NCERT Solutions for Class 11 Biology Chapter 6: Anatomy of Flowering Plants

Meristems are regions of actively dividing cells. Based on their location, they are of three types:

1. Apical Meristem:
   • Location: Found at the tips (apices) of roots and shoots.
   • Function: Responsible for the increase in the length of the plant. This is known as primary growth.
2. Intercalary Meristem:
   • Location: Found in between mature tissues, typically at the base of internodes in grasses.
   • Function: Helps in the elongation of the organ and regeneration of parts removed by grazing.
3. Lateral Meristem:
   • Location: Found along the sides of roots and stems (e.g., vascular cambium, cork cambium).
   • Function: Responsible for the increase in the girth of the plant. This is known as secondary growth.

Yes, the statement is correct, but incomplete. Cork cambium, also known as phellogen, forms tissues on both sides:

• On the outer side: It cuts off cells that form cork or phellem. These cells have suberin deposits, making them impermeable to water.
• On the inner side: It cuts off cells that form the secondary cortex or phelloderm. These cells are parenchymatous.

Together, the cork (phellem), cork cambium (phellogen), and secondary cortex (phelloderm) are called the periderm. So, cork cambium forms the entire periderm, not just cork.

Secondary growth increases the girth of the stem and is caused by two lateral meristems: vascular cambium and cork cambium.

1. Activity of the Vascular Cambium:

1. Formation of Cambial Ring: The intrafascicular cambium (within vascular bundles) and interfascicular cambium (between bundles) join to form a continuous cambial ring.
2. Formation of Secondary Tissues: The cambial ring cuts off new cells. Cells towards the inner side form secondary xylem, and cells towards the outer side form secondary phloem.
3. Growth and Annual Rings: The cambium is more active on the inner side, producing more secondary xylem than phloem. In temperate climates, this activity is seasonal:
   • Spring Wood (Early Wood): Active cambium produces wider vessels. Lighter in colour and less dense.
   • Autumn Wood (Late Wood): Less active cambium produces narrower vessels. Darker in colour and denser.
   These two types of wood form one annual ring, which can be used to estimate the tree's age.

2. Activity of the Cork Cambium (Phellogen):

1. As the stem's girth increases, the epidermis ruptures. A new meristem, the cork cambium (phellogen), develops in the cortex.
2. It cuts cells on both sides: cork (phellem) on the outside and secondary cortex (phelloderm) on the inside.
3. The phellem, phellogen, and phelloderm together constitute the periderm. All tissues outside the vascular cambium are collectively called bark.

(a) Differences between Dicot Root and Monocot Root

(b) Differences between Dicot Stem and Monocot Stem

I would observe its transverse section under a microscope for these key features:

1. Arrangement of Vascular Bundles:
   • Dicot Stem: Vascular bundles are arranged in a distinct ring.
   • Monocot Stem: Vascular bundles are scattered.
2. Presence of Cambium:
   • Dicot Stem: Vascular bundles are open (cambium present between xylem and phloem). This is the definitive feature.
   • Monocot Stem: Vascular bundles are closed (cambium absent).
3. Differentiation of Ground Tissue:
   • Dicot Stem: Ground tissue is differentiated into cortex, endodermis, pericycle, and a central pith.
   • Monocot Stem: No such differentiation is present.

Conclusion: The most reliable features are the ring arrangement and open bundles for a dicot stem, versus scattered and closed bundles for a monocot stem.

The plant material is a Monocot Stem.

Reasons:

• Conjoint, scattered vascular bundles: This is the most prominent feature of a monocot stem.
• Sclerenchymatous bundle sheath: The presence of a hard sheath around each vascular bundle is characteristic of monocot stems.
• Phloem parenchyma is absent: This is another key identifying feature of monocot stems.

Xylem and phloem are called complex tissues because they are composed of more than one type of cell that work together as a single unit.

• Xylem: Conducts water and minerals. It consists of four elements:
  1. Tracheids
  2. Vessels
  3. Xylem Fibres
  4. Xylem Parenchyma
• Phloem: Transports food. It consists of four elements:
  1. Sieve Tube Elements
  2. Companion Cells
  3. Phloem Fibres
  4. Phloem Parenchyma

The stomatal apparatus comprises the stomatal pore, two guard cells, and the surrounding subsidiary cells.

Structure of Stomata:

• Stomatal Pore: A tiny opening that facilitates gas exchange and transpiration.
• Guard Cells: Two specialized, bean-shaped (in dicots) or dumbbell-shaped (in monocots) cells that surround the pore. Their inner walls are thick and elastic, while the outer walls are thin. They contain chloroplasts. The turgidity of guard cells regulates the opening and closing of the pore.
• Subsidiary Cells: Specialized epidermal cells that surround the guard cells and support their function.

The three basic tissue systems are:

1. The Epidermal Tissue System: Forms the outer protective covering.
   • Tissues: Epidermis, Stomata, Trichomes, Root hairs.
2. The Ground Tissue System: Forms the main bulk of the plant.
   • Tissues: Parenchyma, Collenchyma, Sclerenchyma. (Forms cortex, pericycle, pith).
3. The Vascular Tissue System: Conducts water and food.
   • Tissues: Xylem and Phloem (forming vascular bundles).

The study of plant anatomy is useful in several fields:

• Understanding Plant Function: It helps relate internal structures (xylem, phloem) to their functions (transport).
• Crop Improvement: Anatomical traits can be used to breed for stronger, more resistant crop varieties.
- Taxonomic Classification: Features like vascular bundle arrangement help in plant identification and classification.
• Pharmacognosy: It is used to identify medicinal plants and check for adulterants in crude drugs.
• Forensic Science: Plant fragments from a crime scene can be identified using anatomical details.
• Commercial Applications: Wood anatomy helps the timber industry assess wood quality and use.

Periderm is a protective tissue that replaces the epidermis during secondary growth. It consists of phellogen, phellem, and phelloderm.

Formation of Periderm:

1. As the stem's girth increases, the epidermis ruptures. To protect the inner tissues, a meristematic layer called phellogen (cork cambium) develops in the cortex.
2. Phellogen divides to form cells on both sides.
3. Cells on the outer side differentiate into phellem (cork), which are dead and suberised.
4. Cells on the inner side differentiate into phelloderm (secondary cortex), which are living and parenchymatous.

A dorsiventral (dicot) leaf has three main parts:

1. Epidermis:
   • Adaxial (Upper) Epidermis: Has a thick cuticle and fewer or no stomata.
   • Abaxial (Lower) Epidermis: Has a thinner cuticle and numerous stomata.
2. Mesophyll: The ground tissue between both epidermal layers, differentiated into:
   • Palisade Parenchyma: Elongated, compactly packed cells below the upper epidermis. Main site of photosynthesis.
   • Spongy Parenchyma: Irregularly shaped, loosely arranged cells with large air cavities below the palisade layer. Facilitates gas exchange.
3. Vascular System: The vascular bundles (veins) are conjoint and collateral. Xylem is towards the upper epidermis, and phloem is towards the lower epidermis. Each bundle is surrounded by a bundle sheath.

Lenticels are lens-shaped pores on the bark of woody stems, formed by loosely arranged cells from the phellogen.

Function: They permit the exchange of gases between the outer atmosphere and the internal tissues of the stem.

Comparison of Spring Wood and Autumn Wood:

Collective Significance:

Together, one ring of spring wood and one ring of autumn wood constitute an annual ring or growth ring. By counting the number of annual rings, one can estimate the age of the tree. This science is known as dendrochronology.

(i) Identify the plant part shown in the slide.

The plant part is a Dicot Stem.

(ii) What does the term 'open' vascular bundle signify?

It signifies the presence of cambium between the xylem and phloem.

(iii) Where would you find the protoxylem in this specimen? What is this condition called?

The protoxylem is towards the center (pith). This condition is called endarch.

(iv) Will this plant part show secondary growth? Justify.

Yes. The presence of an open vascular bundle (with cambium) allows for the formation of secondary xylem and phloem, leading to an increase in girth.