AOFP 2

Question 1

Types of development of primary xylem
(On the basis of position of protoxylem):-

Answer 1

Centrifugal( endarch)
Protoxylem → Towards central
Metaxylem → Towards peripheral
E.g. Stem of angiosperms & gymnosperms

Centripetal( exarch)
Protoxylem → Towards peripheral
Metaxylem → Towards central
E.g. Roots (Exarch)

Centrifugal & Centripetal
Protoxylem is surrounded by
metaxylem
E.g. Rhizome of fern (underground
stem)

Question 2

types of vascular bundles

Answer 2

CONJOINT VASCULAR
BUNDLES
(stem & leaf)
(Xylem and phloem
are present on the
same radius)

RADIAL VASCULAR
BUNDLES
(root)
(Xylem and phloem
are present separately
on different radius)

CONCENTRIC VASCULAR
BUNDLES
(stem of fern & some monocot)

(Either xylem surrounds
the phloem or phloem
surrounds the xylem,
always closed)

Question 3

types of conjoiint vascular bundles

Answer 3

i)Conjoint, collateral
(Phloem is present towards the
periphery)

ii) Conjoint , bicollateral
(Phloem present on both side
of xylem, always open)
stem of cucurbitaceae

iii)

Question 4

usually in root thr is radial arrangement of vascular bundles. in which oneis it conjoint collateral

Answer 4

Most of the roots (Dicot, monocot, gymnosperm, fern root).
exception: In radish, carrot, turnip and sugarbeet (Beet root)
roots, vascular bundles are conjoint, collateral.

Question 5

explain monocot stem vascular bundle

Answer 5

Many, scattered, oval shaped. Each vascular
bundle is conjoint, collateral and closed and xylem is endarch.

*Each vascular bundle is surrounded by fibrous sclerenchymatous
bundle sheath.

Cavities-
1.Lysogenic - Spaces formed by
breakdown of cells.
2.Schizogenous - Spaces formed
by separation of cell wall.

Xylem: In metaxylem there occurs two large vessels while in
protoxylem there occurs one or two small vessels.
* Vessels are arranged in the form of V or Y.
* Water cavity present within vascular bundles which is
schizolysigenous in origin.
Phloem: Phloem parenchyma is absent.

Question 6

difference between monocot and dicot leaf

Answer 6

dicot leaf is dorsiventral or bifacial. It is also hypostomatic meaning it has more stomata on abaxial surface

monocot leaf is isobilateral or equifacial. It is also amphistomatic meaning there is equal stomata on abaxial and adaxial surface.

Question 7

what is secondary growth

Answer 7

The growth of the roots and stems in length with the help of apical
meristem is called the primary growth.

Apart from primary growth most dicotyledonous plants exhibit an
increase in girth. This increase is called the secondary growth.

*The tissues involved in secondary growth are the two lateral
meristems: vascular cambium and cork cambium.

Question 8

describe secondary growth

Answer 8

*Increase in the circumference or girth of the plant organs.
*Takes place in roots and stem of Dicotyledons & Gymnosperms.
*Not found in leaves and monocots (root &stem).
*In monocotyledons, secondary growth is absent.
*These plants show abnormal secondary growth. Exceptions – Date Palm,
Yucca, Dracaena).

Question 9

explain 2° growth in stelar region ( stem)

Answer 9

*A cambium present inside the vascular bundle is called Intrafascicular
cambium (Primary lateral meristem).
*Some cells of medullary rays become meristematic to form Interfascicular
cambium (secondary lateral meristem).
*Both are collectively known as Vascular Cambium (formed in the form of a
complete ring).
* The vascular cambium ring is particly primary and partly secondary in origin. The cells of medullary rays are secondary while intrafascicular cambium is primary.
*Two types of cells are found in the ring of this vascular cambium –
1.Fusiform initials
2.Ray initials

Question 10

explain the activity of fusiform intials

Answer 10

*Long pointed ends & more in number.
*Continuous periclinal( parallel to plane) divisions takes place in fusiform initials.

*Towards the periphery(towards pericycle) – 2° Phloem or Bast.
*Towards the centre – 2° Xylem or Wood.

*By the pressure of secondary xylem, all the primary tissues- such as primary
xylem, pith are pushed towards the centre.
*The primary xylem however remains more or less intact in or around the
centre. The primary and secondary phloem of the earlier get gradually
crushed due to the continued formation and accumulation of secondary
xylem.

Secondary xylem is formed 8 – 10 times more as compared to
secondary phloem.

Question 11

what is the activity of ray initials

Answer 11

*Spherical / rounded or oval in shape.
- less in number
*Due to periclinal divisions, ray initials form parenchymatous cells (Secondary
medullary rays).
*Vascular rays or Sec. medullary rays which passes through the secondary
xylem and secondary phloem in the radial direction.
*They conduct water and food in radial direction.

Question 12

what are annual rings

Answer 12

Annual rings are formed due to unequal activity of vascular cambium.
*Activity of vascular cambium is affected by physiological and
environmental factors.
*In temperate regions, the climatic conditions are not uniform through the
year.

Question 13

two types of wood

Answer 13

Spring wood or Early wood
The activity of
cambium is faster.
Produces large number of xylem elements.
Xylem vessels/
trachieds have wider lumen.
Wood is lighter in colour and has lower density

Autumn wood or Late wood
Activity of
cambium is
slower
Produces fewer
xylem elements.
Xylem vessels/
trachieds have
narrow lumen.
Wood is darker in colour and has a higher density.

Question 14

describe annual rings

Answer 14

*A ring of autumn wood and a ring of spring wood are collectively known as
Annual ring (gives the idea of the age of the tree).
*The study of determination of age of a tree by counting annual ring is called
Dendrochronology.
*Annual rings are counted from the base of stem because basal part has
maximum annual rings.
*More clear annual rings are formed in that regions where climatic
variations are sharp (E.g.- Temperate plants).
*Distinct annual rings are not formed in tropical plants. Distinct / clear
annual rings are not formed in India except Himalayan regions (Shimla,
Nainital etc.).
*In seashore regions / coastal regions and deserts annual rings are less
distinct.

Question 15

differentiate between alburnum and Duramen

Answer 15



Sap Wood (Alburnum)
Living part of the wood.
It is situated on
the outer side of
wood
It is less in coloured
Very soft in nature
Tyloses are absent
It is not durable and not resistant to microorganisms

Heart Wood (Duramen)
Dead part of the wood.
It is situated in
the centre part of wood
It is dark in coloured
Hard in nature
Tyloses are present
It is more durable and resists
microorganisms

Question 16

why does heartwood not conduct water

Answer 16

1.Cavities of tracheids and vessels are progressively filled with waste
materials.
2.Balloon like in growth of parenchyma cells enter in the lumen of vessels and
tracheids through pits (tyloses or tracheal plugs).

Question 17

explain vascular cambium ring in dicot root (growth in stelar reginon)

Answer 17

In dicot roots, the vascular cambium is completely secondary in origin.
*Conjunctive tissue becomes meristematic and forms separate curved strips of
vascular cambium below phloem bundles.
*Cells of pericycle lying opposite to protoxylem also become meristematic to
form additional strips of cambium.
*Shape of vascular cambium is wavy in the beginning.
*Later it becomes circular due to pressure of secondary xylem.

The portion of vascular cambium which is formed by pericycle is less.

The main portion of vascular cambium is formed by conjunctive tissue.
*Secondary xylem is formed towards the inner side and secondary phloem
is formed towards the outer side by vascular cambium.
*Two types of medullary rays are found in secondary structure of roots.
*The presence of two types of medullary rays is basic characteristic feature of
roots.
*Only secondary medullary rays are formed in dicot stem during the
secondary growth. Both of them conduct water and food in radial
direction.

Question 18

what is responsible for secondary growth in extrastelar region

Answer 18

Secondary growth takes place in extra stelar region due to the activity of
cork cambium. Cork cambium is also known as Phellogen or Extrastelar
cambium.
*The cells of the cork cambium are narrow, thin walled and nearly
rectangular. Cork combium develops usually in cortical region.
*In Stem → Cork cambium is derived from the hypodermis or outer part of
cortex by dedifferentiation. Cork cambium is single or a couple of layers
(mainly) thick. It forms secondary tissue in extra stelar region.
*In Roots → Cork cambium is developed from the pericycle.

Question 19

describe activity of cork cambium

Answer 19

Cork cambium divides periclinally to form some cells towards the outside
(cork or phellem) and some cells towards the inside (secondary cortex or
Phelloderm).

*The outer cells differentiate into cork or phellem while the inner cells
differentiate into secondary cortex or phelloderm.
*The cork is impervious to water due to suberin deposition in the cell wall.
*The cells of secondary cortex are parenchymatous.
*Phellogen + Phellem + Phelloderm → Periderm
*Due to activity of the cork cambium, pressure builds up on the remaining
layers peripheral to phellogen and ultimately these layers die and slough off.

Question 20

what is the old view and new view of bark

Answer 20

*Old view :- All the tissues situated outside the cork cambium is called bark.
According to old view bark includes mainly dead tissues.
*Modern view :- Bark is a non-technical term that refers to all tissues
exterior to the vascular cambium, therefore including secondary
phloem. According to modern view bark includes both living and dead
tissues.

Question 21

acc new view what all tissues fall under bark

Answer 21

periderm (or)
cork
cork cambium
secondary cortex

primary cortex
pericycle
primary and secondary phloem

Question 22

what are lenticels

Answer 22

*At certain regions, phellogen forms closely arranged parenchymatous cells
on outer side instead of cork cell.
*These thin walled, rounded, colorless, parenchymatous cells are called
complementary cells. These cells are not suberized.
*As complementary cells increase in number, pressure is exerted on
epidermis due to which it ruptures, forming a lens shaped opening called
Lenticels.
*Complementary cells are formed by activity of phellogen.
*Lenticels are found on woody stems, also present on some fruits.
*Lenticels are also found in roots but less in number as compared to stem.
*Lenticels are never found on leaves.
*Lenticels are not found in herbaceous dicots and monocots plants.( they have stomata)

Question 23

what is the function of lenticels

Answer 23

1.Exchange of gases-
*Main function
*Permit the exchange of gases between the outer atmosphere and the internal
tissue of stem.
2.Lenticular transpiration

Question 24

explain the process of girdling

Answer 24

*If bark is removed in the form of a ring (girdling) from the base of
main stem, then root dies first due to lack of food.
*Girdling is not possible in monocot stem because vascular bundles are
scattered.
*If complete bark is removed, then plant dies due to excessive water
loss.