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Which Stimulus Causes the Greatest Growth (Tropism) in a Plant?

based on 4 ratings
Author: Christine Ryder Combs
Difficulty of Project

Medium

Grade Level

Middle

Cost

$10-$20

Safety Issues

There are no safety concerns for this project.

Material Availability

Material for this project is readily available at a local gardening store or gardening department.

Approximate Time Required to Complete the Project

2-4 Months 

Objective

To determine what stimulus will cause the greatest growth response in a plant. The goal of the project involves measuring the growth response of a plant as its roots grow in response to water and gravity.

Materials and Equipment

  • 10 Bean plants
  • Potting soil
  • Metric ruler
  • Water

Introduction

A tropism is a growth response a plant undergoes when it is stimulated by an environmental factor. Environmental factors, including light, chemical nutrients, water, and gravity are stimuli that can provoke tropisms in a plant. The plant grows in the direction of the stimulus as hormones inside the stem, root, and leaf systems in a plant aid in the elongation and growth process of the plant toward the stimuli. This investigation explores the relative strength of two stimulants, gravity, and water to compare and evaluate which stimulus will have the greatest affect on the plant.

Research Questions

  • What is a tropism?
  • What causes a tropism?
  • How do plants respond to a tropism?
  • What hormones cause growth responses in plants?
  • What environmental factors affect the growth of plants?

Terms, Concepts and Questions to Start Background Research

  • Auxin
  • Hormone
  • Stimulus
  • Tropism

The diagram illustrates plants set in soil upside down. Barriers of plastic sheaths separate the plants and separate the soil from into regions of saturated soil and dry soil.

Experimental Procedure

  1. Purchase ten fully germinated bean plants.
  2. Record the number of shoots, roots, and buds of each bean plant.
  3. Prepare a bed of soil to plant the ten germinated bean plants by separating the soil to host each bean plant into three different sections divided by a non-permeable plastic sheath.
  4. Place each bean plant upside down and vertically inside the ten prepared holes.
  5. Encase each bean plant with dry soil and a plastic sheath embedded in either side of the plant. Each plant should have their roots exposed above the soil.
  6. Saturate the soil on either side of each plant in the sections separated from the plant by the plastic sheaths. Throughout the entire experiment, this soil should be kept saturated by the addition of water while the soil directly surrounding the plant should be kept dry.
  7. The roots of each plant should begin to grow in response to gravity and the water in the saturated soil. On a daily basis the number of roots and the length and angular direction of the root growth should be measured using a ruler and protractor. To determine which stimulus has the greatest influence on the root growth, the number of roots growing in the direction of the water saturated soil is greater than the number of roots growing in the direction of the dry soil directly beneath the plant, then the water stimulant is greater than the gravity tropism.

Bibliography

  1. http://plantsinmotion.bio.indiana.edu/plantmotion/movements/tropism/tropisms.html
  2. Tropisms by Natalie Sarraute c) George Braziller, June 1967
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