Transportation in plants - class-IX

According to the most widely accepted theory of translocation of organic solutes - 'Mass flow hypothesis' or 'Pressure flow hypothesis', organic substances move in the sieve tubes from the region of higher osmotic concentration to the region of lower osmotic concentration in a mass flow, due to occurrence of a pressure gradient. 

'Mass flow hypothesis' or 'Pressure flow hypothesis' to explain translocation was given by Munch, 1930. 

Translocation of organic solutes in phloem is bidirectional. The transport of prepared food is from source to sink. The source is the area, where sugars are present in larger amount either because they are synthesized there, example leaves or are stored there, example storage organs. 

Translocation of organic solutes in phloem is bidirectional. The transport of prepared food is from source to sink. 

Root pressure is caused in the Xylem vessels due to osmosis. It is a positive pressure which is developed due to Osmotic pressure which is high in the root as compared to the water column above.

Guttation occurs due to high root pressure when transpiration is low. A high DPD (Diffusion Pressure Deficit) in the leaf indicates water stress, which would pull water into the leaf cells rather than forcing it out through hydathodes.

Root pressure is a phenomenon commonly observed in herbaceous plants. Gymnosperms like Pinus do not typically exhibit significant root pressure compared to angiosperms.

• Negative hydrostatic pressure develops at the time of water loss, it occurs when the water is lost through transpiration and creates tension in the cell.
• Transpiration is the process in which water is lost through the upper or aerial plant parts in the form of water. 
• Imbibition is the swelling of cells that occurs due to the absorption of water. 
• Positive pressure is the force in the xylem which exudates drops of liquids through the stoma to the outside and also requires hydathodes which help in the process of guttation (exudation of water droplets mostly from the leaves margins). 
  

Food from the leaves is conducted downwards through the phloem in the stem.conduction of water through the xylem occurs generally upwards.

Xylem transports and stores water and water-soluble nutrients in vascular plants. Phloem is responsible for transporting sugars, proteins, and other organic molecules in plants.

Water and salt travel upwards mainly in the xylem and food substances travel up and down in the phloem.

Leaf has epidermis on both of its surfaces. The photosynthetic tissue of a leaf is sandwiched between its upper and lower epidermis and is well supplied by the vascular system. In a leaf, xylem always faces the upper epidermis i.e. the adaxial side while phloem faces both upper as well as lower epidermis.