Cohesion is the connection made between identical molecules, while adhesion is the connection between different ones.
Water molecules have cohesion forces attracting them to each other due to the hydrogen bonds they have.
When water molecules come in contact with some charge surfaces like glass or the lining of a xylem vessel, they stick to it by the adhesion forces .
Because of the charge on the surface, the water molecules are attracted to the other upper part of the surface where there is no water sticking to it. As a result, water molecules climb up a little.
These water molecules are still connected to other water molecules below them by the hydrogen bond (adhesion) and hence tend to pull them up as well. The process repeats till a water column goes up inside the tube.
This process is limited by the diameter of the tube as the bigger the diameter the smaller the water column that can climb up. The reason is that the bigger the column, the more water molecule that are to be pulled up and hence the more the effect of the gravity which is opposite in direction to the capillary action.
Transport of water and minerals absorbed by roots from the soil and organic food synthesized in green leaves are two main examples of plant transport.
Plant transport is mainly of 2 types:
1. Transport of water and minerals absorbed by rots from the soil.
2. Transport of organic food materials synthesized in green pars of the plant, mainly leaves.
Transport of water and minerals- Water is absorbed from the soil by roots mainly in root hair zone. Minerals, present dissolved in water in soil, are absorbed by the roots mainly in meristematic zone.
The mechanism of absorption of water is different from that of minerals. Water is absorbed by passive mechanism; whereas the minerals are absorbed by active mechanism.
Water and minerals absorbed by the root are transported to different parts of the plant through xylem.
Transport of organic food- Leaves containing chlorophyll are the main sites of synthesis of
simple carbohydrates during photosynthesis. The carbohydrates synthesized in leaves and
other green tissues are transported to roots and other non-green parts of the plant through