In this article we will study about the secondary structures of plant with the help of suitable diagrams.
A. Stem of Aristo-lochia Sp. (Fig. 7.1):
In the primary condition the stem reveals the features of a typical dicotyledonous stem, i.e., presence of uniseriate epidermis, well differentiated cortex, ring arrangement of vascular bundles which are distinctly collateral and open type, and also having a distinct pith.
But with the commencement of secondary growth in thickness, both extrastelar and intrastelar secondary growths are seen, as a result of which periderm and intrastelar secondary wood formation can be recorded.
The detailed tissue arrangement pattern after secondary growth (as seen in T.S. of stem) is:
1. Periderm:
Due to the formation of periderm (a structure formed as a result of extrastelar cambial activity) in the sub-epidermal layers, epidermis gets stretched and ruptures. The periderm is distinctly differentiated into outer thick layer of cork cells (phellem), one or two layers of phellogen (cork cambium) and considerable parenchymatous phelloderm on the inner side. Lenticels are found in good number on the surface layer.
2. Cortex:
It is extremely bounded by periderm layer and internally by perivascular fibres. It consists of parenchymatous cells. The perivascular fibres, which form a continuous ring, become broken into patches as a result of continued secondary growth in the stellar region.
3. Vascular Tissues:
Due to the formation of inter-fascicular cambium and its subsequent joining with fascicular cambium, a cambial ring is formed. The fascicular cambium goes on diving and produces secondary xylem and phloem on the inner and outer sides, respectively. Thus the primary xylem and primary phloem are pushed apart from each other.
The vascular bundles thus increase enormously in size due to continued activity of the fascicular cambium. The inter-fascicular cambium simply produces parenchyma cells on the outer and inner sides. Thus the medullary rays becomes increasingly broader and longer.
4. Pith:
It is parenchymatous, but very much reduced.
Comments on Anatomical Features:
The specimen shows the anatomical features of typical secondary growth of dicotyledonous stem.
The reasons are:
(a) Presence of distinct periderm tissue layers.
(b) Presence of lenticels.
(c) Presence of secondary vascular tissue formation within each vascular bundle.
(d) Presence of medullary rays in the inter-fascicular regions.
(e) Pith distinct, but reduced very much.
B. Stem of Dracaena Sp. (Fig. 7.2):
Normally, due to the absence of cambium, the monocotyledonous stem does not have any sort of secondary growth in thickness. However, some monocotyledons belonging to the family, Liliaceae — mainly species of Dracaena, Yucca, Cordyline, Agave, Aloe etc. — exhibit a peculiar type of secondary growth due to the formation of cambial layer in the sub-epidermal region and also just outside the vascular bundles.
Inner cambial layer produces conjunctive tissues with some scattered secondary vascular bundles, which encircle the primary vascular bundles. The secondary bundles are, as usual, amphivasal. The outer cambial layer, however, produces 3-5 layers of cork cells and lies just below the epidermis.
The detailed tissue arrangement is:
1. Epidermis:
It is single-layered, ruptured here and there.
2. Cortex:
It is mostly parenchymatous, externally bounded by 3-5 layers of cork cells (storeyed cork) and internally by a cambial layer which produces conjunctive tissues.
3. Vascular Bundles:
The bundles are of collateral and closed type. The primary bundles are scattered at the central region of ground tissue. The secondary bundles are differentiated from conjunctive tissues.
4. Pith:
It is completely absent.
Comments on Anatomical Features:
The specimen shows secondary growth of monocotyledonous stem.
The reasons are:
(a) Presence of uniseriate, cuticularised epidermis.
(b) Presence of 3-5 layers of cork cells.
(c) Presence of conjunctive tissues with secondary vascular bundles.
(d) Primary vascular bundles are scattered in central ground tissue region.
(e) Vascular bundles are collateral and closed type.
C. Root of Tinospora Sp. (Fig. 7.3):
Secondary growth in roots of some herbaceous dicotyledons and most of the woody dicotyledons are well-marked. The cambium develops both within the stele as well as outside the stele, i.e., outside the pericycle layer.
A few parenchyma cells beneath each phloem group become meristematic and thus form strips of cambium, the number of strips being equal to the number of phloem groups present. The cells of the uniseriate pericycle lying against the protoxylem groups now divide and form a few layers of cells.
This tissue combines with cambium to form a cambial ring. This layer produces secondary vascular tissues as in stem. Thus a continuous layer of secondary xylem and secondary phloem are formed inside and outside, respectively (with respect to cambial layer). The primary xylem is pushed towards the pith and primary phloem towards the circumference.
Ray cells are formed frequently within the secondary xylem layer. Secondary phloem tissues are, however, formed in radial patches only. Periderm is formed from the outer cambial layer, which is developed just outside the pericycle.
It is composed of outer cork cell layers (phellem), inner phelloderm layer and middle phellogen layer. The pressure developed due to the formation of secondary tissues inside ruptures the cortex with endodermis, which is ultimately sloughed- off. Lenticels may be formed.
The detailed tissue arrangement is:
1. Cork:
It forms the external boundary due to sloughing-off of cortex and epidermis. It is several layers thick.
2. Pericycle Zone:
There are a few layers of parenchymatous cells.
3. Vascular Bundles:
Primary xylem is pushed into the pith region. It is encircled by thick layers of secondary xylem interrupted by ray cells. Secondary phloem lies external to the secondary xylem layers. Primary phloem is limited to certain patches only.
Comments on Anatomical Features:
The specimen shows the features of secondary growth of dicotyledonous stem.
The reasons are:
(a) There is presence of periderm, cork layer is formed in the deeper zone.
(b) Primary xylem is pushed into the pith, which is then externally encircled by secondary xylem layer.
(c) Phloem tissues, however, remain in radial patches.
(d) The root nature is revealed by the presence of radial primary xylem at the centre.
(e) The rays are relatively broad.