TISSUE SYSTEM and VASCULAR BUNDLES

TISSUE SYSTEM

  

In higher plants several tissues work together in the form of a unit to perform a particular function. These tissues have the same origin. Such tissues form a system which is called tissue system. On the basis of location/position & structure / morphology tissues were categorised by Von Sachs (German scientist) into three types of tissue system. Each system usually consist of an association of tissues which perform specific function.

1. Epidermal/dermal tissue system. This system forms the outer most.covering of the whole plant body and comprises epidermis, stomata and epidermal appendages. e.g Root hairs, trichomes.

• Epidermis: It is outermost layer of the primary plant body and made up of elongated. compactly arranged cells. It is usually single layered and made up of parenchymatous cells Cuticle (Waxy thick layer) is present on the epidermis which prevents the loss of water. Cuticle is absent in roots Epidermal cells are with a small amount of cytoplasm lining the cell wall and a large vacuole.

• Trichomes:- The cells of the epidermis of dicot stem produce hairs called trichomes. The trichomes in the shoot system are usually multicellular. They may be branched or unbranched and soft or stiff. They may even be secretory called glandular hairs. The trichomes help in preventing water loss due to transpiration.

Note : In shoot system trichomes are usually multicellular.

Function: The trichomes help in reduction of water loss.

• Root hairs :- The root hairs are formed due to the elongation of the epidermal cells. These have a vacuolated protoplasm. The thin wall is made up of cellulose and pectic materials. Root hairs are always unicellular.

Function: Root hairs play an important role in absorbing water from the soil.

2. Ground tissue system: - It is the largest tissue system. All tissues except epidermis and vascular bundles constitute the ground tissue. It includes hypodermis, general cortex, endodermis, pericycle and medullary rays (pith rays), pith. In leaf G.T.S. consists of mesophyll. GTS is also called fundamental tissue system. G.TS. is made of simple tissues such as parenchyma, collenchyma & sclerenchyma. The GTS forms the main bulk of the plant.

3. Vascular/conducting tissue system:- 

The V.T.S. consists of complex tissues. xylem and phloem, It is also called specific tissue system.

Note : Primary structure of plant organ or primary plant body is mainly composed of parenchyma.

Types of development of primary xylem :

I. Centrifugal - In this type of development, the protoxylem is formed towards the centre (pith) and metaxylem is formed away from the centre, it means towards the periphery In this condition xylem is known as endarch ex. Stem of angiosperms & gymnosperms.

II. Centripetal - In this type of development protoxylem is formed towards the periphery near the pericycle and metaxylem is formed towards the centre (pith). In this condition xylem is called exarch ex. Roots.

III. Centrifugal and Centripetal :- Elements of metaxylem are formed on both sides of the elements of protoxylem. In this condition xylem is known as mesarch. ex Fern rhizome (underground stem).

VASCULAR BUNDLES

Xylem and phloem are collectively termed as vascular bundles. Which may or may not have cambium.

On the basis of arrangement of elements means location of xylem and phloem vascular bundles are divided into three categories.

Types of vascular bundles 

1. Radial vascular bundles When the xylem and phloem are present separately on different radio in alternate manner, then vascular bundles are called radial vascular bundles

In these vascular bundles xylem is exarch. The order of development of xylem in these vascular bundles is centripetal. Example : Most of the roots (Dicot, monocot, gymnosperm, fern root) 

Exception :- In Radish, carrot, turnip and sugar beet (Beet root) roots, conjoint, collateral vascular bundles are present.

2. Conjoint vascular bundles In the localar bundeswomand loan ton same radius of combine into a bundle. These are of two types :-

Collateral (Open) Bicollateral Conjoint collateral in this type of vascular bundle xylem and phloem re present on the same radius and phloem is present towards the periphery

These are two types :

(i) Open - the cambium is present between the xylem and phloem, then it is said to be open vascular bundle. Ex. stem of dicots/dicotyledons and gymnosperms

(ii) Closed- When cambium is absent between the xylem and phloem conjoint vascular buut then it ched closed vascular bundle.

Ex. Monocotyledonous stem and leaves of angiosperms.

In this type of vascular bundle. xylem is endarch and cool development of sum is centrifugal

(2) Conjoint, bicollateral and open vascular bundle -In this type of V.B two patches of phloem, one of each side of xylem and found. There are two strips of cambium (outer and innert), one oneach side of xylem are found. Only outer cambium is functinal.

Order of development of xylem is centrifugal means endarch condition is found.

ex.  stem of family cucurbitaceae and some plants of family solanaceae.

3. Concentric vascular bundles In these type of vascular bundies either xylem surrounds vascular bundles are always closed. They are of two  :

(a) Amphicribral or Hadrocentric In this type of vascular bundle xylem completely, Surrounded by phloem. It means xylem procent the centre of vascular bundle. Such types of vascular bundles are found in ferns rhizome (Underground stem). The order of development of xylem in ferns time is of both centripetal and centrifugal and we mesarch.

(b) Amphivasal or Leptocentric

• In this type of vascular bundle phlegm is completely surrounded by xylem. It means phloem is present centre of the vascular bundle.

• In this type of vascular bundle, xylem is endarch. stem of Dracaena, Yucca etc

THANK YOU FOR VISTING...:)