Opening and Closing of Stomata : 
The immediate cause of the opening or closing of the stomata is a change in the turgidity of the guard cells. The inner wall of each guard cell, towards the pore or stomatal aperture, is thick and elastic. When turgidity increases within the two guard cells flanking each stomatal aperture or pore, the thin outer walls bulge out and force the inner walls into a crescent shape. The opening of the stoma is also aided due to the orientation of the microfibrils in the cell walls of the guard cells. Cellulose microfibrils are oriented radially rather than longitudinally making it easier for the stoma to open. When the guard cells lose turgor, due to water loss (or water stress) the elastic inner walls regain their original shape, the guard cells become flaccid and the stoma closes.

The Pressure Flow or Mass Flow Hypothesis
The accepted mechanism used for the translocation of sugars from source to sink is called the pressure flow hypothesis. As glucose is prepared at the source (by photosynthesis) it is converted to sucrose (a dissacharide). The sugar is then moved in the form of sucrose into the companion cells and then into the living phloem sieve tube cells by active transport. This process of loading at the source produces a hypertonic condition in the phloem. Water in the adjacent xylem moves into the phloem by osmosis. As osmotic pressure builds up the phloem sap will move to areas of lower pressure. At the sink osmotic pressure must be reduced. Again active transport is necessary to move the sucrose out of the phloem sap and into the cells which will use the sugar - converting it into energy, starch, or cellulose. As sugars are removed, the osmotic pressure decreases and water moves out of the phloem.

Phloem tissue is composed of sieve tube cells, which form long columns with holes in their end walls called sieve plates. Cytoplasmic strands pass through the holes in the sieve plates, so forming continuous filaments. As hydrostatic pressure in the phloem sieve tube increases, pressure flowbegins, and the sap moves through the phloem. Meanwhile, at the sink, incoming sugars are actively transported out of the phloem and removed as complex carbohydrates. The loss of solute produces a high water potential in the phloem, and water passes out, returning eventually to xylem.
A simple experiment, called girdling, was used to identify the tissues through which food is transported. On the trunk of a tree a ring of bark up to a depth of the phloem layer, can be carefully removed. In the absence of downward movement of food the portion of the bark above the ring on the stem becomes swollen after a few weeks. This simple experiment shows that phloem is the tissue responsible for translocation of food; and that transport takes place in one direction, i.e., towards the roots. This experiment can be performed by you easily.

Transpiration is the evaporative loss of water by plants. It occurs mainly through the stomata in the leaves. Besides the loss of water vapour in transpiration, exchange of oxygen and carbon dioxide in the leaf also occurs through pores called stomata (sing. : stoma). Normally stomata are open in the day time and close during the night. The immediate cause of the opening or closing of the stomata is a change in the turgidity of the guard cells. The inner wall of each guard cell, towards the pore or stomatal aperture, is thick and elastic. When turgidity increases within the two guard cells flanking each stomatal aperture or pore, the thin outer walls bulge out and force the inner walls into a crescent shape. The opening of the stoma is also aided due to the orientation of the microfibrils in the cell walls of the guard cells. Cellulose microfibrils are oriented radially rather than longitudinally making it easier for the stoma to open. When the guard cells lose turgor, due to water loss (or water stress) the elastic inner walls regain their original shape, the guard cells become flaccid and the stoma closes.
Factors Affecting Transpiration
1Temperature,
2.light,
3.humidity,
4.wind speed.
As water evaporates through the stomata, since the thin film of water over the cells is continuous, it results in pulling of water, molecule by molecule, into the leaf from the xylem. Also, because of lower concentration of water vapour in the atmosphere as compared to the substomatal cavity and intercellular spaces, water diffuses into the surrounding air. This creates a 'pull'.
Importance of Transpiration: Transport of liquids and minerals is facilitated because of transpiration.

Guttation and Transpiration:
Guttation is the appearance of drops of xylem sap on the tips or edges of leaves of some vascular plants, such as grasses. Guttation is not to be confused with dew, which condenses from the atmosphere onto the plant surface.
Process :  
At night, transpiration usually does not occur because most plants have their stomata closed. When there is a high soil moisture level, water will enter plant roots, because the water potential of the roots is lower than in the soil solution. The water will accumulate in the plant, creating a slight root pressure. The root pressure forces some water to exude through special leaf tip or edge structures, hydathodes, forming drops. Root pressure provides the impetus for this flow, rather than transpirational pull.
Transpiration on the other hand happens because of transpiration pull.

Apoplast and Symplast pathways of movement of water in plants.
Within a plant, the apoplast is the free diffusional space outside the plasma membrane. It is interrupted by the Casparian strip in roots, air spaces between pfant ceITsand the cuticula of the plant.
Structurally, the apoplast is formed by the continuum of cell walls of adjacent cells as well as the extracellular spaces, forming a tissue level compartment comparable to the symplast. The apoplastic route facilitates the transport of water and solutes across a tissue or organ. This process is known as apoplastic transport.
The symplast of a plant is the inner side of the plasma membrane in which water (and low-molecular solutes) can freely diffuse. The plasmodesmata allow the direct flow of small molecules such as sugars, amino acids, and ions between cells. Larger molecules, including transcription factors and plant viruses, can also be transported through with the help of actin structures.
This allows direct cytoplasm to cytoplasm flow of water and other nutrients along concentration gradients. In particular, it is used in the root systems to bring in nutrients from soil. It moves these solutes from epidermis cells through the cortex into the endodermis and eventually the pericycle, where it can be moved into the xylem for long distance transport. It is contrasted with the apoplastic flow,which uses cell wall transport.

A. The diagram illustrates the process of wilting, i.e., more water is lost by transpiration and less water is, absorbed.
The plant was subjected to high heat and light conditions that resulted in loss of more water than it can be absorb from the soil. When the water evaporated is not replaced by water absorbed by the roots at the same rate, the tissue becomes flaccid and the plant shows wilting.

Labeling of the parts/ pathways Parts 1 Root hair, 2 Endodermis, 3 Casparian strip,
4 Apoplastic path and 5 Vacuole. 
Functions of parts 2 and 3
2. EndodermisIt is a special layer of living cells that enclose the vascular cylinder of the root. The major function of endodermis in roots is to prevent the
loss of water and minerals.
3. Casparian stripThe Casparian strip present in the wall of endodermal cells is made up of lignosuberin, a waxy substance that prevents movement of water
and minerals via cell wall route.
Symplast pathwayWater moves from cell to cell through living cytoplasm and plasmodesmata.

(i) The solution marked as (1) will be hypertonic (more concentrated) due to which cell shrinks.
(ii) From C to D, figure is showing the process of deplasmolysis as shrinked cell has again regained its original shape.
(iii) In space marked as (2) hypotonic solution and water is present.
(iv) Turgor pressure will be zero in figure Band Cbecause cell is in a flaccid condition.
(v) Wall pressure will be positive in- figure A and D because in these figure cell wall is exerting equal and opposite pressure against the expanding protoplasm.

(i) Plasmolysis occurs when water moves out of the cell and the cell membrane of a plant cell shrinks away from its cell wall. This occurs when the cell is kept in a
solution that is hypertonic (has more solutes) to the protoplasm. Water moves out from the cell through diffusion and causes the protoplasm to shrink away from the walls. In such situation, cell becomes plasmolysed.

When the cell is placed in an isotonic solution. There is no flow of water towards inside or outside. If the external solution balances the osmotic pressure of the
cytoplasm, it is said to be isotonic. When the water flows into the cell and out of the cell or in equilibrium the cell is called flaccid.
(ii) When the plant cell is kept in a solution having high water potential (hypotonic solution or dilute solution as compared to cytoplasm), water diffuses into the
cell causing the cytoplasm to build up a pressure against the wall, called turgor pressure.
The pressure exerted by the protoplasts due to entry of water against the rigid walls is called pressure potential \(({ \psi }_{ p })\) Because of the rigidity of the cell wall,
the cell does not rupture. This turgor pressure is ultimately responsible for enlargement of cells.

Transpiration pull is the phenomenon which takes place when thousands of transpiring mesophyll cells withdraw water from the xylem by generating a negative pressure in the water column and exerting an upward pull over the water column.This pull is further transmitted to the roots in search of more water.