<Li> in secondary xylem, laid down by a meristem called the vascular cambium in woody plants </Li> <Li> as part of a stelar arrangement not divided into bundles, as in many ferns . </Li> <P> In transitional stages of plants with secondary growth, the first two categories are not mutually exclusive, although usually a vascular bundle will contain primary xylem only . </P> <P> The branching pattern exhibited by xylem follows Murray's law . </P>

Describe the structure of xylem vessels and tracheids and relate this to their function