What is secondary growth? How it differs in Dicot and Monocot Plants?
In herbs and
shrubs and trees secondary growth takes place as a result of formation of new
secondary tissues in them. Secondary tissues are formed by two meristems
cambium in the stellar region and cork cambium formed later in the extra
stellar or cortical region. The increase in thickness due to addition of
secondary tissues cut off by the cambium and the cork cambium in the stellar
and extra stellar regions respectively is spoken of as secondary growth.
In Dicot stem
secondary growth begins with the activity of cambium ring. Cambium ring begins
to cut off new cells externally and internally. Those cut off on outer side are
gradually modified into the elements of phloem; these constitute secondary phloem
which consists of sieve tubes, companion cells and phloem parenchyma and often
also some bands or patches of bast fibres. Many textile fibres of jute and bast
fibres of secondary phloem. New cells cut off from cambium on inner side are
modified into secondary xylem. The secondary xylem consists of scalari form and
pitted vessels. Tracheids, numerous wood fibres in radial rows and some wood
parenchyma. Xylem increases more rapidly in bulk than phloem and soon forms
hard occupying major portion of stem.
Here and there
cambium forms some narrow bands of parenchyma, running across the stem in the
radial direction through secondary phloem and secondary xylem; these are
secondary medullary rays. They are one, two or a few layers in thickness and
one to many layers in height.
Cambium in
spring becomes more active and form large pitted vessels. In winter it becomes
less active. The wood formed in springs is called spring wood and in winter is
autumn wood. These two kinds of appear together as concentric ring known as the
annual ring or growth ring each annual ring corresponds to one year’s growth
and by counting total rings the age of plant is determined. In the region of
cortex cork cambium appear, it begins to divide and give off new cells on both
sides of cork on outer side and secondary cortex on inner side. The cells of
secondary cortex are parenchymatous in nature.
In Dicot roots
secondary growth is due to addition of new tissues cut off by the cambium and
cork cambium in interior as well as in peripheral regions. In the root
secondary growth commences a few centimetres behind the apex. Portion of
cambium near inner cambium becomes active. It begins to cut off new cells on
inside. As a result the cambium and phloem are pushed outwards. Wavy band of
cambium soon becomes circular or ring like. New cells cut off by cambium ring
on inner side gradually become differentiated into the elements of xylem and
these new elements together form secondary xylem. Secondary wood increases
rapidly and soon forms main bulk of the root.
New elements cut off by cambium
on outer side become modified into elements of phloem and side become modified
into elements of phloem and all these form secondary phloem. It is thinner
single layered pericycle divides into few rows of thin walled roughly
rectangular cells which form cork cambium or phellogen. Secondary cortex of
root does not contain chloroplasts.
In
monocotyledons normally vascular bundles are closed. The cambium being absent
the secondary growth is absent but in some plants like Dracaena and Yucca
secondary growth takes place.
Dracaena Stem:
Epidermis is
followed by sclerenchymatous hypodermis. Large number of closed collateral
bundles are scattered in ground tissue. Cambium is on inner side and little on
outside where it forms only parenchyma. On inner side it forms xylem and parenchyma
in alternate patches. The inner parenchyamtous cells are called conjunctive
tissue.
Later cambium on
inner side changes and above xylem it starts forming phloem and then again
xylem. Thus phloem becomes encircled by xylem and a ring of lap to centric
vascular bundles is formed. Xylem formed earlier has bigger vessels. Around
each vascular bundle is developed sclerenchymatous sheath.
Later cambium
forms xylem on inner side at those places where it was previously forming
parenchyma in place of xylem.
Again by change
in activity it forms a ring of vascular bundles. Later in cambium
more rings of vascular bundles are formed.
Cork cambium is
formed below by hypodermis and forms Cork and Cork cambium in normal fashion.
In some Monocot
plants like Musa rhizome or palm stems thickness takes place by formation of
primary thickening meristem which initiates in the embryo itself. The
procambial strands mainly develop from this meristem.
Tomilson (1961,
1971) has shown that in palmae the parenchymatous cells in the centre and the
cells of bundle sheath keep on dividing and expanding which results in the
thickening of the stem. He has described such growth as diffuse secondary
growth.
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