ISU Wk 10 How the Change in Mass Affects the Rate of Transpiration Project Summary

Plant Form and Function: Transpiration

A vascular plant must overcome the force of gravity to transport life-sustaining water from the soil throughout its tissues. The particular cohesion-adhesion properties of water are what allow plants to pull water up from the roots against gravity and to distribute it to the leaves and other parts of the plant. This process is known as transpiration, the movement of water and minerals through the xylem. Transpiration is a central mechanism by which plants deal with external conditions and internal needs.

Transpiration occurs when the drier air outside pulls out the water vapor inside the leaf. The water vapor from the air spaces inside the leaf is replaced by the evaporation of water from the mesophyll cells. As water molecules evaporate, they exert a pulling force on the rest of the water in the cells. This pulling force is due to two unique properties of water: water molecules tend to stick to one another (cohesion) and they also tend to stick to the sides of the cells (adhesion). Thus, evaporation pulls water out from the cells, creating tension on the xylem cells to replace this lost water. In this manner, water is pulled up through the xylem as explained by the cohesion-tension hypothesis.

Transpiration rates are affected by a variety of factors. For one, specialized cells called guard cells surround leaf pores called stomata that open and close depending on the environmental conditions surrounding the plant. When water is in excess, light is low, and the temperature is cool, the stomata remain open; when water is scarce, light is in abundance, and the temperature is warm, the stomata are closed. Stomata help regulate water loss from the plant and are critical parts of the transpiration process. Plants in different environments experience different conditions thus have different adaptations that prevent excessive water loss, while still allowing for transpiration to occur.

            In this lab you will design, and your TA will carry out, an experiment in which you ask how do transpiration rates differ in “condition x” compared with “condition y”? By comparing transpiration rates among plants and among environmental conditions, we can understand the process of transpiration and the adaptations involved in transpiration. Your TA will collect the data, but you will analyze the data and write an illustrated summary describing your findings.

Learning Goals of this Lab Activity

1. Design an experiment that allows you to identify the effect of a manipulation of your choosing on the rate of transpiration.

2. Convey findings by authoring an abstract that summarizes your study using references, and includes a figure, caption, and statistics.

DETAILED ASSIGNMENT

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