● As `color{violet}("various ions")` from the soil are actively transported into the `color{violet}("vascular tissues")` of the roots, water follows (its potential gradient) and increases the `color{brown}("pressure")` inside the `color{violet}("xylem.")`

● This positive pressure is called `color{brown}("root pressure,")` and can be responsible for pushing up water to `color{violet}("small heights")`
in the stem.

● `color{violet}("Effects of root pressure")` is also observable at night and early morning when evaporation is low, and excess water collects in the form of droplets around special openings of veins near the `color{violet}("tip of grass blades")`, and `color{violet}("leaves")` of many `color{violet}("herbaceous parts.")`

● Such `color{violet}("water loss in its liquid phase")` is known as `color{brown}("guttation.")`

● `color{violet}("Root pressure")` can, at best, only provide a `color{brown}("modest push")` in the overall process of water transport.

● They obviously do not play a major role in `color{violet}("water movement")` up tall trees.

● The greatest contribution of `color{violet}("root pressure")` may be to `color{brown}("re-establish the continuous chains")` of water molecules in the `color{violet}("xylem")` which often break under the `color{violet}("enormous tensions")` created by transpiration.

● `color{violet}("Root pressure")` does not account for the majority of `color{violet}("water transport;")` most plants meet their need by `color{brown}("transpiratory pull.")`

● Choose a `color{brown}("small soft-stemmed plant")` and on a day, when there is plenty of `color{violet}("atmospheric moisture,")` cut the stem horizontally near the base with a `color{violet}("sharp blade,")` early in the morning.

● You will soon see drops of solution ooze out of the cut stem; this comes out due to the `color{brown}("positive root pressure.")`

● If you fix a rubber tube to the cut stem as a sleeve you can actually collect and measure the rate of `color{brown}("exudation,")` and also determine the `color{brown}("composition of the exudates.")`

● Despite the absence of a `color{violet}("heart or a circulatory system")` in plants, the flow of water upward through the xylem in plants can achieve fairly high rates, up to `color{brown}("15 metres per hour.")`

● A long standing question is, whether water is `color{brown}("‘pushed’")` or `color{brown}("‘pulled’ ")` through the `color{violet}("plant.")`

● Most researchers agree that water is mainly `color{violet}("‘pulled’")` through the `color{violet}("plant,")` and that the driving force for this process is `color{brown}("transpiration")` from the `color{violet}("leaves.")`

● This is referred to as the `color{brown}("cohesion-tension-transpiration pull model")` of water transport.

● Water is `color{brown}("transient")` in plants.

● `color{brown}("Less than 1 percent")` of the water reaching the leaves is used in `color{violet}("photosynthesis")` and `color{violet}("plant growth.")`

● Most of it is `color{brown}("lost")` through the `color{brown}("stomata")` in the leaves.

● This water loss is known as `color{brown}("transpiration.")`

● One has studied transpiration in an earlier class by enclosing a `color{brown}("healthy plant in polythene bag")` and observing the droplets of water formed inside the bag.

● One can also study water loss from a leaf using `color{brown}("cobalt chloride paper")` , which turns colour on absorbing water.