Question

Describe and discuss the transpiration stream in the plant body. Include in your answer:
-description of the paths that water takes entering the root and the roles of the root endodermis.
-description of water and the principles involved in tension and the problem of cavitation and embolism and how this is resolved.
-description of the release of water and the absorption of carbon dioxide through stomata.
-comparison of the “typical” leaf and the leaf of Bigpod Ceanothus and how this leaf increases photosynthetic surface area while reducing leaf size and water loss through transpiration.

Answer 1

Transpiration is the evaporation of water at the surfaces of leaves and it is also accompanied by the loss of water vapour through the stomata. Due to this a pull is formed. This is called transpiration pull. Due to this transpiration pull, water is taken up from the soil via the roots and it moves through the xylem vessels in a continous transpiration stream.So, the general pathway of the water is: Root---->Stem--->Leaf.

The transpiration stream has many functions. Which includes transporting mineral ions, providing water to keep cells turgid in order to supportthe plant, providing water to leaf cells for photosynthesis, and keeping the leaves cool by evaporation.

The water enters through the root hairs in the root. they are long and thin so they can penetrate between soil particles, and they have a large surface area for absorption of water. Water passes from the soil water to the root hair cell's cytoplasm by osmosis. this happens because the soil water has a higher water potential than the root hair cell cytoplasm. the endodermis is the innermost lyer of the root and it is composed of closely packed cells that have within their walls Casparian strips, water-impermeable deposits of suberin that regulate water and mineral uptake by the roots.

In the transpiration stream, water is passively transported into the roots and then into the xylem. The forces of cohesion and adhesion comes into play and it causes the water molecules to form a column in the xylem. Then the water moves from the xylem into the mesophyl cells, evaporates from their surfaces and then leaves the plant by diffusion through the stomata. it is driven by capillary action. The main driving factor of the transpiration stream is the difference in water potential between the soil and the substomatal cavity caused by transpiration.

Sometimes, cavitation occurs in the xyle of vascular plants when the tension of water within the xylem becomes so high that dissolved air within water expands to fill either the vessels or the tracheids. The blocking of a xylem vessel or tracheid by an air bubble or cavity is called as embolism. The repairing of embolism occurs at night when transpiration is low or absent and root pressure is high. Under such situation, root pressure generates positive xylem pressure which reduces tension in xylem water and allows air to re-dissolve in the xylem solution.

Carbon dioxide enters through the stomata nad water and oxygen exits through the stomata. The opening and closing of stomata is governed by increases decreases of solutes in the guard cells, which cause them to take up or lose water, respectively. Carbon dioxide absorption is needed by the plants because of photosynthesis.

The leaves of the bigpod ceanothus are different from the typical leaves and it is thick and shaped in between oval shape and rectangular shape.In them, the stomata are located in epidermal depressions called crypts. Due to this the transpiration is reduced. An the shape of the leaf effectively increases the surface area for the photosynthesis compared to typical leaves.