I have seen a number of these early papers by Hepburn and others.
I'm just not convinced that the production of proteolytic enzymes in S.
purpurea has been conclusively demonstrated since judging from their
experimental protocols I don't see how bacterial contamination can be ruled
out. For example, in the paper by Hepburn and St. John, the authors refer
to "closed pitchers". The problem is how you define a closed pitcher.
>From the samples I've collected, most pitchers big enough to use in an
experiment may look closed, but the opening is not really sealed, and
bacterial contamination is very likely especially if the pitcher gets wet.
Rainwater or dew can easily travel through the tiny opening carrying
bacteria into the pitcher. Additionally, the anoetid mites found in S.
purpurea pitchers migrate to new pitchers and enter as soon as the tiniest
opening appears. They can easily carry bacteria to a new pitcher. Though
Hepburn and St. John tested for the presence of bacteria and found none,
their conclusion was based on the lack of growth on agar. We now know that
only about 0.1 - 1% of all bacteria found in natural samples can be
detected using cultural techniques. Many won't grow simply because we
don't know what their minimum growth requirements are. The lack of growth
on culture media cannot be accepted as proof of their absence.
The Hepburn papers are from the 1920s, so of course I would not
expect their technical abilities to be on a par with modern standards.
However, because of this, I think it's very important to interpret early
papers like these with caution.
Another problem with some of the earlier work is that pitchers were
studied without an understanding of the importance of the organisms living
in the fluid. Juniper et al. describe the importance of low pH in
suppressing bacterial growth in pitchers. From my own research, this
conclusion doesn't hold. Far more important in regulating the bacterial
community is the presence of grazing organisms such as mosquito larvae,
rotifers, and protozoa. Besides, as pH drops, the species of bacteria
present may shift but the overall density of bacteria tends to remain
pretty constant.
Juniper et al. (1989) also refer to work by Parkes (1980) that
reports on a few enzymes have been cytologically localized in digestive
glands in this species, however, since the reference is a M.Sc. thesis I
haven't as yet been able to acquire a copy. BTW, it's from Monash
University in Victoria, Australia. Maybe one of our colleagues from down
under might have seen this.
I'm pretty convinced that other members of the genus *do* secrete
enzymes. If you know of some really definitive studies of enzyme secretion
by S. purpurea, please let me know. I really like to find out how much the
plant actually contributes to digestion, and I'm always open to new ideas
:-).
Regards,
Liane Cochran-Stafira