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Secondary growth in plants

The document discusses the origin of cambium and secondary growth in plant stems, detailing the roles of vascular and cork cambium in thickening stems and roots. It explains how cambium facilitates secondary growth through cell division and the formation of annual rings, as well as variations in cambium duration across different plant species. The text also highlights the structural differences between monocots and dicots in relation to cambium and the formation of protective tissues like cork during secondary growth.

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ORIGION OF CAMBIUM & SECONDARY GROWTH IN STEM Presented by : Sakshi Sharma (M.Sc I) BOTANY PAU, LUDHIANA
Secondary growth is the growth that results from cell division in the cambia or lateral meristems i.e., vascular and cork cambium and that causes the stems and roots to thicken i.e., increase in girth. 1. • To support the extra weight of the tree due to an increase in its height and to prevent it from collapsing. 2. • Add new conducting tissues replacing the old one. 3. • Helps in meeting increased demand of long distance transport of sap 4. • Produces corky bark around the tree trunk that protects the interior from abrasion , heat , cold & infections Sakshi Sharma
Sakshi Sharma
• A CAMBIUM(plural cambia or cambiums), in plants, is a tissue layer that provides partially undifferentiated cells for plant growth. It is found in the area between xylem and phloem. • Known as meristematic layer being responsible for cutting off the vascular tissues : Xylem and Phloem • It is responsible for secondary growth in plants. Sakshi Sharma
ORIGION OF CAMBIUM •Strictly speaking only initials constitute the cambium but frequently the term is used with reference to Cambial zone because it is difficult to distinguish the initials from recent derivatives. It divides in such a way that one out of the 2 identical daughter cells always remains meristematic i.e., the persistent cambium cell the other becomes a xylem or phloem mother cell depending upon its position . Adjacent cambium cells divide nearly at same time and the daughter cell of same tissue this way the continuity of cambium is maintained • Maturing of the cells of the procambium strands form xylem and phloem , the plants which do not possess secondary growth , all cells of the procambium strands mature and develop into vascular tissue. • In the plant which have secondary growth later on, a part of the procambium strand remains meristematic and gives rise to the cambium proper Sakshi Sharma
Tissue between primary xylem and phloem called FASCICULAR or INTRAFASCICULAR cambium Develops into complete ring of Cambial cells Cells of medullary rays lying between 2 adjascent vascular bundle in line with fascicular cambium Becomes meristematic Forming a strip of INTERFASCICULAR CAMBIUM when these strips join with fascicular cambium forms a complete cambium ring Sakshi Sharma
Annual rings 1. An annual ring may be defined as a ring of vascular tissue produced by the fascicular cambium during the course of one year(in different seasons). 2. The activity of the fascicular cambium is more periodical than continuous, As a result of this periodic activity, distinct growth layers are formed in the secondary xylem. 3. In a transverse section of the stem, these layers appear as rings, hence called annual lings or growth rings. Seasons Favorable season(spring) Unfavorable season (winter) More active cambium(bigger vessels produced ) Lesser active Sakshi Sharma
STRUCTURE OF CAMBIUM • There are two types of cambial cells • 1. RAY INITIALS :- more or less isodiametric and give rise to vascular rays • 2. FUSIFORM INITIALS :- The elongate tapering cells that divide to form all cells of vertical system. Sakshi Sharma
CAMBIUM Intrafascicular Cambium: It is primary in origin, present in between primary phloem and primary xylem. Vascular Cambium Ring: Both intrafascicular and Fascicular cambium joins together and forms cambium ring. Interfascicular Cambium: It is true secondary meristem . It originates from parenchyma cells of medularly rays region. It lies in between the vascular bundles Sakshi Sharma
DURATION OF CAMBIUM • Varies greatly in different species and even in different parts of same plant . • As in perennial woody plant the cambium of main stem lives from the time of its formation until death of plant . • In leaves , inflorescence and other deciduous parts , the functional life of cambium is short. Sakshi Sharma
FUNCTIONS OF CAMBIUM 1.Secondary growth of stems and roots (secondary growth occurs after the first season) results in increase in thickness. 2. A cambium may also form within callus tissues—masses of cells that grow over the injured surface of a wound, leading to healing. Sakshi Sharma
Secondary growth in stem Sakshi Sharma
Why not in monocots ? Unlike dicots the vascular bundles in monocots are scattered and they have an atactostele. Each vascular bundle has xylem arranged in the form of Y. Phloem elements are present on the outer side of xylem elements in between the two arms of Y. A sclerenchymatous bundle sheath is present around each vascular bundle to give mechanical strength to stem. E.g., Family : LILIACEAE(Dracaena) Sakshi Sharma
Some cells of parenchymatous cortex gets dedifferentiated forms a ring Cork cambium this is also a meristem so produces new cells on outer side - parenchymatous cells loosely arranged , after their formations cells undergo deposition of SUBERIN Because of this the outer cell dies out as no substances reaches to them due to Suberin Side cells gets crushed during expansion due to pressure we need to maintain those crushed cells if not then any pathogen can enter Therefore we need to syntheize different kind of cells – cork cambium Sakshi Sharma
• The cells of cambium present between primary xylem & phloem is called INTERFASCICULAR CAMBIUM • The cells of medullary rays adjoining these interfascicular cambium becomes meristematic and forms a continous CAMBIUM RING. FORMATION OF CAMBIUM RING • Cambium ring becomes active and begins to cut off new cell both towards the inner and outer side . • Cells towards pith mature into SECONDARY XYLEM and those towards periphery mature into SECONDARY PHLOEM • Cambium more active on inner side than on outer , as a result amount of secondary xylem is more than secondary phloem ACTIVITY OF CAMBIUM RING :- Sakshi Sharma
Sakshi Sharma
Due to activity of cambium and addition of layers a lot of pressure is exerted New lateral meristem needed to be developled in peripheral part To protect the newly formed tissue Phellogen arise in cortex , cuts off new segments towards outer & inner side Produced by lateral meristem called CORK CAMBIUM or PHELLOGEN Periderm is formed ; a protective tissue Outer cells develop into CORK or PHELLEM Inner modified into secondary cortex or PHELLODERM Dead cells lying outside phellogen called BARK (INNER;OUTER) FORMATION OF PERIDERM Sakshi Sharma
Sakshi Sharma
CHANGES AFTER PRIMARY GROWTH • EPIDERMIS • HYPODERMIS • PARENCHYMATOUS TISSUE • CORTEX • ENDODERMIS • PERICYCLE • PRIMARY PHLOEM • CAMBIUM • PRIMARY XYLEM • PITH CHANGES AFTER SECONDARY GROWTH • EPIDERMIS • HYPODERMIS • CORK CAMBIUM • SECONDARY CORTEX • ENDODERMIS • PERICYCLE • PRIMARY PHLOEM • SECONDARY PHLOEM • CAMBIUM • SECONDARY XYLEM • PRIMARY XYLEM (DEAD CELLS) • PITH (reduced) • CRUSHED Sakshi Sharma
Presented by : Sakshi Sharma (M.Sc I) BOTANY PAU, LUDHIANA