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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