I have now discovered a FANTASTIC reference:
JOEL, D.M. (yes, we know him!) & FAHN, A. (1975) Calyx glands of _Plumbago
capensis_ Thunb., their structure and secretion, Isr.J.Bot.24:51
An abstract, followed by a more detailed article in Isr.J.Bot.25:127-139
(1976).
The authors write (1975): "The calyx of _Plumbago capensis_ bears colleters
which secrete a sticky substance. Each colleter consists of a "head" and a
"foot" in the center of which a vascular strand occurs...", later
(1976:128) modified into: "A short vascular strand occurs in some of them,
irrespective of their length."
WOW! I cannot aptly describe how excited I am about reading (and now
writing) this. Do you know what that means? With this finding and the
notion that _P.indica_ apparently always *lacks* vascular strands in the
"feet" of the glandular emergences, PLUS the fact that there are species
(like _P.tristis_) which even lack the glandular "head" of the calyx
emergences, we have a complete series of intermediates from bristle to
tentacle WITHIN ONE SINGLE GENUS RELATED TO THE KNOWN TENTACLE-CPS
(_Drosophyllum_, _Triphyophyllum_, _Drosera_)!!! Glabrous calyces or such
with non-glandular emergences can be found in genera related to _Plumbago_
(viz. _Dyerophytum_ and _Ceratostigma_) so these character states could be
regarded as a plesiomorphy in Plumbaginaceae-Plumbaginoideae. The glandular
(?vascularized?) emergence is inherited to _Plumbagella_, a seemingly
derived (annual/temperate-continental; the other genera are rather
subtropical-tropical perennials), monotypic genus. And it *was* possibly
inherited (much earlier in evolution) to the cps mentioned.
This could mean that the present representatives of _Plumbago_ still
"remember" the intermediates of the evolution of an organ which has gained
peculiar imortance in the carnivorous process found in related (but
nowadays widely distinct) families. We can now formulate a phylogenetic
hypothesis based on present day evidence. Of course, the present species
are only descendants from phylogenetically successive steps, they represent
transformed offspring from now extinct "missing links" (now that we know at
least their relatives they are of course no longer missing!).
An interesting theory about the function of these emergences in _Plumbago_
is found in FAHN & WERKER (in KOZLOWSKI, ed.: Seed Biology 1:151-221, 1972)
where it is stated thet the sticky resin facilitates the dispersal of the
fruits (included in the detached fruiting calyces) by animals. I can verify
from observations on _P.indica_ that the dried fruits break off from the
stem very easily at the point where the pedicel is attached to the calyx.
This could mean that:
1. the evolution of a dispersal mechanism
(glabrous->bristly->glandular->tentacular mucilaginous calyx) could have
led to:
2. an evolution of floral biology ensuring cross-pollination by flying
insects, excluding crawlers by the ability to immobilize (i.e. to catch)
the latter, which could have led to:
3. an evolution of traps devoted to the process of capture and (by means of
further transforming the secretory apparatus) digestion of prey. The shift
of the tentacles from floral parts (external surface of calyx lobes, lower
surface of bracts) to the normal leaves by a process called neoteny:
ontogenetically late organs of phylogenetically older forms (like
inflorescence parts in _Plumbago_) are formed at ontogenetically earlier
stages in phylogenetically younger forms (like leaves or even cotyledons in
_Drosophyllum_). This resulted in an increased trapping efficiency (larger
surface, not dependent on flowering) It may be of significance in this
context (as I have mentioned before) that _Drosophyllum_ has the tentacles
on the lower, abaxial leaf surface (like the _Plumbago_ inflorescence
parts).
4. The trapping apparatus was presumably further transformed within
Droseraceae s.str., where mobility of tentacles or leaf parts, the ability
to perceive tactile stimuli, and a higher integration of trapping and
digestive functions in the tentacle head etc. were adopted (i.e.
mutation/selectioned) as further apomorphies. The translocation of the
trapping apparatus to the upper, adaxial leaf surface being another
palaeo-droseracean development.
So we have intermediates, we have the common (synplesiomorphous)
characters, and we have a direction (or several directions after
diversification). This is what I would call an (of course improvable)
evolutionary theory. And all the facts were known and published before a
single gene was sequenced (but apparently nobody -including myself, I
admit- cared until...). Well, the genetical information can now very
conveniently be fit into a scheme in order to construct "rooted" trees.
Kind regards
Jan