Compare and contrast the anomalous secondary growth in Dicot and Monocot Stem
DICOT STEM
Plants showing
anomalous secondary growth can be studied in two main groups.
(1) Those in
which cambium of normal type is present and persists but by peculiarity or
irregularity in its activity develop vascular tissues of unusual arrangement.
(2) Those in
which the normal cambium either does not develop or in soon replaced by another
cambium. This abnormal cambium may either develop from cortex or pericycle and
shows abnormal activity.
Normal Cambium With abnormal Activity:
Aristolochia
Stem: During secondary growth inter and intra fascicular cambium becomes active
and forms a ring. The inter fascicular cambium cuts vascular tissue and the
inter fascicular cambium forms parenchyma. The strands thus remain discrete.
The growth rings are visible in xylem. With the increase of secondary tissue
pith and rays are partly crushed. This crushing is probably a result of
resistance offered to the expanding vascular system by the perivascular
cylinder (pericycle). This slender also breaks up after some time at the rays.
Adjacent parenchyma cells invade the breaks by intrusive growth and may
differentiate into sclereide. The intra fascicular cambium at places forms
parenchyma which results in the bifurcation of bundles phellogen is formed from
the hypodermis. It is initiated as isolated vertical strips extending from node
to node and takes several years to spread over the entire surface. The cork
developed is layered and considerable amount of phellogen (secondary cortex) is
developed lenticles are also formed.
Anomalous Secondary Growth in Monocot Stem
In monocotyledons
normally the vascular bundles are closed. The cambium being absent the
secondary growth is absent; but in some plants like dracaema and Yucca
secondary growth takes place.
Dracaens Stem:
(1) The young stem
has typical structure i.e. epidermis is followed by sclerenchymatous
hypodermis. A large number of closed collateral bundles are scattered in ground
tissue.
(2) One of the
outer layers of cells from the ground tissue becomes meristematic and functions
as cambium. The cambium formed in the region which has ceased elongating.
(3) The activity
of this cambium is more on the inner side and very little on the outside where
it forms only parenchyma. On the inner side it forms xylem and parenchyma in
alternate patches. The inner parenchymatous cells are called conjunctive
tissue.
(4) After a
short while the activity of cambium on inner side changes and above the xylem
is starts forming phloem and then again xylem. Thus phloem becomes encircled by
xylem and ring of leptocentric (amphibasal) vascular bundle is formed. The
xylem formed earlier has bigger vessels.
(5) Around each
vascular bundle is developed sclerenchymatous sheath.
(6) The cambium
after sometime alters its activity and forms xylem on the inner side, at those
places where it was previously forming the parenchyma and parenchyma in place
of xylem. Similar to earlier case again by change in activity it forms a ring
of vascular bundles.
(7) Activity of
cambium goes on changing regularly and more rings of vascular bundles are
formed.
(8) The last one
or two rings of vascular bundles lie in conjunctive tissue.
(9) Cork cambium
is formed below hypodermis and forms cork and cork cambium in normal fashion.
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