Can carnivorous plants escape Hobson’s Choice?
Where to elect there is but one,
‘Tis Hobson’s choice—take that, or none.
(fromEngland’s reformation, by Thomas Ward; 1710)
The observations that carnivory appears to be energetically costly, that excess nutrients do not lead directly to
increasing photosynthetic rates in existing leaves or traps,
and that photosynthetic nutrient use efficiency of carnivorous plants is extremely low led Ellison and Farnsworth
(2005) to suggest that botanical carnivory is an evolutionary
Hobson’s Choice—the last resort when nutrients are
scarcely available from the soil. Two new lines of evidence
challenge this interpretation, however.
First, two recent studies have shown that the actual
energetic costs of constructing carnivorous traps are significantly lower than the energetic costs of constructing phyllodia
of carnivorous plants (Osunkoyaet al., 2007; Karagatzides
and Ellison, 2009) or leaves of non-carnivorous plants
(Fig. 8). These data include not only ‘passive’ traps (flypaper
traps of Drosera, pitfall traps of NepenthesandSarracenia)
but also the ‘active’ snap-traps ofDionaea. Thus, carnivorous
traps are relatively inexpensive structures that provide substantial nutrient gain for little energetic cost; thus, it would
take very little photosynthetic gain to yield a substantial
marginal benefit
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from a small investment in carnivory.
Not all active traps are equally active, however. The
snap-trap of the Venus’ fly-trap uses a mechanical trigger
(the mechanism of which is still poorly understood)
passively to release elastic energy stored in the fully
hydrated leaf (Forterreet al., 2005). This relatively cheap
trap is rarely reset; rather, after one (rarely two or three)
captures, the trap senesces (Darwin, 1875). In contrast,
Utricularia’s suction trap is used multiple times, and must
be reset after it captures prey (Lloyd, 1942). Pumping out
water is an energetically expensive process, and how
Utriculariabears this cost has come to light only recently.
Jobson et al. (2004) found that the coxI gene in
Utriculariahas a markedly different structure—with two