What are the roles of the following part of a plant:

Guard cells

Stoma

Xylem

Phloem

Guard cells becoming larger or smaller according to the pressure within the cells. stoma any of the minute pore in the epidemics or the leaf or stem of the plant. xylem the vascular tissue in plants that conducts water and dissolved nutrients. phloem is the vascular tissue that conducts sugars downward the leaves.