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both in water transport from roots to leaves and in mechanical support. In
the case of vessels, typical of angiosperms, there are two different types of
specialized cells: the vessel elements, which are responsible for water trans-
port, and the fibres, which provide mechanical support ( Pittermann, 2010 ).
This division of functions provides vessels with higher conductivities and
makes them more efficient at maintaining such conductivities ( Sperry, 2003 ).
In terms of development and physiology, the change from a tracheid-
producing cambium to one forming vessels may only have required a change
in auxin and gibberellin levels during differentiation ( Aloni et al., 2000 ).
Vessels evolved independently in five different groups: selaginellales,
horsetails, ferns, gnetophytes and angiosperms ( Baas and Wheeler, 1996 ;
Fig. 2 ), indicating the important advantages vessels represent in water
conduction. Despite the advantage represented by vessels, many widespread
plants lack vessels, including the gymnosperms. Some authors have sug-
gested that plants containing short tracheids may have greater cavitation
resistance because they can refill more easily ( Ewers, 1985; Sperry, 2003 ),
which could explain the presence of gymnosperm woods in areas where
angiosperms have trouble surviving. Even so, the number of angiosperm
species far exceeds gymnosperm species (255,247 angiosperms vs. 831 gym-
nosperms). The number of gymnosperms species is even lower than that of
fern species (11,380) ( Palmer et al., 2004 ). Given that gymnosperms and
angiosperms may exhibit similar hydraulic efficiencies ( Pittermann et al.,
2005 ), it is surprising that the number of fern species exceeds that of
gymnosperms. All gymnosperms have tracheids as the water-conducting
system but, whereas most ferns have tracheids, some species such Pteridium
and the aquatic fern genus Ceratopteris are known to possess vessels
( Carlquist and Schneider, 2000, 2001 ). Tracheids in ferns only confer a
water transport system and are released from the need to provide mechani-
cal support, which is provided by a ring of sclerenchyma fibres peripheral to
the central axis of the frond ( Rowe et al., 2004 ). This implies that tracheids
can be wider in ferns than in gymnosperms without the need for a rein-
forced cell wall. The wider and longer conduits of ferns allow a high degree
of hydraulic conductivity in the xylem, which can support the water loss in
the absence of an active hormone-controlled stomatal closure ( Pittermann
et al., 2011 ).
B. COMPOSITION AND CELLULAR DISTRIBUTION OF LIGNINS
Lignin is deposited mainly in tracheids, vessels, fibres of the xylem and
phloem and sclerenchyma ( Ros Barcel ยด , 1997 ). However, lignin composition
varies among species, phylogenetic groups, cell types, development stage and
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