En EspaŅol

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
Can't find the answer in our existing FAQs, submit a question to Mr. Smarty Plants.
Need help with plant identification, visit the plant identification page.
 
rate this answer
138 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

Is Esperanza a deciduous or an evergreen plant?
March 08, 2009 - I've read that Esperanza/Tecoma Stans is an evergreen. I planted one last year that seemed very healthy, but it dropped its leaves in late fall and looks (at least) dormant now. Will it come back o...
view the full question and answer

What caused purple heartwood in my Tuliptree?
June 15, 2009 - My Tulip tree was hit by lightning and all bark from the base of the tree up to 50 feet was blown off. The tree also sustained a significant crack through the trunk. When the tree was cut down, we...
view the full question and answer

Fragrant Texas wildflowers from Waco TX
July 28, 2013 - Hi, I'm interested in looking at any Texas Wildflowers which have attractive aromas which I can distill essential oil from. Any ideas? Thanks
view the full question and answer

Petals on Black eyed Susans not developing from Austin
September 04, 2012 - I just read Barbara Medfords response to undeveloped petals on perennial black eyed susans and was disappointed not to find a better explanation. I have had the exact same thing happen to mine, and I...
view the full question and answer

Effectiveness of house plants for removing household air pollution
January 30, 2006 - I have heard that there is a combination of house plants that will increase the oxgyen in your home. Could you please tell me the names of these plants? I think there are a group of four of them. ...
view the full question and answer

Smarty Plants's Facebook profile Support the Wildflower Center by Donating Online or Becoming a Member today.

Mr. Smarty Plants wants you to be his Facebook friend. Click the Facebook icon to add yourself to Mr. Smarty Plants list of friends.
E-NEWSLETTER | BECOME A MEMBER | DONATE NOW | MEDIA | SITEMAP
© 2014 Lady Bird Johnson Wildflower Center