xylem that is formed earlier is called protoxylem and the one that is formed later is called the metaxylem. In endarch condition, the metaxylem lies towards the periphery and protoxylem towards the centre. In exarch condition, the protoxylem lies towards the periphery and metaxylem towards the centre.

Hence the correct option is b.

Q 3. Arrange the following organs with a gradual increase in the number of vascular bundles in them.

I. Monocot Root
II. Monocot stem
III. Dicot root

a. III, I, II
b. II, I, III
c. I, III, II
d. I, II, III

Answer: The number of vascular bundles in the dicot root is two to four (diarch to tetrarch condition). The number of vascular bundles in the monocot root is more than six (polyarch condition). In the monocot stem, there are a large number of vascular bundles scattered throughout the ground tissue. Monocot stem has more vascular bundles followed by the monocot root and then the dicot root.

Hence the correct option is a.

Q 4. Which of the following statements is correct regarding the open collateral vascular bundles

a. The xylem and phloem lie on separate radii
b. Cambium is present
c. Phloem is in direct contact with xylem. 
d. It is present in monocot stems

Answer: Open collateral vascular bundles have xylem and phloem arranged in a bundle, on the same radius, with a strip of cambium lying in between them. Thus, phloem is not in direct contact with the xylem. These are mostly seen in dicot stems. 

Hence the correct option is b.

FAQs

Q 1. What is the age of vascular plants?
Answer: Vascular plants first appeared 425 million years ago. The Silurian period is when the first vascular plants were discovered. Cooksonia is commonly recognised as the earliest known vascular land plant fossil, dating from the late Early Silurian period 425 million years ago. It was a tiny plant, measuring only a few centimetres in height. It's a leafless plant with sporangia at the terminals of dichotomously branched (forking into two) stems. So far, only fragments have been discovered; no rhizomes or below-ground portions have been discovered.

Q 2. Why do vascular plants flourish more?
Answer: The vascular bundles with xylem and phloem allow water and food to be distributed to all regions of the body. These plants also have deeply growing roots, presence of cuticle to conserve water and woody tissue for greater strength and support. Thus, vascular plants can colonise diverse habitats and have flourished more.

Q 3. Which plants commonly show the presence of bicollateral vascular bundles?
Answer: Members of the Cucurbitaceae family have bicollateral vascular bundles. It's an open vascular bundle that's typically observed in Cucurbita or Cephalandra plant stems. It's also found in the Solanaceae family, which includes potatoes.

Q 4. How do vascular bundles differ in their arrangement in monocot and dicot stems?
Answer: The configuration of vascular bundles in monocot and dicot stem during primary growth differs. The vascular bundles in monocot stems are randomly distributed throughout the stem. Dicot stems, on the other hand, have them organised in a circle on the outer border, right below the epidermis.

YOUTUBE LINK: https://www.youtube.com/watch?v=GjfjE5B-P1w&t=1038s

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