Distribution of Cutan in Modern Leaves - Biopolymers: A Molecular Paleontology Approach - page 36

potential source of the aliphatic component of the fossil leaf macropolymer. Cutin
can become crosslinked (Deshmukh et al. 2003 ) or intermolecularly ether linked
(Schmidt and Schönherr 1982 ) making it diagenetically stable. As cutin contains
C 16 and C 18 units these could be the source of the corresponding short chain n -alkyl
component observed in the fossils. As previously suggested by Collinson et al.
( 2000 ) and Finch and Freeman ( 2001 ), long chain waxes (Eglinton and Hamilton
1967 ; Walton 1990 ) can be incorporated into the fossil geomacromolecule to
account for the higher molecular weight long chain n -alkanes and n -alkenes gener-
ated during pyrolysis. Thus, while selective preservation of the biopolymer cutan
cannot explain the preservation of fossil leaves, their aliphatic composition may be
attributed to in situ polymerisation (Briggs 1999 ; Stankiewicz et al. 2000 ) of extract-
able and non-hydrolysable lipid components resulting in an aliphatic geopolymer
(not inherited from the biopolymer cutan), a process that may be of widespread
importance in the fossilization of organic materials.
The Ecology and Physiology of Plants
with Cutan-Containing Leaves
There is no one-to-one correlation between the occurrence of cutan and leaf succu-
lence or thick evergreen leaves with thick cuticles. The leaves of the cutan-containing
Agave and the stems of the cactus Cereus are succulent, but the leaves of Kalanchoe ,
which are also succulent, lack cutan (Finch and Freeman 2001 ; not based on oxidative
isolation, contra Boom et al. 2005 ). The cutan-containing leaves of Clivia , Clusia and
epiphytic orchids are relatively fl eshy but much less succulent. Leaves of Podocarpus
and Prunus laurocerasus are evergreen and relatively thick with thick cutan-contain-
ing cuticles but leaves that lack cutan, such as Citrus limon , are similar in texture. The
leaves of Pinus are evergreen with thick cuticle but lack cutan; nonetheless they are
needle-like with a very low surface area to volume ratio, an adaptation to drought.
Cutan occurs in some CAM plants but it is absent in others, and it is present in
plants using the C 3 photosynthetic pathway. Clusia rosea and C. multifl ora both
contain cutan (Boom et al. 2005 ); the former exhibits C 3 or CAM plasticity, and the
latter is an obligate C 3 plant (Herzog et al. 1999 ; Lüttge 1999 ). Thus, some CAM
plants, some succulent plants and some plants with thick cuticles do not contain
cutan. A clear correlation cannot be made between any of these attributes and the
presence of cutan and, on the basis of the small sample currently available, it is not
yet clear that the presence of cutan in cuticles is an adaptation for drought resistance
( contra Boom et al. 2005 ).
Most importantly, this study shows that the highly aliphatic signal in fossils is not
due to the selective preservation of cutan and we suggest that it derives from in situ
polymerisation of more labile aliphatic components such as waxes, internal lipids,
and cutin. Thus neither the ecology and physiology of plants with cutan-containing
leaves nor the presence or absence of cutan in leaves exert any major bias on the
preservation of leaves in the fossil record.