Look at specimens of different branches and stems, from bamboo (an example of an endogen, which grows inwardly into a hollow stem) to sycamore or even logs for the fire (exogens, which add new tissue in the form of growth rings). This can help develop students' observational skills and introduce the properties of raw materials for hundreds of things we make with plants.
If logs are split in different planes, the arrangements of the tissues can be seen without a lens or microscope. Annual rings, showing the growth made in each year, can be counted too.
Pupils at KS34 could look at how the properties of different woods can be incorporated into investigations of the strength of materials.
Classification of trees can make useful extension activities for able children. Woods as different as ebony, oak and balsa provide a good range for work on floating and sinking - these examples could be used with other materials to study density at KS3.
At KS34 students could extend work on cell structure by pulling out the stringy xylem vessels (the more rigid tubes) from cabbage stalks, stale celery or Busy Lizzie. Get the students to put the stems in coloured water to show the movement of water through the xylem tubes.
At GCSE and A-level, explore taxonomic ideas of variation, genus and species in detail by looking at the range of plants in the rose family - raspberry and blackberry, rowan, whitebeam and mountain mahogany. How the distribution of tissues changes in progressively older stem-sections of specimens such as lime, sycamore or oak can be observed using prepared slides stained with lignin dyes like haematoxylin or gentian violet. These could be compared with the image reproduced here.
A good source of teaching materials can be found at the Kew Gardens website, www.kew.org.