Tag: botany

Atmospheric pressure theory was advanced to explain ascent of sap in plants. The protagonists of this theory have assumed that plants are closed systems. When water escapes by transpiration from the surface of the leaves, it is believed that vacuum will be created within the plant body. As the root system is submerged in soil water, with the atmospheric action on the soil water, in order to fill up the vacuum created in the xylem vessels, water just enters passively; thus, the water is translocated upwards. But it was later on concluded that plants are not closed systems but they exhibit openness, for, the gases can diffuse into and out of the plant system with ease and facility. Added to this, atmospheric pressure can support the water to be lifted only to a height of 34 feet; but there are plants which are taller than this and still there is transport of water. Hence it can be concluded that atmospheric pressure could not be the force for ascent of sap.

Imbibitions theory was proposed (1868) and supported by Sachs (1878). According to this theory the ascent of sap occurs due to imbibitonal activity of the cell walls of the xylem elements.

This theory is discarded as it has been experimentally found that water rises up through the lumen of the xylem and not through the walls.

Transpiration is the process of water movement from soil solution into the plants and then to the atmosphere. The direction and intensity of water movement are determined by the difference in the water potential gradient between the two regions. The water potential in the leaf is lower which allows the movement of water in the upward direction. When the water potential is low, the solute potential is high. 

Guttation is the loss of water in liquid form as a dilute solution from the leaf tips. The uninjured leaf tips have hydathodes or water stomata. Under conditions of low transpiration dilute solution of minerals oozes out through them under the influence of increased root pressure. Water potential depends on the number of water molecules in the system. More the number of water molecules, greater will be the water potential. Under the conditions, when there are more water molecules in the system, water potential is expected to be higher. There is more water molecules will be stuffed up inside leaf in the intercellular spaces giving rise to a positive water potential. There is low transpiration and guttation takes place.

A deficiency of 0xygen always causes a corresponding decrease in the rate of mineral absorption. It is probably due to the unavailability of ATP. The increased oxygen tension helps in increased uptake of salts. So the given statement is false.

Imbibition is a special type of diffusion when water is absorbed by solids- colloids causing an enormous increase in volume. Examples include the absorption of water by seeds and dry wood.