Biology Reference
In-Depth Information
single-celled conduits with lignified and ornamented walls, either banded or
pitted (
Doyle, 1998; Kenrick and Crane, 1997
).
S-type tracheids (named after the genus Sennicaulis) were present in the
members of Rhyniopsida, a group where initial diversification of vascular
plants is hypothesized to have occurred (
Friedman and Cook, 2000
). They
exhibited annular or helical thickenings made of spongy material. Adjacent
to this degradation-prone layer, S-type tracheids showed a thin degradation-
resistant layer next to the lumen (
Friedman and Cook, 2000; Kenrick and
Crane, 1991, 1997
).
G-type tracheids (after the genus Gosslingia) were found in the zostero-
phylls, an extinct group ancestral to extant lycophytes. These tracheids
showed annular or helical thickenings constituted by two different layers:
adjacent to the cell lumen there is a lignified zone with an unlignified hollow
core (
Friedman and Cook, 2000
).
P-type tracheids (after the genus Psilophyton) were typical of the earliest
members of euphyllophytes, which include modern ferns and spermatophytes.
These tracheids showed bordered pits with a degradation-prone layer
surrounded by a degradation-resistant layer (
Friedman and Cook, 2000
).
As regards these three types of tracheids, the degradation-resistant
layer seemed to be lignified, unlike the degradation-prone layer. However,
studies on tracheary elements have been done in extant spermatophytes, so
the available information is partial (
Friedman and Cook, 2000
). In extant
pteridophytes, an unlignified core at the base of secondary cell wall thicken-
ings has been described, which could be the missing degradation-prone layer
in early fossil tracheids (
Bierhorst, 1960; Kenrick and Crane, 1991
).
Developing a vascular system, together with the presence of stomata
(which are only absent in liverworts among land plants), allowed plants
to become homoiohydric rather than poikilohydric (
Sperry, 2003
). Plants
were able to regulate their water dependence, which involved a loss of
desiccation tolerance in vegetative cells (
Oliver, 1996
). Due to the change
into homoiohydric, plants were allowed to develop greater sizes, with
obvious advantages of escaping from shade and improving long-distance
aerial dispersal. This change also required a shift from hydrostatic support
to a lignified cell wall-based support (
Niklas, 1994; Sperry, 2003
), which
was accompanied by the evolution of sclerenchyma tissue surrounding the
vascular system (
Raven, 1987
). The diameter of tracheids increased gradually
until the Devonian period, reaching 80
m
m(
Niklas, 1985
) but, due to physi-
cal restrictions, tracheids could not be wider (
Zimmermann, 1983
). This
problem was resolved by increasing the number of water-conducting cells,
the so-called vessels, allowing greater diameter and length than tracheids
(
Zimmermann, 1983
). Tracheids have a double function: they participate
