Cells displaced towards the outside form Secondary Phloem. This converts a Eustele into an Ectophloic Siphonostele . We will study the process of secondary growth formation in the next lab. It is sufficient that you recognize the secondary tissues for this lab. The Xylem is composed entirely of Tracheids and Ray Parenchyma .
Tracheids are highly elongated cells with overlapping end walls .
They have thick, lignified walls that allows them to conduct water vertically , and provide structural support . The alternation of favorable and unfavorable environmental conditions causes changes in wall thickness and cell diameter. Narrow, thick-walled cells are produced during stressful periods like winter or dry seasons. Tracheids have large pits in their cell walls which are used to conduct water horizontally between cells and different regions of the plant. The pits are more concentrated on the end walls where tracheids overlap .