Anatomy of Flowering Plants

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Cambium is more active on the inner side than on the outer. As a result,

more secondary xylem is produced than secondary phloem and soon

forms a compact mass.

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Primary and secondary phloems get gradually crushed due to the

continued formation and accumulation of secondary xylem. However,

primary xylem remains intact, in or around the centre. At some places,

cambium forms a narrow band of parenchyma, which passes through

the secondary xylem and the secondary phloem in the radial directions.

These are the secondary medullary rays.

Spring wood and autumn wood

Many physiological &

environmental factors

control the activity of

cambium. (NEET 2019)

In temperature regions,

climatic conditions are not

uniform throughout the

year.

In spring season,

cambium is very active

and produces many

xylary elements having

vessels with wider cavities.

This wood is called spring

wood (early wood). It is

lighter in colour and has a

lower density.

01

02

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In winter, cambium is less

active and forms fewer

xylary elements having

narrow

vessels.

This

wood is called autumn

wood (late wood). It is

darker and has higher

density.

These

two

kinds

of

woods that appear as

alternate

concentric

rings

constitute

an

annual ring. This is used

to estimate the age of

tree. (NEET 2013)

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Heartwood and sapwood

Heartwood: It is the hard, dead, dark brown-coloured, highly lignified and

non-functional central part of the secondary xylem of old trees. The dark

colour is due to deposition of organic compounds (tannins, resins, oils,

gums, aromatic substances, essential oils, etc.). These substances make it

hard, durable and resistant to the attacks of microorganisms and insects.

(NEET 2017)