(9) Explain how columns of water move up through the roots, xylem and leaves. What effect does changing the thickness of the leaf boundary layer have on water loss? What adaptation can influence the boundary layer to benefit the plant? What type of root structure might be found in plants growing in low nutrient soils that rely primarily on precipitation for water?

A. Evaporation from leaves pulls water upward from the roots thru water-conducting cells. Cohesion and adhesion properties of water contribute to this transportation by helping countering the downward pull of gravity.
The boundary layer once increases in thickness; it will reduce the transfer of water between the leaves and the atmosphere.
Plants growing in low nutrient soils would increase root production in order to maximize the nutrient uptake from the soil.

B. The cohesion-tension theory
1. Inside a leaf, the area not occupied by cells is filled with moist air. Water diffuses from the inside of the leaf to the atmosphere.
2. As water exits the leaf, the humidity of the spaces inside the leaf drops, causing water to evaporate from the menisci that exist at the air-water interfaces.
3. The resulting tension created at the menisci pulls water that surrounds nearby cells, which in turn pulls water out the xylem.
4. Tension is transmitted from water in leaf xylem through stem all the way to the root xylem by cohesion.
5. Tension pulls water from root cortex cells into root xylem.
6. Tension pulls water from soil into roots.
The thicker the boundary layer the greater the resistance to both transfer of heat, and movement of carbon dioxide and water vapor into and out of leaves. The thickness of the boundary layer depends upon the size and shape of a leaf, the presence or absence of hairs (trichomes) on its surface, and above all, on the speed of air movement around the leaf. Large, broad leaves have a thicker boundary layer than narrow leaves or those with deeply indented lobes on the leaf margin. Surface appendages like trichomes increase the thickness of the boundary layer. In still air, the boundary layer can be several millimeters thick, while a moderate wind will sweep it away entirely.

C. Water can move through roots, xylem, and leaves because water has cohesion between itself. Water sticks to water, so as the plant transpires it pull the water through the roots into other part of the plant. The boundary layer is the moisture around the leaf to help with the water gradient. A thick leaf boundary layer has less transfer of water to the atmosphere than a think boundary layer. A plant will adapt to have a think or think boundary layer to best maximize its use of water. Stomata density can help with the boundary layer as can small hairs on the bottom of leaves. Roots growing in poor nutrient soil would need to spend more energy on root production so it can get more nutrients from precipitation

D.As the water transpires from the leaves, water’s cohesive property pulls the chain of water molecules all the way from the roots through the xylem and into the leaves at a very slow pace. The thicker the leaf boundary layer, the higher the water loss. The steepness of gradient determines how much h2o leaves and influences the transfer of heat from the plant to the surrounding environment. Boundary layer reduces steepness of h2o. The root structure for plants in low nutrient soils and rely on water would be shallow and spread across the ground as far as possible. Leaf size and shape influences the thickness and dynamics of the boundary layer. boundary layers tend to be thicker and more intact in larger leaves