Someone mentioned xylem cells being mostly dead. that certainly could
have a lot to do with your problems. It seems that also HOW you
sterilize your equipment may also be important. If you are merely
autoclaving your glassware, it will not kill off all the RNAses.
Baking of glassware is essential in most RNA preps. Also in some
cases gloves won't do it either. Some need lab coat, hair net, etc.
Hope some of this helps