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Comprehensive Study of Cambium, Vascular Cambium, and Secondary Growth in Plants

Detailed exploration of cambium types, vascular cambium structure, periderm formation, hormonal influence on cambial activity, and xylem development in dicotyledons and gymnosperms.

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Cambium • Cambium is a group of activity dividing cells that divide predominantly periclinally to form permanent tissues both in stem and roots of most dicotyledons and gymnosperms at intra – and extrastelar region. • Cambium is an example of lateral meristem and during secondary growth it comes to form thin cylinder or strip of cells . It may be vascular and cork cambium.
Vascular Cambium • Vascular Cambium occur in the stele. The cells of cambium remain surrounding the primary xylem in both stem and roots. • The vascular cambium is one cell thick permanent secondary meristem. It is often described as cylindrical and developed between primary xylem and phloem. • It is present in stems and roots and in their branches. It is extends from tips to the base of stem and roots. It may even extend into leaf petiole in some Woody plants.
• In the stele of stem, the cylinder of cambium consists of fuscicular and interfascicular cambium. • In the vascular bundle of dicotyledonous stem the fuscicular cambium occurs between primary xylem and phloem. • The interfascicular cambium originates from the parenchyma tissue that occurs between the two vascular bundles. Fascicular and interfascicular cambium together compose a cylinder of cambial cells. • The cylinder of cambial cells , ones complete, divides chiefly by periclinal division. • The inner derivatives differentiated in to a cylinder of secondary xylem and the peripheral derivative cells from a cylinder of secondary phloem.
Periderm • When the secondary thickening goes on, the pressure exerted by the newely formed xylem and phloem often causes the epidermal and some subepidermal cortical cells to break. • This makes their function of protection difficult. • The periderm is a tissue formed by the activity of a lateral meristem and protects the part of plants seen with in it. • Hence it act as a protective layer and replaced the function of epidermis when it breaks. • The formation of periderm is characteristic of Woody dicot plants
• Gymnosperms, and few herbaceous plants. • In monocots this tissue is absent. • Typically periderm consists of three layers of tissues known as phellogen(cork cambium), phellum(cork) , phelloderm(secondary cortex).
• In an experiment (Warring et Al.,1964) the shoot of Acer pseudoplatanus were completely disbudded. • It was treated with IAA only and the formation of xylem was observed.lt was noted that narrow zone of xylem was formed where the vessels were lignifed only. • The other disbudded shoots were treated with GA alone.In this case xylem war produced and they were non lignified and without vessels.But a normal and wide zone of xylem was formed when the disbudded shoot were treated both with IAA andGA. • This reveals that there exists are correlation between hormones in initiating the cambial activity.
• In an experiment plants were grown in klinostat and it was noted that no reaction wood is formed. This proves that reaction wood is formed in response to gravity.
• Pressure- The vascular cambium always remains compressed by the tissues present on internal and peripheral side of it.lt is needed for the normal functioning of cambium. In an experiment with Pinus strobus and Populus trichocarpa(Brown,1964) the pressure was released on the peripheral side. • Releasing a strip of bark from the tree by incisions while the apical end is still attached with the tree did this. Aseptic measures were taken and the operated zone was wrapped in polythene to prevent infection and reduce desiccation. • After a few days it was noticed that an unorganized callus was formed
• On the inner side of the free strip of the bark and the function of cambium was not normal. • In another experiment the pressure was released according to the previous experiment and then the pressure was applied by binding the strip of bark with the tree. Aseptic measures were taken as usual and after few days it was noticed that no callus tissue was formed and the cambium functioned normally. • This implies that pressure influences the normal cambial activity.
Day length. – In short days late wood is formed only. In this wood no vessels are formed. The vessels, if formed , are few in number and with small diameter. • In long days early wood is formed.In this wood many vessels are developed and they have large diameter. • In long days tracheids with large diameter are formed in conifers in contrast to short days where tracheids with narrow diameter are formed. • In long days the needles of conifers elongate and there is no doubt that is associated with auxin production. The production of auxin promotes to form tracheids with large diameter.
• In an experiment (Larson,1964) short day(long night) plants were given a low intensity of light for a short time during the long night period.lt was observed that tracheids of wide diameter were formed.It indicates that the cambial activity is influenced by day length and it is true photoperiodic phenomenon. • Temperature- Temperature influences the activity of cambium. It was observed in Robinia seedling the activity of cambium was continuous for a long period when the plants are kept in high temperature and short days(Waisel and Fahn,1965).
Xylem
Origin of trachids • Tracheids of primary xylem originated from procambium. • In secondary xylem they originated from cambial ring. • A single fusiform initial forms a tracheids. • In general , vascular cryptograms have very long tracheids, gymnosperms processes tracheids of intermediate length. • From ontogenetic viewpoint annular elements proceed helical elements. The helical bands became joined at certain areas thus forming scalariform elements. • In the phylogenetic scale pitted elements are most advanced.
• Origin of xylem fibre – in primary xylem fibres originate from procambium where as they are developed from fusiform initial of cambium in case of secondary xylem. • Fibres and tracheids are phylogenetic ally related and it is suggested that the former evolved from the latter. During evolution the length of the fibre decreased, the bordered pits reduced in size and the thickness of the cell wall increased.
• Ray parenchyma. - cells are of various shapes and the two most common forms are where the longest axis of the cell is either vertical or radial.The former one, where the longest axis is oriented vertically, is called erect or upright ray cells. The latter one is termed as procambial ray cells where the longest axis is oriented radially. Ray cells may be also be square or isodiametric. Square cells are considered as morphologically equivalent to erect cells .In radial longitudinal section the parenchyma cells appear as fine horizontal lines. So the transverse or radial longitudinal section reveals the longitudinal axis of ray cells.
• In angiosperms ray cells may be homogenous (ex. Populus) where all the cells are procumbent. i.e. the longest axis is oriented in radial direction or heterogeneous (olea) where the ray cells are procumbent and square or oriented vertically. These two terms homogenous and heterogeneous are more or less similar to homocellular and heterocellular ray cells respectively of gymnosperms wood. • The ray cells of Pinus have very large pits with very narrow border that appears as more or less circular or rectangular areas throughout the width of the cells. These are known as fenestriform pits whose surface view is obtained in radial longitudinal section.
• Initially ray parenchyma cells are living and serve in storage and aeration. They store carbohydrates and other nutrients. These substances are transported radially with in the wood over short distances. The upright cells have direct connection with axial cells. • When the upright ray parenchyma cell is in contact with axial parenchyma plasmodesmata exist between them. The upright parenchyma cell maybe in contact with the axial tracheary elements (tracheid or vessel.). In such case ray cells have very thin walls with small pits. Pits are the routes of transport of carbohydrates and other nutrients stored in the upright parenchyma cells.
• Axial parenchyma – The distribution of axial parenchyma is very characteristic in dicotyledonous wood. Parenchyma may like independent of vessels or they distinctly associated with them. • These two forms are known as apotracheal and paratracheal respectively. The common apotracheal forms are 1) diffuse parenchyma, parenchyma cells appear as a single cells or a small uni seriate band through out the growth ring.2) banded or metatracheal parenchyma, parenchyma cells appear concentric bands.
• Origin-. Xylem parenchyma cells are present in both primary and secondary xylem. In primary xylem they originated from procambium. In secondary xylem ray parenchyma cells originated from the ray initial of cambium. The fusiform initial of cambium give rise to axial parenchyma .