Did you ever think about how water and minerals ions travel from roots to the leaves? And what happens to them in the leaves? In this lesson, we will be learning about the transport in flowering plants.
They are two conducting tissues present in the plants which transport the water and sugars throughout the plant. Xylem vessels transport water from the root hair cells to the leaf and then out of the leaves. It's a unidirectional (1 way) movement. The other one is phloem, which transports sugars, mainly sucrose. It's a bidirectional (2 way; from roots to leaves and from leaves to roots) movement.
Translocation is the movement of sucrose and amino acids through phloem. It's a bidirectional pathway where sugars can move leaves to roots or to other organs. These sugars and amino acids are sent to those parts of the plant where they are stored such as roots, stem fruit and flowers. These parts of the plant are called sinks. To discuss the structure of phloem, it has sieved end walls and have companion cells beside them which provide energy and help in better functioning of the phloem.
Transpiration stream :
The stream, in which water moves from roots to the leaves in xylem is called transpiration stream. Xylem vessels are elongated, hollow, strong and lignified tubes with no protoplasm and no end walls so the water can move more efficiently. The water first is absorbed by the root hair cells by the process of osmosis. Then the water moves to root and then to the root cortex by maintaining the gradient for osmosis. This process is basically called root pressure when the water absorbed maintains the turgidity of the sap vacuole for osmosis. When water travels to the root xylem, capillary action has to contribute so the water can travel up. It moves water from xylem of the root to xylem of the stem, with the help of cohesion (attraction of water molecules with each other) and adhesion (attraction of water molecules with xylem's walls). Throughout this process, transpirational pull helps to suck the water. Transpiration is basically removal of water from aerial parts of plants through stomata. This removal causes a suction of water from roots to the leaves. This removal causes the need for water. However, excess transpiration can cause wilting.
Wilting:
Wilting is that due to the excess loss of water, the plant doesn't remain turgid and starts becoming flaccid. When guard cells are flaccid, stoma won't open and the rate of transpiration would decrease. Also, this closing causes less absorption of carbon dioxide for the process of photosynthesis and hence, rate of photosynthesis would decrease. To conclude with, transpiration is important as it draws water and mineral salts from roots to the leaves in a transpiration stream. It evaporates the water to reduce the heat in the plant so it can stay cool.
Factors affecting the rate of transpiration:
Factors that affect the transpiration's rate are temperature, wind, sunlight, and humidity. If temperature increases, then enzymes involved in photosynthesis would start working at a faster rate and the rate would increase which would make the plant more turgid. When the guard cells are turgid, stoma would open for transpiration and other exchange of gases.
Wind and humidity are related to each other because wind reduces the humidity around the plant which creates a steep concentration gradient for the process of diffusion. When wind blows, humidity decreases and water molecules diffuse out.
Sunlight is another factor. When there's more sunlight, the rate of photosynthesis would increase and the guard cells would become turgid and the stoma would be made so the water vapors can diffuse out.
Summary:
In conclusion, xylem and phloem are the conducting tissues present in plants that transport the water and sugars around for the better functioning of the plant.
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