Biology Today — December 2017

ANSWER KEY

New MCQs

1. (d) 2. (b) 3. (c) 4. (d) 5. (b)


2. (c) 7. (c) 8. (c) 9. (b) 10. (a)


3. (d) 12. (a) 13. (d) 14. (c) 15. (b)

Exam Section

1. (b) 2. (b) 3. (b) 4. (c) 5. (b)


2. (e) 7. (a) 8. (a) 9. (a) 10. (a)


3. (a,b) 12. (b) 13. (c) 14. (a) 15. (d)

Assertion & Reason

1. (c) 2. (a) 3. (b) 4. (c) 5. (d)

Short Answer Type Questions

1. (i) dilation
   (ii) Dendrochronology

(iii) tyloses

2. The phellogen produces cork or phellem on outer side. It

comprises of dead and compactly arranged rectangular,

suberised cells. Due to the presence of tannins they appear

dark brown in colour. Cork prevents the loss of water by

evaporation and also protects the interior against entry of

harmful microorganisms, mechanical injury and extremes of

temperature. Cork is light, compressible, non-reactive and

resistant to fire. It is also used as stopper for bottles, shock

absorption and insulation.

3. The differences between phellem and phelloderm are as follows:
   Phellem Phelloderm
   (i) Phellem or cork is a
   tissue formed on the
   outer side of phellogen
   or cork cambium.

Phelloderm or

secondary cortex is

produced on the inner

side of phellogen.

(ii) It is composed of dead

cells.

Phelloderm is made of

living cells.

(iii) Phellem is protective in

function.

Its cells take part in

storage of food.

(iv) The cell walls become

impermeable due to

suberisation.

Suberisation is absent.

(v) Phellem cells are filled

with tannins.

Tannins are absent.

(vi) The cells are compactly

arranged except for the

presence of lenticels.

The cells enclose small

intercellular spaces.

(vii) Phellogen is more active

on the side of phellem,

i.e., more phellem is

formed as compared to

phelloderm.

Phellogen is less

active on the side of

phelloderm, i.e., less

phelloderm is formed as

compared to phellem.

(viii) The outer part of the

phellem is peeled off at

intervals.

There is no loss of

phelloderm.

4. T.S. of dicotyledonous stem showing secondary growth is as
   follows.

Epidermis

Single outermost layer with thick cuticle

Cortex

Several layered

parenchymatous cells

Hard bast

The distinct sieve tubes and associated

cells of primary phloem are capped by a

patch of sclerenchyma called hard bast Cork cambium

Arises in primary phloem and cut off cork layers

Bar of sclerenchyma on outside

Secondary phloem Cambium

Consists of bands of sieve tubes, companion

cells and parenchyma alternating with

tangential bars of sclerenchyma Primary ray

Broad, multiseriate, separating the vascular bundles

Pith

(Large and parenchymatous)

Fig.: Secondary growth in stem of Vitis vinifera

Primary xylem

Secondary ray

Distinct and extend upto

secondary phloem

Secondary xylem

Consists of a series of wide vessels

and some amount of parenchyma

Cork

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