Contact Us Host an Event Volunteer Join

Q. Who is Mr. Smarty Plants?

A: There are those who suspect Wildflower Center volunteers are the culpable and capable culprits. Yet, others think staff members play some, albeit small, role. You can torture us with your plant questions, but we will never reveal the Green Guru's secret identity.

Help us grow by giving to the Plant Database Fund or by becoming a member

Did you know you can access the Native Plant Information Network with your web-enabled smartphone?

Share

Ask Mr. Smarty Plants

Ask Mr. Smarty Plants is a free service provided by the staff and volunteers at the Lady Bird Johnson Wildflower Center.

Search Smarty Plants
See a list of all Smarty Plants questions

Please forgive us, but Mr. Smarty Plants has been overwhelmed by a flood of mail and must take a break for awhile to catch up. We hope to be accepting new questions again soon. Thank you!

Need help with plant identification, visit the plant identification page.

 
rate this answer
156 ratings

Monday - February 27, 2006

From: Delhi, India
Region: Other
Topic: General Botany
Title: Why do sunflowers turn towards the sun?
Answered by: Nan Hampton

QUESTION:

Why do sunflowers turn towards the sun?

ANSWER:

Heliotropism is the term used to describe the tracking of the sun by the flower buds of sunflowers. Other plants also have flowers and/or leaves that track the sun. First, let's discuss how the plants are able to track the sun. There is a collection of specialized cells at the base of the flower bud or leaf called a pulvinus that carry the "motor" cells that enable the plant leaf or flower to track the sun. These cells enlarge or shrink according the turgor pressure from the water inside them. In response to blue wavelength light, potassium ion concentration increases in the "motor" cells on the shadow side of the pulvinus. With the increase of potassium ions the osmotic potential in the cells becomes more negative and the cells absorb more water and elongate, turning the face of the flower to the sun.

It is easy to understand why heliotropism is advantageous for leaves. Turning the leaf perpendicular to the sun provides maximum sunlight to power photosynthesis. At mid-day, to avoid overheating, these same leaves may change their orientation to parallel rather than perpendicular so that the edge of the leaves, rather than the surfaces, are facing the sun. It is thought that one reason the buds of the sunflower (and other flowers) track the sun is that insects are attracted to the warmth from the sun and their presence is necessary for pollination to occur. The warming from the sun is especially important for flowers in cold environments such as alpine and arctic regions. It is interesting to note that mature sunflowers that have been pollinated no longer track the sun. A study done on the snow, or Alpine, buttercup, which also exhibits floral heliotropism, suggests that sun tracking also increases pollen development and germination. You can learn more about heliotropism in an article "Sun Stalker—flowers and sun" by Candace Galen in Natural History, May 1999.

 

More General Botany Questions

Disappearing sunlight in Phoenix, AZ
September 29, 2009 - I live in a condo in Phoenix, AZ with a north facing patio that goes out about 10 feet and is 20 feet wide. During the summer months there is a span of 1 foot in the front that goes the 20 foot length...
view the full question and answer

How to distinguish male and female grape vines in Gage OK.
August 19, 2012 - Are there male and female plants for wild grapes? If so, how do we tell the difference?
view the full question and answer

USDA Hardiness Zone of Rancho Bernardo, CA
October 01, 2009 - What plant zone is Poway, Rancho Bernardo CA?
view the full question and answer

Is Poison ivy always rooted in the ground?
November 11, 2015 - Does Poison ivy on a tree always start at the ground and climb up the tree or can it start producing its vine and leaves by itself at the top of the tree or middle?
view the full question and answer

What to do about grass dying under pin oaks in Iowa
December 10, 2008 - We have 2 pin oaks about 15 years old in our front yard. The grass has started dying out under and around them. What can we do?
view the full question and answer

Support the Wildflower Center by Donating Online or Becoming a Member today.