Gravitropism hands-on device Sarantos Oikonomidis, Vassilis Grigoriou, Nikos Kaponikolos, Stavroula Kanavi, George Kalkanis. Science, Technology and Environment Laboratory, Pedagogical Department P.E., University of Athens, Greece

Introduction We represent an apparatus with which we can study the phenomenon of gravitropism; that is the attribute of plants to grow against the effective gravity. We also mention the difficulties and the solutions that we came in. At last, we propose a didactical approach with specific student objectives that our exercise may fulfil.

What is gravitropism? Gravitropism is simply a plant’s response to gravity. Positive gravitropism: When the roots grow with gravity. Negative gravitropism: When shoots grow against gravity. The research of gravity sensing is very exciting right now because the exact mechanism is not known.

Theory According to theory, rotating seeds were supposed to grow parallel to the resultant of their weight and the centripetal force. This means that seeds should not grow vertically to the ground but to make an angle.

Variables Effective gravity  Angular velocity  Distance of the flower pot from the rotating axle dependent Other tropism  Sun  Wind (low angular velocity) independent

The experimental device Our device consists of an electric motor which is connected with a rubber band to a round metal disc. The electric motor is placed in a wooden base. A piece of steel pipe 1m long, is welded at the centre of the metal disc. At the other end of the steel pipe, we placed vertically an 2m long wooden axle. At the wooden axle we put three plates with seeds.

The experimental device Axes’ assembly Operation Final Assembly

The experimental device The metal disc rotates with an angular velocity of 1rad/s approximately. This means that the wooden axis with the seeds also rotates with the same velocity. As a result, the resultant force at the seed is the gravity and the centrifugal force.

Testing and difficulties Selection of seeds. Selection of material to deposit seeds. Motor selection. Transmit motion to axle. Select the distance of the flower pot from the rotating axle. Rotation speed.

Experimental Results Our experimental results comply with our expectations. The seeds did not grow vertical to the ground, but they formed an angle towards the rotation axle.

Educational proposal The experiment should consist of two parts; 1.One with the aforesaid setup, 2.One with the flower pot motionless. Thus, it would be feasible to make comparisons between the two different ways of grow.

Student objectives This exercise may fulfil a large variety of student objectives cognitive and psychomotor.

Cognitive student objectives Students are expected: To predict the normal response of a seed when grown down with gravity. To predict the response of a seed when it is being rotated. To compare the growth patterns of seed that have been rotated to those that are growing down with gravity. To compare the growth and angle of orientation of seeds according to angular velocity of the device. To identify why it is important for a plant to have gravity sensing.

Psychomotorstudent objectives Psychomotor student objectives Students are expected: To explain why it is important to have a control for an experiment. To measure the growth and change in seed angle over time. To interpret data generated in tables and graphs.

Suggestions As a future enhancement, we propose an experiment where students can study various factors that determine seeds’ grow such as light and orientation. With this they will have to handle more variables and study the importance of each one at the seeds grow. The required changes in the experimental setup are small, as four electric bulbs are needed and a mechanism that turns the vertical axis in various angles.