19 Investigating Gravitropism in Seeds

Key Idea: The effect of gravity on the direction of root growth can be easily studied using sprout seeds. The direction of root growth will change if the seedling's orientation is altered. The experiment described below is a simple but effective way in which to investigate gravitropism in seedlings. Using the information below, analyse results and draw conclusions about the effect of gravity on the directional growth of seedling roots.

The aim

To investigate the effect of gravity on the direction of root growth in seedlings.

Results

The students took photographs to record changes in growth during the course of the experiment. One seedling at day 5 and 11 is shown below.

Photo 1: This photo was 5 five days after the seed began to germinate. Photo 2: After photo 1 was taken, the cardboard was rotated 90°. Photo 3: This photo was taken 6 days after the seed was rotated 90°.

Hypothesis

Roots will always grow towards the Earth’s gravitational pull, even when the seedling's orientation is changed.

1 2

Shoot beginning

Method

A damp kitchen paper towel was folded and placed inside a clear plastic sandwich bag. Two sprout seeds were soaked in water for five minutes and then placed in the centre of the paper towel. The bag was sealed. The plastic bag was then placed on a piece of cardboard which was slightly larger than the plastic bag. The plastic bag was stretched tightly so the plastic held the seeds in place, and secured with staples to the cardboard. The cardboard was placed upright against a wall. Once the first root from each seed reached 2 cm long, the cardboard was turned 90° degrees. Daily observations and photographs were made of the root length and direction throughout the duration of the experiment. Photos of one seedling from days 5 and 11 are shown right.

Root

3

Day 5, rotated 90° clockwise

Shoot

Root beginning to bend down

Day 5 Day 11

1. (a) What direction did the root first begin to grow in?

(b) Describe what happened to the root when the students rotated the cardboard 90°:

(c) Explain why this occurred:

(d) Predict the result after six more days growth if the students rotated the seedling in photo 3 90° clockwise. Draw your answer in the space right:

2. During the course of the experiment a shoot developed.

(a) In what direction did the shoot grow at first?

(b) In what direction did the shoot grow after rotation 90° (photo 3)?

(c) Why did this occur?