David R. Hershey dh321 at PGSTUMAIL.PG.CC.MD.US
Mon Oct 13 22:58:51 EST 1997

Skototropism is a term for negative phototropism that occurs in tropical
vines which germinate on the forest floor and then grow toward a tree to
climb. It is probably an unnecessary term because it is actually just a
negative phototropism. English ivy (Hedera helix) and other vines that
climb by clinging rather than twining have stems that are negatively
phototropic, which enables them to climb walls. It also explains why vines
often grow into buildings through cracks, even though it is darker inside
than outside. Their leaves are positively phototropic, however. Roots also
tend to be negatively phototropic.

There is a cultivar of English ivy grown as a houseplant called 'Erecta'
which has vertical stems and is free standing. Casual observation suggests
that its stems are not negatively phototropic.  An interesting experiment
would be to compare 'Erecta' with regular English ivy. 

Because negative phtotropism is a growth away from light rather than
toward dark, you may want to rethink your treatments. Your basic setup of
having light coming from just one direction is a standard approach. A
cardboard box painted flat black on the inside is often used to assure
that light comes from just one direction. Having 10 replicates is also

Another type of experiment would use a lighttight cylinder (oatmeal box) 
with one or more windows of colored cellophane to determine which color of
light the plant is responding to. For example, in a cylindrical box with
equally spaced red, blue, and green windows, which window will the stem
grow away from? 

David R. Hershey

Snail mail: 6700 Belcrest Road #112, Hyattsville, MD 20782-1340

Adjunct Professor, Biology/Horticulture Dept.
Prince George's Community College, Largo, MD 20772-2199

Email: dh321 at


On Mon, 13 Oct 1997, Gary wrote:

> Hello, I am a student in 7th grade, and I am working on a large project
> dealing with "skototropisms," or the plant growth towards darkness. 
> Although I have found quite enough information on tropisms, I have not
> found any books or articles on skototropisms.  Let me explain my
> project, and maybe you can help me. 
> I will construct three "walls," all the same length and height.  These
> walls will be placed in the same place and exactly the same distance
> from a large window.  The first of these three walls will be constructed
> of black construction paper.  The second will be wax paper.  The third
> will be Saran Wrap.  (Keep in mind that there will be a few experimental
> plants that have no wall near them.)  I will have a minimum of 10
> plants, and they will all be in a row, therefore they will also all be
> the same distance from the walls. These plants will get the same amount
> of water, the same type of soil, be put in the same type of pots, and
> have same amount of sunlight. My hypothesis is that the plants will grow
> towards the darkest of the walls.  
> My problem with this experiment is the plants.  What types of plants
> grow towards darkness? Why? How does this affect their photosynthesis?
> Please help! 
> 						Thanks,
> 						Needs Info

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