Movement in Plants

Plants do not show noticeable movements. The movements of higher plants are chiefly in the form of bending, twisting, elongation of certain parts etc. Plant movements occur in response to certain stimuli.

Types of Movements
There are two types of movement in plants.

1.    Automatic movement
2.    Paratonic Movement

1.    Automatic Movement: Movements which occur due to factors inherent inside the plant body itself are known as automatic or spontaneous movements.
Automatic movements are further of two types.

a.    Locomotion Movement
b.    Growth Movement

a.    Locomotion Movement: It is the movement of whole plant body or an organ or material within the plant cells which are in response to inherent factors. These movements include streaming movement of protoplasm, chromosome movement during cell division, ciliary movement and flagellary movement etc.

b.  Growth Movement: The changes in the form and shape of plant organs are called growth movement. It is due to unequal growth on two sides of plant organs like stem, root, tendril, bud etc. These movements are of three types.

i.       Epinasty: The upper surface of lead in bud condition shows more growth as compared with lower surface, as a result the buds open. Other examples are leaves, petals etc.
ii.            Hyponasty: If the growth in the lower surface of the leaf in bud condition is more than that of the upper surface then the bud will remain close.
iii.         Nutation: The growth in apex takes place in zigzag manner due to alternate change in growth rate on opposite side of the apex. These movements are called nutation. A nutation movement may be called circumnutation when an apex makes a rotational growth around the axis.

2.    Paratonic Movements: Movements which occur due to external stimuli are known as paratonic or induced movements. This movement occurs due to external stimuli. There are three types of paratonic movement.
          
    i.        Tactic Movement
        ii.        Tropic Movement
       iii.        Nastic Movement


i.              Tactic Movement
These movements occur in response to certain external stimuli. The tactic movement may be positive or negative. Tactic movements are the movement of locomotion. These are classified on the basis of nature of stimulus.

      a.    Phototactic Movement
It is the movement in response to light e.g. the passive movement of chloroplast due to cyclosis.
b.    Chemotactic Movement
Thigmotropism
It is the movement in response to chemicals e.g. movement of sperms of liverworts, mosses, ferns toward archegonia in response to stimulus of nucleic acid released by the ovum.


ii.            Tropic Movement
(Greek Tropos means Turn) These are curvature movement caused by stimulus coming from one side. These movements are controlled by the direction of the stimulus and occur in one direction due to differences in the rate of growth of the two sides of the moving organ, which results in curvature e.g. a. Phototropism b. Thigmotropism c. Chemotropism d. Hydrotropism e. Geotropism
a.    Phototropism: This movement is in response to light and is caused due to differential growth of root or stem.
b.    Thigmotropism: It is in response to touch e.g. when vines come in contact with some solid object the growth on the opposite side of contact increases and the tendril coils around the support.
c.    Chemotropism: Such movement is in response to chemicals e.g. Hyphae of Fungi.
d.    Hydrotropism: It is in response to water e.g. growth of roots towards water.
Phototropism 
e.    Geotropism: It is also known as gravitropism which is in response to gravity. Roots show positive geotropism and shoots show negative geotropism.


iii.           Nastic Movement
In this type of movement the direction of the stimulus is not fixed. It is of the following types.
a.    Nyctinasty: It is in response to external stimuli leading to differential growth. These are due to turgor and growth changes. It is of two types.
i.              Photonasty: It is due to photoperiod. The flowers open and close due to light intensity.
ii.            Thermonasty: It is due to temperature e.g. the flowers of tulip close at night because of rapid growth in the lower side by upward and inward bending of the petals.
b.    Haptonasty: This movement is in response to contact e.g. closing action of Venus flytrap.
Geotropism
c.    Seismonastic or Turgor Movement: Turgor movement is due to loss or gain of water due to shaking, pressure etc. e.g.  Sleep movement and rapid movement of leaflets.

Sleep Movement: Some plants raise their leaves in the morning and lower them in the evening e.g. bean, Sylvia, some members of legume family. These are known as sleep movements. Pulvinus is the swollen portion of the petiole. It is composed of parenchymatous cells having large intercellular spaces and central stand of vascular tissues. The sleeping movements are due to daily changes in turgor pressure in the pulvinus. When turgor pressure on the lower side of the pulvinus increases the leaves rise and become horizontal. When turgor pressure decreases on the lower side of the pulvinus, the leaves become lower in position.
Sleep movement in Prayer Plant 

 Image: Rapid movement of leaflets

Rapid Movement of Leaflets

The leaves of the plant Mimosa Pudica show sensitivity so it is known as “touch me not”. It is herbaceous plant with bipinnately compound leaves. The base of the petiole is swollen which is called pulvinus and similarly smaller pulvinules are present at the base of leaflets. On stimulation, the main petiole falls, the secondary petiole close together and leaflets fold towards one another in pairs. The movement is caused by differential loss of turgor on two sides of pulvinus. It consists of thin walled loosely arranged parenchymatous cells, with large intercellular spaces and upper half consists of a bit compactly arranged parenchymatous cells. When stimulus reaches pulvinus, the osmotically active water enters cytoplasm of cells present on the lower half of the pulvinus, thus, lowering the osmotic pressure of the cells effectively and forcing out water into the intercellular spaces. These flaccid cells now fail to hold pinnules erect and cause drooping down of pulvinules and ultimately pulvinus. The whole leaf bends downward. After about ten minutes the plant regains turgor and the internal turgidity of the leaf is restored.  

Heartwood and Sapwood

In older trees the inner annual rings called heartwood, no longer functions in water transport. The cells become plugged with deposits, such as resins, gums etc. that inhibit the growth of bacteria and fungi e.g. in red cedar and conifer. The outer water conducting wood is called sapwood. 

Callus 

One of important functions of cambium is to form callus. A callus is an undifferentiated mass of cells. These are rapidly formed on or below the damaged surface of stems and roots. Callus unites the branches during budding and grafting. 

Cork Cambium

It is located beneath the epidermis and is a meristem. When cork cambium begins to divide, it produces tissues that disrupts the epidermis and replaces it with cork cell. Cork cells are impregnated with suberin, a lipid material that makes them waterproof. This is protective in function, and have lenticels for exchange of gases.

Significance of Secondary Growth

The growth due to the activity of lateral growth meristems, which results in in an increase in the girth, is called secondary growth. There are two types of lateral meristem: 

a- The Vascular Cambium which gives rise to secondary xylem and secondary phloem 

b- The Cork Cambium or Phellogen which arises later to replace the ruptured epidermis of the expending plant body. 

secondary growth in dicot woody stem
The secondary growth takes place in perennial (lasting more than two years) plants like trees, shrub and vine. 


Annual Rings 


Each year in temporal climates, growth resumes in the spring. During winter the cambium remains dormant. The autumn wood produced at the end of one year, as growth ceases, will therefore be immediately next to the spring wood of the following year and will differ markedly in appearance. The two together make up one year’s growth or annual ring. The age of a tree can be known by counting the annual rings. 

secondary growth





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Sclerenchyma Cells

Their primary cell wall is heavily thickened with deposit of lignin. The mature sclerenchyma cells are dead. The main function of sclerenchyma is to provide support and mechanical strength to the plant. There are three types of sclerenchyma cells.


Vessels
These are long tubular structures formed by the fusion of several cells, end to end in a row.

Tracheids
These are long and cylindrical, fibers found in the pericycle of stems, forming a solid rod of tissue “capping” the vascular bundle of dicotyledons. They often form a layer in the cortex below the epidermis of stems or roots.

Sclereids (stone cells)
These are shorter than the fibers. Sclereids are generally scattered singly or in groups and are most common in the cortex, pith phloem and in fruits, seed coats, nut shells. They provide rigidity or      firmness to the structures in which they are found. In both fibers and sclereids, simple pits appear in the walls as they thicken.