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CAMBIAL VARIANTS.pptx

The document discusses various types of anomalous secondary growth found in plants, including abnormal cambium activity and position. Some examples provided include folded cambium forming multiple bundles in Bauhinia stems and abnormal cambium activity producing more parenchyma than xylem/phloem in Aristolochia. Other anomalies include successive cambium rings in Mirabilis, vascular bundles arising from the pericycle in Amaranthaceae, and scattered or cortical bundles present in some dicots. Dracaena and Tinospora stems demonstrate unusual secondary growth patterns of additional parenchyma proliferation and discrete vascular bundles, respectively.

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Cambial Variant
Cambial variants • In plant as result of cambium activity secondary growth produce. Due to normal activity of cambium secondary xylem and phloem produce. But some times instead of normal activity of cambium some unusual activity found in plant .The process of formation of unusual structure in plant is called as an abnormal or anomalous. • Due to abnormal activity of cambium various structure are formed in plant vascular bundle . These unusual structure are called as cambial variant . • Anomalous growth reflects a growth condition which is not commonly seen and which is present in a limited number of families or genera. 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 early stage of development and some tissue behave like cambium. This abnormal cambium may either develop from cortex or pericycle and shows abnormal activity.
Various reason behind the development cambial variant . These reason may be enlisted as follows ; (A) anomalous secondary growth in dicots may be due to following reasons: (i) anomalous position of cambium (ii) abnormal behavior of normal cambium (iii)accessory cambium formation and its activity (iv) extrastelar cambium (v) intraxylary phloem (b) absence of vessels in the xylem (c) scattered vascular bundles in dicots (d) presence of exclusive phloem and xylem bundles (e) presence of medullary bundles (f) presence of cortical bundles (g) vascular bundles arranged in a ring in monocots
• (A) (i) anomalous/abnormal position of vascular cambium: normally vascular cambium is circular, but it is folded in stem of some plants. • Later on these folds break and separate from each other. Each fold is responsible to form a complete vascular bundle. Many vascular bundles are formed in stem e.g. Thinouia, serjania, bauhinia. • Thinouria and Serjania belongs to family Sapindaceae and Bauhnia belongs to family fabaaceae Folded cambium in Bauhina
• (Ii) abnormal activity of vascular cambium: generally xylem and phloem is formed from the maximum part of the vascular cambium and medullary rays are formed from the few parts of vascular cambium. • But in some plants parenchyma (medullary rays) is formed from the maximum part of the vascular cambium and rarely in some places xylem and phloem are formed e.g. Aristolochia, vitis vinifera (grape).
Medullary ray T.S Vitis vinifera stem
• (Iii) sequential or successive ring of vascular cambium: in some of the plants, a new ring of vascular cambium is formed each year. This is formed outside the previous ring e.g. Mirabilis, boerhavia, bougainvillea etc. Boerhavia T.S of Boerhavia stem
Bougainvillea T.S of Bougainvillea stem
• (Iv) formation of vascular cambium from pericycle: vascular cambium is formed from the pericycle in plants of amaranthaceae and chenopodiaceae families. A complete ring of vascular cambium is formed from the pericycle. Beet roots belongs to family Amaranthaceae
• (V) intraxylary phloem: this is also called internal phloem. It occurs usually in the form of strands or as a continuous band around the pith. The origin of intraxylary phloem in most plants is primary. The internal phloem develops after the development of external primary phloem. The bundles are treated as bicollateral because of presence of the internal phloem. This type of development is found in solanaceae, apocynaceae, lathyraceae etc.
• (B) absence of vessels in the xylem: vessels are mainly conducting channels for water but in some species like zygozynum, belliolum, drimys and in some aquatic plants like Elodea, utricularia, ceratophyllum and hydrilla etc. Vessels are absent.
• (C) scattered vascular bundles in dicotyledons: vascular bundles are normally arranged in a ring in dicots but in some taxa such as Piper peperomia, Podophyllum, Papaver, Nymphaea etc. Vascular bundles are scattered. Scattered vascular bundles in dicots recall the arrangement of vascular bundles in monocots. Scattered Vascular Bundle Nymphaea Scattered Vascular Bundle Piper
• (D) presence of exclusive phloem and xylem bundles: sometimes vascular bundles are incomplete . A bundle is represented either exclusively by xylem or phloem strand. In Paeonia, in addition to normal vascular bundle incomplete bundles are also present which are exclusively represented by xylem. Similarly in Cuscuta, Boerhaavia diffusa, Ricinus communis, and Antigonon leptopus only phloem bundles are present. • (E) presence of medullary bundles: in some dicots vascular bundles are present in pith and then they are known as medullary bundles. These bundles show a limited amount of secondary growth. These are found in ranunculaceae, amaranthaceae, acanthaceae, cactaceae and chenopodiaceae. Their number varies from one to many but they do not supply to the lateral organs like leaf and branch. Their presence is only because of increased needs of translocation and also to the mechanical role in lianas.
• (F) presence of cortical bundles: in some dicots in addition to the normal ring of stelar bundles some vascular bundles are also present in the cortex known as cortical bundles. Morphologically these bundles are leaf traces which traverse through the cortical region of the stem before entering into the petiole. Casuarina has a ring of normally oriented cortical bundles below the ridges whereas in limonium vulgare there are numerous irregularly scattered vascular bundles.
• (H) vascular bundles arranged in a ring in monocots: in monocots the vascular bundles are scattered in the ground tissue. But in some cases as in Tamus communis the vascular bundles are arranged in two rings around the pith; the outer ring has only two small bundles which are embedded in the sclerenchymatous pericycle and the inner ring has several large vascular inside the pericycle. In the tubular stems of some of the grasses as triticum, hordeum, oryza etc the vascular bundles are arranged in two or more definite rings. Tamus communis
Bougainvillea stem • Bougainvillea is a member of the Nyctaginaceae and is an example of a dicotyledonous stem which displays anomalous secondary growth. • In the T.S. Of Bougainvillea, near the center of the stem, you will see some primary vascular bundles embedded in lignified pith parenchyma. • Move the slide towards the outer regions, and you will notice that there has been fairly extensive production of secondary vascular tissue. • Secondary phloem and secondary xylem lie on either side of it. The secondary xylem is composed of tracheid's, fibers and narrow-diameter vessels. • Interspersed with the secondary xylem you will be able to see small pockets of phloem and look like large-diameter metaxylem vessels. These are reminiscent of the primary bundles towards the center of the stem. .Primary vascular bundles embedded within the secondary xylem ,this is the main fact behind anomalous growth.
• The anomalous growth results as a result of differential cambial activity. • Newly-produced vascular cambia present on the outer lateral site and behave like meristem but it is not active. In certain condition this quiescent lateral meristem active and show there activity. This cambium shows there activity only when the internal vascular cambium (which produces the individual embedded bundles) becomes less active. • Vascular cambia are said to not produce rays in Nyctaginaceae (lateral meristems produce ), but produce vessels and associated, axial parenchyma and sometimes fibers to the inside and variable secondary phloem to the outside
Nyctanthes stem • In Nyctanthes arbortristis stem which is a dicot plant, apart from normal vascular bundles which occur in a ring in the central region, there are four inversely oriented vascular bundles at the four ridges of stem. • These cortical bundles are collateral and open. So in addition to the normal ring of stelar bundle some vascular bundles are also present in the cortex, they are known as cortical bundles. • Morphologically these are the leaf traces which traverse through the cortical region of the stem before entering into the petiole. • These types of vascular bundles are also found in family Crassulaceae, Casuarinaceae and Oleaceae. These cortical bundles are equally active producing cells and helping in secondary growth of the plant Nyctanthes arbortristis
Dracaena stem • Palm trees are monocots that grow quite tall and thick, yet they lack "normal" secondary growth. • Dracaena is a monocot but not a true palm, as palms lack the peripheral meristem but still it is found in some exceptional plant like Dracaena and Cordyline. • Dracaena is an unusual plant, in that the vascular bundles are surrounded by very prominent fiber bundles. In this sense, Dracaena shows anomalous activity . • The stems undergo a specialized secondary growth, which manifests itself in the production of additional parenchymatous elements. Their later growth pattern is termed diffuse secondary growth, and consists mostly of a proliferation of ground parenchyma cells and additional vascular bundles near the periphery. Dracaena
• The young Dracaena stem has typical structure i.e. epidermis is followed by sclerenchymatous hypodermis. A large number of closed collateral bundles are scattered in ground tissue. • 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. 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. After a short while the activity of cambium on inner side changes and above the xylem it starts forming phloem and then again xylem. Thus phloem becomes encircled by xylem and ring of leptocentric (amphivasal) vascular bundle is formed.
• The xylem formed earlier has bigger vessels and around each vascular bundle is developed a sclerenchymatous sheath. • The cambium after sometime alter 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. Activity of cambium goes on changing regularly and more rings of vascular bundles are formed. • The last one or two rings of vascular bundles lie in conjunctive tissue. Cork cambium is formed below hypodermis and forms cork and cork cambium in normal fashion.
Tinospora stem • Tinospora cordifolia commonly known as guduchi is an indian medicinal plant and has been used in ayurvedic preparations for the treatment of various ailments throughout the centuries. • It is a glabrous, succulent, woody climbing shrub native to India. • It thrives well in the tropical region, often attains a great height, and climbs up the trunks of large trees. • The stem is gray or creamy - white, deeply cleft spirally and longitudinally with the space between spotted with large rosette - like lenticels.
• The wood is white, soft, and porous, and the freshly cut surface quickly assumes a yellow tint when exposed to air. • Vascular zone is composed of discrete vascular strands with 10 to 12 or more wedge shaped strips of xylem, externally surrounded by semi circular strips of phloem alternating with wide medullary rays; phloem parenchyma contain calcium oxalate crystals; cambium is of 1-2 layers; xylem consists of vessel elements, tracheids, parenchyma and fibres. Vessel elements cylindrical in shape bearing bordered pits. Medullary rays 15 to 20 cells wide. Pith mostly made up of large thin walled cells containing starch grains. • The presence of discrete vascular strands in the mature stem of Tinospora cordifolia is one of the anomalous secondary structures found in Menispermaceae. The cambium forms secondary vascular tissue only in the fascicular region, whereas in the interfascicular areas parenchyma is produced. Thus in the old stem the xylem becomes fissured due to the development broad parenchymatous rays. In such stem parenchyma acts like a shock absorber. It also enables the stem to resist the pulling and compression due to the pressure of high winds. This anomaly is thus an adaptation to the climbing habit of the plant.