After reading your query in full I feel that it is a genuine question that requires detail answer .
What you call as xylem tubes are in the scientific terminology are called xylem vessels .
The the question is if xylem vessels are present then why xylem tracheids ?
1 ) Xylem tracheids are more primitive than the xylem vessels.
when the plants moved on the land from the water ; they needed water transporting tissue . This tissue first appeared in the form of Tracheids .
That is the reason why the primitive land plants like Pteridophytes ( ferns , horse tails are the representatives ) show only the tracheids in their xylem and no vessels at all !
This feature ramained for a long time and was / has been retained in the Gymnosperms as well . ( Cycads , pines and conifers are the representatives ).
But gradually , a change occured and polygonal tracheids became circular in out line and with larger diameter .
The cross walls between any two adjescent trachids disappeared ; This facilitated better transportation of water .
Thus , xylem vessels ( Xylem tubes ) appeared .
Naturally , therefore , they are seen only in the modern land plants or flowering plants or the Angiosperms .
You can compare this with invention of the motor car followed by constant improvement in its design for better and still better transportation .
More primitive than ptridophytes are the mosses or bryophytes . They lack such a true transporting tissue .
But at the center of moss stem you can still see origin of such tissue !
click on the link below to see it =
http://www2.puc.edu/Faculty/Gilbert_Muth/phot0131.jpgGnetum is a Gymnosperm , so expected to show tracheids .
But it is on the higher evolutionary stage so shows more vessels than tracheids !!!
In fact this group is called a connecting link between Gymnosperm and the modern Angiospems .
http://www.jstor.org/pss/2445861click on the links below to see a section of Gnetum wood and see almost circular and with large diameter that are xylem vessels =
http://sherwincarlquist.com/images_500/gnetales-gg.jpghttp://sherwincarlquist.com/images_500/gnetales-hh.jpgSherwincarlquist wrote = "' ... But most importantly, I saw that wood of Gnetales is essentially conifer wood in which vessels had developed, and it has nothing to do with angiosperms. I was the only one at the day-long symposium on Gnetales at San Diego who presented the conifer concept for Gnetales, and stated it clearly, citing also some early DNA work by Hasebe et al. The remaining participants seemed determined to show or agreeable to the idea that Gnetales were ancestors of angiosperms. Subsequent DNA work has shown that Gnetales are definitely either within conifers or adjacent to conifers. But aside from being ahead of my time in affirming the correct position of Gnetales, I learned an enormous amount about the woods of the group, summarized in a paper published in a special issue of International Journal of Plant Sciences """
2 ) Xylem vessels = These are more common, if not exclusive , in the Angiosperms .
But , some of the angiospermic plants have still retained the primitive feature of xylem tracheids . So they show both !!!
click on the link below to see a section of corn stem=
http://blog.lib.umn.edu/michaels/fall09courseguide/monocot%20stem%202.jpgThank you !
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