Secondary Growth (NCERT) - Free MCQ Practice Test with solutions, NEET

The wood of the older stems (e.g ., Dalbergia, Acacia, etc.) gets differentiated into two zones, ther outer light coloured and functional sapwood or alburnum and the inner darker and nonfunctional heartwood or duramen. The trachelds and vessels of the heart wood get plugged by the ingrowth of the adjacent parenchyma cells into their cavities through the pits. These ingrowths are calles tyloses.

Intrafascicular or fascicular cambium is present in the form of a narrow strip of primary meristematic cells in between the phloem and the xylem of a vascular bundle. Intrafascicular cambium is primary lateral meristem which helps in increasing girth of the plant. It lies in the vascular bundles of dicot and gymnosperm stems.

The primary xylem persists in the center of the axis, secondary xylem forms the bulk of the stem and the primary and older secondary phloem get crushed in a woody dicot stem which shows extensive secondary growth.

Heartwood (duramen) is the central wood of an old stem. It is dark coloured part of secondary xylem. Living cells are absent here. The tracheary elements are plugged by tyloses and have depositions of tannins, resins, gums, etc. Heartwood is heavier. It is more durable due to its little susceptibility to the attack of pathogens and insects. The outer or peripheral portion of the trunk is soft and lighter in colour. It performs the functions of conduction of water and minerals and is known as sapwood or alburnum.

Periderm consists of a group of secondary tissues forming a protective layer which replaces the epidermis of many plant stems, roots, and other parts; composed of cork cambium, phelloderm, and cork. Phellem or cork is a tissue formed on the outer side of phellogen or cork cambium. Phellogen is more active on the side of phellem and Phelloderm or secondary cortex is produced on the inner side of phellogen.

So, the correct answer is 'Phellem→Phellogen → Phelloderm.

The bark of Betula is called as Bhojpatra, Quinine is present in Bark of Cinchona, Cork of Quercus suber used as Insulators (soundproofing) and Bark of Cinnamomum consists of Dalchini as a chemical constituent.

Secondary growth is the increase in thickness of root or stem due to the activity of lateral meristems (vascular cambium and cork cambium). With few exceptions most of the dicots and gymnosperms show secondary growth. Roots and stems of monocot plants do not show secondary growth (due to absence of vascular cambium), with few exceptions e.g., Dracaena, Yucca, etc., which exhibit anomalous secondary growth.

Bark is a non-technical term and used to define all tissues present outside vascular cambium. Bark = Cortex + Pericyle + Primary and Secondary Pholoem.

In some grasses, on epidermis, there are some larger thin-walled cells with vacuoles called bulliform cells or motor cells or bubble like cells, which in dry conditions lose water and thus the rolling up of leaves occurs, e.g., in psamma, Poa Agropyron, Amopyron, Amophilla, etc.
Each stoma or pore is guarded by two specialized kidney or dumb-bell shaped guard cells, which regulate along and closing of stomata. The guard cells are also surrounded by other specialized epidermal cells called subsIdiary cells or accessory cells. 
Lenticels are some loosely arranged areas in the periderm. These are the aerating pores present in the bark of plants. Due to more activity of certain portions of phellogen, the /Agent cells are cut off very rapidly and hence these cells are loosely arranged with much intercellular spaces.

During secondary drowth in a dicot stem, intrafascicular combium and interfascicular cambium get connected to form a complete ring of vascular cambium. The cells of this vascular cambium are of two types, elongated spindle-shaped fusiform initials and shorter isodiametric ray initials. Ray initials give rise to vascular rays or secondary medu8llary rays. Fusiform initials divide to form seconodary phloem on the outer side and secondary xylem on the inner side.

Xylem is also known as wood. It consists of four types of cells, viz., tracheids, vessels (both tracheary elements), xylem or wood parenchyma and xylem or wood fibres. Phloem is also called bast. It consists of four types of cells, viz, sieve tubes, companion cells, phloem parenchyma and fibres. Heberland phloem and hadrome for the conducting part of xylem.

The phellem or cork cells are dead these are impervious tow water due to suberin deposition in the cell wall. Cork prevents the loss of water by evaporation. It also protects the interior against entry of harmful microorganisms, mechanical injury and extermes of temperature. cork is light compressible, nonreactive and sufficiently resistant to fire. it is used as stopper for bottles, shock absorption and insulation.

In spring, cambium is very active and produces many xylary elements having vessels with wider cavities. The wood formed during this season is called spring wood or early wood. Such a bark is soft.

In winter, the cambium is less active and forms fewer xylary elements that have narrow vessels, and this wood is called autumn wood or late wood. Such a bark is hard.

Cork cambium is formed by the activity of the pericycle. 
During secondary growth in a dicot root, the vascular cambium, which is responsible for producing secondary xylem and phloem, becomes active. As the vascular cambium produces more secondary xylem towards the inside and secondary phloem towards the outside, the root increases in diameter. This increase in diameter causes the outer tissues, such as the epidermis, hypodermis, and cortex, to get crushed and eventually rupture.

To replace these damaged outer tissues and create a protective barrier, a new meristematic tissue called cork cambium (or phellogen) develops. Cork cambium arises from the pericycle, which is a layer of cells found between the endodermis and the vascular tissues (xylem and phloem) in the root.

Soft wood is the technical name of gymnosperm wood.
Because it is devoid of vessels it is also called as nonporous wood. Several of the soft woods are very easy to work with (e.g., Cedrus, Pin us species), however all of them are not 'soft'. The softness depends upon the content of fibres and vascular rays. 90 - 95% of wood is made of tracheids. Vascular rays constitute 5 - 10 % of the wood.
Hard wood is the name of dicot wood which possesses abundant vessels. Due to the presence of vessels, the hard woods are also called porous woods. Content of tracheids is very low (less than 5%).

Anomalous secondary growth is abnormal type of secondary growth that occurs in some arborescent monocots (e.g. Dracaena, Yucca, Agave) and storage roots (e.g., beet, sweet potato etc.).

The only adequate cure of malaria i.e:, quinine is obtained from the thick bark of Cinchona officinalis.

Each mature sieve tube element has thin or thick cellulose wall surrounding a highly albuminous and viscous content. Cytoplasm occurs in the form of thin lining enclosing a big central vacuole. The nucleus, plastids, mitochondria, endoplasmic reticulum and dictyosomes are absent. Vacuole is filled with albuminous substances.

• Stele: It is the central core of the stem and root of a vascular plant, consisting of the vascular tissue (xylem and phloem) and associated supporting tissue. 
• Endodermis: It is an inner layer of cells in the cortex of a root and of some stems, surrounding a vascular bundle.
• Casparian strips: It is a band of cell wall material deposited on the radial and transverse walls of the endodermis and is chemically different from the rest of the cell wall, the cell wall is made of lignin and without suberin, whereas the Casparian strip is made of suberin and sometimes lignin.
• Bark: Bark is the outermost layers of stems and roots of woody plants. Plants with bark include trees, woody vines, and shrubs.