Test: Xylem & Phloem Transport (Old NCERT) - JAMB MCQ

According to transpiration pull theory, due to transpiration, the water column inside the plant comes under tension. This is called 'transpiration puli'. On account of this tension, the water column is pulled up passively from below to top of the plant (almost like a rope). A tension of one atmosphere is sufficient to pull water to a height of about 20 meters.

There are two types of vascular tissues in plants, xylem and phloem. Xylem translocation is mainly from roots to aerial parts. It passes water with mineral salts, some organic nitrogen and hormones. Phloem translocates organic substances and some inorganic solutes first from leaves to all other parts of the plant and storage organs. Storage organs re export organic nutrients to those parts which require the same, such as newly formed leaves and fruits.

Cohesion tension theory(Cohesion-tension and transpiration pull theory) explaints the ascent of sap best. It was put forward by Dixon and jolly in 1894.It was further improved by Dixon in 1914. Therefore the theory is alos named after him as Dixon's theory of ascent of sap. Today most of the woekers believe in this theory.

There is a continuous column of water from root through the stem and into the leaves. The water column is present in tracheary elements. The column of water does not fall down under the impact of gravity because forces of transpiration provide both energy and necessary pull. Cohesion, adhesion and surface tension keep the water in place.

Phloem sap consists primarily of water with sugars (sucrose) dissolved in it. It flows from the source or supply end (region of manufacture or storage) to the region of utilisation or sink.

• The food is processed in the leaves through which it first travels to the main stem and then is distributed in other parts of the plant.
• The food reserves are transported to the new tuber maximally and not to the old tubers since old tubers contain adequate food storage. And the brands focus on transferring the food to the newly developing tubers.

Water passage from root hair to xylem and then to other parts is undirectional (unidirectional upwards in xylem). The dirction of movement of organic solutes in phloem can be upwards or downwards i.e. bi-directional.

The sugars synthesized in leaves (as a result of photosyntesis) are translocated downwards, upwards and laterally to storage organs mainly through phloem. These sugars are translocated in the form of sucrose.

In girdling or ringing experiments (Malpighi,1675), a ring fo bark is cut from the stem. It also removes phloem. Nutrients collect above the ring where the bark also swells up and may give rise to adventitious roots. Growth is also vigorous above the ring The tissues below the ring not only show stoppage of growth but also begin to shrivel Roots can be starved and killed if the ring is not healed after some time. Killing of roots shall kill the whole plant clearly showing that bark or phloem is involved in the movement of organic solutes towards root.

Mass flow or pressure flow hypothesis was put forward by Munch (1927,1930). According to this hypothesis, organic substances move from the region of high osmotic pressure to the region of low osmotic pressure in a mass flow due to the development of a gradient of turgor pressure.