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Chapter: 11th Botany : Chapter 10 : Secondary Growth

Answer the following questions

Botany : Secondary Growth - Book Back and Important Questions Answers - Brief Questions Answers, Short Questions Answers

Plant Anatomy (Structural Organisation) 

Secondary Growth 


Answer the Following

6. In a forest, if the bark of a tree is damaged by the horn of a deer, How will the plant overcome the damage?

(Bark include all tissues outer to cambium)

1. The innate wound healing mechanism get activated.

2. In the wounded site, there is rapid cell division, in the cambium to seal the wound to prevent any microbial infection.

3. A reaction zone is created, which alters the chemistry of the wood, in the wound site to avoid microbial infection.

4.  Barrier zone or callus separates wounded area from other, it over grow the wound and seal it, there by new wood grow beneath to complete the healing.

7. In which season the vessels of angiosperms are larger in size, why?

1. Spring is a season with favourable climatic conditions, such as Optimum temperature, light intensity, good soil water source, which faciliate growth.

2. So to optimize photosynthesis and inturn growh, cambium is very active producing large number of xylary elements such as vessels and tracheids with wide lumen - (spring wood) thus increases up take of water and minerals to large scale.

8. Continuous state of dividing tissue is called meristem. In connection to this, what is the role of lateral meristem?

The secondary growth in dicots and gymnosperms is brought about by two lateral meristems.

1. Vascular cambium and 2. Cork cambium

1. Vascular cambium:

The vascular cambium is the lateral meristem that produces the secondary vascular tissues. i.e... secondary xylem and secondary phloem.

Origin and Formation of Vascular Cambium:

1. A strip of vascular cambium originate from the procambium is present between xylem and pholem of the vascular bundle. This cambial strip is known as intrafascicular or fascicular cambium.

2. In between the vascular bundles, a few parenchymatous cells of the medullary rays that are in line with the fascicular cambium become meristematic and form strips of vascular cambium. It is called interfascicular cambium.

A. Organization of Vascular cambium:

1. The active vascular cambium possesses cells with large central vacuole (or vacuoles) surrounded by a thin, layers of dense cytoplasm.

2. The most important character of the vascular cambium is the presence of two kinds of initials, namely fusiform initials and ray initials.

Fusiform Initials:

1. These are vertically elongated cells. They give rise to the longitudinal or axial system of the secondary xylem (tracheary elements, fibres, and axial parenchyma) and pholem (sieve, elements, fibers, and axial parenchyma).

2. Based on the arrangement of the fusiform initials two types of vascular cambium are recognized. Storied (Stratified cambium) and Non-storied (Non-stratified cambium)

1. If the fusiform initials are arranged in horizontal tiers, with the end of the cells of one tier appearing at approximately the same level, as seen in tangential longitudinal section (TLS) it is called storied (stratified) cambium. It is the characteristic of the plants with short fusiform initials.

2. In plants with long fusiform initials, they strongly overlap at the ends, and this type of cambium is called non-storied (non stratified) cambium.

Ray Initials:

These are horizontally elongated cells. They give rise to the ray cells and form the elements of the radial system of secondary xylem and pholem.

Activity of Vascular Cambium:

1. The vascular cambial ring, when active, cuts off new cells both towards the inner and outer side. The cells which are produced outward form secondary phloem and inward secondary xylem.

2. Due to the continued formation of secondary xylem and phloem through vascular cambial activity, both the primary xylem and phloem get gradullay crushed.

B. Phellogen (Cork Cambium)

1. It is a secondary lateral meristem. It comprises homogenous meristematic cells unlike vascular cambium. It arises from epidermis, cortex, pholem or pericycle (extrastelar in origin). Its cells divide periclinally and produce radially arranged files of cells.

2. The cells towards the outer side differentiate into phellem (cork) and those towards the inside as phelloderm (secondary cortex).

9. A timber merchant bought 2 logs of wood from a forest & named them A & B, The log A was 50 year old & B was 20 years old. Which log of wood will last longer for the merchant? Why?

1. In wood, the older it is, the stronger it becomes.

2. Log A-Which was 50 years old is stronger and it will last longer.

3. In a tree the central part of the wood will be darker in colour dead in nature known as Heart wood or Duramen, and the outer sad wood is lighter in colour, living and conducting water

4. In the central Heart wood the conduction is blocked by the formation of tyloses from the nearby parenchyma cells, and dead.

5. In the fully developed tyloses, starch crystals, resings, gums, oils tannins and coloured substances are found and it become very hard and durable.

6. It is more resistant to the attack of microbes and insects like termites.

7. Older woods have more heart wood than sap wood.

8. Here log 'A' is older, has more heart wood and it is stronger and will last longer

10. A transverse section of the trunk of a tree shows concentric rings which are known as growth rings. How are these rings formed? What are the significance of these rings?

Growth (or) Annual Rings:

1. In spring season cambium is very active and produces large number of xylary elements called Early wood or Spring wood.

In Winter season cambium is less active and. form few xylary elements - Late wood or Autumn Wood.

2. The spring wood is lighter in colour and has a lower density whereas the autumn wood is darker and has a higher density. The annual ring denotes the combination of carly wood and late wood and the ring becomes evident to our eye due to the high density of late wood. Sometimes annual rings are called growth rings

3. Pseudo-Annual Rings:

Additional growth rings are developed within a year due to adverse natural calamities like drough, frost, defoliation, flood, mechanical, injury and biotic factors. Such rings are called pseudo-or false annual rings.

4. Dendrochronology:

Each annual ring corresponds to one year's growth and on the basis of these rings, the age of a particular plant can easily be calculated. The determination of the age of a tree by counting the annual rings is called dendrochronology.

Q. A cross section of tree trunk contains 60 lighter and 60 darker rings. Determine the age of the tree and justify

Q. Central part of the wood is always dark. Why?

Q. Differentiate the wood formed in Pinus from that of Morus

Q. When the plants shed their leaves how do they respire?

Q. What is wood botanically?

Q. There are many tissues produced outside the vascular cambium in the stem. Explain them

Q. When you go to a timber mart to collect timber for your construction of a new house, how will you select good timber?

Q. Explain artificial seasoning.

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