SUPPORT IN PLANTS

Root and shoot are two major parts of body of higher plants. They are identified on the basis of morphology and physiology. These plants require some special tissue, which are present in their whole body. Young stem has outer-layer called epidermis. Beneath epidermis cortex is present and central portion called stele vascular tissues. This type of stem depends for its mechanical support on the following tissues:

• Thin walled parenchyma-having turgidity.
• Thick walled living tissues collencyma and dead tissues like scelerenchyma.
• Stele as cylindrical core of vascular bundles.

Parenchymatous tissues are found in epidermis, cortex and pith. Bryophytes are made up of these tissues. Parenchyma are turgid, exert and internal pressure called turgor pressure.

Collenchyma is another type of simple tissue important for the support in plants. They perform functions of support in young plants and in older plants in stem and leaves.

Scelerenchyma tissues are mostly dead on their maturity. Tracheids are found in the xylem while vessels are long tubular scelerenchyma cells join end to end to form long water conducting pipe in xylem. Fibers and sclereids are two types of scelerenchyma cells. Fibers are tough and strong, but flexible while sclereids are variable, mostly irregular in shape. Sclereids are formed in nuts and hard parts of seeds. The simple unbranched sclereids are generally called stone cells. Jute and hams having fibers of scelernchyma cells, which are used for making ropes.

Significance of secondary tissues:

              Tissues formed by the activity of vascular cambium and  cork cambium are called secondary tissues. it takes place most in dicot and rare in monocot species. The cambium cells within the vascular bundles are called fusiform intials. The part of ring where changes occurred called heart-wood. Whereas outer part is called sap-wood. For the protection of exposed tissues and to increase in diameter, a layer of meristematic cells the cork cambium or phellogen, arises in the cortex. They deposit a waxy material called suberin in their walls  and then die. Masses of loose cells, through which gases and water vapours readily pass out are known as lenticels. Secondary growth requires more water, minerals and food conduction for growing plant hormone control and coordinate the plant responses in two ways:

1. Hormones control movement and various metabolic process in plants.
2. They control growth in different parts of plants body.

Plant movement:

         In plants there is no proper and distinct movement like animals, but in their body growth of certain organs, curved growth and bending processed are considered as movement. The movement of plant occurs in response to certain stimulus, in a particular direction of that stimulus. Such responses are called tropism. Tropism is a Latin word, which means turn of body part. There are three kinds of tropism:
   

       1.Phototropism:  It is movement of plant towards light.

       2.Geotropism: This movement is due to force of gravity.

       3.Thigmotropism: This movement is due to touch of plant body.

 

 Biological clock and circadian rhythm:  

 There is a particular clock system in plant and animals, which control physiological and metabolic functions according to time. It is known as biological clock. Movement of plant organs due to light & temperature are controlled by it.

      Performing of function by living organism according to a particular time called biological rhythms or biorhythms. When this process is repeated after 24 hours is known as circadian rhythms. The circadian rhythm in plants is independent of climatic meters, such as in Bean plant sleep movement occurs in the presence and also in absence of light,
                   In Oat (Ararot) plant growth is by biological clock and in Bryophyllum plant CO2 metabolism takes place by biological clock.
        In plants when all function and movements are controlled by internal biological clock system, then these are called endogenous rhythms.
         This clock system is not changed umtil some abnormal change takes place in body of living organism.

 Photoperiodism: 

                  The period or lenght required by plant body to increase the ability for growth, production of fruits and seeds is called photoperiodism. It may also be defined "response of plant to relative period or lenght of the day and night." The light period is different in different plants.


5. Response to environmental stresses:
                 The unfavourable conditions of environment are called environmental stresses. Water, light, temperature, CO2 and other substance are necessary for the plants, when there is shortage of these essential substances, the plant face great difficulty, when their functions. the environmental stresses for plants are as follow:

1. )  Water shortage or drought conditions
2. )   Less supply of O2
3. )   High concentration of salts in soil
4. )   High temperature 
5. )   Herbivory / over grazing