6. ANATOMY OF FLOWERING PLANTS
SECONDARY GROWTH
-
The growth of the roots and stems in length
with the help of apical meristem is called the primary growth.
- Secondary
growth is the increase in girth of dicot plants.
- Tissues
involved in secondary growth are the two lateral meristems: Vascular cambium
& cork cambium.
Vascular Cambium
- It
is the meristematic layer responsible for cutting off vascular tissues (xylem
and phloem).
- In
the young stem, it is present in patches as a single layer between xylem & phloem.
Later it forms a complete ring.
Formation of cambial ring
- In
dicot stems, cells of cambium present between primary xylem & primary phloem
is intrafascicular cambium.
- Cells
of medullary cells, adjoining this intrafascicular cambium become meristematic
and form interfascicular cambium. Thus, a continuous ring of cambium is
formed.
Activity of the cambial ring
- The
cambial ring becomes active and cut off new cells, both towards the inner and
outer sides. The cells cut off towards pith, mature into secondary xylem. The
cells cut off towards periphery mature into secondary phloem.
- 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.
- 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.
Secondary growth in a dicot stem – stages
in transverse views
Spring wood and autumn wood
- Many
physiological & environmental factors control the activity of cambium.
- 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.
- 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 (Dendrochronology).
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.
Function: It
gives mechanical support to stem.
- Sapwood:
It
is the peripheral region of secondary xylem. It is living and lighter in
colour. It is involved in the conduction of water and minerals from root
to leaf.
Cork Cambium
-
As the stem continues to increase in girth
due to the activity
of vascular cambium, the outer cortical &
epidermis layers get broken. It is to be replaced to provide new protective
cell layers. Hence another meristematic tissue called cork cambium (phellogen)
develops, usually in the cortex.
-
Phellogen is a couple of layers thick. It
is made of narrow, thin-walled and nearly rectangular cells.
- Phellogen cuts off cells on both sides.
The outer cells differentiate into cork (phellem) while the inner cells
differentiate into secondary cortex (phelloderm). Cells of secondary
cortex are parenchymatous.
-
The cork is impervious to water due to
suberin deposition in the cell wall.
-
Phellogen, phellem, and phelloderm are
collectively known as periderm. Due to activity of cork cambium,
pressure builds up on the remaining layers peripheral to phellogen and
ultimately these layers die and slough off.
- Bark is
a non-technical term that refers to all tissues (periderm & secondary
phloem) exterior to the vascular cambium. It is 2 types:
o Early
(soft) bark: It is formed early in the season.
o Late
(hard) bark: It is formed towards end of season.
-
Function: Lenticels
permit gas exchange of between the outer atmosphere and the internal tissue
of the stem.
Secondary Growth in Roots
- In
dicot root, vascular cambium is completely secondary in origin. It originates
from the tissue located just below the phloem bundles (a portion of pericycle)
above the protoxylem forming a complete and continuous wavy ring. It later
becomes circular.
- Further
events are similar to those of a dicotyledon stem.
- Secondary
growth also occurs in stems & roots of gymnosperms. But it does not occur
in monocotyledons.
Different stages of the secondary growth
in a typical dicot root
