Re: Are philodendrons really skototropic?

Complete Question:

Is it true that philodendron seedlings in the jungle seek the trunks of trees 
by growing toward the DARK? I read this somewhere, and the source was emphatic 
that they were not growing AWAY from light but TOWARD the dark. This does not 
seem possible since darkeness is like a vacuum - the absence of something 
rather than a physical property. Experiments were supposedly done where the 
highest light intensity was at 90 degrees to the area of least light intensity 
and the plant grew directly toward the dark spot. Can this be true? Also, 
supposedly, this ‘scototropism’ reverts to normal phototropism after the 
philodendrons climb the tree trunks the reach the canopy  of the forest. What 
is the explanation for this behavior?

Reply:

I agree with your assessment that skototropism is actually just a negative 
phototropism, and the botany text by Kaufman et al. (1989) also calls it a 
negative phototropism. The term was possibly coined because scientists are fond 
of new terminology, and a scientist who coins a new term memorializes 
themselves in a way. There doesn't seem to be a need for the term given that it 
is inaccurate and negative phototropism covers the phenomenon. There has been 
an effort to replace the term geotropism with the more accurate gravitropism 
because the response is to gravity, not to the earth (geo). It would also seem 
logical to use the term negative phototropism in place of skototropism.

Stems of many vines have to be negatively phototropic or they would grow away 
from walls, tree trunks and other solid objects rather than toward them. I have 
seen English ivy (Hedera helix) exhibit a negative phototropism by growing into 
a dark basement through a crack in a foundation and continue to grow away from 
the basement window, rather than toward it, even though the basement window was 
the only source of light. 

References

Kaufmann, P.B. et al. 1989. Plants: Their Biology and Importance. New York: 
Harper and Row.

Strong, D. R. and T. S. Ray. 1975. Host tree location behavior of a tropical 
vine ( Monstera gigantea) by skototropism. Science, 190: 804--06.