However, in early plants, tracheids were too mechanically vulnerable, and retained a central position, with a layer of tough sclerenchyma on the outer rim of the stems.
42.
End walls excluded, the tracheids of prevascular plants were able to operate under the same hydraulic conductivity as those of the first vascular plant, " Cooksonia ".
43.
Because tracheids have a much higher surface to volume ratio compared to vessel elements, they serve to hold water against gravity ( by adhesion ) when transpiration is not occurring.
44.
A tracheid, once cavitated, cannot have its embolism removed and return to service ( except in a few advanced angiosperms that have developed a mechanism of doing so ).
45.
In particular, they did not have tracheids : elongated cells that help transport water and mineral salts, and that develop a thick lignified wall at maturity that provides mechanical strength.
46.
In coniferous or softwood species the wood cells are mostly of one kind, tracheids, and as a result the material is much more uniform in structure than that of most hardwoods.
47.
The different preservation of the Kazakhstan specimens allowed the presence of a central strand of tracheids to be demonstrated, showing that " C . sphaerica " is a vascular plant.
48.
Tracheids end with walls, which impose a great deal of resistance on flow; vessel members have perforated end walls, and are arranged in series to operate as if they were one continuous vessel.
49.
The stem presents nodes unilacunar ( with one trace ), with internal phloem absent, secondary thickening developing from a conventional cambial ring, xylem with tracheids; The sieve-tube plastids are S-type.
50.
The conducting elements of the xylem, the tracheids, were of the so-called'P-type'in which the walls were strengthened by ladder-like ( scalariform ) bars with circular openings between them.
