Biology Reference
In-Depth Information
Pteridium and Phyllitis (
Espi ˜ eira et al., 2011
). Interestingly, some fern
species do not follow this previously established pattern. For example, Azolla
caroliniana is an aquatic floating fern which has been described, by means of
pyrolysis and thermally assisted hydrolysis, as not containing lignin in leaves
or in roots (
Nierop et al., 2011
). However, some polyphenolic compounds
have been found, which show resemblance to tannins, and the authors
hypothesized these polyphenolic compounds might have a structural role
similar to lignin (
Nierop et al., 2011
).
Ceratopteris constitute a group of aquatic ferns that possess both trac-
heids and vessels in the conducting tissues (
Carlquist and Schneider,
2000
). The evolutionary and functional significance of the presence of
vessels in an aquatic fern with maximum availability of water still remains
unsolved.
Espi
˜
eira et al. (2011)
studied the lignin composition, through
thioacidolysis and nitrobenzene oxidation analysis, of two species of the
genus Ceratopteris. Ceratopteris thalictroides contained only guaiacyl lig-
nin in their cell walls, whereas Ceratopteris cornuta also contained syr-
ingyl units (
Fig. 7
), reaching 48% of the total lignin building blocks.
However, C. cornuta is not the sole exception among ferns, as syringyl
lignins have been reported in Dennstaedtia bipinnata, a tree fern, by
means of the less diagnostic cupric oxide alkaline oxidation (
Logan and
Thomas, 1985
). Neither the habit nor the ecology of the species seems to
be, at first sight, connected with the distribution of this biochemical
characteristic.
Horsetails are a monophyletic group among monilophytes, with a vascular
system forming an eustela consisting of bundles surrounding a hypothetical
central axis. Equisetum telmateia, only showed lignins of the guaiacyl type
(
Espi ˜ eira et al., 2011
), whereas other Equisetum species, such as Equisetum
fluviatile, have already been described as containing syringyl lignins (
Logan
and Thomas, 1985
).
The presence of non-expected compounds in Equisetum cell walls is also
reflected in polysaccharide composition. The mixed-linkage (1
4)-
b
-
D
-glucan (MLG) which was thought to be restricted to poales cell walls has
recently been found in the horsetail Equisetum arvense (
Sørensen and Willats,
2008
). MLG is thought to have emerged from convergent evolution (
Popper
and Tuohy, 2010; Sørensen et al., 2010
).
!
3),(1
!
b. Spermatophytes (seed plants). The spermatophytes are by far the most
diverse lineage within vascular plants, with about 270,000 living species. All
the spermatophytes share the production of secondary xylem (at least ances-
trally) through the activity of the cambium (
Judd et al., 2008
). The first
spermatophytes appeared in the late Devonian period (more than 360 mya;
