Name __________________________________________________________________________ Period _________

How Do Hormones Affect Plant Growth?


Living systems produce many chemicals that influence or regulate specific cell processes in organisms.  One type of chemical is called a hormone.  Hormones are a type of protein that regulates chemical process and reactions in plants and animals.   For instance, in humans, a hormone called adrenaline surges through the body when confronted with fear; this response is called the “flight or fight” response. 

Plant hormones, in general, are growth regulators.  Some hormones affect the length of cells, and thereby the length of plant parts, such as stems or roots.  Other hormones may cause both rapid cell division and elongation of cells. 

Plant hormones include gibberellins and auxins.  Gibberellins stimulate the production of new cells and promote the elongation of cells.   Auxins are produced in the root tips and stem tips of complex plants. 

The hormones called auxins bring about the responses made by plants to stimuli such as light or gravity.  Examples of plant responses to auxins are: turning of leaves toward light, turning downward into the ground by roots, or turning upward by shoots and stem tips.  Such responses of plants to environmental stimuli are called tropisms.  They contribute to the survival of the plant.

Tropisms are named for the stimulus that causes the response.  For example, a response to light is called a phototropism, or a response to gravity is called a geotropism.  Tropisms occur because environmental stimuli cause the hormone, auxins, to become unequally distributed in the plant tissues.  The tissues of plant organs such as stems and roots then grow at unequal rates, causing turnings in the stems or roots.  A tropism may be positive, a turning toward the stimulus, or it may be negative, a turning away from a stimulus. 


·         Record the positions and directions of growth of new roots and stems for four corn seeds over a period of time.

·         Make a hypothesis that explains how the direction an seed faces influences the growth of the stem and roots

·         Compare the direction of growth shown by young corn roots and stems when positioned in different orientations to the force of gravity. 



Petri dish

Cotton balls


Corn seeds (soaked)






1.      Obtain a Petri dish. 

2.      Use tap water and thoroughly soak several cotton balls. 

3.      Place cotton balls in the Petri dish, until it is completely full.  Make sure you can close the petri dish over the cotton balls. 

4.      Obtain 4 soaked corn seeds.  Place one seed in each of the four quadrants in the appropriate position.  See figure #1. 

5.      Close the Petri dish and tape it shut. 


7.      Label the Petri dish “top” and “bottom.”  Make sure you look at figure #1 to see which seed is at the top and which is at the bottom. 

8.      Label your Petri dish with your group number and period. 

9.      Make a hypothesis that explains the relationship of the position and direction of a seed to the growth of its roots and stem.  Write your hypothesis in the space provided. 

10.  Place your dish in the appropriate place as directed by your teacher. 

HYPOTHESIS: _______________________________________________________________________________ ____________________________________________________________________________________________________________________________________________________________________________________





1.       What is the direction of root growth from the seed with the pointed end facing:

a.      Down? _____________________________________________________________________

b.      Up? _______________________________________________________________________

c.       Left?  ______________________________________________________________________

d.      Right?  _____________________________________________________________________

2.      What influence does the direction in which a seed is pointing have on a direction of root growth? ____ ___________________________________________________________________________________ ___________________________________________________________________________________

3.      Do roots seem to grow toward or away from the force of gravity?  _____________________________ ___________________________________________________________________________________  ___________________________________________________________________________________

4.      Is this a positive or a negative response to gravity? __________________________________________

5.      What is the direction of stem growth from the seed with the pointed end facing:

a.      Down? _____________________________________________________________________

b.      Up?  _______________________________________________________________________

c.       Left?  ______________________________________________________________________

d.      Right?  _____________________________________________________________________

6.       What influences does the direction in which a seed is pointing have on the direction of stem growth? _____________________________________________________________________________________ _____________________________________________________________________________________

7.      Do stems grow in the direction toward or away from the force of gravity? _________________________ _____________________________________________________________________________________

8.      Is this a positive or a negative response to gravity? ____________________________________________

9.      What is affecting the way roots and stems grow in response to the environmental stimuli of gravity? _____________________________________________________________________________________ ______________________________________________________________________________________ ______________________________________________________________________________________

10.  Do roots and stem cells respond to the presence of auxins in a similar or different manner?  Explain _____ ______________________________________________________________________________________ ______________________________________________________________________________________