Actually, I think the pitcher contents do sometimes smell pretty bad. N.
rafflesiana, at least, in the wild have a very thick sickly-sweet pitcher
fluid and in cases where a pitcher has a big snail or something the
contents smell awful. Some species seem to keep a very low pH (high acid)
in the pitcher, so that should keep down bacteria. A paper by Higashi et
al. (1993, J. Plant Res. 106:47-54) suggested that the enzymes secreted by
the pitcher work best at low pH (about 3.0), while the enzymes secreted by
the bacteria in the pitcher work best at high pH (7.0-9.0). I thought it
might be possible that pitchers cycle between pH states, first letting the
bacteria do the work, then dropping the pH, killing the bacteria, and
eating them and the products of the prey breakdown. I found no evidence of
cycling in wild plants in Singapore. Higashi et al. were working with a
hybrid Nepenthes (N. hybrida), so maybe their results reflect some
breakdown of the feeding strategy of "true" species.
As to the top dying back when the pitchers dry out or age, I think
this is a very important observation. I would couple that with the logic
that if the plant kept filling the pitcher over its lifespan, how would it
know when the pitcher is full? In the field, I often saw pitchers with a
hole bored in the side. If the pitcher kept adding fluid, it might work
something like a tap left running (not good!). Evaporation of the fluid or
punctures of the pitcher seem more likely occurrences in the wild than the
pitcher being emptied out; thus it would seem that a good evolutionary
strategy would be to fill the pitcher once and as the fluid decreases, open
the opportunity to catch rainwater by retracting the lid. If that doesn't
work, as in if there is a prolonged drought, then maybe the plants can't
really afford the water loss through the pitchers anyway. They dry up and
the plant makes new one's when conditions are more favorable or the plant
relies more on shaded pitchers than those in the hot, dry sun. The point
that Perry brought up that emptied pitchers might "know" to fill up again
if prey is added is extremeley interesting.
So many questions...
Chris
-----------------------------------------------------------
Chris Frazier
Dept. of Biology, UNM
Albuquerque, NM, USA 87131
(505) 277-0683
Homepage: http://redtail.unm.edu/