Botanists are very familiar with the Cucurbita stem since it presents a clear demonstration of cell differentiation in the vascular system of a herbaceous plant. The phloem, a translocation system of the pumpkin plant, has sieve plates with large pores, sieve tube elements with companion cells, and phloem parenchyma cells (thin-walled, living photosynthetic and storage cells). The sieve tube cells, specialized for the conduction of metabolites and the products of photosynthesis, are either clear or contain a dark substance known as slime or p-protein. The xylem (support tissue) is composed of protoxylem and metaxylem. Pumpkin stems, however, are atypical for dicots since they have phloem on both sides of the vascular bundle instead of just outside. Most Cucurbita stems are hollow and pentangular.