Botany lecture notes

Types of Stelar Systems and its Evolution in Pteridophytes and Higher Plants with PPT


types of steles in pteridophytes

Stelar Evolution in Vascular Plants
(Origin and Evolution of Stele in Pteridophytes and Higher Plants)

What is stele? What are the components of stele?

Ø  Stele is the central cylinder or core of vascular tissue in higher plants.

Ø  The stele consists of xylem, phloem, pericycle and medullary rays and pith if present.

Ø  The term ‘stele’ was for the first time used by Van Tieghem and Douliot in 1886 in their ‘Stelar Theory’.

What is ‘stellar theory’?

Ø  Proposed by Van Tieghem and Douliot in 1886.

Ø  Major highlights in stellar theory are:

$.  The stele is a real entity and present universally in all axis of higher plants.

$.  The primary components of stele are xylem and phloem.

$.  Tissues like pericycle, medullary rays and pith are also the components of stele.

$.  ‘Stelar theory’ also says that the cortex and the stele are the two fundamental parts of a shoot system.

$.  Both these components (stele and cortex) are separated by the endodermis.

$.  In higher vascular plants (Pteridophytes, Gymnosperms and Angiosperms), the leaf traces are large, and it appears that they play an important role in the vascular system of the axis.

$.  The whole set-up of leaf traces appears as a composite structure in these plants.

$.  Such composite structures do not remain within the limits of stellar theory of Van Tieghem and Douliot.

What are the different types of steles in plants (Pteridophytes and higher plants)?

Ø  On the basis of ontogeny and phylogeney, there are THREE broad categories of steles in vascular plants.

Ø  They are:

(1).  Protostele

(2).  Siphonostele

(3).  Solenostele

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Botany lecture notes

Receptacular vs Appendicular Theory of Inferior Ovary Development in Flowers


Origin and Evolution of Inferior ovary

Receptacular vs Appendicular Theory
(A Comparative Approach)

Based on the position of ovary there are three types of flowers- (1) Hypogynous, (2) Perigynous, and (3) Epigynous.

(1). Hypogynous Flower: The most primitive type with convex shaped Thalamus (torus). The ovary is superior and all other floral parts (calyx, corolla and androecium) arise from the base of the ovary.

(2). Perigynous Flower: An intermediate or transient type between Hypogynous and Epigynous flowers. The thalamus is more or less cup shaped and the ovary is half inferior, located at the centre of the thalamus cup. All other floral parts arise from the rim of the thalamus cup. Sometimes the thalamus cup forms a long tube like structure called hypanthium.

(3). Epigynous Flower: The most advanced type of flower. The ovary is inferior and all other floral parts arise from the above portion of the ovary.

Ovary Position Classification

(image source: cc wikipedia)

Ø  The origin and evolution of inferior ovary is a well debated question in the phylogenetics of Angiosperms.

Ø  Comparative morphological, anatomical and paleo-botanical studies suggest that the inferior ovary has evolved many times among different groups of Angiosperms in different ways and in different times in the remote past.

Ø  In order to explain the formation of inferior ovary, two theories have developed by the evolutionary biologists.

Ø  The two theories are:

(1). Appendicular Theory

(2). Receptacular (axial) Theory

Ø  Both these theories are proposed on close attention to the organization of the course of vascular bundles supply to the ovules in the flower.

Ø  The two theories also considered that the hypogynous flower is the most primitive one. Moreover, the epigynous condition evolved from a hypogynous condition through the transient perigynous state.

(1). Appendicular Theory

Ø  Proposed by Eames in 1961.

Ø  According to this theory, extensive fusion (both connation and adnation) of the outer lower portion floral whorls to one another and to the ovary wall has occurred. This result in the formation of an inferior ovary (epigynous condition).

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Botany lecture notes

Sclerenchyma: Structure, Classification and Functions with PPT


sclerenchymatous tissue ppt

Sclerenchyma
(Structure, Types and Functions of Sclerenchymatous Cells in Plants)

What are Sclerenchymatous Cells?

Ø  Sclerenchyma is a simple permanent tissue in plants.

Ø  Sclerenchymatous cells are dead at their maturity.

Ø  Cells do not have protoplast when they completely developed.

Ø  They have thick secondary cell wall.

Ø  The secondary cell wall is lignified and very hard.

Ø  Most of the sclerenchymatous cells show intrusive growth.

Different Types of Sclerenchymatous Cells in Plants:

Ø  Based on size, two types of sclerenchyma are described.

  (I).   Sclereids

(II).    Fibres

(I). Sclereids:

Ø  Sclereids are short sclerenchymatous cells.

Ø  They are also called as stone cells.

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Botany lecture notes

Collenchyma Cells in Plants: Structure, Classification and Functions with PPT

Functions of collechyma

Collenchyma
(Structure, Types and Functions of Collenchymatous Cells in Plants)

What is collenchyma?

Ø  Collenchyma is a simple permanent tissue in plants.

Ø  They are living cells with prominent nucleus and all the cell organelles.

Ø  Each collenchymatous cell is with a large and prominent vacuole in the centre.

Ø  The vacuole is filled with many secondary metabolites.

Ø  Unlike parenchyma, the collenchyma cells possess thick primary cell wall.

Ø  Thick walls are NOT lignified.

Ø  The thick wall is due to the deposition of hemicellulose and pectin along with cellulose.

Ø  Wall thickening in collenchyma is greatly affected by the extent of mechanical stress.

Ø  A plant part which in severe stress or motion due to high wind are more likely to possess more thickened collenchyma.

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Botany lecture notes

Parenchyma Cells in Plants: Structure, Classification and Functions (PPT)


functions of parenchyma

Parenchyma
(Structure, Classification and Function of Parenchyma)

What is simple tissue?

Ø  The tissue (a group of cells with particular function) composed of single type of cells.

Ø  Three types of simple tissue system in plants:

(1).  Parenchyma

(2).  Collenchyma

(3).  Sclerenchyma

What are the characteristics of Parenchyma (Parenchymatous Cells)?

Ø  Parenchyma is a simple permanent tissue.

Ø  They are living cells which contains plenty of water.

Ø  Cells are nucleated with prominent nucleus.

Ø  They are thin walled cells.

Ø  Cell wall composed of cellulosic primary cell wall only.

Ø  No lignin deposition in the cell wall of parenchyma.

Ø  Parenchymatous cells are relatively undifferentiated

Ø  Parenchyma is the least specialized along simple permanent tissues in plants.

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