Sugar are made in the leaves using the water transported by the xylem vessels and the carbon dioxide which diffuses into the leaves from the air. These sugar are carried to every other part of the plant that needs them, such as the roots, buds and younger leaves, by the phloem tubes. The phloem tubes also carry other substances made by the plant cells such as the amino acids.
Unlike xylem (which is composed primarily of dead cells), the phloem is composed of still-living cells that transport sap. The sap is a water-based solution, but rich in sugars made by the photosynthetic areas. The phloem tube is made of cellulose and perforated end walls called sieve plates. It is permeable, and are surrounded by companion cells.
Together xylem and phloem form the vascular tissue, often also referred to as the vascular bundle. The two types of vessel are always found together, but they occupy slightly different locations in the root and the stem.
