Biology Reference
In-Depth Information
Conifers are the most diverse group among gymnosperms, with about 600
species.
Gnetales contain only 75 living species, divided into three lineages: Ephe-
dra (distributed in the deserts around the world), Gnetum (found in tropical
forests) and Welwitschia (restricted to south-western Africa). Gnetales share
some characteristics with angiosperms, such as opposite leaves, vessels, a
short life cycle and insect pollination. However, gnetophytes never became
a significant component of vegetation and underwent a dramatic decline
during the late Cretaceous period (
Crane, 1996; Crane et al., 1995
).
Until recent years, the general hypothesis was to consider that gymnosperms
contain mainly guaiacyl lignin, with a variable proportion of p-hydroxyphenyl
units. In gymnosperms, the presence of S lignin has been widely reported, but
in residual amounts, representing 1-2% of total lignin (
Gross, 1980
). However,
G. biloba suspension cell cultures contain S lignin in a proportion as high as
11%, as detected by thioacidolysis and nitrobenzene oxidation (
Fig. 8
).
However, the woody tissues of Ginkgo are not able to synthesize S lignin
(
Novo-Uzal et al., 2009
). A species belonging to Gnetopsida, Ephedra viridis,
has been reported to contain 60% S units and only traces of H units, a pattern
analogous to angiosperms (
G´mez Ros et al., 2007a
). This similarity has been
linked to the presence of angiosperm-like vessels in gnetales (
Gross, 1980
). The
xylem in gnetales emerged through convergent evolution (
Chaw et al., 2000
).
Angiosperms. Angiosperms (flowering plants) contain 257,000 species and
account for most of the green plants, land plants and seed plant diversity.
This monophyletic group underwent a major radiation starting in the early
Cretaceous period (
Crane et al., 1995; Doyle and Donoghue, 1993
). The
oldest angiosperm fossils are pollen grains from 135 mya. Angiosperms
developed some new and important features that probably had a deep impact
on flowering plants diversity: seeds covered with a carpel, reduced female
gametophyte, double fertilization and the presence of vessels as a water-
conducting system. However, the first angiosperms did not exhibit vessels,
as suggested by the presence of tracheids in the basal angiosperm Amborella
(
Field et al., 2000
). Angiosperms did not become widespread until the early
tertiary period, when the radiation of modern birds and mammals occurred;
the evolution of large colourful fruits and seeds are associated with the
evolution of these groups.
Angiosperm lignins are constituted mainly of guaiacyl and syringyl units,
with a minor proportion of p-hydroxyphenyl units. Thus, angiosperms,
whose radiation came after divergence between angiosperms and gymnos-
perms, have always been considered as the sole phylogenetic group containing
syringyl lignin. Nevertheless, in recent years many reports have seemed to
