Initiation And Activity Of Vascular Cambium

About Initiation And Activity Of Vascular Cambium

The vascular bundles in dicot roots are radial and diarch to hexarch. Xylem and phloem are arranged in a ring.

During the secondary growth, parenchymatous cells of conjunctive tissue lying below the phloem patches become meristematic and form crescent shaped strips of cambium.

As the activity of these cambium strips proceeds, the cells of the pericycle external to each xylem strand also become meristematic and small strips of cambium are also formed outside the xylem strands.

The first formed strips of the cambium inner to the phloem strands join with the newly formed strips of cambium outside the xylem strands. Thus a complete but wavy ring of cambium is formed which runs outside the xylem inside the phloem strands. The first formed cambial strips start functioning earlier than the later formed cambium.

Due to the formation of secondary xylem opposite the primary phloem strands the cambium in these regions is pushed outwards. The wavy cambium ring becomes circular in outline. This cambium ring produces secondary phloem on the outer side and secondary xylem on the inner side.

INITIATION AND ACTIVITY OF CORK CAMBIUM

• A ring of cork cambium or phellogen is differentiated in pericycle or cortex. It produces phellem (cork) towards the outside and phelloderm (secondary cortex) towards the inner side. The first formed phellogen may persist for a considerable period of time. When it becomes inactive, it is replaced by successively more deep seated phellogens.
• The three tissues, phellem, phellogen and phelloderm together constitute the periderm. The cells of phellogen undergo divisions both on the outerside as well as on the inner side. The derivatives formed on the outside are rectangular and compactly arranged, become suberised, tannin is deposited in the cell interiors.
• Cork, being suberised is impermeable to water. Being dead and tannin containing air filled tissue, cork protects the root interior from pathogens, changes in temperature and mechanical injury.
• At places phellem is interrupted by special pores called lenticels. A lenticel is filled with complementary cells that enclose spaces for gaseous exchange.
• Due to formation of impervious cork, all the tissue present outside cork gets starved, killed and peeled off. They include endodermis, cortex and epiblema.
• A few layered secondary cortex is formed on the inner side of the cork cambium, called phelloderm. Secondary cortex (phelloderm), phellogen (cork cambium) and cork (phellem) are collectively called periderm or secondary ground tissue.

Fig. Secondary growth in Dicot Root

• The part of the axis of the plant which is usually ascending and aerial and also bears leaves and reproductive structures is called stem. The stem has conspicuous nodes and internodes and differs fundamentally from the root in its vascular structure.
• Vascular bundles are conjoint, collateral or bicollateral or even concentric, open or closed.
• A transverse section of young stem shows the following structures:

Epidermis

It is represented by the outermost layer of the stem which is protective in function. It is made up of a single layer of closely packed rectangular cells with relatively thin primary walls. The epidermis is covered on the outer surface by a layer of cuticle. The continuity of epidermis is broken by the presence of stomata. Multicellular hairs (trichomes) are frequently found on the epidermis.

Fundamental (ground) tissue system

In dicotyledonous stems the ground tissue is clearly differentiated into cortex and pith. But in monocotyledonous stems there is no such distinction in the ground tissue.

In dicotyledonous stems the cortex is usually differentiated into three distinct regions, an outer collenchymatous hypodermis, a middle parenchymatous general cortex and an inner endodermis.

• Hypodermis: It consists of a few layers of collenchymatous cells immediately below the epidermis. It usually forms a continuous ring in cylindrical stems, but in angular stems, it occurs in patches (e.g., Cucurbita).
• General Cortex: It is parenchymatous with polygonal, round or oval cells having intercellular spaces. Chloroplasts may be present in the outer layers of the cortex. Latex tubes, resin ducts, crystals of calcium oxalate and reservoirs of waste products are frequently present in the cortex.
• Endodermis: It is the innermost limiting layer of the cortex which separates vascular tissue from the cortex. It is made up of vertically elongated cells which appear barrel shaped in cross section. These cells, however, do not develop casparian thickening like those of the true endodermis. The cells of this layer contain abundant starch and hence it is also called starch sheath.
• Pericycle: The layer of cells which occur inner to the endodermis and outside the vascular tissue form pericycle.
• It is usually made up of closely fitting fibres, which occur in isolated patches or form a complete cylinder. It is multilayered and may be parenchymatous (e.g., most of the dicots) or sclerenchymatous (e.g., cucurbits) or both (e.g., Asteraceae)
• Pith: It is the ground tissue present in the centre of the stem. It is made up of large parenchymatous cells with distinct intercellular spaces. Laticifers, secretory cells, sclereids, idioblasts, crystals of various kinds and other ergastic substances are present in the pith. The outer part of the pith is sometimes distinct in having smaller cells with thicker walls. Such a morphologically distinct outer pith is called perimedullary zone or medullary sheath.
• Medullary rays: These are narrow, vertically elongated bands of parenchymatous cells which extends transversely from the pith towards the periphery. The first rays, formed as a result of the growth of the apical meristem, are known as primary medullary rays.

Anatomical differences between dicotyledonous and monocotyledonous stems

Vascular tissue system

The vascular tissue is in the form of distinct vascular bundles. In dicotyledonous stems, vascular bundles are arranged in a ring. They are conjoint, collateral, bicollateral or concentric and open. Such stele in which vascular bundles are arranged in ring is called eustele. In monocotyledonous stems vascular bundles are scattered and such stele is called atactostele. Usually the outer bundles are smaller and gradually become larger as they pass inward. The bundles are collateral and closed.

• Xylem: It consists of vessels, tracheids, xylem parenchyma and xylem fibres. The protoxylem lies towards the inner side of the bundle (endarch condition). The cells constituting the protoxylem are smaller in diameter than those of the form of 'Y'.
• The two metaxylem vessels are situated at the tips of the arms of 'Y' and the other two at the base of 'Y'. The protoxylem often disintegrates to form a schizolysigenous cavity (protoxylem lacuna).
• Phloem: It consists of sieve tubes, companion cells, phloem parenchyma and phloem fibres. In monocots, the phloem, which is situated in between the arms of 'Y', does not have phloem parenchyma.
• Cambium: It is a band of thin walled cells which lie between the phloem and xylem. The cells constituting cambium are thin walled and rectangular in cross section. They have prominent nuclei and dense cytoplasm. During secondary growth, by rapid division in cambial cells, secondary phloem and secondary xylem are formed. Cambium is absent in monocots.

• Secondary growth may be defined as an increase in girth or diameter due to addition of secondary permanent tissues formed with the help of lateral meristems. i.e., cambium and cork cambium in stelar and extra stelar regions, respectively.
• Secondary vascular tissues are produced by vascular cambium while secondary ground tissues are formed by phellogen or cork cambium.
• Monocotyledonous stems normally do not have secondary growth but the stems of perennial dicots increase in girth by a special type of growth process.

Secondary growth in dicot stems can be studied under the two heads: secondary growth in stelar region and secondary growth in extrastelar region.