Biology today

which form secondary medullary rays. They are usually one

to few layers in thickness and one to several layers in height.

The medullary rays form the radial system responsible for

radial conduction of solutes. They maintain connection

between pith and cortex.

Formation of periderm
• In order to provide an increase in girth and prevent harm to
outer ground tissues, dicot stems produce a cork cambium
(or phellogen) in the outer cortical cells. Phellogen cuts
derivatives on the outer side to form cork (or phellem) and
on the inner side to form secondary cortex or phelloderm.
Cork, phellogen and phelloderm are collectively referred to
as periderm.
• At places, phellogen produces aerating pores instead of cork.
These pores are called lenticels. Each lenticel is a mass of
loosely arranged suberised cells called complementary
cells.
• Bark is a non-technical term that refers to all the tissues
outside the vascular cambium viz. secondary phloem and
periderm, in a woody stem in which secondary growth has
taken place. Bark formed in early growing season is early
or soft bark whereas bark formed towards end of growing
season is late or hard bark.

Annual ring (growth ring) is the wood formed in a single year. It

consists of two types of wood, spring wood and autumn wood.

The spring or early wood is much wider than the autumn or

late wood. It is lighter in colour and of lower density. Spring

wood consists of larger and wider xylem elements. The autumn

or late wood is dark coloured and of higher density. The

transition from spring to autumn wood in an annual ring is

gradual but the transition from autumn wood to the spring

wood of the next year is sudden. Therefore, each year’s growth

is quite distinct. The number of annual rings corresponds to the

age of the plant, the annual rings also give some clue about

the climatic conditions of the past through which the plant has

passed. Dendrochronology is the science of determining age of

trees by counting and analysing annual growth rings of trees.

Annual rings

sapwood and heartwood

•    The wood of the older stems (Dalbergia, Acacia) formed by

secondary growth gets differentiated into two zones, the

outer light coloured and functional sapwood or alburnum

and the inner darker and non-functional heartwood or

duramen.

Table : Differences between sapwood or alburnum

and heartwood or duramen

Sapwood

(Alburnum)

Heartwood

(Duramen)

(i) It is outer wood of an

old stem.

It is the central wood of

an old stem.

(ii) It is light coloured. Heartwood is dark

coloured.

(iii) Living cells are present. Living cells are absent.

(iv) Sapwood is the

functional part of the

secondary xylem or

wood.

Heartwood is the

non-functional part

of secondary xylem or

wood.

(v) The tracheary elements

are not plugged by

tyloses.

The tracheary elements

are plugged by tyloses.

(vi) Tracheary elements

do not possess any

deposition in their

lumen.

Tracheary elements have

deposition of tannins,

resins, gums, etc.

(vii) Sapwood is lighter. Heartwood is heavier.

(viii) It is less durable

because it is susceptible

to attack by pathogens

and insects.

It is more durable due to

its little susceptibility to

the attack of pathogens

and insects.