(sieve tube member) A relatively advanced vascular cell in the phloem of angiosperms, characterized by the presence of a * sieve plate , a nonlignified secondary cell wall, and a living enucleate protoplast. Sieve tube elements are joined together at their ends to form * sieve tubes , their end walls having become modified to form the sieve plates. Sieve tube elements usually have specialized parenchyma cells associated with them. These are known as * companion cells, the protoplast of the companion cell being connected to that of the sieve tube member by plasmodesmata. Compare sieve element , sieve cell. |
