Anatomy of Flowering plants

Plant Anatomy:

The branch of botany which deals with study of internal structures and organization of plants or plant organs (plant parts) is known as plant anatomy/study of internal structure of plants is called plant anatomy.

N.Grew (Nehemiah Grew) is known as father of plant anatomy.

K.A. Chaudhary is known as father of Indian plant anatomy.

Note: Book- "The Anatomy of seed plants" was written by Katherine Esau (K.Esau). It was published in 1960. It was referred to as Webster's of plant biology- it is encyclopedia.

TISSUE:

A group of cells having a common origin and usually perform a common function is called tissue.

 The term tissue was coined by Nehemiah Grew.

The tissue were divided into two groups by Karl Nageli.

Meristem: Growth in plants is largely restricted to specialised regions of active cell division called meristems/ A meristem is a localised region in which actual cell division occurs.

Meristem term was given by Nageli. It is derived from a Greek word meristos (means Divided/Divisible).

CLASSIFICATION OF MERISTEMATIC TISSUE MERISTEM/MERISTEM:

MERISTEM BASED ON ORIGIN AND DEVELOPMENT

On the basis of origin and development meristems can be divided into following three types:-

PROMERISTEM/PRIMORDIAL MERISTEM/URMERISTEM

This meristem develops in the beginning during embryonic stage. It forms primary meristem.

eg. Embryonic meristem

PRIMARY MERISTEM

• Meristematic cells developed from promeristem are known as primary meristem.

• It appears early in the life of a plant and contribute to the formation of the primary plant body.
• Cells are always in division phase and form primary permanent by the process of differentiation. eg. Apical meristem, intercalary meristem, intrafascicular cambium (Fascicular vascular cambium).

SECONDARY MERISTEM

• Secondary meristem develops from primary permanent tissue by the process of dedifferentiation. 
• Secondary meristem appears later than primary meristem. 
• By the activity of secondary meristem, secondary growth takes place. eg. Interfascicular cambium & cork cambium of dicot stem, vascular cambium & cork cambium of dicot root.

[B] MERISTEM BASED ON LOCATION (POSITION) IN PLANT BODY

On the basis of position, meristems are divided into three types:

(a) APICAL MERISTEM : It is an example of primary meristem.

The meristems which occur at the tips of roots and shoots and produce primary tissues are called apical meristers. They are responsible for increase in the length of plant organs. It means they are responsible for primary growth. Examples of apical meristem - Root apex/root apical meristem, shoot apex/shoot apical meristem.

HABERLANDT DIVIDED EUMERISTEM (it is a primary Meristem i.e. apical Meristem) into three region  on the basis of function.

Protoderm : It is the outermost layer of meristem By the activity of protoderm epidermal tissue system is formed. E.T.S. includes epidermis, stomata, stem hair (shoot hair/trichomes) etc.

Procambium : It is made up of elongated cells and it gives rise to the vascular tissue system V.T.S. includes Xylem, phloem (vascular bundles).

Ground Meristem : The cells of this region are thin walled and isodiametric. Ground tissue system is formed by the activity of these cells. G.T.S. includes hypodermis, general cortex. endodermis, pericycle, pith-rays (medullary rays) and pith(medulla). During the formation of the primary plant body, specific regions of the apical meristem produce dermal tissues, ground tissues and vascular tissues.

(b)INTERCALARY MERISTEM :

Is an example of primary meristem.

It is present at the base of internode of monocuts stems e.g.grasses,bamboo,sugarcane etc it is also present at the base of leaves. 

By the activity of this Meristem, length of leaves increases.

Intercalary meristem occurs between mature tissues.

By the activity of this meristem length of the plant organs increases.

They occur in grasses and regenerate parts removed by the grazing herbivores.

Both apical meristems & intercalary meristems are primary meristems because they appear early in the life of a plant and contribute to the formation of primary plant body

(c) LATERAL MERISTEM:

• Lateral meristem occurs on lateral side of plant organs.
• Activity of lateral meristem increases the circumference/ girth/thickness of plant organ.

• All secondary meristems are lateral meristems.

• Lateral meristems are both primary and secondary in origin. (mostly secondary in origin).

Primary lateral meristems:

1.Marginal meristem

2. Intrafascicular cambium

1.Marginal meristem :- It occurs at the margin of young leaf. Its activity causes the width o f not thickness). For total growth of head only primary meristems are responsible.

2. Intrafascicular cambium /fascicular cambium : This cambium occurs inside the vascular bundles of dicot stem  and gymnosperms stems.

Secondary lateral meristems :-Interfascicular cambium and cork cambium (phellogen) of dicot stem and gymnosperm stem. Cork cambium and vascular cambium of dicot roots.

Note : Generally lateral meristems are cylindrical.

Note: The meristem that occurs in the mature regions of roots and shoots of many plants, particularly those that produce woody axis and appears later than primary meristemis  called the secondary meristem.

COMPOSITION/ORGANISATION OF APICAL MERISTEM IN DIFFERENT PLANTS

• Apical meristem is absent in most of the algae and fungi. All the cells of these plants are divisible, so they do not show apical growth. Thus such type of growth in these plants is called diffused growth Diffused growth also occurs in animals.

• In some algae (eg.- brown algae), bryophytes and some pteridophytes one meristematic cell present  at the apex. Generally the shape of apical cell is pyramid like.
• In ferns, gymnosperms and angiosperms apical meristem consists of many cells.

HISTOGEN THEORY:

• This is most valid theory for root apex organisation.

• This theory was proposed by Hanstein (1870).

According to Hanstein, the apical meristems (root and shoot apices) are distinguished into three histogens (meristematic regions). These are as follows.

1.Dermatogen. This is the outermost histogen & composed of single layer of cells. These cells form uniseriate (single layered) epidermis.

2.Periblem - This region is situated Just below the dermatogen. It forms cortex (hypodermis, general cortex and endodermis).

3.Plerome - This is the innermost histogen. Stele formation takes place by division of these cells. It means formation of pericycle, vascular bundles, pith rays (medullary rays) and pith(medulla).

This theory is true only for root apex. It is not applicable for shoot apex of higher plants because in most of the gymnosperms and angiosperms, shoot apex is not differentiated into three histogens.

Including above described three histogens, a fourth histogen is also present in monocotyledon root apex. This is known as calyptrogen. Root cap is produced by calyptrogen in monocots. Root cap & epiblema/epidermis are produced by dermatogen in dicotyledons.

Due to presence of root cap, position of root apical meristem is sub terminal/sub apical, so maximum growth in root takes place behind the apex.

• In hydrophytes root cap is absent eg. Pistia. In place of root cap, root pockets are present.

• Generally, root cap is single but in Pandanus (screw pine), multiple root cap is present.

• Root cap is living, it contains large amount of golgi bodies which secrete mucilage which makes the roots slimy.

• In monocot root, epidermis and calyptrogen are derived from dermatogen.

Note :- Root cap is formed by calyptrogen (If dicot or monocot is not given)

QUIESCENT CENTRE:-

Quiescent centre term was coined by "Clowes". Quiescent centre was discovered by Clowes in maize root.

A group of inactive or less active cells present between the dermatogen and calyptrogen of monocot root is called quiescent centre. The cells of quiescent centre contain less amount of DNA, RNA, light cytoplasm, small nuclei and synthesis of protein is also less.

Function: The quiescent centre in the root meristem serves as a reserve for replenishment damaged cells of the meristem or Inactive cells of quiescent centre become active when previously active initials of calyptrogen get damaged. Quiescent centre is crescentic shape.

TUNICA CORPUS THEORY

This theory was proposed by Schmidt (1924).It is most valid theory for shoot apex organisation of angiosperms. It is based on planes of division. According to this theory two zones are found in the shoot apex:-

(1) TUNICA

• This is peripheral layer. In turica cells, anticlinal division takes place only in one plane Epidermis is formed by tunica.
• Generally, tunica is single layered, but sometimes it is multilayered, than the outer most layer forms the epidermis and remaining layers of tunica form rest types of tissues with the association of corpus.

CORPUS

•  The mass of cells present below the tunica is called corpus. The cells of this zone divides in all directions (all planes) due to which volume increases. The cells of corpus are usually larger than the cells of tunica.
• Function : Formation stele of ground tissue system and vascular tissue system or Formation of cortex and stele.

CYTO-HISTOLOGICAL ZONATION THEORY:- 

According to Foster shoot apical meristem bested into two regions on the basis of rate of division

1. Summit

2 Flanks

VEGETATIVE SHOOT APEX

SUMMIT:

The rate of division is slow in this region. This region is located at the apex

FLANKS:

The rate of division is very fast in this region. This region lies behind/below the summit and leaf primordia are formed by this region.

• Time period between initiation of two successive leaf primordia is called "Plastochron".
• Shape of vegetative shoot apex- dome (mainly) or conical shaped.
•  Shape of reproductive shoot apex- Broad & flat
• Shoot apex is terminal in position.
• Growth of leaf primordium - first apical then marginal.
• Function of leaf primordium-Provide protection to shoot apex.

REPRODUCTIVE SHOOT APEX :

During reproductive phase i.e., at the time of flowering, vegetative shoot apex transforms into reproductive shoot apex. This change of shoot apex is induced by florigen & light.

• In reproductive shoot apex, summit zone is more active (rate of cell division fast) and it forms stamens (androecium) & carpels (gynoecium) and flanks zone is less active rate of cell division slow) and it forms sepals (calyx) and petals (corolla).

• During the formation of leaves and elongation of stem, some cells "Left behind" from shoot apical meristem. constitute the axillary bud. Such buds are present in the axils of leaves and are capable of forming a branch or a flower.

• Root apical meristem occupies the tip of a root while shoot apical meristem occupies the distant most region of the stem axis.

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